The Pennsylvania State University

The Graduate School

College of Earth and Mineral Sciences

SEDIMENTARY RECORD OF THE EVOLUTION OF THE

KUMANO FOREARC BASIN, OFFSHORE SOUTHWEST

JAPAN

A Thesis in

Geosciences

by

Yang Xu

© 2017 Yang Xu

Submitted in Partial Fulfillment

of the Requirements

for the Degree of

Master of Science

May 2017

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The thesis of Yang Xu was reviewed and approved* by the following:

Elizabeth A. Hajek

Assistant Professor of Geosciences

Thesis Co-Adviser

Demian M. Saffer

Professor of Geosciences

Associate Head for Graduate Programs and Research

Thesis Co-Adviser

Donald M. Fisher

Professor of Geosciences

*Signatures are on file in the Graduate School

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ABSTRACT

Forearc basins have been poorly understood due to their complex formation history and

the loading patterns that affect sediment dispersal and topography on active margins. Compared

to foreland basins and passive margins where the subsidence histories follow consistent patterns,

forearc basins exhibit a wide range of subsidence characteristics, owing to multiple driving

mechanisms such as sedimentary and tectonic loading. In addition, accretionary forearc basins

are particularly sensitive to changes in margin geometry related to wedge dynamics. Sequence

stratigraphic analyses of forearc basin fills can be used to constrain the tectonic and depositional

controls on basin evolution in this type of setting. Here, sequence stratigraphic analysis of a

high-resolution 3-D seismic volume in the Kumano forearc basin located offshore SW Japan was

used to decipher the relative timing and pattern of infill in the lower portion of the basin.

Sediments in this lower unit were deposited in a paleo-outer wedge setting and sit

stratigraphically above the present-day inner accretionary prism and below upper forearc

sediments. Stratal terminations were mapped to identify depositional patterns and unconformities

that signify major basin reorganizations. In-depth mapping of a lower drape-like sub-unit

revealed three distinct stages of downlap that each span ~0.5 Myr. Evidence from these stratal

patterns suggests that, in addition to the fill-and-spill model of ponded basin assemblages such as

those caused by salt tectonics in the Gulf of Mexico, episodic uplift and deformation of the

accretionary prism significantly affected early evolution of the Kumano Basin. This emphasizes

the interplay between the dynamic generation of accommodation in accretionary prism

depositional systems and sediment distribution, and contrasts with passive margins where

eustatic sea level change plays a more significant role.

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TABLE OF CONTENTS

LIST OF TABLES ................................................................................................................................. vi

LIST OF FIGURES .............................................................................................................................. vii

ACKNOWLEDGEMENTS ................................................................................................................... x

CHAPTER 1. INTRODUCTION .......................................................................................................... 1

OBJECTIVES ........................................................................................................................................... 1

OVERVIEW OF SEQUENCE STRATIGRAPHY .................................................................................. 3

BASIN MECHANICS AND ARCHITECTURE ..................................................................................... 6

CHAPTER 2. APPLICATION AND METHODS ............................................................................. 12

NANKAI REGION STUDY AREA....................................................................................................... 12

KUMANO FOREARC BASIN .............................................................................................................. 13

NanTroSEIZE KUMANO DATASET ................................................................................................... 16

Seismic volume ......................................................................................................................... 16

Lithology and biostratigraphy ................................................................................................... 17

Methods .................................................................................................................................... 19

CHAPTER 3. RESULTS ...................................................................................................................... 27

SEISMIC STRATIGRAPHY OF PRE-KUMANO BASIN PRISM DRAPING UNIT ......................... 27

FLATTENING OF HORIZONS ............................................................................................................ 31

WHEELER DIAGRAM ANALYSIS ..................................................................................................... 31

CHAPTER 4. DISCUSSION ................................................................................................................ 54

COMPARISON OF PKBPD UNIT WITH A MINIBASIN DEPOSITIONAL MODEL ...................... 54

EARLY KUMANO BASIN EVOLUTIONARY MODEL .................................................................... 56

PKBPD unit development ......................................................................................................... 56

Evolution of the upper Kumano Basin vs. the PKBPD unit ..................................................... 60

Sedimentation patterns in the Kumano Basin ........................................................................... 62

CHAPTER 5. CONCLUSIONS ........................................................................................................... 69

REFERENCES ...................................................................................................................................... 71

APPENDIX A ........................................................................................................................................ 78

ARC-TRENCH SYSTEMS .................................................................................................................... 78

SUMMARY OF IODP REPORTS ON LITHOLOGY AND WELL LOGS ......................................... 79

HORIZON COMPARISON TO PREVIOUS STUDIES ....................................................................... 82

APPENDIX B......................................................................................................................................... 88

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SEISMIC DATA INTERPRETATION INVENTORY.......................................................................... 88

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LIST OF TABLES

Table 1. Summary table of observed vs. theoretical features found in example foreland basins,

passive margins, and forearc basins around the world. 1 NW European Passive Margins (Shannon

et al., 2005); 2 Inventory of subsidence patterns from various margins worldwide (Xie and

Heller, 2009); 3 Quantitative modelling of passive margin deposition (Jervey, 1988); 4 Alpine

Foreland Basin, SE France (Joseph and Lomas, 2004); 5 Experimental modelling of stratigraphy

in a passive margin (Paola et al., 2001); 6 Tectonics in sedimentary basins (McCann & Saintot,

2003); 7 Kumano Basin lithologic core descriptions (Expedition 315 Scientists, 2009); 8

Modelling of a confined turbidite system in a forearc minibasin, NE Nankai Trough (Egawa et

al., 2013); 9 Thrace Forearc Basin, NE Greece (Maravelis et al., 2015); 10 Kumano Basin

(Ramirez et al., 2015); 11 Numerical modelling of sedimentation over growing subduction wedge

(Fuller et al., 2006); 12 Depositional elements in deep-water settings (Posamentier & Kolla,

2003); 13 Xigaze Forearc evolution and facies architecture (Einsele et al., 1994); 14 Foreland

basin systems (DeCelles & Giles, 1996); 15 Western Europe deep-water foreland basins (Covault

and Graham, 2008); 16 Modelling passive margin stratigraphy (Steckler et al., 1993). ............... 10 Table 2. Summary of drill holes with relevant data to this study. See Expedition 314 Scientists

(2009), Expedition 315 Scientists (2009), Expedition 316 Scientists (2009), and Expedition 319

Scientists (2010)............................................................................................................................ 81 Table 3. Description of horizons within this thesis and comparison to previously mapped

horizons by Gulick et al. (2010) and Ramirez et al. (2015). Bold horizons indicate common

horizons. ........................................................................................................................................ 87 Table 4. Inventory of horizons, faults, surfaces, seismic cross sections and thickness maps

generated in the Petrel project. ...................................................................................................... 90

Table 5. Well tops for Site C0002 and Site C0009. ..................................................................... 91

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LIST OF FIGURES

Figure 1. Regional map of the Nankai Trough and Kumano Basin located offshore Southwest

Honshu. Pink polygon delineates 3-D seismic coverage and green circles highlight wells drilled

at Site C0002 and C0009 used for lithologic interpretation. ........................................................ 21 Figure 2. Sub-regional bathymetric map of the NanTroSEIZE dataset seismic coverage with

IODP well locations. Yellow polygon outlines the Nankai 3-D volume. Red dots are Kumano

Basin drill sites. Blue dots are locations of wells trench-ward of the Kumano Basin. White line

shows location of inline 2520 with bolded portion as the 2-D extent depicted in Figure 6.

Modified from Moore et al. (2009) and Ramirez et al. (2015). .................................................... 22 Figure 3. Regional seismic transect through NanTroSEIZE 3-D volume showing morphotectonic

zones, large-scale structural elements of the Nankai arc-trench system, and well locations.

Modified from Moore et al. (2009). .............................................................................................. 23 Figure 4. Summary lithostratigraphic chart with nannofossil events from core cuttings,

interpreted logging units, and depths from Sites C0002 and C0009. Major surfaces are shown in

dashed lines. TAP = top of the accretionary prism. Modified from Expedition 315 Scientists

(2009); Expedition 319 Scientists (2010); and Ramirez et al. (2015). ......................................... 24 Figure 5. Representative inline and crossline through 3-D volume showing upper Kumano Basin

and PKBPD subunits LBU1, LBU2, and LBU3 as delineated in Ramirez et al. (2015). Dotted red

line indicates horizon TAP (top of accretionary prism), the boundary between the inner

accretionary prism and PKBPD sediments. .................................................................................. 25 Figure 6. Examples of stratal terminations observed in the PKBPD sediments in inline 2520.

Location of inline shown in Figure 2. A) Instance of growth strata. Note that reflector thickness

increases towards the depocentre of the minibasin. B) Examples of onlap, downlap, and

truncation. ..................................................................................................................................... 26

Figure 7. Isochore maps of lower basin units LBU1, LBU2, and LBU3 in stratigraphic order.

LBU1 is the thickest unit that first filled minibasin depocenters and reached the SE portion of the

basin. LBU2 draped LBU1 sediments and continued filling topographic lows. LBU3 is a

localized unit characterized by a distinct terrigenous input consisting of woody fragments and

lignite analyzed from core samples (Expedition 319 Scientists, 2010). ....................................... 34 Figure 8. Base map on TAP. Labeled lines correspond to interpreted cross sections throughout

Chapter 3 and Chapter 4. Line 1 and 2 cut along strike of the minibasin depocenters while Line 3

and 4 are perpendicular to structure. Line 5 indicates the well correlation section (refer to Figure

9). .................................................................................................................................................. 35 Figure 9. Well correlation cross section through Sites C0009 and C0002 with IODP logging

units. A) Major sub-units delineated by colored polygons. Dashed line represents the

unconformity separating the inner accretionary prism and Kumano Basin sediments. B) (Next

page) Interpreted seismic section through Site C0009 and C0002 including key horizons

throughout the PKBPD unit and biostratigraphy data from IODP reports. Note that all labeled

unconformities (PKBPD-1, PKBPD-2, and PKBPD-3) are time transgressive. Ages of strata

increase from Site C0009 to C0002. Fault interpretations are from Ramirez et al. (2015) and

Boston et al. (2016). ...................................................................................................................... 36 Figure 10. Arbitrary seismic cross section of arbitrary Line 1. Location is shown in Figure 8.

Line 1 is oriented along strike through depocentre D2. ................................................................ 38 Figure 11. Arbitrary Line 2 seismic cross section oriented along strike of depocenter D1. ........ 39 Figure 12. Arbitrary Line 3 seismic cross section oriented perpendicular to structure. .............. 40

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Figure 13. Arbitrary Line 4 seismic cross section oriented perpendicular to structure. .............. 41 Figure 14. Isopach map of section between unconformity PKBPD-1 and LB2-B. ..................... 42 Figure 15. Isopach map of section between horizon LB2-B and LB2-C. .................................... 43 Figure 16. Isopach map of section between LB2-C and PKBPD-2 unconformity. ..................... 44

Figure 17. Cross sections showing stratal relationships within LBU2, bounded by PKBPD-1 and

PKBPD-2. A) Zoomed in seismic section of Line 1 along structure D2. Black arrows show

apparent, progressive downlap onto PKBPD-1, LB2-B, and LB2-C from SW to NE. B) Zoomed

in cross section of Line 2 along structure D1. Downlap relationship is less apparent. Black

arrows onlap at either ends of depocentres between PKBPD-1 AND LB2-B and LB2-C.

Downlap direction between LB2-C and PKBPD-2 is from SW to NE. C) Inline 2300 showing

direction of apparent downlap from SE to NW. ........................................................................... 45 Figure 18. Line 2 flattened on PKBPD-1. Red arrows highlight stratal terminations. Toward the

NE, arrows terminate onto the same surface indicating the filling of topographic lows followed

by an expansion of accommodation away from the depocenter. .................................................. 46 Figure 19. Line 5 flattened on LB2-B. Strata are mostly concentrated in topographic lows and in

some areas step out from the SE to the NW. ................................................................................ 47 Figure 20. Line 1 flattened on LB2-C with stratal terminations on the flattened surface

highlighted by red arrows. ............................................................................................................ 48 Figure 21. Wheeler diagram of Line 5 (C0002 to C0009 well correlation). Colors correspond to

interpreted seismic units in figures from Chapter 3. Dashed lines delineate time-transgressive

unconformities and bold black lines indicate time horizons traced from wells. ........................... 49 Figure 22. Wheeler diagram of Line 1 cross section oriented along strike of structure. Colors

correspond to interpreted seismic sections in figures from Chapter 3. Dashed lines are time-

transgressive unconformities, and bold lines indicate time horizons traced from wells. ............. 50 Figure 23. Wheeler diagram of Line 2 oriented along strike of structure. Colors correspond to

interpreted seismic sections in figures from Chapter 3. Dashed lines are time-transgressive

unconformities, and bold lines indicate time horizons traced from wells. ................................... 51 Figure 24. Wheeler diagram of Line 3 oriented perpendicular to structure. Colors correspond to

interpreted seismic sections in figures from Chapter 3. Dashed lines are time-transgressive

unconformities, and bold lines indicate time horizons traced from wells. ................................... 52 Figure 25. Wheeler diagram of Line 4 oriented perpendicular to structure. Colors correspond to

interpreted seismic sections in figures from Chapter 3. Dashed lines are time-transgressive

unconformities, and bold lines indicate time horizons traced from wells. ................................... 53

Figure 26. Comparison of Kumano minibasin architecture with a minibasin model from

Sylvester et al. (2015). A) D2 zoomed in from Line 3 shows clear large-scale onlap termination

points at basin edges based on seismic interpretation. B) Minibasin model scenario of constant

sediment input with initial deep basin topography. The first panel shows the same basin-edge

onlap patterns as in (A) with the corresponding Wheeler diagram indicating the basin broadening

over time. ...................................................................................................................................... 65 Figure 27. Side-by-side comparison of GOM Brazos-Trinity Basin and a minibasin within the

KFB. (A) Brazos-Trinity Basin seismic reflection profile with GR log superimposed. Courtesy of

Sylvester et al. (2015). (B) interpreted Kumano Forearc minibasin exhibiting similar morphology

to (A). ............................................................................................................................................ 66 Figure 28. Early Kumano basin evolutionary schematic depicting stages of development from

~5.6 Ma until 0.9 Ma. Panels on the left side zoom in on PKBPD and upper KFB sediments.

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Panels on the right side show a zoomed out depiction of basin fill in a regional context from the

subduction zone. ........................................................................................................................... 67 Figure 29. Seismic cross section of inline 2620 depicting interpretation of sub-units in the upper

KFB. Bolded horizons labeled in red separate UB sub-units. Red arrows highlight onlapping

terminations onto major horizons to illustrate the internal geometries that build each wedge. .... 68 Figure 30: (A) Screenshot of inline 2529 to compare and contrast with horizons mapped in the

Gulick et al. (2010) study. Several horizons are equivalent to one another across the studies. For

example, UB-D in this study is equivalent to K4 and UB-A is equivalent to K6 in Gulick et al.

(2010). (B) Below: Inline 2529 from Gulick et al. (2010) study (Figure 6) showing interpreted

horizons in the upper Kumano Basin and normal faults ............................................................... 82 Figure 31. Comparison of crossline 6850 between horizons derived from biostratigraphy tops

and Gulick et al. (2010). (A) Crossline 6850 made in Kingdom Suites for comparison to Figure

11 in Gulick et al. (2010). Some horizons (labeled in red) are the same between both figures. For

example, UB-A, which was interpreted in this study as an onlap surface is equivalent to K6

horizon in (B). The dated horizon 0.905_9 is equivalent to K4. (B) Figure 11 in Gulick et al.

(2010). ........................................................................................................................................... 84 Figure 32: Comparison of inline 2532 from Figure 5 of Moore et al. (2015). All horizons from

Moore et al. (2015) were replicated from the Gulick et al. (2010) study. Many of the horizons in

this study (solid lines in (B)) do not match the horizons from Moore et al. (2015) and Gulick et

al. (2010) in (A), which means their studies did not rely primarily on biostratigraphy to choose

horizons. ........................................................................................................................................ 85

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ACKNOWLEDGEMENTS

I would like to express my sincere gratitude to my advisor, Dr. Elizabeth Hajek, for

guiding me throughout my years as a graduate student, for providing astute insight into our

meetings regardless of subject matter, and for her unwavering support. A special thanks to my

co-advisor, Demian Saffer, for his assistance from the very beginning and willingness to impart

his wealth of knowledge on the Nankai region. Without him, I would not have had the chance to

work with such a rich suite of data.

I would also like to extend my gratitude to IHS and Schlumberger for providing Kingdom

Suites and Petrel, respectively, as the primary seismic interpretation tools for this thesis and for

their easy- access online support. Without it, I would have spent numerous days troubleshooting

in the latest hours of the night.

This research was supported by the generous grants from Shell and Chesapeake Energy

Corporation through the Shell Geosciences Energy Research Facilitation Award and the

Chesapeake Energy Scholarship, respectively. Last, but not least, I extend many thanks to the

Department of Geosciences at The Pennsylvania State University for giving all the moral support

and resources I could ever need.

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CHAPTER 1. INTRODUCTION

OBJECTIVES

Sedimentary basin analysis is used to interpret the evolution and depositional history of

basins and reveal the underlying mechanisms that control basin architecture. Basin analysis

studies often use sequence stratigraphy as a tool to a) determine the presence and extent of

hydrocarbons through petroleum exploration and b) better predict facies architecture through the

investigation of stratal geometries. Sequence stratigraphy is most commonly applied in passive

margin, foreland basin, and rift basin settings. In contrast, few studies have investigated

sequence stratigraphy in deepwater forearc basins. Applying sequence stratigraphy in these

settings is not straightforward in part because tectonic and depositional controls that directly

influence stratigraphy in passive margins or foreland basins (e.g., eustatic sea level or sediment

supply to a coastline) are not as tightly coupled to the transport and storage of sediments in

deepwater active margins.

Subsidence and sedimentation mechanisms in forearc basins differ from the processes

that control passive margins and foreland basins where, for example, subsidence on passive

margins is primarily follows seafloor cooling trends and subsidence in foreland basin systems

show distinct episodes of subsidence driven by thrust-sheet loading (e.g. Xie and Heller, 2009;

Ingersoll, 2012). The subsidence mechanism of forearc basins are especially difficult to constrain

due to the lack of quantitative paleobathymetric controls in the rock record and the challenge of

establishing the isostatic balance of masses within the dynamic arc-trench system, arc massif,

and the subducting oceanic lithosphere (e.g. Dickinson, 1995). Previous studies have indicated

that forearc basin subsidence is primarily driven by a combination of sedimentary loading,

tectonic loading, and thermal re-equilibration of the arc massif (e.g. Dickinson, 1995; Ingersoll,

2012). Sedimentation in active margins varies widely depending on the interplay of sediment

supply, sediment routing mechanisms, and proximity to sediment sources with variable

preservation potential (e.g. Ingersoll, 2012; Noda, 2016). For these reasons, uniquely identifying

the main controls on stratigraphic architecture in forearc basins can be challenging. However,

sequence-stratigraphic analyses of forearc basins can help answer a range of important

outstanding questions : 1) Is there evidence in the sedimentary record that alludes to allocyclic

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forcings on early forearc basin development both spatially and temporally? 2) What spatial

scales of sedimentation mechanisms can we deduce from the basin fill? 3) How applicable is

sequence stratigraphy in analyzing an active margin setting where the effects of tectonics and

sedimentation are often confounded?

In order to address these questions and to understand how sediments record regional

tectonic activity and the early stages of forearc basin evolution, I conducted a detailed sequence

stratigraphic analysis of the Kumano Forearc Basin (KFB) utilizing a high-resolution 3-D dataset

from the Integrated Ocean Drilling Program. The unique architecture of this well-studied forearc

basin provides an excellent example of the complex interactions between tectonic forcing and

sedimentation, which both exerted influence in the basin’s nascent stages of formation (e.g.

Gulick et al., 2010; Buchs et al., 2015; Moore et al., 2015; Ramirez et al., 2015). Much of the

previous work in the basin has focused on the structural framework of the KFB and influences

from large-scale tectonic driving mechanisms – mainly a transient megasplay fault that has been

active since the Quaternary (e.g. Park et al., 2002; Moore et al., 2007; Bangs et al., 2009;

Strasser et al., 2009; Gulick et al., 2010; Kimura et al., 2011). Apart from previous studies that

focus on the KFB strata, this thesis provides a finer-scaled analysis of lower forearc architecture

using a modified, process-based approach to sequence stratigraphy and seismic stratigraphic

frameworks established by Ramirez et al. (2015) and Gulick et al. (2010). Specifically, in-depth

seismic stratigraphic interpretations of a thin paleo-slope unit in the present-day lower forearc

basin reveal that distinct phases of tectonic forcings possibly acted on the region over million-

year timescales during early basin development. These new seismic interpretations show that

relatively thin stratigraphic units in this particular basin setting can represent large timescales

that signify important tectonic reconfigurations. Results from seismic stratigraphy are used to

compare with a simplified minibasin model from the Gulf of Mexico (GOM) and analyses from

previous studies of the region to decipher how the fundamental controls on the formation of the

KFB is recorded. In addition, findings from this study point to not only the advantages of

applying sequence stratigraphy commonly used in passive margins to complicated forearc

systems but also the need to use the geometric relationships of strata as a first order, qualitative

method for deciphering how tectonic processes and sedimentation mechanisms manifest in the

rock record.

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OVERVIEW OF SEQUENCE STRATIGRAPHY

Since the mid 1970’s, the field of sequence stratigraphy has been a primary tool for

geologists to analyze the depositional history and facies distributions of the sedimentary record

(Vail et al., 1977; Posamentier and Vail, 1988; Van Wagoner et al., 1988). From its conception

in the 1960’s on the basis of detailed observations of stratal relationships in the cratonic interior

of North America (Sloss, 1963), sequence stratigraphy has been evolving through studies that

integrate field data, seismic data, and well logs (e.g. Van Wagoner et al., 1988; Van Wagoner,

1995; Hart, 1999). Sequence stratigraphy uses a chronostratigraphic framework to study the

relationship between what is observed in the rock record and the physical processes and driving

mechanisms that produce those observations (e.g. Jervey, 1988; Van Wagoner et al., 1988;

Catuneanu, 2006). Multiple schools of thought have evolved over time, each developing its own

method for categorizing depositional packages, hierarchies, and surfaces (Van Wagoner, 1995;

Catuneanu et al., 2009). Despite the exhaustive list of terminology in the field of sequence

stratigraphy, there is still no present-day consensus on precise definitions. However, there is

general agreement that the positions and geometries of rock bodies in relation to one another are

tied to physical driving mechanisms (e.g. Catuneanu, 2006). Sequence stratigraphy essentially

provides the framework to analyze the present-day depositional regime and serves as a tool to

address temporal and spatial variations throughout geologic history (e.g. Van Wagoner et al.,

1988; Catuneanu, 2006).

Theoretically, the process by which sediment is deposited and preserved in the

sedimentary rock record at any given margin hinges on two primary factors: sediment supply and

accommodation (Vail et al., 1977; Jervey, 1988; Van Wagoner et al., 1988; Catuneanu, 2006;

Steckler et al., 1993; Catuneanu et al., 2011). Variations in sediment supply can depend on a

plethora of factors ranging from the fluvial drainage basin area, proximity to hinterland source,

and rate of physical denudation of the landscape (e.g. Catuneanu, 2006). Depending on the

environment of deposition, the sediments can then be mobilized and reworked by forces such as

fairweather waves and storm surges in shallow-water settings or deep ocean currents and mass

transport mechanisms in deepwater clastic systems (Catuneanu, 2006; Catuneanu et al., 2009). In

order for sediment to be deposited and preserved, there has to be space available for it (e.g. Coe

and Church, 2003). Accommodation refers to the amount of physical space present for sediments

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to accumulate (Jervey, 1988; Steckler et al., 1993; Coe and Church, 2003; McCann and Saintot,

2003). In shallow marine environments like the continental shelf, accommodation is particularly

sensitive to relative sea level variations (e.g. Coe and Church, 2003; Catuneanu, 2006, 2011) but

is theoretically inexhaustible in the open ocean because there is always space in the water

column for sediments to accumulate. Since accommodation in shallow marine settings is

governed by changes in relative sea level, it implies that eustatic sea level fluctuations, tectonic

activity and subsidence all contribute to changes in the creation of space (e.g. Posamentier and

Kolla, 2003). As they act independently, eustasy and tectonic processes add complexity to

stratigraphic interpretations (McCann and Saintot, 2003; Catuneanu, 2006), especially in

tectonically active, deepwater basins where the effects of one driving mechanism is often

obscured by the other (Van Wagoner, 1995; Shannon et al., 2005). This frequently leads to

disparities between depositional units and facies along strike and produces diachronous

boundaries with limited lateral extent (Shannon et al., 2005; Catuneanu, 2006; Catuneanu et al.,

2011).

A typical example of the piecemeal stratigraphic record is the temporal and spatial

discontinuity between shallow-water sediments and their deepwater counterparts (e.g. Jervey,

1988; Catuneanu, 2006). Instead of accommodation being the limiting factor, diminished

sediment supply in the deepwater leads to incomplete stratigraphic records compared to their

shelfal analogs. Sediments deposited close to the source at the continental shelf do not reach the

open waters due to the loss in kinetic energy from friction and gravity (Jolliffe, 1978). Basin-

ward, this translates to either non-deposition or the formation of condensed sections consisting of

hemipelagic and pelagic sediments that represent very low sedimentation rates (e.g. Loutit et al.,

1988; Catuneanu et al., 2011). Condensed sections in the basin may equate to an entire sequence

deposited over hundreds of thousands of years at the continental shelf and typically result from

transgressions or a landward shift of the depocenter that starves the basin plain of coarse-grained

sediment (e.g. Loutit et al., 1988). Even in periods of high sediment flux, transport to the deep-

water environment can still be extremely limited to only the finest sediments (Catuneanu et al.,

2011). This often poses a problem for the correlation of facies and bounding surfaces. Another

challenge is that mass transport processes and gravity flows often dominate deepwater settings,

which result in variable lateral stacking rather than a representative vertical stratigraphic record

(Catuneanu et al., 2011). Sediments that are sequestered at the continental shelf-margin may not

5

get transported to the basin until slope failure occurs with subsequent sea level falls that are

sufficient to partially expose the shelf, erode the shoreline, and carry eroded sediment to the

distal basin plain (e.g. Posamentier and Kolla, 2003).

Tectonism can also produce vertical stacking patterns unrelated to depositional processes

occurring at the shelf margin (e.g. Catuneanu et al., 2011), which is exhibited in several basins

surrounding the Japanese Island Arc (e.g. Ingersoll, 2012). A number of forearc and back-arc

basins in the NE Japan Arc and SW Nankai Trough region are predominantly filled by fine-

grained turbidites, hemipelagites, and pelagic sediment with compelling evidence showing

tectonic influence on stratigraphy (e.g. Stow and Tabrez, 1998; Expedition 315 Scientists, 2009;

Expedition 316 Scientists, 2009; Expedition 319 Scientists, 2010; Egawa et al., 2013; Tokano et

al., 2013). Forearc basin fill located along the NE Japan Arc shows facies progressions that

indicate a confined forearc setting controlled by the evolution of the trench slope break, which

acted as a topographic barrier and regulated sediment routing patterns throughout the basin’s

evolution (Tokano et al., 2013). Another study conducted by von Huene and Arthur (1981) along

the Japan Trench off northern Honshu Island concluded that vertical tectonism and relative sea

level changes heavily influenced the Pliocene and Pleistocene strata along the trench-to-forearc

transect, causing local erosion and redistribution of large amounts of sediment across the basin.

Just northeast of the Kumano study area, Pleistocene Tokai-oki-Kumano-nada forearc basins are

dominated by submarine fan turbidite systems that are thought to be deposited as a result of

tectonic evolution of the margin and variations in sediment supply based on distinct stages of

basin configurations (Tokano et al., 2013). The deposits in many of these offshore basins

surrounding the Japanese Island Arc, including the KFB, indicate basin formation below sea

level throughout most of their development, with the exception of the Sorachi and Yubari

subbasins in NE Japan Arc that contain fluvial and lacustrine deposits (Takano et al., 2013) and

the Japan Trench, which shows evidence of subaerial exposure of the outer forearc area in its

early evolutionary phase (von Huene and Arthur, 1981).

To partially resolve the lateral incongruity of strata in active margins, Loutit et al. (1988)

demonstrated that biostratigraphic dating with planktonic-microfossils, combined with seismic,

outcrop, and well log data, serve as powerful tools in stratigraphically linking shallow and deep-

water sediments. As we demonstrate in this study, seismic stratigraphic analysis frequently relies

on biostratigraphy to validate well correlations, build the depositional history of a system, and

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constrain the timing of stratigraphic events (Coe and Church, 2003; Expedition 315 Scientists,

2009; Expedition 316 Scientists, 2009; Expedition 319 Scientists, 2010).

A majority of these sequence stratigraphic concepts is taken from observations in passive

margins where the relationship between sea level change and sedimentation is more apparent,

and tectonic effects on depositional sequences are subdued (e.g. Van Wagoner, 1995). Concepts

such as well-defined hierarchies of sedimentary packages bounded by surfaces of erosion or non-

deposition (Vail et al., 1977; Van Wagoner et al., 1988; Van Wagoner, 1995; Catuneanu, 2011)

tend to be inadequate when applied to forearc basins in subduction zones because deepwater

active margins are the sites where the lateral discontinuity of depositional sequences compound

with tectonics.

BASIN MECHANICS AND ARCHITECTURE

The ultimate challenge for stratigraphers is using the rock record to differentiate between

autocyclic and allocyclic influences since both factors operate on different spatiotemporal scales

in sedimentary basins. Autogenic processes control energy redistribution within a sedimentary

system and can include delta lobe shifting, channel avulsion, and stream meandering (e.g.

Hampson, 2016). Allogenic controls act externally on the sedimentary system, such as eustatic

sea level fluctuations, tectonics, and compactional subsidence (e.g. Hampson, 2016). Hence, a

significant amount of research has focused on modelling basin architecture at different scales to

better understand the fundamentals of these feedback mechanisms that produce the observed

stratal geometries (e.g. Heller et al., 1988; Jervey, 1988; Steckler et al., 2003; Paola and Martin,

2012; Sylvester et al., 2015). Paola (2000) comprehensively outlined the progress of quantitative

basin modelling over its decades of development and emphasized that the ultimate goal of

modelling is to connect qualitative observations with the quantitatively measurable processes and

boundary conditions that we believe are acting on the basin. Sloss provided an early framework

for the modern stratigraphic model in 1962 in a conceptual rather than quantitative manner by

outlining the main variables that shape the architecture of the stratigraphic record (Paola, 2000).

His simplified take on basin modelling represents a generalized view of the widespread causes of

sedimentary signals that modern models reflect. For example, the simple yet flexible models

provided in the works of Jervey (1988), Steckler et al. (1993), Paola and Martin (2012), and

7

Sylvester et al. (2015) highlight the importance of numerically integrating only the fundamental

factors that control sequence formation: sediment supply and accommodation (which can

encompass eustasy, tectonics, and thermal subsidence) and are both controlled by autocyclic and

allocyclic forcings. These aforementioned studies aimed to predict facies distribution and stratal

architecture by numerically varying boundary condition parameters, namely sea level,

subsidence, and sediment flux (Jervey, 1988; Steckler et al., 1993; Paola and Martin, 2012;

Sylvester et al., 2015). My thesis specifically compares Kumano Basin stratigraphy with the

models provided in Sylvester et al. (2015), which produced distinct stratal architectures of

minibasin fill by varying only subsidence and sediment supply to mimic minibasin strata in

deepwater GOM. Since accommodation in the deepwater environment is indirectly influenced by

sea level changes, the unfilled basin volume below the basin spillpoint was treated as the

available accommodation (Sylvester et al., 2015).

One way to simplify basin modelling is to apply it to passive margins (Jervey, 1988; Van

Wagoner, 1995; Paola and Martin, 2012) where the tectonic driving mechanisms are more

predictable due to their characteristic subsidence curves accompanying basin evolution: initial

rapid synrift subsidence followed by slow post-rift thermal subsidence (Xie and Heller, 2009).

Unlike foreland and forearc basins, passive margins typically form in tectonically quiescent

transition zones between continental and oceanic lithosphere. Table 1 summarizes some of the

key features observed in passive margins along with characteristics of other basins worldwide. In

a conventional passive margin example, sediment supply is primarily driven by hinterland

erosion and transport of sediment via fluvial drainage systems and aerial processes. These

deposits may form fundamental stratal units or depositional sequences that are bounded by

unconformities and their correlative conformities further out in the basin.

In foreland basins, similar depositional conditions control sediment input into the basin

but with the added complexity of tectonic loading from contractional orogenic belts formed by

the collision of plates (Xie and Heller, 2006) (Table 1). As discussed below, accommodation in

foreland basins is more dynamic. Passive, vertical isostatic response and active tectonic response

from enhanced exhumation of adjacent orogenies compound to affect the overall subsidence

history of foreland systems (e.g. Miall, 1995; DeCelles and Giles, 1996; Willett, 2010). In

addition, the temporal evolution of the thrust belt produces a time lag associated with the

propagation of the sediment load from orogenic buildup on the order of a few million years

8

across the foreland basin (Xie and Heller, 2009). Classic foreland basins have identifiable

morphotectonic zones that manifest from progressive stages of thrust belt evolution (e.g.

DeCelles and Giles, 1996). Decelles and Giles (1996) defined wedge-top, foredeep, forebulge,

and back-bulge as the four distinct depozones of foreland basin systems. The asymmetric

geometry of foreland basins arises from disproportionate flexural subsidence closest to the thrust

belt (the wedge-top) followed by decreasing sediment load moving further away from the

orogenic wedge. Sediment sources originate from the orogenic belt and may be transported into

the basin aerially or sub-aerially. Depositional facies vary across the entire foreland system and

heavily depend on proximity to the migrating fold and thrust belt in addition to the mode of

sediment dispersal, which is broadly classified into transverse and axial flow (Miall, 1995).

Forearc basins, previously coined “outer-arc basins” and “midslope basins”, lie between

the trench and the parallel magmatic arc within the arc-trench system (Dickinson, 1995).

Consequently, forearc basins are products of dynamic interactions within convergent plate

boundaries and play a crucial role in recording the history of early margin evolution in their basal

fill (Dickinson, 1995; Noda, 2016). The mode of sediment transport varies from margin to

margin, but hemipelagic sedimentation, submarine transport mechanisms, and recycling of

accretionary prism sediments can all be found to some extent within the stratigraphic succession

of forearc basins (Table 1).

Forearc basins also exhibit high variability in subsidence, which is largely controlled by

the interaction of several driving mechanisms (Xie and Heller, 2009). Dickinson and Seely

(1979) recognize four types of forearc basins that lie between the island arc and subduction zone:

a) intramassif basins that lie unconformably on basement terranes of the arc massif, which

includes the entire volcanic sequence, underlying plutons and associated metamorphic country

rock; b) residual basins that sit on top of oceanic or transitional crust; c) accretionary basins that

lie above accreted sediment scraped off by the overriding plate; and d) hybrid basins which

include a combination of characteristics from the basins defined previously. On the basis of

filling conditions and morphology, Dickinson further classified forearc basins into eight different

types depending on whether the basin is overfilled or underfilled and whether the configuration

of the basin is sloped, ridged/terraced, ridged/shelved or ridged/benched (Dickinson, 1995).

These academic classifications are more encompassing and complicated than the distinctions

9

recognized in the field of hydrocarbon exploration and therefore reflect the uncertainty in

categorizing forearc basins generally (McCann and Saintot, 2003).

Evidently through Dickinson’s classification scheme, forearc basins encompass a wide

range of morphological characteristics that reflect the dynamic interaction between

sedimentation and the evolution of the accretionary prism (e.g. Fuller et al., 2006). The

interchange between the actively deforming outer wedge and less deformed inner wedge of

accretionary prisms heavily influences the seismicity of the arc-trench system as well its own

evolutionary configuration (e.g. Wang and Hu, 2006). Linkage between basin-centered

asperities, or coseismic slip, and the position of basins suggests that forearc subsidence may be at

least partly responsible for focusing slip (Song and Simons, 2003; Wells et al., 2003). Fuller et

al. (2006) proposed an interesting example of the feedback between forearc basin sedimentation

and its stabilizing effect on the accretionary wedge through numerical models that demonstrated

how sedimentary loading hindered internal deformation below the forearc. A combination of

processes may lead to wedge stability, but the takeaway is that seismic coupling is correlated

with forearc basins and the geometry of the wedge taper (Fuller et al., 2006). The Nankai

subduction zone is a seismogenic region that has been the focus of such studies over the past

several years, and the KFB provides one of the keys to understanding the mechanisms that

govern its development.

10

OBSERVED FEATURES

FORELAND PASSIVE MARGIN FOREARC

Internal

stratigraphic

architecture

General coarsening upwards succession recording

initial marine units that progressively shallow

upward into non-marine continental deposits6

Apenninic foredeep – erosional unconformities

with packages of thick turbidites up to hundreds of

meters thick in wedge top depozone15

Transition from deep-marine sedimentation to

shallow-marine reflecting origin from oceanic

trench to continental crust emergence14

Hierarchical successions of

strata bound by regional

unconformities1

Mass transport complexes

occurring at the toes of

prograding packages that

intersperse with basinal strata1

Progradational clastic wedges1

Commonly underfilled and

sediment-starved in deep-water

setting1

Kumano Basin – broadly separates into 2 units: lower

forearc and upper forearc10 (Figure 5)

Thrace Basin – divided into older shallow-marine units

and younger sand-rich submarine fan deposits; mature,

two-sided forearc succession with input from trenchward

prism9

Xigaze Basin – Fining upward megasequence comprised

of upper submarine fan, middle fan and outer fan deposits13

Lithologies/

Basin-fill

succession

Alpine Basin –combination of shallow marine

sediments, hemipelagic mudstones with carbonate

input, and gravity flow deposits4

Apenninic foredeep – coarse grained sand and

gravel within turbidite successions that fine

upward; shelf sediments15

Variable successions of fluidized

flows (i.e. debris flows and

turbidites), hemipelagic and

contouritic marine muds1,6

Kumano Basin– distal sedimentation of mainly

hemipelagic mud; intervals of glauconite, rare volcanic ash

beds; thin, silty turbidites; and mass transport complexes7

Thrace Basin – marine and submarine fan systems from

eroded Rhodopian arc comprised of 3 facies: sandstone

with minor mud, sandstone interbedded with mud, and

mudstone9

Xigaze Basin – conglomerates at the base of sequences,

volcaniclastic sandstones of varying grain size, and

hemipelagic calcareous marls capping sequences13

Basin

architecture

Elongate depressions with four possible distinct

depozones: wedge top, foredeep, forebulge, and

backbulge14

Basin fill hundreds of meters to a few km thick15

Sediments in wedge top zone show more

deformation, synorogenic deposition, and

unconformities14,15

Wedge top zones extend tens of km in length

parallel to transport14

NW European margins–

Underfilled with steep basin

margin slopes due to rapid initial

subsidence1

50-250 km in width and 50-500 km in length6

Sediment thickness ranges from 1-10 km 6

Nankai Trough basins – minibasin architecture and

ponded basins as basal unit above accretionary prism8

Kumano Basin – underfilled, ridged basin separated into

lower minibasin unit draped by slope fill and an upper,

landward tilting wedge ~1 km thick7,10

Xigaze Forearc – long and narrow synclinorium with 5

km of flysch sequence basin fill13

Table 1. Summary table of observed vs. theoretical features found in example foreland basins, passive margins, and forearc basins around the world. 1 NW European Passive Margins

(Shannon et al., 2005); 2 Inventory of subsidence patterns from various margins worldwide (Xie and Heller, 2009); 3 Quantitative modelling of passive margin deposition (Jervey,

1988); 4 Alpine Foreland Basin, SE France (Joseph and Lomas, 2004); 5 Experimental modelling of stratigraphy in a passive margin (Paola et al., 2001); 6 Tectonics in sedimentary

basins (McCann & Saintot, 2003); 7 Kumano Basin lithologic core descriptions (Expedition 315 Scientists, 2009); 8 Modelling of a confined turbidite system in a forearc minibasin,

NE Nankai Trough (Egawa et al., 2013); 9 Thrace Forearc Basin, NE Greece (Maravelis et al., 2015); 10 Kumano Basin (Ramirez et al., 2015); 11 Numerical modelling of

sedimentation over growing subduction wedge (Fuller et al., 2006); 12 Depositional elements in deep-water settings (Posamentier & Kolla, 2003); 13 Xigaze Forearc evolution and

facies architecture (Einsele et al., 1994); 14 Foreland basin systems (DeCelles & Giles, 1996); 15 Western Europe deep-water foreland basins (Covault and Graham, 2008); 16

Modelling passive margin stratigraphy (Steckler et al., 1993).

11

THEORETICAL/INTERPRETED FEATURES

FORELAND PASSIVE MARGIN FOREARC

Subsidence patterns

Steep, convex-up profile with

intermittent kinks representing time-

transgressive reactivation of thrust load

from orogenic belt; highly dependent

on orogenic growth adjacent to basin2

Initial rapid subsidence during syn-rift phase

followed by reduced rate of subsidence in the

post-rift phase, eventually mimicking

subsidence of the seafloor2

No distinctive subsidence trends and poorly

constrained2

Variable shapes of curves reflect complex

driving mechanisms2

Background subsidence driven by isostatic

response to emplacement of dense oceanic

crust subducting beneath forearc region and

sediment load within depocenter13

Mechanism of sediment

dispersal/

Source area

Asymmetric sediment supply from a

few point sources that may coalesce to

form a pseudo-line source from

orogenic belt6

Aerial erosion6

Transit from subaerial hinterland

source to deep-water canyon-channel

systems14,15

Sediment gravity flows14,15

Submarine fan and canyon systems driven by

gravity in deep-water (vs. deltaic and fluvial

fans in coastal, shallow-water setting)6

Deep-water current circulation 1

Hyperpycnal flows12

Gravity-driven flows from transverse and

axial submarine canyon systems fed from

forearc high6,8,9,10,13

Hyperpycnal flows12

Basin architecture from

numerical modelling

Minibasin fill-and-spill sedimentation

with complex 3-D sediment dispersal

over temporal variations throughout

tectonic evolution4

Prograding clastic wedge with clinoforms

dipping basinward (refer to Jervey, 1988 for

model parameters)3

Well-defined systems tracts and

unconformities in a simple bowl-shaped

model with subsidence increasing at

depocenter5

Basin geometry most influenced by sea level

changes but sediment supply and subsidence

rate can produce similar geometries16

Confined, bowl-shaped minibasin

recovered from structural unfolding and

backstripping the sediment load8

Presence of negative-α basins (i.e. sediment

infilling depressions caused by deformation

of the subduction wedge) stabilizes the

critical taper leading to stable underlying

wedge 11

Table 1 (continued). Theoretical features observed in foreland basins, passive margin and forearc basins. Refer to previous page for sources.

12

CHAPTER 2. APPLICATION AND METHODS

NANKAI REGION STUDY AREA

The Nankai Trough, located offshore Japan south of the island of Honshu, Japan, has

been the focus of several studies due to the seismogenic nature of the region. The Philippine Sea

Plate is currently subducting beneath the Eurasian Plate at a rate of 4 – 6.5 cm/yr and at an

azimuth of ~300-315 degrees (Seno et al., 1993; Miyazaki and Heki, 2001) (Figure 1). This

present-day plate configuration initiated between 10 to 4 Ma (Seno and Maruyama, 1984) with

the modern Nankai Trough subduction zone existing at least since 7.5 Ma (Fergusson, 2003).

Sediments of the fan-shaped Shikoku Basin, which formed by back-arc spreading behind the Izu-

Bonin arc, are subducted below the Eurasian Plate and accreted landward of the Nankai Trough.

Initial spreading began in the Oligocene and continued until 15 Ma (Okino et al., 1994). The

Japanese island arcs are comprised of four segments: the western Kuril, Honshu, Ryukyu, and

Izu-Bonin (Taira, 2001). The Nankai Trough subduction zone is part of the accretionary arc-

trench system in the Honshu segment where sediment from the subducting plate is actively

accreting onto the overriding plate as opposed to an erosive margin, which is defined as a

trenchward migration of a fixed point along the forearc due to tectonic erosion (Clift and

Vannucchi, 2004). The development of this accretionary arc-trench system has produced

characteristic morphotectonic zones in which several wells were drilled to assess the structural,

lithostratigraphic, biostratigraphic, and logging while drilling data (Expedition 314 Scientists,

2009; Expedition 315 Scientists, 2009; Expedition 316 Scientists, 2009; Moore et al., 2009;

Expedition 319 Scientists, 2010) (Figure 3 and Table 2 in Appendix A). Sediment delivery to the

trough is relatively high, owing to active collision between the Honshu Arc and the Izu-Bonin

Arc (Underwood et al., 2003). Most of the sediment from the continental shelf gets funneled

through submarine canyons like the Suruga Canyon and Tenryu Canyon (Buchs et al., 2015).

A large out-of-sequence thrust (OOST) branches from the active décollement at ~10 km

depth below the seafloor, as observed in the bottom panel of Figure 3 (Park et al., 2002; Moore

et al., 2007; Moore et al., 2009; Underwood and Moore, 2012). 2-D seismic lines in the region

show that this fault, termed the “megasplay,” extends 100 km along strike, corresponding to

underthrusted sediment that covers an area of 33 x 100 km2 (Bangs et al., 2009). Wells C0004C

13

and C0004D surrounding this megasplay fault penetrate the slope apron sediments, the upper

accretionary prism, and the underthrust slope facies that represent lower trench-slope

environment at ~400 mbsf (Expedition 316 Scientists, 2009) (Figure 2 and 3, Table 2). A

prominent bathymetric ridge that extends greater than 120 km along strike of the trough

characterizes the shallowest, updip portion of the megasplay (Park et al., 2002), coined by Moore

et al. (2009) as the Kumano basin edge fault zone (KBEFZ) (Figure 2 and 3). Slip along the

splay fault may have thrusted the seaward tip of the forearc basin upward, creating the outer

ridge (Park et al., 2002; Moore et al., 2007; Bangs et al., 2009). The topography of the outer

ridge also suggests that slip along the splay fault has been a repeating occurrence (Park et al.,

2002). Interpretations of 3-D seismic data and well data from Site C0004 and C0008 of shallow

fault systems in the slope fill by Kimura et al. (2011) date the first signs of megasplay activity to

manifest around 2 Ma.

KUMANO FOREARC BASIN

Directly above the megasplay branch lies the Kumano Forearc Basin, the primary area of

focus for this study which builds upon the works of numerous scientists who have made efforts

to understand the development of the forearc within the context of the Nankai trough

seismogenic zone. If we apply the forearc basin classification scheme from Dickinson (1995) to

the Nankai margin, the present-day KFB is morphologically considered a submerged ridged, and

underfilled forearc. The basin sits unconformably above a thick, late Miocene accretionary

complex that has been significantly shortened during the Pleistocene (Ashi et al., 2009; Tobin et

al., 2009). This shortening contrasts with the extensional regime dominating the younger 1 km

thick Quaternary upper forearc sediments, which are populated with normal faults (e.g. Tobin et

al., 2009; Gulick et al., 2010; Sacks et al., 2013). Structural analysis and seismic interpretation

also exhibit evidence of extension in the lower forearc and the drilled segment of the inner

accretionary prism (Ashi et al., 2009; Tobin et al., 2009). The most up-dip portion of the

megasplay fault coincides with the KBEFZ and marks the trench-ward border of the KFB, ~50

km landward of the trench (Moore et al., 2007).

14

Sediments of the KFB are broadly distinguished as upper and lower forearc units

separated by a regional time-transgressive unconformity onto which strata appear to downlap

(Expedition 315 Scientists, 2009; Expedition 319 Scientists, 2010; Ramirez et al., 2015) (Figure

5). Site C0002 located trenchward and Site C0009 located further landward of C0002 were both

drilled down into the upper accretionary prism within the KFB with a coring TD of 1052 mbsf in

hole 315-C0002B and 1604 mbsf in 319-C0009A (Figure 2, 3, and 4). The origin of the lower

sediment packages below this regional unconformity characterized by downlap is still a subject

of debate, but is inferred to be early forearc basin sediment or slope sediments deposited out on

the paleo-outer accretionary wedge (Expedition 314 Scientists, 2009; Expedition 315 Scientists,

2009; Moore et al., 2015). Studies such as Moore et al. (2015) refer to the lower forearc units as

“slope sediments” because these packages are interpreted to be slope apron deposits on the paleo

outer wedge before slip on the megasplay fault initiated ~2 Ma (Expedition 319 Scientists,

2010). To distinguish between the present-day slope sediments resting unconformably above the

outer accretionary wedge, we refer to the entire lower forearc basin sitting on top of the modern

inner accretionary wedge as the pre-Kumano-Basin-prism-draping (PKBPD) unit hereafter to

systematically distinguish it from the present day slope sediments.

The upper KFB comprises of a series of arcward-tilting packages that Gulick et al. (2010)

have attributed to late Quaternary (1.3-1 Ma) landward tilting from a major fault propagation

event along the megasplay. These packages downlap in the arcward direction onto the regional

unconformity and either onlap or pinch out updip at the trenchward side as a result of the

depocenter shifting progressively from SE to NW (Gulick et al., 2010; Moore et al., 2015)

(Figure 5). A major shift and expansion of the basin depocenter occurred in the Middle

Pleistocene in response to progressive arcward tilting of the forearc (Gulick et al., 2010; Moore

et al., 2015). Because of the arcward tilt, upper KFB sediments at C0002 are older than their

correlative packages at C0009 (Expedition 319 Scientists, 2010; Moore et al., 2015). Sediments

in the upper Kumano basin generally contain a higher fraction of silty turbidites and coarser

grained material than the lower forearc basin fill (Expedition 315 Scientists, 2009; Expedition

319 Scientists, 2010). Detailed logging while drilling data from Expedition 314 Scientists (2009)

have been interpreted to exhibit a cyclic pattern of upper KFB distal turbidites associated with

eustatic sea-level changes and Northern Hemisphere glaciation, although age control at Sites

15

C0002 and C0009 are insufficient to resolve these time scales (Guo et al., 2013; Buchs et al.,

2015; Moore et al., 2015).

Mass transport deposits have been clearly identified within the upper KFB (Moore et al.,

2015; Moore and Strasser, 2015) and in present-day slope sediments at sites (Strasser et al.,

2011). Correlative seismic reflectors show that the locally generated mass transport deposits are

all younger than 1.24 Ma, range in size from several km wide to hundreds of meters across, and

are identified primarily by their internal chaotic and sometimes non-reflective seismic character

(Moore and Strasser, 2015). Mass transport deposits within the present-day slope sediments lying

unconformably above the outer accretionary prism are mostly older (Strasser et al., 2011).

Generation of mass transport complexes along this margin has been correlated to slope failure

resulting from motion along the megasplay fault (Strasser et al., 2011) and earthquake shaking at

shorter time scales (Moore and Strasser, 2015; Moore et al., 2015).

Sediment routing in the Quaternary KFB is broadly linked to submarine canyons and

routing systems from SW Honshu and controlled by climatic and tectonic factors that spatially

and temporally affect the sediment source to sink (Fergusson, 2003; Usman et al., 2014; Buchs et

al., 2015). Detailed pyroxene provenance analysis has revealed that the amount of sediment

routed through transverse canyons from the Inner (Ise Bay) and Outer Zones (e.g. Kumano

River) to the KFB has progressively increased from pre-basin tilting to present day

configurations as a result of turbidites getting confined by an early Quaternary tilting event

(Buchs et al., 2015). These sediments occasionally spilled over to the adjacent slope basins in the

outer wedge environment before the outer arc high restricted down-slope transport (Usman et al.,

2014; Buchs et al., 2015). Longitudinal transport from the distant Izu Collision Zone to the

Nankai Trough persisted throughout the Quaternary, allowing the accretionary wedge to continue

building (Usman et al., 2014; Buchs et al., 2015). Similar mechanisms of flow routing have been

found just northeast of our study area in which a northeasterly flow of confined turbidites was

predicted to have been morphologically trapped within simple U-shaped minibasins and

deflected by topographic highs (Egawa et al., 2013).

PKBPD sediments exhibit the same overall landward younging pattern as in the upper

KFB, though age control in this part of the basin infill is sparse (Expedition 315 Scientists, 2009;

Expedition 315 Scientists, 2010). Ramirez et al. (2015) mapped three sub-units (LB1, LB2, and

LB3) within the PKBPD section based on seismic reflection characteristics and stratal

16

geometries. These units are used in this thesis as a framework for subsequent in-depth mapping.

Each of the sub-units are bounded by basin-scale unconformities and may correspond to facies

changes supported by core analysis (Expedition 319 Scientists, 2010; Ramirez et al., 2015).

NanTroSEIZE KUMANO DATASET

Seismic volume

The Integrated Ocean Drilling Program’s Nankai Trough Seismogenic Zone Experiment

(NanTroSEIZE) is built upon years of study in the Nankai Trough region and encompasses a

high-resolution 3-D survey, several multi-stage drilling sites, and multidisciplinary expeditions

(Moore et al., 2009; Tobin et al., 2009). The 3-D Nankai seismic data set was acquired by the

Petroleum Geo-Services in 2006 with the M/V Nordic Explorer using an acoustic network of two

arrays that were fired alternately at 37.5 m shot intervals giving an inline spacing of 18.75 m

(Moore et al., 2009). Four receiver cables spaced 150 m apart resulted in a 12.5 m crossline

spacing (Moore et al., 2009). The seismic area covers roughly 12 km x 56 km giving a total area

of ~ 585 km2 and extends NE to SW from the seaward portion of the Kumano Basin to the

Nankai trough along the dip direction (Figure 3). Seismic processing was carried out in three

stages. The first stage was done by Petroleum Geo-Services as a first pass at the overall data to

understand the regional seismic reflection characteristics. A 3-D prestack time migration was

then carried out by Compagnie Générale de Géophysique followed by a 3-D prestack depth

migration. This final stage resulted in a high-resolution view of the faults and finer-scale

structures that were not previously visible in the prestack time migration data set (Moore et al.,

2009).

Resolution of seismic data generally decreases with depth into the subsurface due to

acoustic attenuation of the seismic wavelet as it travels through earth’s layers (Sheriff, 1997).

Due to the large uncertainty in the P-wave velocities traveling through the sedimentary layers,

vertical resolution of the seismic data does not resolve fine-scaled features smaller than ~10-20

m in the deepest KFB sediments and ~90-125 m in the deeper accretionary prism and oceanic

crust region (Moore et al., 2009).

17

Lithology and biostratigraphy

Reports from IODP in Expeditions 314, 315, and 319 document the lithologic,

depositional, and structural characteristics of sediments in the Kumano Basin and upper

accretionary prism. These reports provide the foundation for the research in this thesis. Core and

log data from the Kumano basin succession were taken from C0002, which is located

approximately 4 km NW of the KBEFZ (arcward) and C0009 located near the most arcward

extent of the survey, ~20 km NW of Site C0002 (Figure 3, lower panel). Logging while drilling

data were collected during Expedition 314 for Site C0002 while Expedition 319 at Site C0009

collected both wireline data and core cuttings (Expedition 314 Scientists, 2009; Expedition 319

Scientists, 2010). Expedition 315 primarily collected cores from Site C0002 (Expedition 315

Scientists, 2009). Both of these sites penetrated what is interpreted to be the upper Miocene

accretionary prism just below the lower forearc unit (Expedition 315 Scientists, 2009; Expedition

314 Scientists, 2009; Expedition 319 Scientists, 2010). Site C0002 from Expedition 314 and Site

C0009 from Expedition 319 were drilled for wireline data including density, porosity, seismic

velocity and rock strength parameters (Expedition 314 Scientists, 2009; Tobin et al., 2009;

Expedition 319 Scientists, 2010).

We used the lithology and biostratigraphic data from core and cuttings to constrain the

timing of depositional events and correlate regional seismic units. The Expedition 315 and 319

scientists recognized four primary lithologic units at C0002 and C0009 from core and cuttings

analysis based on changes in grain size, layer thickness, internal structures, fossil assemblages,

and mineralogy (Expedition 315 Scientists, 2009; Expedition 319 Scientists, 2010) (Figure 4).

These same lithologic units can be correlated to similar units at C0009 and are clearly time

transgressive. For example, Unit I is older than 1 Ma at Site C0002, but younger than 0.9 Ma at

Site C0009, indicating an arcward younging pattern (Expedition 315 Scientists, 2009; Expedition

319 Scientists, 2010). Unit IV at both sites is composed of mudstone with thinly-bedded

turbidites, which are highly deformed at Site C0009 (Expedition 315 Scientists, 2009; Expedition

319 Scientists, 2010). This unit at Site C0002 was interpreted to be accretionary prism sediments

(Expedition 314 Scientists, 2009; Expedition 315 Scientists, 2009). Similarly, Unit IV at Site

C0009 shows similar lithology and mineralogy, but is only weakly deformed. This unit was

18

interpreted to be trench-slope deposits, accreted trench sediments, or the earliest Kumano Basin

sediments (Expedition 319 Scientists, 2010). Unit III overall exhibits variable thickness

throughout the seismic volume and primarily fills minibasin depocenters. In this study, Unit III

includes LBU1, LBU2, and LBU3, collectively referred to as the PKBPD unit. Mudstone with

occasional layers of silty claystone, abundant nannofossils, and localized zones of glauconite

characterize Unit III (Expedition 315 Scientists, 2009). This lower forearc unit was interpreted in

Expedition 315 Scientists (2009) as early forearc or slope sediments. Unit IIIB at C0009,

however, contains an abundance of terrigenous organic material (e.g. wood and lignite

fragments) that is not found at Site C0002, leading to the sub-unit distinction and conclusion that

this localized unit was sourced by different transport pathways than the rest of the PKBPD sub-

units and produced by bathymetric reactivation (Expedition 319 Scientists, 2010; Ramirez et al.,

2015). Units I and II at both drill sites have similar compositions that grade from interbedded silt

and sand layers within silty mud in Unit II to progressively more abundant layers of sand and silt

going up-section into Unit I (Expedition 315 Scientists, 2009; Expedition 319 Scientists, 2010).

Unit I at C0009 is sandier and more turbidite-rich than its equivalent unit at Site C0002

(Expedition 319 Scientists, 2010).

Primary age control of sediments was determined by dating calcareous nannofossils and

planktonic foraminifera from core cuttings, as summarized by the Expedition 315 Scientists

(2009) and Expedition 319 Scientists (2010). As a whole, core recovery was moderate to poor at

Site C0002, particularly where the logging while drilling character indicated sandy intervals. At

the C0002 and C0009 drill sites, dates that are >1 Myr apart straddle the unconformity separating

Unit III and Unit IV, suggesting that a major hiatus occurred from at least ~5.04 – 3.8 Ma at the

time of its formation (Expedition 315 Scientists, 2009; Expedition 319 Scientists, 2010). The

cause of this hiatus has been speculated to be associated with propagation of an out-of-sequence

megasplay fault, basement ridge subduction, or underplating of hemipelagic sediments

(Expedition 319 Scientists, 2010). According to Moore et al. (2015), the timing of megasplay

fault propagation post-dates the hiatus defining the unconformity based on isopach analysis of

upper Kumano sediments that constrain the timing of depocenter shift. Unit III at both sites is

relatively coeval, representing ~4.7 Myr of deposition at C0009 and ~3.92 Myr at C0002

(Expedition 319 Scientists, 2010). This translates to ~53 m/Myr rate of deposition in Unit III at

Site C0009 and ~26 m/myr rate of deposition at Site C0002. Compared to rates of ~400 - 800

19

m/Myr in the upper basin at Site C0002, sedimentation was much slower in the PKBPD unit and

suggests a starved distal basin setting consistent with the dominant hemipelagite facies from core

descriptions (Expedition 315 Scientists, 2009; Expedition 319 Scientists, 2010). Younger

nannofossil events within Unit III at both sites were encountered stratigraphically below older

events in the Pliocene section, an indication that the sediments were partially reworked after

deposition (Expedition 315 Scientists, 2009; Expedition 319 Scientists, 2010). Resolution of

nannofossil dating is not high enough to calculate depositional rates within PKBPD sub-units

and smaller-scale interpreted seismic sections. Frequency of nannofossil occurrence was

generally higher in the PKBPD unit compared to overall recovery in the upper forearc basin fill

(Figure 4).

Methods

I interpreted the seismic volume within the PKBPD unit using conventional seismic

stratigraphic analysis and analyzed relationships between stratal geometries and seismically

resolvable packages. Figure 5 shows examples of how this was systematically done throughout

the dataset. I used onlap, downlap, truncation, and toplap stratal geometries to identify major

unconformities and sequences. Horizons with high-amplitude reflectors were preferentially

mapped throughout the basin, particularly within the minibasin depocenters where there was a

lack of absolute age control from Site C0002 and C0009. Near the wells, the reflectors closest to

biostratigraphy data points were mapped and carried out as far as possible before the horizon

terminated against another reflector or encountered an area of transparent or chaotic facies where

it could not be continued without tying from another inline or crossline. These surfaces provide

the age constraints for our Wheeler diagrams discussed in Chapter 4.

I then flattened on key horizons to minimize the effects of structural deformation and

post-depositional influence. Flattening the seismic section provides a confidence check of the

interpretations and, more importantly, helps to visualize the depositional environment at a given

point in time. This method was crucial in distinguishing downlap versus onlap onto

unconformities throughout the seismic volume and helped clarify the depositional processes

associated with these stratal distinctions.

20

Construction of the Wheeler diagrams was treated similarly to fence diagrams whereby

intersecting cross sections are tied together in space (X vs. Y vs. Z), but with one key difference:

the Wheeler diagrams are tied by age based on the well-to-well cross section with biostratigraphy

for a 3-D (X vs. Y vs. time) representation of the KFB units. I first used the C0002 to C0009

well tie interpretation (Line 5) to establish absolute age boundaries for the y-axis. The top of the

accretionary prism (TAP) at Site C0002 is dated between 5.04-5.12 Ma and 3.65 Ma where there

is a marked unconformity from core cuttings (Expedition 315 Scientists, 2009). Site C0009

narrows the age of the TAP surface to be between ~5.6 – 3.8 Ma (Expedition 319 Scientists,

2010). Thus, 5.6 Ma is used as the earliest age boundary for the TAP assuming that some

deposition of LBU1 began to occur elsewhere from the well sites. The latest KFB age used in

this study is 0.9 Ma. Other mapped age horizons from the two well sites were added to complete

the Wheeler diagram for Line 5. Line 1 through 4 were tied to Line 5 using its dated horizons.

Due to the lack of age data in the majority of the PKBPD units, each polygon “strip” in the

Wheeler diagram represents an equal amount of time within each sub-unit. The x-axis of the time

strips are from actual horizon interpretations to show depositional evolution throughout time.

Colors of strips and dashed unconformities match those shown in interpreted seismic cross

sections.

The horizon nomenclature used in this thesis is based on sub-units and absolute ages from

Site C0002 and C0009 where they are mappable. The number that comes after “LB” and “UB”,

which stands for lower basin and upper basin respectively, corresponds to the stratigraphic sub-

unit followed by a letter to distinguish the top of each package within the sub-units. Where

biostratigraphy data points were available at well locations, the reflector closest to the posted

data point was picked as an absolute age horizon named after its age followed by an underscore

and either a “9” for a data point originating from Site C0009 or “2” if it originated from Site

C0002.

21

10 mi

JAPAN S. KOREA

EURASIAN PLATE

PHILIPPINE PLATE

~4 - 6.5cm/yr

Figure 1. Regional map of the Nankai Trough and Kumano Basin located offshore Southwest Honshu. Pink polygon delineates 3-D

seismic coverage and green circles highlight wells drilled at Site C0002 and C0009 used for lithologic interpretation.

22

Figure 2. Sub-regional bathymetric map of the NanTroSEIZE dataset seismic coverage with IODP well locations.

Yellow polygon outlines the Nankai 3-D volume. Red dots are Kumano Basin drill sites. Blue dots are locations of

wells trench-ward of the Kumano Basin. White line shows location of inline 2520 with bolded portion as the 2-D

extent depicted in Figure 6. Modified from Moore et al. (2009) and Ramirez et al. (2015).

23

Figure 3. Regional seismic transect through NanTroSEIZE 3-D volume showing morphotectonic zones, large-scale structural elements of the Nankai arc-

trench system, and well locations. Modified from Moore et al. (2009).

24

Figure 4. Summary lithostratigraphic chart with nannofossil events from core cuttings, interpreted logging units,

and depths from Sites C0002 and C0009. Major surfaces are shown in dashed lines. TAP = top of the accretionary

prism. Modified from Expedition 315 Scientists (2009); Expedition 319 Scientists (2010); and Ramirez et al. (2015).

25

Figure 5. Representative inline and crossline through 3-D volume showing upper Kumano Basin and PKBPD subunits LBU1, LBU2, and LBU3 as delineated in

Ramirez et al. (2015). Dotted red line indicates horizon TAP (top of accretionary prism), the boundary between the inner accretionary prism and PKBPD

sediments.

26

rrr

Figure 6. Examples of stratal terminations observed in the PKBPD sediments in inline 2520. Location of inline shown in Figure 2. A) Instance of

growth strata. Note that reflector thickness increases towards the depocentre of the minibasin. B) Examples of onlap, downlap, and truncation.

27

CHAPTER 3. RESULTS

My analysis of the KFB hinges upon the results of a basin-wide seismic interpretation of

the PKBPD unit and the integration of mapped surfaces into 3-D Wheeler diagrams. Overall, the

goal of this chapter is to present the interpretations in a three-dimensional and chronological

context so that aspects of the lower basin architecture are clarified.

SEISMIC STRATIGRAPHY OF PRE-KUMANO BASIN PRISM DRAPING UNIT

I identified and mapped the same three sub-units in the PKBPD unit that Ramirez et al.

(2015) mapped in their study to provide the framework for further in-depth mapping using a set

of arbitrary lines (Figure 8 and Figure 9). The isopachs of LBU1, LBU2, and LBU3 are shown in

Figure 7 in ascending stratigraphic order. LBU1 is the oldest PKBPD sub-unit sitting

unconformably above the inner accretionary prism and constitutes the main minibasin infill.

LBU1 is bounded by the TAP and a regional unconformity, PKBPD-1 (Figure 9a and Figure 10).

The TAP separates discontinuous, chaotic reflectors of the inner accretionary prism below it and

continuous, onlapping LBU1 reflectors above. LBU2 is a much thinner unit that drapes over the

existing topography of LBU1, and LBU3 is a localized wedge of sediment in the NW corner of

the seismic volume. These strata are generally older at Site C0002 because PKBPD-1 and

PKBPD-2 are time–transgressive surfaces that young to the NW (toward Site C0009).

The morphology of the PKBPD sub-units is defined by a series of synclines and

anticlines bounded by inactive, buried thrust faults (Figure 9b) (Ramirez et al., 2015; Boston et

al., 2016). All of the synforms trend NE-SW, mimicking the present-day structural trends. From

the isopach and structure maps, I identified three primary depocenters in the dataset, which are

labeled in Figure 7 and Figure 8. The size and depth of the synforms decrease trenchward. The

main synform, D2, is the deepest and contains the thickest succession of sediment. The northern

boundary of D2 is restricted by a seaward verging thrust fault and to the south by an arcward

verging thrust. Sediment fill in D2 indicates syndepositional deformation concentrated around

the buried thrust fault located NW of the synform (Figure 9, 12, 13). From isopach analysis and

seismic interpretation, D3 contains a localized wedge of sediment, LBU3, that exists only in the

28

NW corner of the survey and is equivalent to the terrigenous facies at Site C0009 containing

woody and lignite fragments (Ramirez et al., 2015) (Figure 7). Depocenter D1 consists of a

series of smaller, shallower synforms that are also divided by inactive thrust faults. Seismic

reflection patterns indicate that contraction-related events acted on sediments during deposition,

with deeper strata appearing more deformed than those in shallower sections. The depocenter at

the northwestern edge of Line 3 (Figure 12) illustrates the increasing degree of folding from

LB1-O down to LB1-J. Judging by how the horizons in LBU1 conform to the folding of the

thrusts but are not disconnected by them, we conclude that the faults created fault-propagation

folds during time of LBU1 deposition. Observations of growth strata within D2 also support this

interpretation.

LBU1 is the oldest sub-unit above the TAP and consists of a series of deformed packages

capped by the PKBPD-1 unconformity (cyan line in seismic sections). Figure 9 illustrates the

series of seismic packages in D2 that onlap the NE-SW trending structural high towards C0009.

Horizons LB1-A through LB1-M are all confined within D2 towards the NW. To the SE, most of

the seismic packages that fill D2 continue trenchward towards C0002 until they either truncate

against PKBPD-1 (e.g. LB1-C, LB1-D, and LB1-E) or pinch out into the condensed section at

C0002 where these correlative packages are not seismically resolvable (Figure 9, 12, and 13).

Thus, the horizons within D2 lack any kind of absolute age control since they do not extend to

either Site C0002 or C0009. Interpolating between absolute ages from C0002 and C0009, we

conclude that LBU1 sediments are younger than ~5.6 Myr and older than 2.52 Myr since

PKBPD-1 lies in between 2.52_9 and 2.87_9 at C0009. Each of these packages follows the

present day morphology of the TAP (Figure 10 and Figure 11), suggesting either a syntectonic

deposition or deformation of the accretionary prism and PKBPD sediments post-deposition.

Mapped units within D1 and D2 thicken towards the depocenter and thin toward the anticline

hinge, resembling characteristics of growth strata (Figure 9a, Figure 10 and Figure 12).

Several LBU1 packages in D3 are isolated from D2 and onlap the trenchward-verging

structural high to the SE. Figure 12 highlights the confinement of sediment in the NW corner of

the seismic volume. LB1-J, LB1-K, and LB1-O onlap the antiform, existing only within D3. I

was only able to extend LB1-L into D2, before it truncated against PKBPD-1 (Figure 12). LB1-L

is discontinuous further towards the NE corner where the NE-SW trending structural high

separating D1 and D2 is much shallower and most likely acted as a barrier to deposition during

29

the time of LB1-L formation (Figure 13). In theory, LB1-O and LB1-M could be the same

temporal surface, but the resolution of the seismic data in that region made it difficult to

conclusively connect the two horizons.

A wedge of sediment was identified in the NW corner of the seismic volume, consistent

with what Ramirez et al. (2015) mapped and logging results from Expedition 319 Scientists

(2010) (Figure 9a and Figure 13). This localized package, LBU3, coincides with the early

Pliocene section recognized in Expedition 319 that contains abundant woody and lignite

fragments not found in any other logging units. It unconformably overlies LBU1 at its

northwestern edge and LBU2 further SE. This is most evident in Figure 12 in which the lower

boundary of LBU3 is PKBPD-1 (cyan line) at the NW edge of the cross section and transitions

into PKBPD-2 (magenta line) going SE towards D2. It is bounded at the top by PKBPD-3, the

regional unconformity extending only as far as LBU3 exists. The unit is ~600 m at its thickest

and only covers roughly 3 km of the NW corner of the seismic volume. At C0009, the age of

LBU3 is estimated to be between ~1.34 and ~0.9 Myr. Internally, LBU3 is distinguishable by its

gently-dipping, high-amplitude reflectors that onlap PKBPD-2 towards the SE (Figure 12 and

Figure 13).

Above the PKBPD-1 unconformity, LBU2 is a much thinner unit that drapes LBU1 and

continues to generally follow LBU1 topography (Figure 7). The LBU2 sub-unit is bounded by

PKBPD-1 at its base and PKBPD-2 at its top (Figure 9a). PKBPD-2 is another regional

unconformity characterized by truncated reflectors terminating against it below and apparent

downlap onto the surface (Figure 10 and Figure 11). Like LBU1 strata, LBU2 packages at C0002

pinch out but extend slightly further than LBU1 before truncating against UB sediments (Figure

9 and Figure 12). Generally, LBU2 covers a greater area and is more uniformly distributed

throughout the basin than LBU1 and LBU3 (Figure 7). The youngest LBU2 deposit can be no

younger than ~1.34 Ma at C0009 and no younger than ~1.67 at C0002. The age of individual

LBU2 packages are difficult to determine due to the condensed section at C0002 compounded

with the closeness of data points spaced at an irresolvable level of seismic resolution. Figure 4 in

the previous chapter and Figure 9b both illustrate that the entire ~90-meter-thick PKBPD unit at

C0002 spans approximately 3.5 Myr. However, we deduced that the PKBPD unit at C0002

cannot be any younger than ~1.67 Myr old since that age corresponds to the age of the oldest UB

30

data point available at the well. Realistically, the age if the youngest strata at the trenchward site

is most likely closer to ~2.06 Myr old.

Interesting stratal relationships are observed in LBU2 when the sub-unit is dissected in

detail. Within LBU2, I identified and mapped two key horizons extensively throughout the

seismic volume. LB2-B and LB2-C are major downlap surfaces and exhibit characteristics of

unconformities (Figure 17). Each of these horizons extends across and drapes the central portion

of the basin (Figure 14, 15, and 16). The isopach maps highlight the drape-like character and the

noticeably gentler topographic relief of LBU2 packages, which does not exceed greater than 300

m in average thickness and are bounded by unconformities. A key observation in Figure 17a is

that reflectors appear to downlap onto PKBPD-1, LB2-B, and LB2-C from the SW to the NE,

with the exception of a few reflectors concentrating between PKBPD-1 and LB2-B in a small

topographic low towards the SW. The dips of LBU2 reflectors decrease updip and gradually

conform to the topography of the downlap surface above, resembling toplap geometry and

indicating minor erosion. This stratal pattern is also observed in Line 2 with reflectors first

concentrating in small depocenters and terminating onto the same surface followed by migrating

reflectors away from topographic lows (Figure 17b). However, the direction of downlap is not

straightforward when only looking from one orientation. To get a better idea of the overall stratal

architecture of LBU2, I used inline 2300. Figure 17c shows reflectors downlapping from a

southeasterly direction towards the northwest. Thus, my interpretation is that sediment initially

ponded in topographic lows before filling the southern edges of the basin and stepping

northward.

From a sediment supply and accommodation perspective, the source of sediment in the

thin LBU2 sub-unit is assumed to be mostly uniform hemipelagic drape that blanketed wherever

there was space available and the gradient was low enough to retain sediment. This assumption

of sediment source is primarily based on correlating LBU2 to Unit IIIA at Site C0009 (Figure 9),

which is composed of silty mudstone with rare silty beds (Figure 4) and the relative isopachous

nature of the packages between PKBPD-1, LB2-B, LB2-C and PKBPD-2 (Figure 14, 15, and

16). Because hemipelagic sedimentation implies a combination of vertical settling down the

water column and slow lateral advection in a deepwater, low energy environment (Stow and

Tabrez, 1998), there is an overall lack of traction-dominated sediment transport that is associated

with channelized flows through submarine canyons (e.g. Posamentier and Kolla, 2003). In this

31

case, the accommodation for hemipelagic sediments in the KFB can be taken as wherever a

depocenter exists for sediment to accumulate without being mobilized by a significant gradient.

If the depocenter shifts, then the concentration of sediment shifts as well and the pattern of

migration should manifest in the stratigraphic record if preservation is high.

FLATTENING OF HORIZONS

In order to further check the validity of the stratal geometries observed in LBU2, I used

horizon flattening on PKBPD-1, LB2-B, and LB2-C. The downlapping reflectors seen in Figure

17b between PKBPD-1 and LB2-B are vertically exaggerated and reproduced in Figure 18. The

red arrows that terminate onto the same surface, PKBPD-1, indicate that deposition was

concentrated at a topographic low at the NE end, and then expanded outwards away from the

depocenters. Flattening on LB2-B reveals similar patterns where strata form arches above the

flattened surface, indicating the filling of low spots followed by sediment stepping outwards

(Figure 19). Moving stratigraphically upward, direction of deposition is more apparent in Figure

20 when LB2-C is flattened. All of the reflectors originate from the SW and progressively

downlap onto LB2-C towards the NE. Horizon flattening confirmed that LBU2 is composed of

smaller sub-units with arcward stepping strata bounded by unconformities.

WHEELER DIAGRAM ANALYSIS

To dissect the overall pattern, spatial distribution and timing of deposition, I produced a

series of Wheeler diagrams from seismic cross sections that summarize the spatio-temporal

relationships within the KFB. LBU1 is unique because it is comprised of largely deep, localized

packages of sediment that represent the minibasin stage of early PKBPD unit development. This

is most apparent in cross sections oriented perpendicular to structure in Figure 21, Figure 24, and

Figure 25 where the main LBU1 fill is concentrated in two main depocenters (D2 and D3) and

spans more than 2 Myr of deposition. The highest sedimentation rate extrapolated for LBU1 is

~360 m/Myr in D2 where the unit is the thickest. This rate is comparable to the 400-800 m/Myr

32

rate in the UB (Expedition 315 Scientists, 2009; Expedition 319 Scientists, 2010), which is

considered high for such a distal setting.

Above LBU1, LBU2 drape exhibits interesting stratal patterns reminiscent of what was

documented in the UB. I identified three distinct occurrences of migrating packages in LBU2

(Figure 21 through Figure 25). In Figure 21, the first set of these migrations began ~3.3 Ma at

C0002 and gradually shifted NW towards C0009 until ~3.0 Ma before being capped by the LB2-

B unconformity. The next set, bounded by LB2-B and LB2-C, began again at C0002 and

migrated across the basin to C0009 where packages concentrated at the NW end until ~2.0 Ma.

As the Wheeler diagram of Line 5 suggests in Figure 21, the early stages of LBU2 were

deposited synchronously with LBU1 sediments. The spatial pattern of the migrations in the dip

profiles is generally from SE to NW (Figure 21, Figure 24, and Figure 25). However, the pattern

is less definitive for profiles along strike. Line 1 Wheeler diagram shows LBU2 packages

concentrating mostly in the SW before stepping towards the NE in three stages (Figure 22).

Arbitrary Line 2 actually indicates the opposite direction of migration. LBU2 sediments first

pond in the NE during the first stage (bounded by PKBPD-1 and LB2-B) before stepping SW

(Figure 23). Spatial variations exist even within the LBU2 unit. In fact, the progression of the

oldest LBU2 stage between LB2-C and PKBPD-2 horizons shows sediment ponding in the NE

and stepping SW, which is the opposite direction of sediment accumulation in the first stage. A

synthesis of the Wheeler diagrams yields a three-dimensional depiction of LBU2 sediment

initially accumulating at the southern edges and migrating northbound in three distinct stages

that were punctuated by sub-regional unconformities within LBU2. Each of the migrating stages

in LBU2 span about 0.3 – 0.5 Myr, but these time spans are largely unconstrained due to the

absence of correlatable biostratigraphic data in LBU2 at both drill sites. The only two age points

at C0009 were traced from seismic data and then extrapolated trench-ward to C0002 following

the observation of migrating packages that imply the time-transgressive nature of the PKBPD-1,

LB2-B and LB2-C surfaces (Figure 21).

Apart from the lower basin units, the Wheeler diagrams also reveal complex stratigraphic

relationships in the upper Kumano Basin. The diagrams compiled from seismic sections

perpendicular to structure show the stratal variations most apparently. Sedimentary packages

younger than 1.24 Myr appeared to undergo phases of backstepping in the arcward direction with

onlapping onto older strata followed by a trench-ward migration back to the most distal point of

33

deposition (green units in Figure 21, Figure 24, and Figure 25). With the exception of Line 4,

Line 3 and Line 5 show that the lateral extent of each of the UB onlapping strata did not change

significantly as indicated by the preserved length of green strips that are shifted in the Wheeler

diagrams (Figure 21 and Figure 24). Line 4, on the other hand, displays strata that have laterally

shortened and then expanded over time (jagged edges bounding green unit, Figure 25), indicating

a possible adjustment of accommodation space.

34

Figure 7. Isochore maps of lower basin units LBU1, LBU2, and LBU3 in stratigraphic order. LBU1 is the thickest unit that first filled minibasin depocenters and

reached the SE portion of the basin. LBU2 draped LBU1 sediments and continued filling topographic lows. LBU3 is a localized unit characterized by a distinct

terrigenous input consisting of woody fragments and lignite analyzed from core samples (Expedition 319 Scientists, 2010).

C0002 C0002

35

Figure 8. Base map on TAP. Labeled lines correspond to interpreted cross sections throughout Chapter 3 and

Chapter 4. Line 1 and 2 cut along strike of the minibasin depocenters while Line 3 and 4 are perpendicular to

structure. Line 5 indicates the well correlation section (refer to Figure 9).

36

Figure 9. Well correlation cross section through Sites C0009 and C0002 with IODP logging units. A) Major sub-units delineated by colored polygons. Dashed

line represents the unconformity separating the inner accretionary prism and Kumano Basin sediments. B) (Next page) Interpreted seismic section through Site

C0009 and C0002 including key horizons throughout the PKBPD unit and biostratigraphy data from IODP reports. Note that all labeled unconformities

(PKBPD-1, PKBPD-2, and PKBPD-3) are time transgressive. Ages of strata increase from Site C0009 to C0002. Fault interpretations are from Ramirez et al.

(2015) and Boston et al. (2016).

A

37

Figure 9 (continued). Refer to figure caption on previous page.

B

38

Figure 10. Arbitrary seismic cross section of arbitrary Line 1. Location is shown in Figure 8. Line 1 is oriented along

strike through depocentre D2.

39

Figure 11. Arbitrary Line 2 seismic cross section oriented along strike of depocenter D1.

40

Figure 12. Arbitrary Line 3 seismic cross section oriented perpendicular to structure.

41

Figure 13. Arbitrary Line 4 seismic cross section oriented perpendicular to structure.

42

Figure 14. Isopach map of section between unconformity PKBPD-1 and LB2-B.

43

Figure 15. Isopach map of section between horizon LB2-B and LB2-C.

44

Figure 16. Isopach map of section between LB2-C and PKBPD-2 unconformity.

45

A

B

NW SE

C

Figure 17. Cross sections showing stratal relationships within LBU2, bounded by PKBPD-1 and PKBPD-2. A) Zoomed in

seismic section of Line 1 along structure D2. Black arrows show apparent, progressive downlap onto PKBPD-1, LB2-B, and

LB2-C from SW to NE. B) Zoomed in cross section of Line 2 along structure D1. Downlap relationship is less apparent. Black

arrows onlap at either ends of depocentres between PKBPD-1 AND LB2-B and LB2-C. Downlap direction between LB2-C

and PKBPD-2 is from SW to NE. C) Inline 2300 showing direction of apparent downlap from SE to NW.

46

Figure 18. Line 2 flattened on PKBPD-1. Red arrows highlight stratal terminations. Toward the NE, arrows terminate onto the same surface

indicating the filling of topographic lows followed by an expansion of accommodation away from the depocenter.

47

Figure 19. Line 5 flattened on LB2-B. Strata are mostly concentrated in topographic lows and in some areas step out from the SE to the NW.

48

Figure 20. Line 1 flattened on LB2-C with stratal terminations on the flattened surface highlighted by red arrows.

49

Figure 21. Wheeler diagram of Line 5 (C0002 to C0009 well correlation). Colors correspond to interpreted seismic units in figures from Chapter 3. Dashed

lines delineate time-transgressive unconformities and bold black lines indicate time horizons traced from wells.

50

Figure 22. Wheeler diagram of Line 1 cross section oriented along strike of structure. Colors correspond to

interpreted seismic sections in figures from Chapter 3. Dashed lines are time-transgressive unconformities, and bold

lines indicate time horizons traced from wells.

51

Figure 23. Wheeler diagram of Line 2 oriented along strike of structure. Colors correspond to interpreted seismic

sections in figures from Chapter 3. Dashed lines are time-transgressive unconformities, and bold lines indicate time

horizons traced from wells.

52

Figure 24. Wheeler diagram of Line 3 oriented perpendicular to structure. Colors correspond to interpreted seismic sections in figures from Chapter 3. Dashed

lines are time-transgressive unconformities, and bold lines indicate time horizons traced from wells.

53

Figure 25. Wheeler diagram of Line 4 oriented perpendicular to structure. Colors correspond to interpreted seismic

sections in figures from Chapter 3. Dashed lines are time-transgressive unconformities, and bold lines indicate time

horizons traced from wells.

54

CHAPTER 4. DISCUSSION

Results from the seismic data analysis show that at least one driving mechanism at the

tectonic scale was responsible for creating the basin-wide accommodation shifts associated with

tilting during the evolution of both the lower and upper KFB. The stratigraphic relationships

examined so far thus substantiate the discussion of some plausible causes of regional

reconfiguration of the incipient Kumano Basin.

COMPARISON OF PKBPD UNIT WITH A MINIBASIN DEPOSITIONAL MODEL

Identifying the stratal geometries within the PKBPD unit allows us to dissect the

architecture of the early basin fill. One way to understand and compare the processes that

produced the observations to what is known about the system is to use quantitative models.

Models can bridge the gap between these observations and the driving mechanisms that

produced them. Therefore, it is important to compare observations with simplified models in

order to identify plausible depositional scenarios that fit the tectonic history of the region.

Since a mass balance approach is the simplest way to compare basin systems, we

analyzed the KFB using the mass balance model introduced by Sylvester et al. (2015) that varied

subsidence and sediment supply to control basin development. Stratal architecture in LBU1 was

directly compared to the large-scale architecture in the models (Figure 26). The model used large

temporal scales with time steps ranging from tens to thousands of years, and sediment supply

was treated as an averaged volumetric discharge. Subsidence rates were varied spatially, with

maximum rates at the middle of the basin. Depositional topography was largely ignored. For

detailed methods and model parameters, refer to Sylvester et al. (2015). The parameters in this

model broadly fit conditions within the PKBPD based on extrapolated depositional rates and

inferred time scales from biostratigraphy.

A key finding is that the location of stratal termination points is a direct indication of how

subsidence and sediment supply interact (Sylvester et al., 2015). Specifically, the behavior of the

termination points is dictated by whether there is enough subsidence to create space available for

incoming sediment. If the volume of sediment exceeds available accommodation, then

termination points migrate toward the edge of the basin, creating onlap (Figure 26). In contrast, if

55

sediment flux is less than the growth of accommodation, termination points move toward the

basin center. Throughout the KFB, seismic cross sections oriented in the NW-SE direction of all

minibasins exhibit onlap onto the basin edge and is most evident around the buried anticline in

the NW portion of the seismic volume (Figure 9, 12, 13). Compared to the “Case 2: Constant

Sediment Input, Deep Basin: Onlap” scenario in Sylvester et al.’s (2015) model, the geometries

are strikingly similar (Figure 26). For example, all mapped reflectors in the D2 minibasin

terminate against the folded TAP surface at the northwestern boundary in Figure 26a but

continue over the basement high towards the SE, indicating differential uplift or an active thrust

fault in the NW at the time of LBU1 deposition. This is consistent with interpretations by

Ramirez et al. (2015) and Boston et al. (2016).

If we apply the Case 2 model assumptions that a depocenter existed prior to sediment

flux into the system to the PKBPD unit, it posits that some kind of tectonic process had to have

created initial accommodation, and/or background subsidence was occurring over a period with

little or no sediment input. The onset of LBU1 deposition directly above the accretionary prism

is comparable to the Case 2 scenario. The TAP represents a time gap – a hiatus that spans more

than 1 Myr (Expedition 315 Scientists, 2009; Expedition 319 Scientists, 2010) (Figure 4). A

significant bathymetric high at the seaward edge during this time period may have effectively

blocked sediments from entering the basin, allowing shortening and background subsidence to

form topography (Expedition 315 Scientists, 2009). Comparison of the inner accretionary prism

to the modern day outer accretionary prism reveals similar features resembling buried thrust

faults and hanging-wall anticlines (Boston et al., 2016), which indicate that shortening could

have produced initial accommodation for sediment accumulation. The modern day outer

accretionary prism contains ponded basin assemblages in small depocenters flanked by imbricate

thrusts. The top of the accretionary prism at the megasplay fault zone is dated to be 1.95 Myr at

Site C0008 and <2.87 Myr at Site C0004, suggesting that these locations were at the frontal-

prism-toe position since the late Pliocene (Strasser et al., 2009). Morphologically, the present-

day outer accretionary prism may be a suitable analog for the inner accretionary prism; however,

there are key mechanistic differences that are discussed later.

Although the tectonic setting of minibasins in the Gulf of Mexico (GOM) may be

different than the one governing basin evolution in the KFB, the morphologies are strikingly

similar. At a first glance, the PKBPD fill resembles those within the salt-tectonic-driven ponded

56

basins in the GOM, in which differential loading caused by salt withdrawal control growth of

accommodation and sediment dispersal (e.g. Prather et al., 1998). A side-by-side comparison of

previously discussed cross sections from Chapter 3 with a high-resolution seismic profile of the

Brazos-Trinity Basin in GOM in Figure 27a shows analogous broad, U-shaped depocenters with

basin-edge onlap. Even without looking at well log data, it is obvious in both systems that

variations in sedimentary deposits throughout the minibasin succession produced density

differences manifested in alternating transparent to high-amplitude reflectors. Both systems

contain alternating packages of transparent to chaotic reflectors and high amplitude reflectors.

Based on the similarities of seismic reflection characteristics of the two minibasins, I anticipate

that a log through the minibasin depocenter in Figure 27b would show similar oscillating gamma

ray values as the one in the Brazos-Trinity Basin. Sediments within intraslope basins like the

Brazos-Trinity were deposited by a variety of deepwater processes (Prather et al., 1998), similar

to sediment transport in the KFB. The main difference is that the latter setting contains abundant

hemipelagites and much finer sediment in a low-energy environment.

EARLY KUMANO BASIN EVOLUTIONARY MODEL

PKBPD unit development

The capstone of my analysis is an early basin evolutionary scheme constructed from the

synthesis of my findings and previous studies on the Kumano Basin’s initial development. The

model addresses Pliocene minibasin formation and focuses on the evolution of LBU2 since a) in-

depth analysis of the sub-unit yields interesting perspectives on the overall basin evolution and b)

existing studies have not specifically distinguished this sub-unit.

The model for the developmental stages of the LBU1 minibasins depicts how the

accommodation and geometries evolved through time, producing the patterns observed today in

the seismic data. Deposition of LBU1 occurred between ~5.6 – 3.3 Ma above the outer

accretionary wedge during a time of active prism growth resulting from the buildup of sediment

scraped off of the subducting Philippine Plate. I speculate that by ~5.6 Ma, space existed for

LBU1 sediments to accumulate within D2, D3 (the most arcward synforms filled in blue, Figure

28), and D1 further trench-ward, which started off as an elongated shallower depocenter. As

57

ongoing contraction caused by subduction continued to act on LBU1 sediments throughout the

Pliocene and early Pleistocene, fault propagation folds increasingly steepened minibasin

topography by creating relief at the anticlinal hinges (Figure 28, panels A through N) (Boston et

al., 2016). Additionally, deformation from plate convergence gradually divided the D1

depocenter after most of the LBU1 sediment was deposited, resulting in heavily folded strata

with preserved thicknesses. Thus, deposition and preservation of LBU1 was made possible by

accommodation created from initial subsidence and tectonic activity. Boston et al. (2016)

performed structural restoration using a kinematic trishear model along the TAP to confirm the

existence of a shallow synclinal structure prior to any sedimentation in the PKBPD.

Unlike the other PKBPD sub-units, LBU2 displays persistent drape-like morphology that

is recognizably different from LBU1’s confined minibasin architecture and LBU3’s localized

wedge, and possibly indicates a shift of the tectonic or depositional regime capable of creating

accommodation. Following deposition of LBU1, sediments in the first stage (yellow packages in

Figure 28) of LBU2 were deposited in the most trenchward depocenter above the paleo-outer

wedge. The initial depocenter was likely created by imbricate thrust faulting and folding due to

ongoing subduction during the Pliocene. I use evidence of the imbricate thrust faults flanking the

present-day slope sediments, contraction of the accretionary prism near Site C0001, and the

gently dipping profile of the present-day outer accretionary prism to support this conclusion

(Ashi et al., 2009). By approximately 3 Ma, the first stage of LBU2 sedimentation had reached

the arcward limit of the Kumano seismic volume (panel E and F, Figure 28). The second and

third stages of LBU2 were deposited in a similar manner from ~2.6 to ~2.06 Ma. Each stage of

LBU2 strata appear to backstep from trenchward to arcward onto the same surface. However, the

depositional process that is normally regarded as retrogradational packages backstepping onto a

transgressive surface in passive margins is not analogous to the migrating strata in LBU2.

Instead, I propose that some kind of allocyclic forcing was responsible for gradually tilting the

outer wedge and driving the migration of accommodation space. Therefore, the apparent

downlap terminations onto PKBPD-1, LB1-B and LB1-C were originally onlapping the

unconformities at the time of deposition and resembled the present-day stratal geometry of

sediments in the LBU1 minibasins before tilting occurred (Figure 28).

These large-scale patterns within LBU2 signify a driving mechanism that forced

distinguishable stages of depocenter shifts to manifest across the entire basin followed by a

58

“system reset” after each stage in which sedimentation moved back to its original depocenter

trenchward. For a migration of strata to manifest in the sedimentary record on a basin-wide scale,

the system would have had to experience regional, albeit subtle, tilting related to either relative

uplift from a hinge arcward of the outer wedge or subsidence relative to a hinge in the vicinity of

the trough. From either point of reference, the stratal geometries suggest a progressive flattening

of the outer wedge during the first stage of LBU2 deposition followed by steepening of the outer

wedge and relative flattening again (Figure 28). Although no evidence of this kind of

depositional pattern is found in LBU1 or LBU3, the inferred time interval of each LBU2 stage

roughly matches the duration of pronounced activity along the megasplay fault that caused a

landward tilting event in the late Quaternary UB unit (Park et al., 2002; Moore et al., 2007;

Bangs et al., 2009; Gulick et al., 2010). Based on seismic data and stratigraphic analysis from

cores, activity along this OOST fault has been dated back to ~2 Ma with subsequent reactivation

occurring throughout the late Quaternary (Strasser et al., 2009; Kimura et al., 2011). Our

findings suggest that a tectonic driver capable of tilting the PKBPD units landward was active

from as early as the late Pliocene.

Mechanistically, the geometry of LBU2 strata can be explained by a range of possible

tectonic driving mechanisms that may have caused accommodation to expand from the confines

of LBU1 and shift gradually northward in multiple stages. For example, if we assume that the

entire PKBPD unit was deposited above the paleo-outer accretionary wedge, as analyses by

Strasser et al. (2009) and Kimura et al. (2011) support, then one theory of evolution for LBU2 is

that shortening from continued accumulation of subducting plate material allowed the system to

reconfigure accommodation as it deformed. Evidence from previous studies suggest that

deformation within the accretionary prism was neither temporally nor spatially uniform (Ramirez

et al., 2015; Boston et al., 2016). I agree with the interpretation by Ramirez et al. (2015) that as

sediment slowly filled the basin, the outer wedge continued to actively build and internally

deform, creating additional space for hemipelagic drape and the occasional blanketing of mass

wasting products (Ramirez et al., 2015). Slope instability during deposition of LBU2 also helps

explain the presence of internal unconformities (LB2-B and LB2-C) related to minor erosion

from mass wasting events. Locally diminished activity within the outer accretionary prism may

have given the system enough leeway to relax and the minibasins time to deepen from sediment-

load-induced subsidence. However, it is difficult to imagine that outer prism deformation

59

accommodated by imbricate thrusts could solely be responsible for tilting the entire outer prism

landward, since these seaward-verging thrusts show only small offsets (Moore et al., 2007).

Timing of deformation in the accretionary prism is also largely unconstrained since

biostratigraphy is sparse (Expedition 315 Scientists, 2009; Expedition 319 Scientists, 2010;

Boston et al., 2016) and the resolution of seismic data at these greater depths does not resolve

finer-scaled structural features (Moore et al., 2009).

A more plausible theory to explain an arcward tilt of the entire outer accretionary prism is

the subduction of seamounts. Kimura et al. (2011) suggested that prior to ~1 Ma, a seamount was

outboard of the subduction zone and collision commenced during the development of the

imbricate thrust zone. Seamount subduction has been used to explain the regional uplift of the

KFB in the southwestern area, having changed the bathymetric surface as the seamount was

carried below the imbricate fault zone (Kimura et al., 2011). Bangs et al. (2006) have shown that

the outer wedge thickened and the imbricate thrust faults steepened resulting from the basement

relief of a colliding seamount meeting the accretionary prism along the Muroto Transect just to

the southwest of our study area. Reconfiguration of forearc basins and accretionary wedges due

to subduction of bathymetric highs on the subducting plate has been recognized in many other

active margins around the world. For example, evidence of uplift and arcward tilting from ridge

subduction coupled with subsequent unconformities formed by mass wasting in the Tonga

forearc have been documented (Clift and MacLeod, 1999) as well as in the central Chile margin

(Laursen et al., 2002). Ridge or seamount subduction is often paired with enhanced subsidence

caused by subduction erosion in which the offscraping of material on the overriding plate leads

to sinking of the margin above the underthrusted obstacle (Clift and MacLeod, 1999; Laursen et

al., 2002). Additionally, the amount of sediment and entrapment of fluids within the subducted

material scraped off by the bathymetric ridge could increase overall pore fluid pressure and lead

to transient decreases in the taper angle of the accretionary wedge (Bangs et al., 2006).

Here, I simply consider a hypothetical scenario based on the present-day spacing of

seamounts to infer the potential effect of seamount subduction on the timescales that I have

proposed. Present-day bathymetry data illustrate that seamounts on the Philippine Plate are

spaced approximately 30 to 50 km apart when measured perpendicular to the azimuth of

subduction. I use these estimated distances with the low and high-end rates of plate convergence

to give a range of possible timescales for seamount subduction. It is worthy to note that the

60

estimated Miyazaki and Heki (2001) subduction rate of 6.5 cm/yr is more widely accepted, while

the 4 cm/yr rate from Seno et al. (1993) is based on indirect estimates and subject to large

uncertainties. Assuming that these plate convergence rates have not changed significantly since

the Pliocene, which paleogeographic reconstructions of the region support (Seno and Maruyama,

1984; Kimura et al., 2005), the average time it takes for the seamounts to reach the Eurasian

Plate and begin subduction could range from ~0.75 – 1.5 Myr if calculated with the minimal rate

of 4 cm/yr. Using the upper bound on plate convergence rate of 6.5 cm/yr yields a timescale that

ranges from ~0.5 – 0.8 Myr, which fits more closely with my interpretations. Once a seamount

reaches the trench and begins subduction, it would take an additional ~0.5 – 0.75 Ma for the

seamount to pass under the entire wedge if the present-day transverse distance of ~30 km is used

the outer wedge. Therefore, the total time it would take for a seamount to subduct entirely

beneath the outer Nankai wedge is ~1 Myr. Taking this into account and the assumption that

seamount subduction did not exclusively affect upper KFB development, the occasional collision

of these features could have contributed to the cycles of relative basin uplift and subsidence

during the formation of the lower basin sub-units, as it is hypothesized to have done in the upper

KFB.

Evolution of the upper Kumano Basin vs. the PKBPD unit

Pleistocene sediments in the upper Kumano Basin mark a drastic shift in depositional

character and record high but intermittent megasplay fault activity. The geometry of strata in the

UB are noticeably different from the successions in older units: the entire upper basin is a thick

wedge of sediment that was deposited rapidly and simultaneously tilted arcward throughout the

Quaternary (Figure 28, panels O through R). Depositional rates in the UB are also the highest

within the system (Expedition 315 Scientists, 2009; Expedition 315, 2010). Recent extensive

work on the upper basin units has shown that recurrent slip along the megasplay fault is the main

catalyst for the wedge-shaped basin configuration (Moore et al., 2007; Bangs et al., 2009;

Strasser et al., 2009; Moore et al., 2009; Gulick et al., 2010). Additionally, oblique subduction of

a seamount may have contributed to localized uplift in the southwest corner of the basin and

61

adjusted the local orientation of convergence as suggested by the asynchronous activity of the

eastern and western domains of the megasplay fault (Kimura et al., 2011).

For the purpose of comparison, I analyzed the stratal architecture of the upper Kumano

Basin above PKBPD-2 to distinguish internal variations that correspond to cycles of megasplay

activity. The upper Kumano wedge can be split further into smaller wedges based on onlapping

terminations (Figure 29). With the exception of the oldest unit, UB1, each shaded wedge in green

represents a stratigraphic unit that is separated by a major surface onto which younger strata

onlap. UB1 is capped by the surface 1.34_2 that was mapped using the biostratigraphy data point

at C0002 and represents the first succession of fill that was rapidly deposited at a rate of ~1000

m/Myr (assuming the first UB sediments were deposited 2.05 Ma). Prior to tilting, the basin floor

at the trench-ward edge initially formed a small depocenter, which was subsequently filled until

~1.34 Ma. Subsidence was not fast enough to accommodate all of the incoming sediment,

resulting in onlap at the basin edges analogous to Sylvester et al.’s (2015) Case 2 model (Figure

21). The thicknesses of strata in UB1 are relatively constant between each time horizon, which

suggests that sediment filled accommodation fairly quickly prior to significant landward tilting.

UB2 through UB4 exhibit noticeably different configurations: they resemble wedge-like shapes

that taper trench-ward and thicken landward. UB-A, 0.9_9, and UB-E surfaces are characterized

by a landward-stepping surface followed by progressively trench-ward stepping strata (red

arrows in Figure 29).

Factors leading to basin tilting and rotation in the development of the UB are conceivably

more numerous due to the linkage between the timing of initial megasplay fault activity and

depositional patterns in the UB. Although the initiation of megasplay slip ~1.95 Ma does not

coincide with the beginning of rapid UB sedimentation (1.56 –1.67 Ma), this time discrepancy

can be attributed to a delay between accommodation creation and a reconfiguration of sediment

routing pathways (Expedition 315 Scientists, 2009; Strasser et al., 2009). My observations of the

landward to trench-ward onlapping patterns support the idea that the upper basin also went

through “relaxed” phases in between major tilting events, similar to the ones that LBU2

sediments experienced, but with uplift and tilting primarily caused by episodic megasplay

activity. Each wedge is bounded by surfaces of onlap (bolded lines in Figure 29) and essentially

represents a cycle of megasplay activation followed by diminished activity and relaxation. The

lower onlap boundary of each wedge forms when the system rebounds after initial slip, allowing

62

accommodation to broaden because of relative seafloor flattening. Slip along the megasplay

causes the locus of sedimentation to shift arcward, producing a backstepping of strata towards

Japan, as illustrated by the red onlap termination arrows immediately above each bolded onlap

surface in Figure 29. Subsequent strata above this initial arcward onlap migrate in a southeasterly

direction trenchward and signify the beginning of diminished megasplay activity. Thus, it

follows that the bolded horizons mark the final point of diminished OOST activity before the

system is uplifted and tilted again by a pronounced megasplay slip event. Reconstruction of the

splay-fault system during the Quaternary demonstrates that fault activity progressively

diminished between ~1.95 and ~1.55 Ma followed by a shorter period of high activity between

~1.55 and 1.24 Ma (Strasser et al., 2009; Kimura et al., 2011). Thus, I can validate that the upper

Kumano system endured at least two cycles of megasplay reactivation early in its evolutionary

timeline. Each of the landward stepping stages of LBU2 spans ~0.3 – 0.5 Myr, which I consider

analogous to the onlapping wedge in the upper Kumano Basin. Even though the duration of each

UB wedge is not regularly 0.3 – 0.5 Myr, I establish that a closer sequence stratigraphic analysis

of the fill discloses important features relating to basin formation not previously found.

Sedimentation patterns in the Kumano Basin

The regional tectonic signal driven by megasplay activity, however, may have been

largely masked by the fact that sediment accumulation rates during the late Pliocene were much

slower compared to rates in the early Pleistocene after ~2 Ma (Expedition 315 Scientists, 2009;

Expedition 319 Scientists, 2010; Ramirez et al., 2015; Buchs et al., 2015). The interpretations

from our proposed scenario do not imply a specific direction of sediment source but rather

uniform hemipelagic drape and very fine-grained turbidites blanketing areas wherever

accommodation was available. Hemipelagic deposits dominated sedimentation in LBU2 with

slow rates of deposition ranging from 18 to 30 m/Myr (Expedition 315 Scientists, 2009;

Expedition 319 Scientists, 2010; Ramirez et al., 2015). Persistently high gamma ray values (~70-

100 API), high neutron densities, and low porosities in Unit III at Site C0002 further support the

pervasiveness of a muddy, fine-grained facies (Expedition 314 Scientists, 2009). Rates in the UB

are more than a magnitude higher, and sediment composition consists of not only hemipelagic

mud but also an abundance of silty turbidites with sandy interbeds (Figure 4). The seismic

63

character of LBU2 varies in reflection strength, but no mass transport complexes like the ones

clearly visible in the UB were identified in the sub-unit. The chaotic to transparent seismic

reflections in LBU2 resemble the same reflection characteristics that Posamentier and Kolla

(2003) interpreted as deep-water frontal splay and gravity-flow sheets from offshore Indonesia,

GOM, and Nigeria, which further supports my conclusion that mainly fine-grained sediment

filled the draping units. Slow sedimentation during deposition of LBU2 might also explain why

mass wasting events coincide with periods of enhanced megasplay fault activity in the late

Pleistocene section when sedimentation drastically increased by ten-fold but not in the late

Pliocene to early Pleistocene PKBPD sequence because there was not only a lack of coarse-

grained material to be remobilized, but an overall lack of sediment in the starved basin.

The abrupt onset of high sediment flux of this magnitude around 2 Ma suggests a shift to

some degree in the sedimentary regime that could be related to the intensification of the northern

hemisphere glaciation (INHG). The onset of glaciation has been traced back to as early as ~6

Ma, according to recent evidence from benthic foraminifera δ18O records, but the INHG did not

begin until ~2.75 Ma with sudden spikes in heavy δ18O values correlated to a dramatic increase

in ice rafting in the Northwest Pacific (Maslin et al., 1996). It is now widely accepted that late

Cenozoic climate forcing correlated with the enhanced global denudation of orogens (Willett,

2010). Distinguishing how and when sediment flux responds to such drastic climate fluctuations

is no easy feat, but several regions around the world provide well-preserved stratigraphic records

that can be used to deduce conditions for the NW Pacific around Japan. A prime example that

illustrates the complex interactions between erosion, active tectonics and the dynamic Neogene

climate evolution is the Gulf of Alaska, which contains an incredibly thick succession of

Miocene to Pleistocene glacimarine deposits (Jaeger et al., 2014). Cores recovered from

Expedition 341 recorded accelerated sediment flux from middle to late Pleistocene and

correlated to the INHG phase that affected Alaska ~2.56 Ma with regional seismic

unconformities (Jaeger et al., 2014). Thus, it took approximately <1 Myr for sedimentation to

respond to tectonics and climatic forcing in the Alaska margin. Applying this timescale to SW

Japan would mean that if the earliest INHG was ~2.75 Ma according to Maslin et al. (1996), then

the onset of high sediment flux in the upper KFB beginning ~2 Ma is well within this timespan.

Sediment delivery pathways within the Kumano basin is largely unconstrained and still a

matter of debate, but regional studies of the Japan margin and active margins elsewhere give us

64

conceivable mechanisms of sediment routing to apply to this study area. Regardless of how

sediment was deposited in the region, it is obvious from this analysis and previous studies that

geomorphology and paleobathymetry significantly influence sediment distribution and routing in

basins that are sediment-starved and exhibit high topographic relief. Egawa et al. (2013)

modelled turbidite distribution in a minibasin located in the northeast Nankai Trough area and

found that northeasterly and southeasterly flows of confined turbidites were the most plausible

transport directions after decompaction and structural unfolding of the paleominibasin. Sand

accumulations depended on topographic highs and lows, with flow reflection and deflection

moderated by high topography and ponding of sediment in topographic lows (Egawa et al.,

2013). Their model also implied a load-induced subsidence or relative basin margin uplift of

approximately 300 meters (Egawa et al., 2013). Although I cannot directly infer the same

directions of turbidite flow for the KFB units, the paleobathymetry surface heavily influenced

sediment transport in the Nankai margin based on the results presented in this study. I infer that

similar modes of transport must have existed. Therefore, it is plausible to imagine a scenario for

the deposition of PKBPD units in which blanket turbidites travel downslope from the continental

margin or via transverse canyons, are hindered by high seafloor topography, and subsequently

stratify so that only fine-grained, buoyant material spill over bathymetric highs. Numerical and

experimental turbidity flow models developed over the past few decades establish this concept

(Kneller and Buckee, 2000). Core samples from offshore Honshu further southwest of the

Kumano Basin show the same pervasiveness of hemipelagic deposits and silty layers in ponded

successions along the frontal wedge to slope basin environments (von Huene and Arthur, 1981).

65

Figure 26. Comparison of Kumano minibasin architecture with a minibasin model from Sylvester et al.

(2015). A) D2 zoomed in from Line 3 shows clear large-scale onlap termination points at basin edges

based on seismic interpretation. B) Minibasin model scenario of constant sediment input with initial deep

basin topography. The first panel shows the same basin-edge onlap patterns as in (A) with the

corresponding Wheeler diagram indicating the basin broadening over time.

66

Figure 27. Side-by-side comparison of GOM Brazos-Trinity Basin and a minibasin within

the KFB. (A) Brazos-Trinity Basin seismic reflection profile with GR log superimposed.

Courtesy of Sylvester et al. (2015). (B) interpreted Kumano Forearc minibasin exhibiting

similar morphology to (A).

67

Figure 28. Early Kumano basin

evolutionary schematic depicting

stages of development from ~5.6 Ma

until 0.9 Ma. Panels on the left side

zoom in on PKBPD and upper KFB

sediments. Panels on the right side

show a zoomed out depiction of basin

fill in a regional context from the

subduction zone.

68

Figure 29. Seismic cross section of inline 2620 depicting interpretation of sub-units in the upper KFB. Bolded horizons labeled in red separate UB sub-units. Red

arrows highlight onlapping terminations onto major horizons to illustrate the internal geometries that build each wedge.

69

CHAPTER 5. CONCLUSIONS

Several mechanisms of Kumano Basin evolution were proposed in this study based on

qualitative observations of seismic stratigraphic relationships within seismic units. I used the

relationships of stratal terminations to infer depositional patterns within the PKBPD to provide

insight into constructing a detailed picture of early basin development and to speculate on

possible mechanisms that produced these patterns. Similar to the arcward-tilting sequences in the

upper KFB, the PKBPD unit shows clear evidence of phases of structural reconfiguration. The

architecture of the lower KFB is divided into three sub-units that each have their own distinct

morphology and evolutionary history. LBU1 represents the first phase of slope sedimentation on

the paleo-outer wedge and consists of a series of deep, interconnected minibasins that were

subsequently dissected by deformation along imbricate thrust faults. LBU2 thinly drapes the

topography of the minibasins and represents a period of very slow sedimentation. A third unit,

LBU3, is the localized wedge of sediment in the northwest corner of the modern basin that

contains terrigenous material and likely signifies a distinct depositional event sourced from the

Kii Peninsula area. The most interesting results come from mapping individual surfaces within

LBU2, which reveals three distinct episodes of depocenter migration from trenchward to arcward

that each lasted approximately 0.4 Myr from ~3.3 Ma until ~2.06 Ma, according to my synthesis

of Wheeler diagrams.

Within each stage of arcward migration, stratal terminations show apparent downlap onto

unconformities PKBPD-1, LB2-B and LB2-C. Reflectors at the base of LB2-B and LB2-C

exhibit toplap and occasionally truncation, suggesting minor erosion along these surfaces. These

migrating packages likely signify the influence of tectonic driving mechanisms that are capable

of tilting the outer accretionary wedge during deposition of LBU2. One speculated theory is that

the growth and deformation of the outer accretionary wedge created accommodation over time as

material progressively accreted at the frontal thrust zone and formed irregular topography.

However, this does not explain the degree of arcward tilting. Seamount subduction has been

proposed as a catalyst for uplift of the upper KFB. I infer that the same process could have

contributed to uplift and subsidence of the paleo-outer wedge as a seamount subducted, which is

broadly consistent with present-day seamount spacing and subduction rates that suggest

seamount collision every ~1 Myr. The upper and lower KFB appear comparable in terms of

70

relative timescales, with the exception of a noticeable discrepancy in sediment flux. High

sediment accumulation rates in the upper KFB are probably related to climatic forcings of the

INHG that began ~2.75 Ma and accompanied by a sea level drop and a delay of sediment

delivery to continental margins. Moreover, deepwater sediment routing patterns are highly

influenced by the bathymetric surface. Therefore, the evolution of the Nankai margin likely

altered the transport of turbidity currents and their blanket-like deposits by inherently changing

the surface topography over time. This study also underscores how sequence-stratigraphic

approaches applied to different tectonic settings and carried out over fine scales can yield new

insight into tectonostratigraphically complex systems like the Kumano Basin. Additional detailed

and quantitative studies of the aforementioned mechanisms in the lower Kumano Basin are

needed to further comprehend this margin and also to understand the dominant controls on

forearc basins in general.

71

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78

APPENDIX A

ARC-TRENCH SYSTEMS

The arc-trench system is the product of convergent plate junctions and typically consists

of distinctive morphotectonic elements (Dickinson, 1973; Dickinson, 1995; Hamilton, 1988).

Dickinson (1973, 1995) delineated five major elements in the arc-trench system: 1) the trench,

which marks the deepest bathymetrical feature; 2) the subduction zone – the interface in which

the denser oceanic plate subducts below either a continental plate or a less dense oceanic plate;

3) arc-trench gap, which includes the region that a forearc may inhabit; 4) the magmatic arc in

which intra-arc basins may be found; and 5) the backarc region in which either an interarc basin

overlying oceanic crust and separated from the back of the arc by a normal fault system exists, or

a retroarc basin overlying continental basement and separated from the back of the arc by a thrust

fault system can be found. The lateral continuity and symmetry between the volcanic arc and

trench imply close linkage of tectonic deformation along a representative transect perpendicular

to the trench axes. Several other morphotectonic categorizations exist for the arc-trench system,

such as a simpler three-zone classification: the subduction zone, the arc-trench gap, and the arc-

rear area (Hamilton, 1988; Dickinson, 1995). On the other hand, more defined sub-categories are

specific to their arc-trench settings, like the elements illustrated by Moore et al. (2009) for the

Nankai subduction zone. Regardless of the specificity of geomorphological classification

schemes, the main point is that there are distinct phases of tectonic development along a

representative transect of the active continental margin that can be identified using surficial

(Dickinson, 1973; Dickinson and Seely, 1979; Hamilton, 1988; Dickinson, 1995; Graindorge et

al., 2008), and oftentimes, subsurface evidence.

79

SUMMARY OF IODP REPORTS ON LITHOLOGY AND WELL LOGS

Unit IV - Site C0002

Starting stratigraphically from the bottom of the core, Unit IV is composed of silty

claystone to clayey siltstone with siltstone and sandstone. The mudstone in this section is highly

fractured and gray to greenish gray and dark gray. Internally, the mudstone contains local wavy

laminae defined by darker green colors that indicate higher clay content. The mudstone has a

mottled appearance. Siltstone bands are rare, but sandstone layers occur occasionally and are

sometimes cemented by calcium carbonate. The silt and sand grains are mostly composed of

quarts, feldspar and an array of heavy minerals including epidote, green amphibole, blue

amphibole and zircon. Compared to Unit III above, the presence of calcareous nannofossils is

significantly lower in comparable mudstone sections (Expedition 315 Scientists, 2009).

Logging results show that both uniform intervals and intervals of high variability are

present in Unit IV. Some of the highly variable units are similar to the fining upward cycles

found in Unit II while the uniform intervals of high gamma ray and density values correspond

with considerably less variable resistivity readings and a mottled appearance in the deep

resistivity image log. The log readings and image log within the homogeneous section can be

interpreted as dismembered remnants of clay-rich horizons. Another possible interpretation is

that this unit may consist of deformed interbeds of sand and mudstone with local mass flow

deposits. This is supported by bedding that are significantly steeper (~18° - 60°) in Unit IV from

both the image logs and core data. Unit IV is interpreted from the seismic data to be the upper

accretionary prism fill of trench turbidites (Expedition 314 Scientists, 2009 and Expedition 315

Scientists, 2009).

Unit III – Site C0002

An abrupt change in structural and lithologic characteristics marks the boundary between

Unit III and Unit IV. Unit III is composed primarily of condensed mudstone – a sharp contrast

with the interbedded mudstone, siltstone and sandstone in Unit IV. The dominant lithology is

greenish gray, gray, and gray-brown mudstone (silty claystone) with abundant calcareous

nannofossils within the mudstone. Bioturbation is widespread and diverse. Rare bedding and

irregular volcanic ash lenses were noted. Glauconite presence was confirmed with smear analysis

of green particles ranging from sand to gravel size within zones of intense green mineralization.

Other notable features include sharp-topped zones as evidence of scouring and reworking of

compacted clay-rich sediment and subvertical sigmoidal clay-filled “vein structure.” The veins

seem to correlate with changes in lithology; veins are spaced wider apartin softer gray and gray-

brown mudstone and more closely spaced within the mineralized green intervals (Expedition 315

Scientists, 2009).

Relatively uniform gamma ray, bulk density, and neutron porosity readings throughout

Unit III at Site C0002 indicate the homogeneity of Unit III lithology compared to the units above

and below. These log characteristics are consistent with core interpretations of hemipelagic mud.

The few occurrences of breakouts in gamma ray, PEF, and density log values occur in the middle

portion of logging Unit III. Sharp decreases in gamma ray, PEF, and density correlate with peaks

80

in resistivity. These are interpreted to mark the occurrence of thin silt and sand beds (Expedition

314 Scientists, 2009).

Unit II – Site C0002

The dominant lithology from core recovery is greenish gray to grayish green mud (silty

clay to clayey silt). Thin interbeds and irregular patches of silt, sandy silt, and sand were

occasionally found. The mud found in Unit II is significantly coarser than the units

stratigraphically below it. The mud contains plane-parallel laminae with absence of features

locally. There is occasional, mild bioturbation with sparse calcareous nannofossils in the clayey

silt intervals. Thin turbidite interbeds are typically <5 cm thick with sharp bases, faint laminae,

normal size grading, diffuse tops. Heavy minerals are abundant in these intervals. The EOD of

Unit II is a basin-plain type environment with frequent deposition of turbidites via submarine

canyons and gullies (Expedition 315 Scientists, 2009). Logs generally show higher sand content

than what was recovered from core.

Unit I – Site C0002

Unit I is composed of similar lithology to Unit II but distinguished from it primarily due

to compaction characteristics. The boundary between Unit I and Unit II shows a clear gamma ray

shift from low gamma to higher gamma values, an abrupt lowering of neutron porosity and

increase in resistivity. Unit I is approximately 140 m thick at Site C0002 and consists of silty

clay to clayey silt, sand, and silt turbidites with thin volcanic ash layers. The major lithology is

greenish gray mud enriched in foraminifers. Secondary lithologies contain thin interbeds of

greenish gray medium to fine sand, silty sand, sandy silt, and silt. The coarser interbeds are

interpreted to be turbidites.

Overall, the unit exhibits a fining upward pattern and the environment of deposition

(EOD) is interpreted to be in a distal basin plain environment with submarine canyons and

gullies occasionally feeding fine-grained turbidites into the system (Expedition 315 Scientists,

2009). Low gamma ray readings throughout Unit I and the homogeneously conductive character

from resistivity logs agree with this interpretation. Neutron porosity decreases with depth

indicating an increasing compaction trend going stratigraphically down into Unit II. Logging

data shows a sharp increase in neutron porosity, sonic velocity, and bulk density (Expedition 314

Scientists, 2009). Unit I is interpreted to be upper forearc basin deposits.

81

Table 2. Summary of drill holes with relevant data to this study. See Expedition 314 Scientists (2009), Expedition 315 Scientists (2009), Expedition 316

Scientists (2009), and Expedition 319 Scientists (2010).

Hole Location

TD (drillers depth

below rig floor, m)

Water depth

(mbsl) LWD Wireline Core Cuttings Biostrat. Magnetostrat.

Ex

ped

itio

n 3

14

C0002A 33°18.0192′N,

136°38.1810′E 3337.5 1936 X

C0004A

(pilot hole)

33°13.2424′N,

136°43.3349′E 3032 2632

C0004B 33°13.2264′N,

136°43.3461′E 3037 2637 X

Ex

ped

itio

n 3

15

C0002B 33°17.9928′N,

136°38.2029′E 3023 1937.5 X X X

C0002C 33°18.0026′N,

136°38.1869′E 1978.9 1936.6 X X

C0002D 33°18.0075′N,

136°38.1910′E 2169.62 1937.12 X X X

Ex

ped

itio

n 3

16

C0008A 33°12.8229'N,

136°43.5997'E 3137.25 2751 X X X

C0008B 33°12.7313′N,

136°43.6727′E 2835 2797 X

C0008C 33°12.7313′N,

136°43.6727′E 3001.7 2797 X X X

C0004C 33°13.2278′N,

136°43.3312′E 2790.5 2627 X X X

C0004D 33°13.2190′N,

136°43.3287′E 3059 2630.5 X X X

Ex

ped

itio

n 3

19

C0009A 33°27.4704′N,

136°32.1489′E 3686 2054 X X X

82

Figure 30: (A) Screenshot of inline 2529 to compare and contrast with horizons mapped in the Gulick et al. (2010) study. Several horizons are

equivalent to one another across the studies. For example, UB-D in this study is equivalent to K4 and UB-A is equivalent to K6 in Gulick et al.

(2010). (B) Below: Inline 2529 from Gulick et al. (2010) study (Figure 6) showing interpreted horizons in the upper Kumano Basin and normal faults.

HORIZON COMPARISON TO PREVIOUS STUDIES

Horizons from the Gulick et al. (2010) and Moore et al. (2015) studies were replicated to distinguish the difference between the

horizons in their studies and the horizons that originate from biostratigraphy and stratal terminations that were used in this study. The

next few figures document these comparisons by mimicking the seismic amplitude, vertical exaggeration, and horizon location of

Gulick et al. (2010) and Moore et al. (2015) study’s figures as closely as possible. Horizon data from Gulick et al. (2010) were not

available for direct input into the seismic volume. Thus, these comparisons are largely qualitative. Table 3 documents all horizons

picked in this project compared to the horizons from Gulick et al. (2010) and Ramirez et al. (2015).

2500 m

VE = ~8X

(A)

83

(B)

84

1 km

(B) (A)

Figure 31. Comparison of crossline 6850 between horizons derived from biostratigraphy tops and Gulick et al. (2010). (A) Crossline 6850 made in

Kingdom Suites for comparison to Figure 11 in Gulick et al. (2010). Some horizons (labeled in red) are the same between both figures. For example, UB-A,

which was interpreted in this study as an onlap surface is equivalent to K6 horizon in (B). The dated horizon 0.905_9 is equivalent to K4. (B) Figure 11 in

Gulick et al. (2010).

85

Figure 32: Comparison of inline 2532 from Figure 5 of Moore et al. (2015). All horizons from Moore et al. (2015) were replicated from the Gulick et al. (2010)

study. Many of the horizons in this study (solid lines in (B)) do not match the horizons from Moore et al. (2015) and Gulick et al. (2010) in (A), which means

their studies did not rely primarily on biostratigraphy to choose horizons.

(B) (A)

86

My

horizons

Gulick et

al. (2010)

horizons

Ramirez et

al. (2015)

horizons

HORIZON CHARACTERISTICS

seafloor K1 first strong (+) amplitude encountered; marks transition from water to

seafloor K2 continuous, weak (+) amp. reflector

K3 continous, (+) amp. reflector

UB-D K4 strong (-) amp. reflector; onlap surface

0.905_9 K5 weak (+) amp.; decreases in reflectivity going NW; downlaps onto

PKBPD-2/KL/S3 and PKBPD-3/S4

UB-C continuous (-) amp. reflector; onlap surface; downlaps onto PKBPD-

2/KL/S3

UB-B weak (+) amp. onlap reflector; discontinuous to transparent character

in some areas of volume below -2500 m; downlaps onto PKBPD-

2/KL/S3

UB-A K6 continuous (-) amp. reflector; onlap surface; downlaps onto PKBPD-

2/KL/S3 K7 continuous (+) amp. reflector

K8 strong (+) amp. reflector

1.24_2 weak (-) amp. reflector; transparent and chaotic below -2500 m

1.34_2 K9 (+) amplitude reflector; downlaps onto PKBPD-2/KL/S3

1.46_2 weak (-) amp. reflector; transparent and chaotic throughout volume

K10 continuous, strong (+) amp. reflector

1.57-1.62_2 continuous strong (-) amp. reflector

K11 (+) amp. reflector

1.67_2 continuous (+) amp. reflector

K12 strong (+) amp. reflector

2.52_9 (-) amp. reflector; onlaps PKBPD-1/S2

2.87_9 discontinuous (+) amp. reflector; onlaps TAP/S1

3.65_9 discontinuous (+) amp. reflector; onlaps TAP/S1

PKBPD-3 KL S4 unconformity; reflectors below surface are either truncated or exhibit

toplap; reflectors above downlap onto this surface

LB3-C continuous, strong (-) amp. reflector; truncates against PKBPD-3/S4

LB3-B continuous, strong (-) amp. reflector; onlaps PKBPD-2/KL/S3

LB3-A weak (+) amp. reflector; chaotic reflection in some areas; onlaps

PKBPD-2/KL/S3

PKBPD-2 KL S3 unconformity; amplitude changes across volume; reflectors downlap

onto surface and are truncated below this horizon

LB2-D continuous, weak (-) reflector; confined to western edge of D2

LB2-C unconformity; (+) reflector; terminates against PKBPD-2/KL/S3 to

SE; onlaps PKBPD-1/S2 to the NW; truncates reflectors below;

reflectors downlap onto surface from above

LB2-B unconformity; weak, (+) reflector; truncated by LB2-C in some areas;

truncates reflectors below; reflectors downlap onto surface from

above

LB2-A weak, (+) reflector; confined to D2

PKBPD-1 S2 unconformity; amplitude changes across volume; reflectors are

truncated below surface; onlap/downlap onto surface

87

LB1-O chaotic to transparent in the very NW extent; (-) reflector confined to

D3

LB1-N continuous, (-) reflector; truncates against PKBPD-1/S2 to the SE;

onlaps TAP to the NW; mostly confined to D2

LB1-M continuous, strong (-) reflector; truncates against PKBPD-1/S2 to the

SE in D2; onlaps LB1-L or becomes untraceable due to irresolvable

chaotic reflectors to the NW

LB1-L continuous (-) reflector except when transparent/chaotic towards the

NW to its very SE extent (D3); truncates against PKBPD-1/S2 in SE

(D1)

LB1-K weak, discontinuous (-) reflector; onlaps TAP to the SE; almost

transparent to the NW

LB1-J transparent, weak (+) reflector; only exists in D3

LB1-I discontinuous, (+) reflector; only exists in D3

LB1-H weak, discontinuous (-) reflector; truncates against PKBPD-1/S2

towards SE; onlaps LB1-E to the NW

LB1-G discontinuous (+) reflector; truncates against PKBPD-1/S2 towards

SE

LB1-F continuous, weak (+) reflector; exists only in D2

LB1-E discontinuous (-) reflector; truncates against PKBPD-1/S2 towards

SE and is tranparent in some areas

LB1-D (+) reflector; discontinuous in some areas; truncates against PKBPD-

1/S2 and becomes more tranparent in D1 (SE)

LB1-C continuous (+) reflector in D1 and D2; more transparent character in

D1 and truncates against PKBPD-1/S2 to the SE

LB1-B continuous (+) reflector mostly confined in D2

LB1-A continuous (+) reflector confined in D2

TAP S1 unconformity; amplitude changes across volume; characterized by

chaotic to transparent, highly dipping beds below and high amplitude

reflectors above

Table 3. Description of horizons within this thesis and comparison to previously mapped horizons by Gulick et al.

(2010) and Ramirez et al. (2015). Bold horizons indicate common horizons.

88

APPENDIX B

SEISMIC DATA INTERPRETATION INVENTORY

Location and access of Petrel seismic project in Sed. Lab workstation

Locations of files in Sed. Lab workstation (519 Deike Bldg, University Park, PA 16802)

o HAJEK_HDD1_2015 (M:) > Nana_Kumano > Petrel Project

Two versions of exported horizons under “Petrel_exported_horizons_NX”

Kingdom 3-D interpretation lines (ASCII file type) under folder

named “Horizons_Kingdom 3-D versions”

IESX 3-D interpretation lines (ASCII file type) under folder named

“Horizons_IESX 3-D versions”

Well logs (LAS files) under “Well Logs” in Petrel Project folder

Project name: Kumano_final_version

o Petrel project file “.pet” extension

o Petrel “.ptd” folder

Overview of Petrel project contents

o Horizons

o Faults

o Surfaces (interpolated from horizons)

o Well locations of C0002 and C0009 and associated well logs

o C0002 and C0009 well tops

o Location and interpretations along arbitrary seismic lines

o Well-to-well cross section correlation

Location and access to Petrel seismic project on Scholarsphere (open access)

Go to http://www.scholarsphere.psu.edu

Search for user “Yang Xu” or “Xu_Kumano_Petrel_project” in search bar

All files are located in collection “Xu_Kumano_Petrel_project”

Open readme file in collection for specifics on how to navigate through the Petrel project.

Refer to Table 3 in Appendix A and Table 4 and 5 in Appendix B for inventory of data

interpretations

89

HORIZONS FAULTS SURFACES SEISMIC CROSS

SECTIONS

THICKNESS

MAPS

seafloor Fault interpretation 1 PKBPD-3 surface -

smoothed 1x1:combo

C0002 to C0009

well correlation

Thickness between

PKBPD-2 surface

and LBU2-C surface

UB-D Fault interpretation 2 PKBPD-3 surface Arbitrary Polyline

1_along depocentre

Thickness between

LBU2-C surface and

LBU2-B surface

0.905_9 Fault interpretation 3 PKBPD-2 surface Arbitrary Polyline

2_along depocentre

Thickness between

LBU2-B surface and

PKBPD-1 surface

UB-C Fault interpretation 4 LB2-C surface Arbitrary Polyline

3_perp structure LBU1

UB-B Fault interpretation 5 LB2-B surface Arbitrary Polyline

4_perp structure LBU2

UB-A PKBPD-1 surface LBU3

1.24_2 TAP_surface_smoothed_3x3

1.34_2 TAP_surface

1.46_2

1.57-1.62_2

1.67_2

2.52_9

2.87_9

3.65_9

PKBPD-3

LB3-C

LB3-B

LB3-A

PKBPD-2

LB2-D

LB2-C

LB2-B

LB2-A

PKBPD-1

LB1-O

LB1-N

LB1-M

LB1-L

LB1-K

LB1-J

LB1-I

LB1-H

LB1-G

90

LB1-F

LB1-E

LB1-D

LB1-C

LB1-B

LB1-A

TAP

Table 4. Inventory of horizons, faults, surfaces, seismic cross sections and thickness maps generated in the Petrel

project.

91

C0002 WELL TOPS C0009 WELL TOPS

Z (m) MD (m) SURFACE Z (m) MD (m) SURFACE

1954.85 1954.85 0.063_2 2521 2521 I/II_IODP

1987.8 1987.8 0.3-0.4_2 2766 2766 0.43_9

1987.8 1987.8 0.436_2 2836.7 2836.7 0.905_9

2086.5 2086.5 1.04_2 2856.7 2856.7 II/IIIA_IODP

2107.9 2107.9 1.078_2 2954 2954 1.34_9

2422.3 2422.3 1.24_2 2961.7 2961.7 1.6_9

2450.4 2450.4 1.34_2 2961.7 2961.7 1.6_9

2550.9 2550.9 1.46_2 2984.2 2984.2 2.06_9

2616.5 2616.5 1.57-1.62_2 3009.2 3009.2 2.39_9

2799.5 2799.5 1.67_2 3036.7 3036.7 2.52_9

2812.7 2812.7 2.06_2 3036.7 3036.7 2.5

2824.7 2824.7 2.87_2 3112 3112 2.87_9

2873.7 2873.7 3.65_2 3291.7 3291.7 3.65_9

2880.8 2880.8 TAP_IODP 3331.7 3331.7 3.79_9

2888.3 2888.3 5.59_2 3331.7 3331.7 3.79

2963.3 2963.3 5.9_2 3341.7 3341.7 5.59_9

3341.7 3341.7 5.6

3342 3342 TAP_IODP

3541.2 3541.2 7.1_9

3657.2 3657.2 7.9-8.5

Table 5. Well tops for Site C0002 and Site C0009.