Basic Building Blocks & Process Variability of a Cretaceous Delta - JSR, 2007

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Journal of Sedimentary Research, 2007, v. 77, 284–302

Research Article

DOI: 10.2110/jsr.2007.023

BASIC BUILDING BLOCKS AND PROCESS VARIABILITY OF A CRETACEOUS DELTA: INTERNAL FACIESARCHITECTURE REVEALS A MORE DYNAMIC INTERACTION OF RIVER, WAVE, AND TIDAL PROCESSES

THAN IS INDICATED BY EXTERNAL SHAPE

M. ROYHAN GANI* AND JANOK P. BHATTACHARYA{

Geosciences Department, University of Texas at Dallas, P.O. Box 830688, Richardson, Texas 75083-0688, U.S.A.

e-mail: [email protected]

ABSTRACT: Although delta-building processes and the resultant facies and facies associations have been studied for overa century, there remains a lack of well-documented facies architectural studies of ancient deltas. In this study, architectural-element analysis is applied to determine the basic building blocks of a Cretaceous delta at the top of the upper Turonian WallCreek Member exposed along the western flanks of the Powder River Basin, Wyoming. We document the bed-scale basic

building blocks of this ancient delta, and show them in the context of their facies and stratal variability, and their hierarchicalspatial arrangement within a single deltaic parasequence. We also test the idea that external sand-body shape may not reflectthe internal facies complexity, as has recently been suggested in studies of several modern deltas, including the Burdekin inAustralia, the Brazos in the Gulf of Mexico, and the Baram–Trusan in Borneo.

Five orders of bounding surfaces separate six facies architectural elements in the prodelta and delta front deposits. Theseelements are prodelta fines (PF), frontal splay (FS), channel (CH), storm sheet (SS), tidally modulated deposit (TM), and baraccretion (BA). Seasonal to decadal river floods are thought to represent the main building phases of the delta, producingchannels, bars, and frontal splay elements. During intervening periods, the delta was reworked by waves, storms, and tides,producing tidally modulated and storm sheet elements. Due to the complex interactions of river effluents with waves, storms andtides, the delta is interpreted as mixed-influenced. Regional sandstone isolith, facies, and paleocurrent maps help to reconstructthe paleogeography and show how the various architectural elements varied across the delta front. The channels, bars, frontalsplays, and tidally modulated elements are found only near the distributary mouth, whereas the storm sheets occur extensivelyaway from the distributary mouth.

The plan-view morphology of the studied delta shows a smooth-fronted, arcuate to cuspate shape, which should indicate wave

dominance. In contrast, our analysis of the internal facies architecture shows that the delta was constructed rapidly duringmajor river floods with significant tidal reworking. Additional outcrop mapping shows that storm-wave-reworked sands areattached to the flanks of the system. Previously published delta models predict incorrectly that the tidal reworking of a river-flood dominated system should result in a more bird-foot shape versus the lobate geometry that is actually mapped. Ouranalysis of internal facies versus external shape matches similar observations on several modern deltas and suggests thatexternal shape is a poor indicator of internal facies complexity.

INTRODUCTION

Plan-View Morphology vs. Internal Facies Architecture of Deltas

The still popular tripartite classification of deltas into wave-, tide-, and

river-dominated end members (Galloway 1975) was based largely on

knowledge about the present-day fluvial, wave, and tidal regime as well as

the variable plan-view shape of the deltas as observed from air photos or

coastline maps (Coleman and Wright 1975). Although the process-based

tripartite classification of deltas (Galloway 1975) has been extended to

include changeof relative sealevel(Dalrymple et al.1992) andvariabilityof

grain size (Orton and Reading 1993), only very limited information about

facies variability was available, primarily from scattered cores or surface

distribution of the modern sediments, in developing these models. Asa consequence,thereremain almost no studies of moderndeltasystems that

show how internal facies vary laterally (Hori et al. 2002; Ta et al. 2005) andnone that document the bed-scale facies architecture (Gani and Bhatta-charya 2005a). Although there have been numerous seismic examples from

Quaternary systems that show bed-scale architecture, these generally lackcore data, and facies and lithologic variability are thus difficult to address

(e.g., Suter and Berryhill 1985; Sydow and Roberts 1994; Hart and Long1996; Hiscott 2003; Roberts and Sydow 2003; Anderson and Fillon 2004;

Sidi et al. 2004; Cattaneo et al 2003; Correggiari et al 2005). In addition,most other subsurface and outcrop studies emphasize the more regional-

scale sequence stratigraphy, as opposed to the internal facies architecture(e.g., Bhattacharya and Walker 1991; Plint 2000; Bhattacharya and Willis

2001; Willis and Gabel 2001; Porebski and Steel 2003; Plink-Bjorklund andSteel 2005). Examples of lacustrine deltas studied with ground penetrating

* Present address: Energy & Geoscience Institute (EGI), University of Utah,

423 Wakara Way, Suite 300, Salt Lake City, Utah 84108, U.S.A.

{ Present address: Department of Geosciences, University of Houston, 312,

Science and Research 1, 4800 Calhoun Road, Houston, Texas 77204-5007, U.S.A.

Copyright E 2007, SEPM (Society for Sedimentary Geology) 1527-1404/07/077-284/$03.00



radar reveal bed-scale geometry but also lack core and facies calibration

and are not applicable to marine systems affected by tides (Jol and Smith

1991; Smith et al. 2005).

In subsurface analysis of ancient systems the shape of deltaic sandbodies is routinely mapped using well logs (e.g., Fisher et al. 1969; Busch

1971; Duncan 1983; Bhattacharya and Walker 1991; Maguregui and

Tyler 1991; Bhattacharya and Willis 2001), but core data (which give key

facies information) may be limited in availability. A basic assumption in

applying models of modern deltas to such ancient systems is that theexternal plan-view morphology faithfully reflects the internal framework

facies (Galloway 1975). The inverse is also true. In basins with verylimited well control and a few cores, or where only outcrop are available,

facies composition might be used to predict sand-body geometry away

from control points.

Recent facies-oriented studies of several modern deltas have challenged

the long-held assumption that external shape predicts internal facies. The

modern Brazos and Burdekin deltas, for example, show smooth-frontedplan-view shapes, traditionally classified as wave-dominated end mem-

bers, but recent detailed internal facies analyses of these deltas (Rodriguez

et al. 2000; Fielding et al. 2005) suggest a dominance of river processes.On the other hand, the modern Baram and Trusan delta in NW Borneo

shows wave-dominated and river-dominated shoreline geometry, re-

spectively, but both are characterized by tide-dominated internal faciesdistribution (Lambiase et al. 2003). Clearly what is needed are more

studies of systems that allow integration of internal bed-scale faciesarchitecture with plan-view morphology.

Outcrops of deltaic deposits in the Wall Creek Member of Wyoming

provide such an opportunity. The outcrops are exposed on the well-

drilled flanks of the adjacent Powder River Basin. The arid climateaffords superb cliff exposures through these systems, allowing detailed

mapping of facies architecture and lateral facies variability. In addition,

adjacent well logs allow us to constrain the 3D geometry of the sedimentbody and then to compare internal facies variability with external

morphology in much greater detail than can be achieved by looking at

a modern system. The goal of our research is thus to integrate the study of

the internal process sedimentology with architectural-element analysis

placed in the broader context of overall delta morphology.

Concept of Facies Architecture

Stratigraphic architecture describes the stacking patterns of strati-

graphic units at the regional scale (e.g., sequence stratigraphy), whereas

facies architecture focuses on the stacking patterns of facies units within

a depositional system at a local scale (e.g., architectural-element analysis).The concept of facies architecture and architectural-element analysis was

originally developed for fluvial and eolian rocks (Jackson 1975;

Brookfield 1977; Allen 1983; Miall 1985, 1996). The internal architecture

of fluvial (e.g., Miall 1996; Bridge 2003) and deep-marine (e.g., Boumaand Stone 2000; Mutti et al. 2003) deposits have been studied far more

extensively than their deltaic and shallow marine counterparts. The

architectural-element models of fluvial (e.g., Miall 1985, 1996) anddeepwater (e.g., Ghosh and Lowe 1993; Pickering et al. 1995) depositional

systems are also well established. On the other hand, although general

facies and facies associations of deltas have been summarized (Postma

1990; Bhattacharya and Walker 1992; Reading and Collinson 1996),detailed facies architectural studies of deltaic deposits in outcrops are few

(Willis et al. 1999; Mellere et al. 2002; Anderson et al. 2004; Johnson and

Graham 2004; Olariu et al. 2005; Plink-Bjorklund and Steel 2005). Deltaicdepositional systems show greater facies architectural complexity and

process variability than fluvial and marine depositional systems, because

deltas mark the crucial link between the latter two depositional systems.

The present study is the first of its kind to propose an architectural-

element model for open-marine deltaic deposits in an outcrop.

Architectural elements are the 3D building blocks of a depositional

system. These architectural elements commonly represent a, or part of a,

morphological element. In modern depositional environments important

sandy morphological elements include channels, bars, large bedwaves,

and splays. These morphological elements migrate and/or grow to

produce architectural elements in the rock record. The 3D shape of

a morphological element could be significantly different than that of

a corresponding architectural element, and depends on how a morpho-

logical element evolves, deposits, and is modified through time. Forexample, migrating hummocky bedwaves may produce a storm sheet

sand. In some cases morphological elements may be preserved in the rock

record. Although architectural elements can vary in type from one system

(e.g., delta) to another (e.g., fluvial), or even within the same system in

time and space, there should be a finite number of architectural-element

types in any given depositional system. How far a given deposit should be

broken down in terms of facies architectural elements involves a balance

of how thoroughly but meaningfully the architecture of a deposit can be

described. Building blocks (architectural elements) bind with each other

in 3D space via bounding surfaces. Permutation and combination (both

linear and non-linear) of building-blocks can give rise to an entire

depositional system.

Elucidation of architectural elements in shallow marine and deltaic

systems is important for several reasons, first, architectural elements linkto specific morphometric features, such as bedwaves, mouth bars, and

channels, which typically scale to a specific aspect of flow conditions and

are thus potentially useful in hydrodynamic analysis (e.g., Bhattacharya

and Tye 2004; Willis 2005). This may be critical in developing more

quantitative measures of the controlling parameters in predicting delta

morphology and growth, which historically emphasizes in a qualitative

way the degree of fluvial versus wave and tidal influence, among other

factors. Second, surface-bounded geobodies, and specifically architectural

elements, are the building blocks routinely used in reservoir and aquifer

characterization and fluid-flow modeling (Reynolds 1999; Tye 2004).

Thirdly, surface-bounded, bed-scale architecture provides a fundamental-

ly different view of how subsurface facies should be correlated, as is

described in detail by Gani and Bhattacharya (2005a), versus the layer-

cake correlations that are typically presented in evaluation of many

modern delta systems. The architectural-element approach has been

enormously useful in the analysis of fluvial and deepwater deposits

(Posamentier and Walker 2006) but remains nascent with respect to

application to shallow marine and deltaic systems, and we believe that

this stems from a lack of well studied examples and models.

Objectives of This Study

The objectives of this paper are to identify the basic architectural

elements (building blocks) and bounding-surface hierarchies in an ancient

delta; to examine the relationship between external geometry and internal

facies composition by reconstructing the paleogeography of this delta;

and to compare it with modern deltas to understand the process

variability. The availability of subsurface data adjacent to the outcrops

as well as multidirectional cliff exposures led us to focus on the WallCreek Member of the Frontier Formation (Late Cretaceous) in central

Wyoming (Figs. 1, 2).

A companion study (Lee et al. 2007, this issue) used 2D and 3D ground

penetrating radar to investigate the 3D geometry of the bar and channel

elements described in greater sedimentological detail in this paper.

REGIONAL GEOLOGY

The Cretaceous Western Interior Seaway (WIS) stretched roughly

north–south across central North America, and developed as part of an

eastward migrating and asymmetric retroarc foreland basin lying between

BUILDING BLOCKS AND PROCESS VARIABILITY OF A DELTA 285J S R



FIG. 1.— A) Cretaceous paleogeography of the Western Interior Seaway during the middle Turonian. During this time the Wall Creek Member was deposited ina foreland basin sourced from the Sevier belt in the west (modified from Bhattacharya and Willis 2001, and others). B) Location map showing the positions of outcropand subsurface data, and the Wall Creek outcrop belt at the western margin of the Powder River Basin, Wyoming. The present study focuses at Raptor Ridge. C) RaptorRidge site, showing the studied cliff sections and well locations behind the cliffs. Note the position of Figures 2B, 5A, B, C, D, and 6.

286 M.R. GANI AND J.P. BHATTACHARYA J S R



the Cordilleran volcanic arc to the west and the hinterland to the east

(Fig. 1A; Lawton 1994). A pronounced and widespread Cenomanian–

Turonian transgression led to the joining of an epeiric seaway with the

Tethyan Sea towards the south (Leckie et al. 1998). The event was

followed by the deposition of a clastic wedge (the Wall Creek Member;

Fig. 2A) of Late Turonian age (Cobban 1990; Gardner 1995a, 1995b) in

a ramp setting in Wyoming. The Wall Creek Member was sourced from

an uplift in the west linked to thrusting associated with the Sevier

Orogeny (Fig. 1A), and later itself was deformed into a gently tilted series

of anticlines during the Maestrichtian–Tertiary Laramide Orogeny

FIG. 2.— A) Cretaceous stratigraphy of the Powder River Basin, Wyoming (modified from Willis et al. 1999). B) Outcrop type section of the Wall Creek Member atRaptor Ridge site, where Sandstone 7 was not deposited. For location of the sections see Figure 1C. This study deals with Sandstone 6. C) Lithologic and ichnologicsymbols used in logs.




(Wiltschko and Dorr 1983; Cobban et al. 1994). Numerical paleo-oceanographic modeling coupled with facies observations of the seaway

(Ericksen and Slingerland 1990; Slingerland et al. 1996), particularlyduring the Turonian, suggests that: (1) the seaway was generally wave-dominated with mainly southward-directed longshore drift along its

western margin; (2) both winter storms and summer storms (hurricanes)affected the seaway in such a way that storm-driven currents were shore-parallel, predominantly to the south, but occasionally to the north; (3)

most parts of the seaway were in a microtidal setting, but tides along thewestern shores were enhanced locally.

Seven major ‘‘Sandstones’’ (informal nomenclature; Fig. 2) depositedin shallow marine environments under cyclic regressions and transgres-sions during lowstand have been identified in a regional subsurfacestratigraphic study of the Wall Creek Member (Sadeque et al. in press),although several additional younger sandstones have been mapped

farther to the south and east. Each of the seven sandstones of the WallCreek Member shows one or more upward-coarsening facies successions,called parasequences, bounded by a marine flooding surface commonlymarked by a chert-pebble and shark-tooth lag (Fig. 2; Gani et al. in press;

Sadeque et al. in press). The lower parasequences (Sandstones 1 to 3 and5; Fig. 2) are present only locally and look like classic storm-wavedominated shoreface deposits. Sandstone 4 is far more extensive, and

extends into the subsurface, but shows a rather uniform internal faciescharacter in outcrop. It is also interpreted as a wave-dominated shoreface

type deposit. Sandstone 6 is less extensive than Sandstone 4 but is exposedcontinuously for many tens of kilometers along the outcrop belt. AtRaptor Ridge, both strike-oriented and dip-oriented exposures are

available. Sandstone 6 shows a depositional pinchout in the subsurfacebut within 20 km of the outcrop belt. Reconnaissance mapping of theunit showed it to have much greater facies variability than Sandstones 1to 5, with indications of fluvial, tidal, and wave influence. The youngerSandstone 7 is exposed only locally and occurs primarily in subsurface.

Although the Wall Creek Member was previously interpreted as classic‘‘wave-dominated’’ shoreface deposits (e.g., Winn 1991), Sandstone 6(Fig. 2B) shows varying amounts of wave, storm, tide, and river influenceas demonstrated by a ichnologically based study (Gani et al. in press).Subaerial deposits are notably absent in these deposits because of

ravinement during repeated marine transgressions. In the Powder RiverBasin, the seven sandstones of the Wall Creek Member have beencorrelated regionally in outcrop and in subsurface (Lee et al. 2005;Sadeque et al. in press).

Because much of the Cretaceous is thought to represent ice-free

greenhouse conditions, the origin of these stratigraphic cycles hasvariously been attributed to Milankovitch climatic cycles (Scott et al.1998), foreland basin tectonics related to thrust loading, forebulgemigration (Gardner 1995b; Bhattacharya and Willis 2001), or intraplate

stresses (Vakarelov et al., 2006). However, Miller et al. (2004) haverecently claimed that continental ice sheets paced sea-level changes duringthe Late Cretaceous. Accordingly, the stratigraphic cycles of theCretaceous seaway may represent a composite in-phase and/or out-of-phase interplay between glacio-eustatic and tectonic forcing.

Among the seven ‘‘Sandstones’’ of the Wall Creek Member, Sandstone6 is considered to be the most representative of a delta, inasmuch as itshows greater facies variability and architectural complexity than theunderlying wave-dominated parasequences. Sandstone 6 also crops out inreadily accessible and multidirectional cliff sections, making it suitable for

a detailed facies architectural investigation. The regional stratigraphicsections of Lee et al. (in press) and Sadeque et al. (in press) demonstratethat Sandstone 6 pinches out both northward and eastward, and the

availability of adjacent well logs allows us to fully constrain the shape of the seaward margin, allowing examination of the relationship betweenexternal morphology, as mapped using well logs, versus the internalarchitecture as seen in the cliff sections.

DATA AND METHODS

A series of gullies dissect the Wall Creek outcrop belt and allow

detailed study of Sandstone 6. The strata are gently tilted southeast with

a 6u structural dip. The base of the Cody Shale (a maximum flooding

surface) was used as a datum (Fig. 2D) to map the sand bodies within the

Wall Creek Member using well logs and cores (Sadeque et al. in press). A

series of bentonite horizons both below and above the Wall Creek

Member and a conglomerate lag in the Emigrant Gap Member stand out

as regionally traceable distinct well-log signatures (Sadeque et al. in

press). In this study, gamma and resistivity logs of 28 of these exploratory

wells (Fig. 1B) were used to calculate the thickness of sandstones in

Sandstone 6. Although none of these wells has cores, the boundaries

between mudstones and sandstones were picked in the well logs at

a precision of , 1 m. Along the outcrop belt, a total of 28 outcrop logs

were measured to document grain-size variations, sedimentary structures,

ichnology, and paleocurrents. The ichnofacies approach (e.g., Pemberton

1992; Pemberton et al. 2001) is taken in grouping and interpreting

identified ichnotaxa. The bioturbation index (BI) of Taylor and Goldring

(1993) is used to semiquantitatively measure the degree of bioturbation in

the sediments (BI 0 5 0% bioturbation, BI 1 5 1% to 4% bioturbation, BI

2 5 5% to 30% bioturbation, BI 3 5 31% to 60% bioturbation, BI 4 5

61% to 90% bioturbation, BI 5 5 91% to 99% bioturbation, and BI 6 5

100% bioturbation). A portable scintillometer was used to collect gamma

ray profiles along each of the measured sections.

The concept of facies architecture and architectural-element analysis of

Jackson (1975), Brookfield (1977), Allen (1983), and Miall (1985, 1996)

was followed in this study. However, whether an established scheme of

bounding-surface hierarchies and architectural elements (e.g., Miall 1985,

1996) should be used in every facies architectural analysis is debatable

(e.g., Bridge 1993; Fielding 1993). Miall (1996) defined ‘‘architectural

elements’’ as ‘‘units enclosed by bounding surfaces of third- to fifth-order

rank.’’ However, in this study we used the term ‘‘architectural element’’ in

a more general sense defined as a single stratum or a package of strata

that can be regarded as the smallest but meaningful unit (i.e., building

blocks) enclosed by distinct, through-going surfaces of a depositional

system. Moreover, an independent scheme of ordering and numbering of bounding surfaces was applied in this study. The principles of

superposition and crosscutting relationships were used to trace bounding

surfaces. Bounding surfaces were ranked starting with the lowest (zero)

order and systematically moving up to the highest order, following the

general rule that no surfaces truncate against surfaces of lower rank. It is

the key to produce a ‘‘complete’’ bedding diagram in order to identify

basic building blocks confidently. For example, while tracing beds it was

made sure that every bed was traced completely until it onlaps or

downlaps on and/or truncates against another bed.

In the study area, one particular ridge, here named ‘‘Raptor Ridge’’

(Fig. 1B) exposes the most complex bedding geometry and the most

diverse facies variability of Sandstone 6. At Raptor Ridge, high-

resolution (centimeter-scale) cliff maps (bedding, facies, and shale maps;

Willis et al. 1999) for both depositional dip-oriented and strike-oriented

cliff faces (Fig. 1C) were generated on photomosaic overlays in front of

the outcrop. Ten well cores were taken behind these cliffs (Fig. 1C) to link

with related studies of the ichnology (Gani et al. in press), ground-

penetrating radar, and flow-inhibiting calcite concretions (Lee et al. in

press). To reconstruct the paleogeography of Sandstone 6, paleocurrent

and facies changes were documented along the Wall Creek outcrop belt

for about 20 km around Raptor Ridge. Every measurement was plotted

on a map with the aid of a hand-held GPS receiver (5 m accuracy). The

morphological elements of the Burdekin Delta, thought to be a close

modern analog, were studied using ASTER (Advanced Spaceborne

Thermal Emission and Reflection Radiometer) images (with 15 m




resolution) that were digitally processed using standard remote-sensingtechniques (Jensen 1996).

SEDIMENTOLOGY

Sandstone Isolith Map

Using subsurface well logs and outcrop measured sections, a sandstoneisolith map of Sandstone 6 at Raptor Ridge was generated (Fig. 3). Thethickness data were contoured both manually and independently using anautomated contouring algorithm. In both cases, the overall shape issimilar. Figure 3 shows a representative map combining the two.Paleocurrent data, measured from dune-scale cross-stratification, of thisstudy and using dipmeter data from Sadeque et al. (in press), indicatea northeast–southwest trend of the paleo-shoreline. In Figure 3, the sand

body shows a lobate geometry with a slight shore-parallel elongation(Fig. 3), indicating a possible deltaic origin (e.g., Coleman and Wright1975; Bhattacharya and Walker 1992). The lithofacies, ichnology, andbedding geometry of Sandstone 6, presented later, strongly support thisdeltaic interpretation. The sand body, henceforth referred to as theRaptor Delta, shows two distinct bulges indicating two possible sedimentinput points.

Facies and Facies Architecture

Six depositional facies were recognized in Sandstone 6 (Table 1;Fig. 4). These facies correspond closely to six basic architectural elements

(Table 1; Fig. 5) identified in the Raptor Delta. Cliff mapping at RaptorRidge shows consistently seaward-inclined bedding in the depositionaldip sections (Fig. 5A, C), and parallel to subparallel bedding in the strikesections (Fig. 5B, D). The bedding planes in prodelta mudstones show 6u

structural dip towards the southeast, whereas major bedding planes(interpreted as clinoforms) in delta-front sandstones have dips of up to10u in the same direction. Therefore, the depositional dip of theclinoforms is approximately 10u – 6u 5 4u.

Five orders of bounding surfaces have been established (Fig. 5). Zero-order surfaces include incremental-growth surfaces of sedimentarystructures representing ripple cross-lamination, dune-scale cross-stratifi-cation, parallel lamination, and hummocky cross-stratification. First-

order surfaces include dune and hummocky cross-set boundaries, and

accretion stratification within channels. Second-order surfaces bound sets

of first-order surfaces (except accretion stratification), and sets of parallel

lamination. Third-order surfaces bound channels and sets of second-

order surfaces. Most of the second-order surfaces downlap and/or onlap

on to third-order surfaces. The fourth-order surface at the top is a marine

ravinement surface that truncates lower-order surfaces and eroded delta-

plain deposits. This is a regionally traceable surface (extending . 20 km)

strewn with granules and pebbles consisting of chert, sharks’ teeth, shell

fragments, mudstone rip-ups, and sandstone. Six basic architectural

elements (building blocks) have been identified with the help of high-

resolution cliff mapping (Fig. 5).

Element PF: Prodelta Fines.— This element (Fig. 5) consists mainly of

laminated sandy mudstones of Facies 1 (Table 1; Fig. 4A, J) and,

subordinately, interbedded sandstones and mudstones of Facies 2

(Table 1; Fig. 4B).

A diverse Cruziana ichnofacies and high BI (Gani et al. in press) of

Facies 1 suggest an open marine environment of deposition (e.g.,

Pemberton et al. 2001). Significant (, 15%) disseminated sand grains

suggest possible frequent river input, but at low enough rates to be

homogenized by bioturbation. This is typical of a distal prodelta to

offshore environment (e.g., MacEachern et al. 2005).

Interbedded mudstones and sandstones of Facies 2 (Table 1; Fig. 4B)

suggest fluctuating flow conditions. Paleocurrent trends of asymmetrical

ripples are indicative of river currents carrying sands farther seaward.

Rare symmetrical ripples suggest some wave activity. Coal chips indicatea nonmarine source of sediments delivered to the sea via a possible

distributary channel. The trace-fossil assemblage, moderate BI (e.g., Gani

et al. in press; MacEachern et al. 2005), and the occurrence of Facies 2

just above Facies 1 (Fig. 5J) suggest that Facies 2 was deposited in

a proximal prodelta to distal delta-front environment.

Element PF is interpreted to have been deposited in relatively low-

energy conditions with repetitive development of suspension-deposited

strata with rare starved-rippled strata. Transition into overlying delta-

front facies is sharp to gradational with locally developed Glossifungites

ichnofacies (Fig. 6). Local, laterally sharp facies change into the delta

FIG. 3.—Sandstone isolith map shows thatSandstone 6 is lobate with slight shore-parallelelongation. Paleocurrent diagram includes alloutcrop data and shows a northeast–southwestregional shoreline trend. Paleogeography of this

deltaic sandbody is presented in Figure 9. Y–Z isthe outcrop correlation line of Figure 7.






front is indicative of occasional high sediment supply and/or decrease in

local accommodation.

It may not be easy to separate deposits of ‘‘fluid muds’’ (eventdeposition) from background suspension muds, especially if fluid-muddeposits are thin enough to be obliterated by trace makers, which issuspected to be the case for the Raptor Delta. If identifiable, based onsuggestions made by earlier workers (e.g., Dalrymple et al. 2003; Gani2004), these fluid-mud deposits should be treated as a separate

architectural element.

Element FS: Frontal Splay.— This element consists of structureless toparallel laminated sandstone beds of Facies 3 (Table 1; Fig. 4C). Sharp toerosional bases, floating clasts, and structureless to parallel laminationwithin a single bed suggest deposition from waning sediment gravity flows(Collinson and Thompson 1989; Gani 2004, and references therein).Suspension-feeding structures, such as Ophiomorpha and Skolithos,indicate opportunistic colonization of the event beds, followed by thereestablishment of a low-energy, fairweather community generatingdwelling or deposit-feeding structures Planolites, Chondrites, andHelminthopsis (e.g., Pemberton et al. 2001; MacEachern et al. 2005).

Unless truncated, elements FS are mostly continuous across the cliff sections, but they thin basinward (Fig. 5). These elements are interpreted

to originate from non-channelized sediment gravity flows. These flowscould originate from high sediment discharge during river floods, slumpfailures at a river mouth, and/or storm resuspension of bottom sediments.In the first two cases, individual elements are probably lobate-shaped(e.g., Galloway and Hobday 1996) indicative of a point source. However,

they can also form wedge-shaped aprons if sediment gravity flowsoriginate by spilling over subaqueous levees (e.g., Fig. 5B) of distributary

channels, or by storm resuspension. A lack of hummocky crossstratification and wave ripples suggest that storm resuspension is anunlikely mechanism.

Element CH: Channel.— This element consists of channelized sand-

stones of Facies 4 (Table 1; Fig. 4D, E, F, J). Erosional U-shapedepressions and thin lateral wings (Fig. 5B, C) indicate channelized flow,running down the clinoforms towards the southeast (Fig. 5A, C). Low-

angle accretion surfaces (Fig. 4E) dip roughly 90u from inferred channelflow directions, suggesting lateral migration. Abundant floating clasts(Fig. 4F) indicate high (probably . 30% by volume; Gani 2004) sedimentconcentration of the generating flows, and convolute bedding indicatesdewatering shortly after deposition (Collinson and Thompson 1989; Gani2004). In general, beds are devoid of burrows apart from a very few thatpenetrate into the bed from the top surface. This probably indicates thateach channel fill was deposited rapidly from sediment gravity flows.

Internally lateral-accretion stratification of element CH (Fig. 5) areconsidered first order surfaces, as, unlike fluvial lateral accretion surfaces,they formed rapidly from sediment gravity flows. In strike section,channels show lateral wings (Fig. 5B). Channels show a gradualtransition into mouth-bar elements (BA) in a landward direction(Fig. 5A). Locally, they form two-story amalgamated sand bodiesreaching 4 m in thickness. Dimensions of rarely preserved channel formssuggest that these channels were probably , 3 m deep and , 15 m wide.

The depositional mechanism of element CH is considered analogous tothat of channels on a submarine fan (e.g., Peakall et al. 2000; Abreu et al.

2003), although our examples are much smaller in scale compared to thesubmarine examples. Channelized sediment gravity flows can developunit bars within sinuous channels, probably because of the decreasedshear stress within inner banks. If these bars migrate, they produce bar-accretion surfaces, presumably similar to those observed in CH (Figs. 4E,5). The channels described here are probably chute channels, similar tothose described in the delta-front region of fan deltas (Prior et al. 1981;Nemec 1990). If these channels are linked upstream to distributary

channels, they should be called ‘‘terminal distributary’’ channels (Olariuet al. 2005). The relatively steep slopes (4u) of the delta front indicate thatthese channels should show low sinuosity (e.g., Hart et al. 1992, theirfig. 2). These channels probably originated during high sedimentdischarge as bottom-hugging undercurrents. Periodic alternation of CHwith other elements (Fig. 5A) indicates a probable periodicity in channeloccurrence, which could be the annual to decadal (ENSO type; Rodriguezet al. 2000) high water discharge associated with an upstream river flood.

Lack of intra-element bedding surfaces indicates that individual CHelements were probably rapidly filled within a few weeks, probably during

the short-lived peak discharge of a trunk distributary (e.g., similar toBurdekin River discharge; Amos et al. 2004; Fielding et al. 2005).

Element SS: Storm Sheet.— This element consists of primarilyhummocky cross-stratified (HCS) sandstones of Facies 6 (Table 1;Fig. 4I). HCS has been interpreted to be generated by the combined

effect of strong oscillatory flows (dominant) and unidirectional currents(subordinate) during storms (e.g., Myrow and Southard 1991). Gutter

casts, and mud and coal clasts indicate strong basal scouring. Waveripples at tops of HCS beds suggest waning storm waves. The wide rangeof BI is related to the occurrence of this facies distal (BI 3–5) or proximal(BI 0–2) to river source in the direction of both depositional dip and

strike. Based on ichnofacies assemblage and other observations, Facies 6is interpreted to have been deposited in a distal to middle delta front orshoreface environment. Rarely, Facies 6 was found sandwiched withinFacies 1.

At Raptor Ridge, element SS is rare and patchy (Figs. 5, 6). However,northeast from Raptor Ridge, SS contains stacked HCS beds and

develops as extensive sheets (Fig. 7). Element SS records deposits of seasonal and/or rare storms in the sea.

Element BA: Bar Accretion.— This element consists mostly of unidirectional trough cross-stratified sandstones of Facies 5a (Table 1;Fig. 4G, H, J). The trough cross-stratification suggests deposition due to

migration of simple and/or compound (i.e., superposed) dunes of variablesizes. Rare ripple cross-lamination and parallel lamination record local

decreases or increases in flow velocity, respectively. Very low BI is

interpreted to indicate proximity to a river mouth (e.g., Gani et al. inpress; MacEachern et al. 2005). Dominance of the suspension-feedingstructures Ophiomorpha and Skolithos indicates strong currents at the seabed (e.g., MacEachern et al. 2005). Based on these observations, Facies 5ais interpreted to have been deposited in a lower to upper delta-frontenvironment, and is indicative of mostly riverborne sediments andcurrents during periods of active mouth-bar growth.

In element BA, inasmuch as the set (in the case of simple dunes) orcoset (in the case of superposed dunes) boundaries dip at angles of 1u to

10u seaward (Fig. 5A), they are interpreted to represent accreting bar-front surfaces (Fig. 8). These elements are developed extensively in the

upper delta-front region closer to distributary mouths. This suggests thatBAs are produced mainly by river-fed unidirectional currents andsediments. Minor tidal influence can be inferred from locally observedlandward-directed cross-stratification. However, these tidal facies arepatchy and not organized enough to be treated as a separate element (seeTM below). Individual BA elements are interpreted to be formed byannual to decadal growth of bars.

Element TM: Tidally Modulated Deposit.— This element consists

primarily of bipolar dune cross-stratification of Facies 5b (Table 1;Fig. 4H), interpreted to have been deposited from migration of simpleand/or compound (i.e., superposed) dunes of variable sizes. Abundantmud clasts indicate strong currents capable of eroding underlying beds.Abundant herringbone cross-bedding and double mud drapes suggesta tidal influence (e.g., Visser 1980; Nio and Yang 1991). Facies 5b is




FIG. 4.— Representative facies photos of Sandstone 6 at Raptor Ridge. A) Facies 1: sandy mudstones with BI 4–5 including trace fossils Phycosiphon (Phy),Helminthopsis (He), Terebellina (Ter), Planolites (Pl), and Zoophycos (Zo). B) Facies 2: heterolithics comprising interbedded mudstones and rippled sandstones. C) Facies3: massive to parallel-laminated sandstones with erosional bases. Note pencil (14 cm long) for scale. D) Facies 4: channelized sandstones with erosional U-shapedepressions. E) Facies 4: channelized sandstones with lateral-accretion surfaces. Note hammer (30 cm long) for scale. F) Facies 4: mudstone clasts in channel sandstone.Note hammer (30 cm long) for scale. G) Facies 5a: Dominantly seaward-directed (to the right) trough cross-stratified sandstones. H) Facies 5b: Landward-directed (to theleft) trough cross-stratified sandstones with double mud drapes and mud clasts. I) Facies 6: hummocky cross-stratified sandstones (HCS) overlying facies 1. J) Acoarsening-upward succession showing vertical association of facies 1 to 5 prograding towards sea (to right). This is the same location as log LE4 in Figure 5A.




typically associated laterally with seaward-dipping strata of mouth-bar

deposits (element BA) and is thus interpreted to represent a high degree of

tidal modulation of the associated bar fronts.Element TM is the product of tidal modulation of bar-front sediments

with deposition mostly from tidal currents. Basal erosion suggests periods

of elevated tidal energy. Periodic occurrence of TM, particularly in the

upper part of the sand body (Fig. 5), probably indicates periodic

enhancement of tidal activity during overall delta-front progradation.

This periodicity should be greater than neap–spring cycles, and may

indicate semiannual tidal maxima (Kvale et al. 1998), although not

necessarily every maximum is preserved.

Strike Variability of the Raptor Delta: Facies and Paleocurrents

The farthest accessible outcrop of the Raptor Delta (Sandstone 6)

towards the southwest was measured in a gully 1 km west of Raptor

Ridge. At this site, tidal facies (i.e., Facies 5b and element TM) and

channel facies (i.e., Facies 4 and element CH) are conspicuously absent

(Fig. 7, log a). Upward-coarsening facies successions show all other facies

with a relatively high BI (3–4) in prodelta and lower delta-front deposits,

and a low BI (rarely exceeds 1) in middle to upper delta-front deposits.

Dominant paleocurrents are towards the south (Fig. 7). Clinoforms are as

steep as at Raptor Ridge (4u). In contrast, the entire northeastern flank of

the Raptor Delta, from Raptor Ridge northeastward (Fig. 7), shows

a predominance of HCS facies (i.e., Facies 6 and element SS) alternating

with parallel-laminated facies (Facies 3) (Fig. 7; logs c, d). Again, tidal

facies (Facies 5b) and channel facies (Facies 4) are absent. Upward-

coarsening facies successions show a relatively high BI from bottom to

top (BI 4 at prodelta to lower delta front, and BI 2–3 at upper delta

front). Although the mean paleocurrent is towards 158u, a shore-parallel

component (northeast–southwest) is prominent (Fig. 7). Clinoforms areless steep (, 2u) than in Raptor Ridge (4u). These lateral variations of

facies, paleocurrents, and BI of the Raptor Delta are discussed in the next

section.

DISCUSSION

Delta-Building Processes and Deltaic Architectural Elements

(Building Blocks)

In the Raptor Delta, six types of architectural element were identified

in prodelta and delta-front deposits (Figs. 5, 6). Figure 8 shows these

deltaic architectural elements in idealized forms both in depositional dip

and strike sections. These elements bonded with each other via bounding

surfaces (five orders were labeled; Fig. 5) to create the entire delta.

Prodelta deposits consist mainly of blanketing sandy mudstones (element

PF) at the base of the overall parasequence. Storm sheets (SS) and frontal

splays (FS), produced by event deposition, may be found locally

sandwiched between muddy PF elements (Fig. 6). In a normal prograda-

tional succession and at the transition between the delta-front and

prodelta regions, intercalation of delta-front and prodelta elements give

rise to facies interfingering (cf. Gani and Bhattacharya 2005a; Fig. 8,

element PF).

Delta-front deposits show an alternation of elements FS, SS, TM

(tidally modulated deposit), CH (channel), and BA (bar accretion).

Processes related to each of these elements have been described previously

in the ‘‘Sedimentology’’ section. Broadly, interaction between river and

FIG. 4.—Continued.




FIG. 5.—Outcrop panels (photomosaics with bedding and facies maps) of Sandstone 6 at Raptor Ridge. For location of cliff sections, see Figure 1C. Structural dipmeasured at paleo-horizon surfaces (one such surface is indicated in prodelta deposits of Fig. 5A) is 6 u towards the southeast. Detailed identification of sedimentaryfeatures, bedding planes, and facies in several depositional dip and strike sections allowed to establish bounding-surface hierarchies and architectural elements (marked onbedding diagrams of Fig. 5A, B) of the Raptor Delta. Note individual bar boundaries in the depositional strike section of Figure 5D. See text for discussion. (Parts 5Cand D are in color in the digital version of this article.)




basinal (tide and wave) processes develops these building blocks that fillthe accommodation. Both FS and SS represent event deposits that

generally smooth out delta-front topography. For example, Kostic et al.

(2002) showed that sediment gravity flows can reduce the angle of delta-

front clinoforms by 20%.

A conspicuous and geologically significant building block found in

delta front-deposits of the Raptor Delta is the element CH (Figs. 5, 8),

interpreted as chute or terminal channels. Terminal distributary channels

(i.e., subaqueous distributary channels) have been recognized only

recently in ancient delta-front deposits (Olariu and Bhattacharya 2006).

The present study demonstrates that distributary channels in deltas do

not necessarily stop at the delta plain; rather, they can continue

subaqueously down the delta-front region as terminal channels or chute

channels. Elements CH of the Raptor Delta were rapidly filled with

sediment-gravity-flow deposits. Because slump scars, slide and slump

deposits, and synsedimentary faults are largely absent, the origin of thesechannelized sediment gravity flows is likely to be extreme river floods, like

those seen in the modern Burdekin River (Alexander et al. 1999; Amos et

al. 2004), where much of the annual water and sediment discharge to the

delta is delivered in less than five days, and which produces coarse-

grained, sharp-based, 1–3 m thick sand beds (cf. Fielding et al. 2005).

Elements BA and TM consist mainly of packages of bedwave deposits

originating from river and tidal currents at the delta front. BA

corresponds to incremental growth of individual mouth bars. In the

Raptor Delta, mouth bars are characteristically identified by the

bidirectional downlap of bedding surfaces in strike section (Fig. 5D).

The minimum width of a single bar is c. 50 m. The size of the mouth bardeveloped at the end of a distributary channel should be proportional to

the width of the channel and to the horizontal spreading angle of the river

effluent. Generally, as the distributary channels bifurcate seaward their

widths decrease. Therefore, deltas with high orders of distributaries tend

to develop small bars. Bars may coalesce together to form bar

assemblages, which are recognized in the depositional strike section by

onlapping and/or converging strata of a younger bar onto an older bar

(Fig. 5D). Element TM indicates periodic reworking of riverborne

sediments by strong tidal currents that normally slows down seaward

growth of bars. TM can develop both within and outside of a bar. In the

Raptor Delta, a discrete facies association of trough cross-stratified

sandstones (BM—bedwave migration) was observed that are not part of

any previously described elements. Because the nature of bounding

surfaces of BM has not been established, BM could not be ranked as an

architectural element. BM can develop within the inter-bar region, and onthe wave-dominated upper shoreface, away from distributary mouths.

The dune-scale cross-stratified sandstones of logs c and d (Fig. 7),

developed at the upper part and in the eastern flank of the Raptor Delta,

probably represent upper-shoreface deposits of facies association BM.

The most prominent feature of deltaic deposits is seaward-dipping

inclined beds known as clinoforms (Rich 1951; Bhattacharya and Walker

1992; Gani and Bhattacharya 2005a; Bhattacharya 2006). In the Raptor

Delta, clinoforms are characteristically observed in the cliff sections

(Fig. 5). The depositional dip sections show persistent seaward-dipping

clinoforms (Fig. 5A, C), whereas the strike sections show parallel to

FIG. 5.—Continued.




subparallel clinoforms (Fig. 5B, D). The height (from the top ravinement

surface to the prodelta mudstones) of a single clinoform is, on average,

9 m. Using a compaction factor of 80% (normal for sandstones) the pre-

compaction height of a clinoform would have been 11 m, which is

interpreted to be the minimum water depth into which the delta front of the Raptor Delta prograded.

Knowledge of facies associations, general basinward-dipping clinoform

geometry, and the modern geomorphic units of deltaic systems are well

known (Bhattacharya 2006). However, the understanding of internal

geometrical complexity and variability of a single deltaic sandbody, and

the polyphase evolution history (in terms of variability of river, wave, and

tidal processes) of a single delta during basinward progradation, are

handicapped by the lack of well-documented example of how to identify

and interpret unit building blocks of deltaic deposits. In the studies of

modern deltas, a major challenge is depicting the internal geometry of

deltas by correlating the limited subsurface cores (cf. Gani and

Bhattacharya 2005a, and references therein). This study has identified

the basic architectural elements of the delta-front and prodelta

environment within a mixed river, wave, and tide influenced delta from

a detailed outcrop study. The information of the external morphologies(e.g., shape, placement, aspect ratio, preferred orientation) and the

internal bedding geometries (e.g., stratal dip and truncation, facies

heterogeneity) of these 3D building blocks (Fig. 8) can be used as

predictive tool while correlating the subsurface data, as shown in

Figure 6. Once the facies are identified from the cores and/or wireline-

logs, the knowledge of deltaic architectural elements should guide the

subsurface correlation of bedding surfaces.

Dimensional and geometrical data of a sandbody plays a critical role in

quantitative reservoir modeling (North 1996; Willis and White 2000; Tye

2004; Miall 2006). This study reveals detailed and useful geometrical and

dimensional data of unit building blocks of deltaic reservoirs below thelevel of channel and mouth-bar sands, particularly for object-basedmodeling (cf. Tye 2004). Incorporation of the facies architectural modelpresented in this study in characterizing deltaic reservoirs would result infar more complex but realistic reservoir behavior (e.g., compartmental-ization, heterogeneity, sweep efficiency). For example, the oppositelydipping stratal geometry of BA versus TM elements could complicate the

sweep efficiency of deltaic reservoirs.

Paleogeographic Reconstruction

The shape of the sandstone isolith map of the Raptor Delta (Fig. 3)reflects the general paleogeography of the delta. A slight shore-parallelelongation of this lobate sand body may be interpreted to indicate anoverall wave influence (e.g., Coleman and Wright 1975). There are twothicks in this lobate sand body. Outcrops of the northeastern thick(Raptor Ridge), particularly the identification of elements CH and BA,clearly indicate the presence of a major, subaerial distributary channel inthis region. Although no outcrop data are available for the southwestthick, it can be logically concluded that this region was also close toa major distributary channel. Therefore, these two thicks may represent

two depositional loci of the Raptor Delta. The sand isolith maps of theHolocene Burdekin delta in NE Australia (Fielding et al. 2005) and of the

Rhone delta in France (Galloway and Hobday 1996, p. 111) are smooth-fronted like the Cretaceous Raptor Delta. The Rhone Delta is being builtby two main distributary channels of the Rhone River, as has beeninferred for the Raptor Delta. However, whether the two maindistributaries of the Raptor River coexisted, or one is slightly youngerthan the other (i.e., lobe switching) is not certain.

In this study, a significant criterion in paleogeographic reconstructionis the way paleocurrent data are utilized. When entire paleocurrent data

FIG. 6.—Core-to-outcrop correlation of part of the Raptor Delta. Subsurface stratal correlation was guided by the knowledge of overall shape and internal stratalgeometry of the architectural elements established in the outcrops. For location of line of correlation, see Figure 1C.




of the region are lumped and plotted together (Fig. 3), they fail to show

the local but significant variations of paleocurrents (Fig. 7) that arediagnostic of dominant processes. For example, in the Raptor Delta, the

local plots of paleocurrents show that (1) river and tidal processes were

dominant around the Raptor Ridge, (2) tidal processes were absent in the

southwest of the Raptor Ridge, and (3) wave processes were prominent inthe northeast of the Raptor Ridge (Fig. 7).

The regional outcrop section of the Raptor Delta (Fig. 7) shows

a predominance of element SS developed as an extensive sheet in thenortheast flank (Fig. 7, logs c and d). In this region, comparatively high

BI and an increase in the shore-parallel component of paleocurrentssuggest that the northeast flank of the Raptor Delta had less river

influence, and hence may lie on the updrift side of the distributary mouth,forming an asymmetric wave-influenced delta, as predicted by the model

of Bhattacharya and Giosan (2003).

Incorporating the above observations, a true-scale paleogeographic

reconstruction of the Raptor Delta (Fig. 9) is made. Figure 9 suggeststhat the Western Interior Seaway, at least in the studied region, was

mainly wave- and storm-dominated, had shore-parallel currents domi-

nantly towards the south and occasionally towards the north, and hadvery localized tides. All of these observations are consistent with the

findings of paleo-oceanographic modeling of the seaway (Ericksen and

Slingerland 1990; Slingerland et al. 1996) mentioned earlier. Like other

parasequences of the Wall Creek (Fig. 2), an extensive marine ravinement

surface was observed at the top of Sandstone 6. A relative rise in sea-levelis thought to have produced this transgressive erosion surface, which

eroded delta-plain deposits and left the Raptor Delta top-truncated.

Limitation of Tripartite Classification of Deltas

As introduced earlier, several recent studies of modern deltas, including

the Brazos and Burdekin deltas, have questioned their placement in end-member fields (Galloway 1975) of the tripartite classification of delta

(Gani and Bhattacharya 2005b; Bhattacharya 2006). The Brazos delta in

the Gulf of Mexico, for example, has long been cited as a classic exampleof a wave-dominated delta, on the basis of its plan-view morphology, but

recent coring studies show that, internally, much of the delta was built

during major river-floods, and it is now classified as a river-flood-dominated delta, with some wave reworking of the surficial sediments

(Rodriguez et al. 2000). The Baram River delta in NW Borneo has also

been historically classified as wave-dominated, on the basis of its smooth-

fronted plan-view shape, but recent studies of the internal facies show tide

FIG. 7.— Regional cross section (depositional-strike-oriented) with gross facies distribution and paleocurrent data for the Raptor Delta along outcrop belt. For

location of Y–Z, see Figure 3. Although facies are documented continuously along the outcrop belt, only representative lithologs are presented here. Architecturalelements are PF (prodelta fines), SS (storm sheet), FS (frontal splay), CH (channel), TM (tidally modulated), and BA (bar accretion), with facies association BM(bedwave migration). Percentage of river-, wave-, and tide-dominated facies are plotted across the section. Note that both river- and wave-dominated facies can varylaterally from 0% to 100%.




dominance (Lambiase et al. 2003). The Burdekin delta also shows

a smooth-fronted morphology that has caused it to be classified as ‘‘wave-

dominated,’’ but detailed internal facies analysis show sharp-based river-

dominated mouth bars that record river-flood processes (Fielding et al.

2005). Although the sandstone isolith map of the Burdekin delta (Fielding

et al. 2005, their figure 20) shows a shore-parallel elongation,

a considerably higher river influence was documented in cores and

outcrops of this delta (Fielding et al. 2005). The plan-view morphology of

any given modern delta reflects dominant surficial process but may not

necessarily reflect the short-lived constructional processes of river floods

that dominate the internal facies architecture.

Similarly, the external shape of the Raptor Delta might be seen as

controlled primarily by waves (Figs. 3, 9), but the internal facies

architecture reveals a far more complex interaction of river, waves,

storms, and tidal processes. For example, in any given vertical section,

both river- and wave-dominated facies can vary laterally from 0 to 100

percent (Fig. 7). The estimated proportion of wave versus tidal versus

river facies may change dramatically, depending on where vertical

samples are taken. Worse, some of the facies changes are not clearly

imaged by logs, and can be determined only where there is good outcrop

or core control.

Rather than force-fitting the Raptor Delta into an end-member type of

the tripartite delta classification (Galloway 1975), we prefer to described

it as a ‘‘mixed-influenced delta’’ to indicate the complex delta-building

processes that are reflected in the internal facies. Table 2 of Galloway

(1975) suggested different framework facies for each of the three types of

delta. However, in any given delta these framework facies are not

mutually exclusive; hence, mixed-influenced deltas may be the norm

rather than the exception. Like the modern Brazos Delta (Rodriguez et al.

2000) and the modern Burdekin Delta (Fielding et al. 2005), the main

building phases of the Raptor Delta were likely seasonal to decadal river

floods producing elements CH, FS, and BA. During intervening periods

(e.g., between two floods) the delta was reworked by waves, storms, and

tides, producing mainly SS, TM, and BM, which are reflected in the plan-

form morphology (Fig. 9).

Modern Analog and Morphological Elements

Processes responsible for depositing the architectural elements in

ancient deltaic deposits are better understood when comparison is made

to modern deltaic systems. Although the sharp lateral facies change

between the tidal and wave regime of the Raptor Delta (Fig. 6) can be

compared with that of the modern Orinoco delta in NE Venezuela

(Warne et al. 2002), the latter is almost an order of magnitude larger. The

Burdekin Delta (, 40 km by 60 km) is thought to be the closest scaled

analog for the ancient Raptor Delta (, 30 km by 60 km), particularly in

terms of delta morphology and river-mouth processes. Like the Raptor

Delta, wave- and storm-produced beach ridges lie on the flank of the

Burdekin Delta away from the mouth of two main distributary channels

(Fig. 10). Subaerial distributary channels extend subaqueously as

FIG. 8.—Summary diagram of six basicarchitectural elements (building blocks) of pro-delta and delta-front deposits of the RaptorDelta. These elements are among the basicbuilding blocks of all deltas. Although thedimensions of these elements can vary widelyfrom one deltaic system to another, the likelyranges are tabulated.




terminal distributary channels. These terminal channels (width , 100 m)

meander gently on the delta front and taper basinward. Both mouth bars

and in-channel bars coalesce to form bar assemblages close to the main

distributary mouths. Tides produce a series of large dunes on bar fronts.

The migration and/or growth of these morphological elements should

produce architectural elements like those described in the Raptor Delta.

In fact, in all deltaic systems, major morphological elements include

channels, bars, large bedwaves, beach-ridges/cheniers, and splays.

Channel types include: trunk distributary, above first avulsion point;subaerial distributary; terminal distributary, below mean sea level; delta-

front chute channel; and tidal creek and channel, both subaerial and

subaqueous. Bars are associated with each of these channels. The number

and size of mouth bars are directly proportional to the number and size of

distributary channels, respectively. Beach-ridge/chenier plains develop

when riverborne sediments accrete to the coast away from a river mouth

but within the same river delta system. In the rock record, these

morphological elements produce architectural elements similar to those

described here.

CONCLUSIONS

This study, for the first time, develops an architectural-element model

for open-marine deltaic deposits by investigating the Cretaceous Raptor

Delta in the Wall Creek Member, Wyoming. High-resolution (centimeter

scale) cliff mapping of bedding and facies reveals six architectural

elements in the prodelta and delta-front deposits of a single prograda-

tional parasequence—PF (prodelta fines), FS (frontal splay), CH

(channel), SS (storm sheet), TM (tidally modulated deposit), and BA

(bar accretion). Element CH represent subaqueous terminal or chute

channels filled rapidly with deposits of sediment gravity flows showing

lateral accretion surfaces. The depositional mechanism of these channels

is probably analogous to that of laterally migrating channels in

a submarine fan. The main building phases of the Raptor Delta were

probably seasonal to decadal river floods producing elements CH, FS,

and BA. During intervening periods, the delta was reworked by waves,

storms, and tides producing mainly elements SS and TM. It is suggested

that these elements are among the basic building blocks of all deltas, and

can be used as a guideline for high-resolution object-based modeling of

deltaic reservoirs and as a predictive tool in delineating internal geometry

of modern and ancient deltas with limited subsurface data. However, the

relative proportions, combinations, and bonding styles of these elements

may vary from one system to another, and even within the same system

depending on where vertical samples are taken. Vertical facies proportioncurves may vary over kilometer spacing and should be considered when

using these types of data for reservoir modeling and extrapolation of

facies variability in a subsurface setting.

The sandstone isolith map of the Raptor Delta shows a lobate

geometry with a slight shore-parallel elongation suggesting some wave

influence. The smooth inferred plan-form morphology vs. the internal

facies complexity of the Raptor Delta raises concern about the basic

assumption of the tripartite classification of deltas. The well studied

modern deltas of the Brazos, Baram–Trusan, and Burdekin show similar

observations that the main constructional processes are not reflected in

the external morphology of deltas. Therefore, only a detailed facies

architectural analysis may reveal the complexity of interactive delta-

building processes. Rather than force-fitting into an end-member type,

the Raptor delta is classified as mixed-influenced to indicate its inherent

complexity.

ACKNOWLEDGMENTS

Funding for this research was provided by the U.S. Department of EnergyDOE Grant # DE-FG0301ER15166. Additional support was provided bythe University of Texas Quantitative Sedimentology Research Consortium, of which BP and Chevron are members. Nahid Gani helped greatly in the fieldand in the lab. Thanks are also due to Chuck Howell and Randy Griffin forhelping in the field. An earlier version of the manuscript was reviewed byTony Rodriguez and William Helland-Hansen. We thank Journal reviewersAndrew Miall and Brian Zaitlin, Associate Editor Joe Lambiase, Editor Colin

FIG. 9.—3D reconstruction of the RaptorDelta based on mapping of sandstone isolith,facies, and paleocurrents shown in Figures 3 and7. Asymmetry of wave and tidal energy isindicated with the variation of arrowhead size.




North, and Corresponding Editor John Southard for their helpful suggestionsto improve the manuscript.

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FIG. 10.— ASTER (Advanced Spaceborne Thermal Emission and ReflectionRadiometer) images (3-2-1 band ratio) of modern Burdekin Delta (NE Australia)showing major morphological elements that produce architectural elements in therock records, like those identified in the Cretaceous Raptor Delta (see text fordiscussion).




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