SEDIMENTARY GEOLOGY

ELSEVIER Sedimentary Geology IO4 ( 1996) 89-98

Slope turbidite packets in a fore-arc basin fill sequence of the Plio-Pleistocene Kakegawa Group, Japan: their formation

and sea-level changes

Tetsuya Sakai a, Fuji0 Masuda b a Graduate School of Science, Osaka University, Toyonaka, Osaka 560, Japan

b Department of Earth and Space Science, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan

Received 11 February 1994; revised version accepted 2 February 1995

Abstract

The Plio-Pleistocene Kakegawa Group crops out in western Shizuoka, Japan. It consists of submarine fan and slope to alluvial deposits and forms a third-order sequence. The slope deposits of the upper Kakegawa Group (upper Horinouchi and Hijikata formations) are characterized mainly by alternations of sand and silt (turbidites). Individual sand layers commonly show either small lobe or channel features. In the upper Horinouchi (TST) and middle Hijikata formations (HST), sand tnrbidite packets (2-15 m) consist of small channel and lobe complexes.

The slope sand turbidite packets are different between TST and HST. The differences result from the dissimilar net amplitude of sea-level excursions when fourth- or fifth-order sea-level changes superimposed on a third-order sea-level change with different trend. Other important variables are rate of sediment supply and width of shelf.

1. Introduction

The Plio-Pleistocene Kakegawa Group crops out in western Shizuoka, Japan. It consists of a third- order fore-arc basin fill sequence formed in the period of 2.6 to 1.0 Ma (Masuda and Ishibashi, 199 1; Masuda, 1994).

An example of slope sand turbidite packet has been recognized in the upper part of the group. The sand packets repeatedly appear in slope silt of both the transgressive systems tract (TST) and highstand systems tract (HST) of the third-order Kakegawa sequence.

The purpose of this study is to describe slope turbidite packets and to discuss their difference of characteristics within TST and HST, respectively.

2. Geologic and sequence stratigraphic frame- work

The Kakegawa Group was deposited in a fore-arc basin (Sagara-Kakegawa Basin), formed during the Neogene by oblique subduction of the Philippine Sea Plate beneath the Eurasia Plate (Sakurai and Sato, 1983). This group consists of submarine fan and slope to alluvial fan deposits (Ishibashi, 1989) and is not disrupted by large faults and folds, although it was deposited near the collision zone of the Honshu and Izu-Bonin arcs (Fig. la).

2.1. Lithostratigraphic framework

The Kakegawa Group unconformably overlies pre-Pliocene basement rocks of Cretaceous to Paleo-

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90 T. Sakui, F. Masuda/Sedimentary Geology 104 (1996) 89-98

gene deep sea sediments (accretionary prisms) and older fore-arc basin fill deposits (Ishibashi, 1989). This group is unconformably overlain by the Pleis- tocene Ogasa Group, composing coastal alluvial fans and fill of submarine channels (Muto, 1985; Muto and Blum, 1989).

The upper Kakegawa Group (Figs. 1 and 21, discussed in this paper, encompasses eight forma- tions: the Nobe, Dainichi, Ukari, Aburayama and Soga formations (Makiyama, 1925; Makiyama and Sakamoto, 1957; Tsuchi, 1961; Aoshima and Chinzei, 1972) of alluvial and nearshore to shelf origin, and the upper Horinouchi, Tamari and Hijikata forma- tions (Makiyama, 1925; Chitani, 1926) of slope ori- gin (Figs. lc and 2a).

The Nobe and lower member of the Soga Forma- tion consist of poorly sorted granules to cobbles and

interbedded sand and silt. The maximum thicknesses of the Nobe and the lower member of the Soga Formation are approximately 200 m and 25 m, re- spectively. Gravel beds are characterized by trough cross-stratification and horizontal stratification which typify braided fluvial systems of alluvial fans. Sand and silt beds interbedded in the gravel beds have lenticular shapes (OS-20 m wide and 0.1-2 m thick) and contain abundant roots and inversely grading structures. Inversely graded beds are diagnostic of overbank and flood plain deposits in Japan (Iseya and Masuda, 1985; Suzuki, 1994). The sand and silt beds are, therefore, interpreted as overbank and flood plain deposits.

The Dainichi, Aburayama and the upper member of the Soga Formation consist mainly of hummocky cross-stratified fine-grained sand beds and are inter-

LEGEND Pre-Pliocene Basement

Nobe Formation

Dainichi Formation C

) 5km

1

Ukari Formation

Horinouchi Formation % Ogasa Group

Hijikata Formation m lwatahara Gravel

Aburayama Formation B Tuff Layers

Soga Formation - Main Route

Fig. 1. (a) Location of the Kakegawa Group. Solid lines indicate the plate boundaries. EP = Eurasia Plate, NAP = North American Plate, PP = Pacific Plate, PSI’ = Philippine Sea Plate. (b) Location map of the study area. (c) Geologic map of the study area. (d) Main routes of the columnar sections.

T. Sakai, F. Musuda / Sedimentary Geology 104 (1996) 89-98 91

preted as shoreface to inner shelf deposits. The maximum thicknesses of these formations and mem- ber are approximately 150 m, 50 m and 25 m, respectively.

The Ukari Formation consists of bioturbated sandy silt and interbedded sand beds. The maximum thick- ness is approximately 250 m. Benthic foraminifers and molluscan shells in the sandy silt suggest a depth range of 100-200 m (Aoshima, 1978; Chinzei, 1980). Hence, the sandy silt is interpreted as an outer shelf deposit. The interbedded sand beds consist of fine or very fine sand and resemble turbidite. These sand beds are interpreted as storm sand layers.

terbedded sand layers have elliptical lobate shape in three dimensions. Slumped beds and debris-flow de- posits are intercalated in the silt (Fig. 3a). Benthic foraminifers in the silt suggest a depth of approxi- mately 600 m (Sharma and Takayanagi, 1982). Hence the upper Horinouchi Formation is interpreted as a lower slope deposit (e.g. Shirai and Kimiya, 1987).

The Horinouchi Formation comprises a major part of this group. The maximum thickness is more than 2000 m. The major part of this formation (lower and middle part) is characterized by alternations of sand and silt (turbidite), composing submarine fans (Ishi- bashi, 1989; Sakai and Masuda, 1995).

The northwestern Hijikata and Tamari formations consist mainly of bioturbated silt with interbedded parallel-laminated sand beds. Slump scars (Fig. 3b) are abundant in the silt. Molluscan shells and benthic foraminifers in the silt suggest a depth range of 200-500 m (Chinzei, 1980; Ishibashi, 1985; Nobuhara, 1993). Sand-dominated turbidite intervals repeatedly appear in the silt of the middle part of the northwestern Hijikata Formation. This silt passes gradually into sandy silt of outer shelf deposits to- ward the northwest (landward). Therefore, these for- mations are interpreted as upper slope deposits.

The upper Horinouchi Formation, discussed in The southeastern Hijikata Formation consists of this paper, is characterized by repeated sand- and bioturbated silt. Slumped beds and debris-flow de- silt-dominated intervals of turbidite successions. The posits are interbedded in the silt. Benthic foraminifers maximum thickness is approximately 200 m. In- and molluscan shells in the silt suggest a depth range

a

Alluvial Fan

Outer Shelf

Channel & Levee

Slope Sand Packet

Incised Valley Fill

b SE Composite 6 “0

Ma 2 Sigma Values

- ,Hijikata F. ,

Fig. 2. (a) Schematic cross-section of the upper Kakegawa Group. Numbers in the figure are main routes given in Fig. Id. (b) Oxygen isotope curves from foraminiferal tests (Williams, 1990). The sequence of the Kakegawa Group might be affected by a third-order eustatic sea-level change represented in this curve. SB = sequence boundary. CZ = condensed zone of the sequence.

92 T. Sahi, F. Mnsuda/Sedimentary Geology 104 (1996189-98

Fig. 3. Outcrop pictures of slope deposits in the Kakegawa Group. (a) Lower slope deposits in the upper Horinouchi Formation. The outcrop is 5 m high. Slumped beds are common in the lower slope deposits. (b) Upper slope deposits in the northwestern part of the Hijikata Formation. The outcrop is 13 m high. Channel-like discontinuity in the picture is a slump scar. (c) Typical slope turbidite packet in the upper Horinouchi Formation. The outcrop is 8 m high. The slope turbidite packets consist of alternations of sand beds with convex-up (lobe) and concave-up (channel) geometry and silt beds. (d) TST slope turbid& packet in the upper Horinouchi Formation. The outcrop is 12 m high. (e) HST slope turbiditc packet in the Hijikata Formation. The outcrop is 7 m high. PB = pamsequence boundary; CS = condensed section of the parasequence.

T. Sakui, F. Masuda/Sedimentary Geology 104 (1996) 89-98 93

of 400-800 m (Sharma and Takayanagi, 1982; Nobuhara, 1993). Facies and fauna suggest that it is a lower slope deposit.

2.2. Sequence stratigraphic framework

The upper Kakegawa Group, as described above, is characterized by an overall upward-fining fol- lowed by an upward-coarsening facies succession (Fig. 2a). Based on its age span (1.0-2.6 Ma; Nishimura, 1977; Ishida et al., 1980; Ibaraki, 1986a, b; Masuda, 1994), it forms a third-order sequence. The lower sequence boundary shows the type 1 character of Van Wagoner et al. (19881, because (1) an erosional surface can be traced into slope deposits (Tamari Formation, Fig. 21, and (2) it is incised landward of the shelf break which is inferred at the southeastern extremity of the Dainichi Formation where the top of the basement steeply drops basin- ward (Fig. 2a). The upper sequence boundary is also an erosional surface overlain by coastal alluvial fan and submarine channel fill deposits of the Pleis- tocene Ogasa Group.

Individual systems tracts are identified by means of onlap and downlap patterns of volcanic ash layers by Masuda and Ishibashi (1991). The lowstand sys- tems tract (LST) is characterized by slope-apron, submarine channel and submarine fan facies associa- tions (Masuda and Ishibashi, 1991; Sakai and Ma- suda, 1995). The overall upward-fining unit of the upper Kakegawa Group is a transgressive systems tract (TST) and an overlying upward-coarsening unit corresponds with a highstand systems tract (HST). The TST contains the Nobe, Dainichi, lower Ukari, lower Hijikata and upper Horinouchi formations (Fig. 2a). The HST contains the Aburayama, Soga, upper Ukari and upper Hijikata formations (Fig. 2a). These systems tracts are separated by massive silt of con- densed zone with finer sediments (5-15 m; maxi- mum thickness was observed at Route 13 in Fig. Id). However, the rate of sediment supply during the maximum flooding was so rapid that a maximum flooding ‘surface’ could not be formed (Sakai and Masuda, 1996).

The upper Kakegawa sequence is attributed to a third-order eustatic sea-level change, because (1) the upper Kakegawa cycle is in phase with global oxy- gen isotope curves (Fig. 2b), which reflect glacio-eu- static sea-level changes, and (2) a transgressive and

regressive cycle is also identified in other basins during the same period (e.g. Kazusa and Uonuma groups; Ito, 1992; Ito and Katsura, 1993; Masuda, 1994).

2.3. Parasequence

The parasequences comprise both the TST and HST. The parasequences consist of upward-shallow- ing facies successions of tempestites (5-30 m thick). The strata overlying the top boundaries of the suc- cessions show abrupt increases in paleo-water depth. For instance, the boundary is recognized at the con- tact between HCS sand (inner shelf) and overlying sandy silt (outer shelf). The boundaries are marine flooding surfaces.

The parasequences might be also attributed to eustatic sea-level changes with periods of about sev- eral tens of ka (fourth- or fifth-order) caused by Milankovitch cycles, especially obliquity, because eight parasequences are identified in the strata de- posited in the period of 1.9 Ma and 1.6 Ma (Sakai and Masuda, 1992, 1996).

The parasequences can be traced into slope de- posits along the tuff layers (Sakai and Masuda, in prep.). They are characterized by upward thinning turbidite intervals (fourth- or fifth-order transgressive deposits), massive silt or silt-dominated turbidite in- tervals (condensed zones) and upward-thickening turbidite intervals (highstand to regressive deposits). In general, parasequences are hardly recognized in the slope or basin plain deposits because of slow rate of sediment supply (Van Wagoner et al., 1990). In the Kakegawa Group, however, a relatively high rate of sediment supply and a narrow shelf (ca. 10 km: Ishibashi, 1989) allowed sediments to be transported basinward at a sufficient rate to form distinct parase- quences (Sakai and Masuda, 1996).

3. Slope turbidite packets

The sand beds in the slope deposits (upper Hori- nouchi and Hijikata formations) show different fea- tures from typical turbidite sand in the lower Hori- nouchi Formation, especially their geometry. They consist of well sorted fine- or very fine-grained sand and show convex-up and concave-up geometry.

94 T. Sakai, F. Masuda /Sedimentary Geology 104 (1996) 89-98

The sand beds with convex-up geometry have parallel stratification or gently inclined cross-stratifi- cation and are elongated toward the paleo-slope (southeast to south), forming elliptical lobate de- posits. The lobate turbidite beds are several hundreds of meters long and several to several tens of meters wide. Gently inclined cross-stratification is discor- dant with the top of the sand bed. This geometry suggests deposition by lateral accretion.

The sand beds with concave-up geometry possess very small channel features. The thick sand beds of channel deposits (up to 200 cm) thin laterally and change into poorly sorted very fine-grained sand with abundant plant debris of associated levee de- posits across lo-20 m wide outcrops. The channels are several meters to several tens of meters wide. The channel fill sand is parallel-stratified (b of the Bouma division) and levee deposits have fine lami- nation (d of the Bouma division).

The turbidite sand beds, described above, are in the sand-dominated intervals of the turbidite succes-

NW

LEGEND

m Alluvial Fan

m Shoreface - inner Shelf

m Outer Shelf

Kl Channel & Levee

0 Slope

m Slope Sand Packet

-+ Paleocurrents

sions in the upper Horinouchi and middle Hijikata formations. Lateral shingling of the lenticular sand beds is commonly observed at the large outcrops. The turbidite intervals consist of small channel and lobe complexes (Fig. 3c)

4. Parasequence and slope turbidite packet

4.1. Transgressive systems tract (TST)

The parasequences (SO-SO m) in the lower slope deposits are characterized by sand-dominated (slope turbidite packet; Fig. 3d), massive silt and silt- dominated turbidite intervals (Fig. 4). They are inter- preted as fourth- or fifth-order transgressive, con- densed and highstand to regressive deposits, respec- tively. Very small turbidite packets (2-5 m thick) are also intercalated in the silt-dominated intervals.

Fig. 4. Columnar sections along the Hosoya and Moridaira tuffs in the transgressive systems tract. Sandy mrbidite packets are recognized at the base of pamsequences. PB = pamsequence boundary.

T. Sakai, F. Masuda / Sedimentary Geology 104 (1996) 89-98 95

Volcanic ash layers are quite useful to analyze the formative processes of the slope turbidite packets. The parasequences containing widely traceable acidic volcanic ash layers, the Hosoya Tuff (1.9 Ma by fission-track dating; Nishimura, 1977) with white colored tuff and pumice beds (l-5 m thick), and Moridaira Tuff (Ishibashi, 1988) consisting of four white colored tuff beds (each 3-5 cm thick), are noted. In the parasequence which contains the Moridaira Tuff, the volcanic ash layer is interbedded in the proximal slope turbidite packet (Fig. 4). The ash occurs at the top of the sand turbidite packet in which a sand-dominated interval gradually changes into massive silt (condensed zone of the parase- quence). Tracing this tuff layer towards distal de- posits, it occurs in the silt-dominated interval with the packet several meters below (Fig. 4). This fact indicates that slope turbidite packets were initially formed in a distal environment and the center of deposition moved landward. Therefore, the turbidite packets were backstepping.

Small channel and levee deposits several meters deep and wide, are recognized near the base of the

LEGEND

m Alluvial Fan

m Shoreface - inner Shelf

m Outer Shelf

m Channel & Levee

0 Slope

m Slope Sand Packet

---) Paleocurrents

parasequences in the upper slope deposits (Fig. 4). These channels are filled with clast-supported gravel with abundant shells and mud clasts. The shells in the channels are composed mainly of shallow marine species (e.g. Glycymeris sp., Scapharca sp., Dosinia sp.). Shell fragments of these species are also found in the sand beds of the slope turbidite packets, so that these sand might have been supplied through the channels.

4.2. Highstand systems tract (HST)

The parasequences in outer shelf to upper slope deposits of the HST consist, from base to top, of upward-thinning silt-dominated intervals, massive silt, upward-thickening silt-dominated and overlying sand-dominated turbidite intervals (Figs. 3e and 5). They separate components approximately 5 m, 5 m, 30 m and 5 m thick, respectively.

The basal upward-thinning intervals might have been deposited during fourth- or fifth-order trans- gressions, accounting for a decrease in sediment supply. The overlying massive silt is interpreted as a

IElf 8

Fig. 5. Columnar sections of the highstand systems tract of the third-order sequence. Sandy hubidite packets are interbedded in the top of the parasequences.

96 T. Sakai, F. Masuda /Sedimentary Geology 104 (1996) 89-98

condensed zone. The thickening-upward intervals and slope turbidite packet is interpreted as a fourth- or fifth-order highstand- to regressive-deposits.

At the southeastern extremity of the basin, the third-order highstand deposit consists of massive silt. Here, parasequences can not be distinguished in the lower slope deposits.

4.3. Differences between TST and HST slope tur- bidite packets

Characteristics of slope turbidite packets are dif- ferent between TST and HST. These sand packets are recognized at the base of parasequences (i.e. fourth- or fifth-order transgressive deposits) in the TST (Fig. 6b) and at the top of parasequences (i.e. fourth- or fifth-order highstand- to regressive-de- posits) in the HST (Fig. 6a). Larger turbidite packets occur in lower slope deposits of the TST and smaller ones occur in outer shelf to upper slope deposits in the HST.

The reason why sand turbidites are deposited during fourth- or fifth-order transgressions of the third-order TST can be explained by the asymmetric amplitude of the fourth- or fifth-order transgressions (Fig. 6b). The amplitude of fourth- or fifth-order transgressions superimposed on a third-order trans- gression should, on average, be larger than those during a third-order highstand (Fig. 6). Intense shoreface erosion caused by the enhanced fourth- or fifth-order transgressions would produce more abun- dant sediments. Because of the narrower shelf at this time, these sandy deposits might be transported bas- inward through channels on the upper slope to form lower slope turbidite packets.

During fourth- or fifth-order highstands to regres- sions superimposed on a third-order highstand, shelf sands tend to prograde to a more distal position than during the preceding third-order transgression. Therefore, sandy sediment is frequently transported to the outer shelf and upper slope in this phase and sand turbidite packets develop (Fig. 6a). Due to the absence of strong erosional phase (i.e. deep ravine- ment), the volume of sandy sediment derived to the slope was smaller than during fourth- or fifth-order transgressions of a third-order transgression, be- cause, in the HST, most sandy sediment is trapped in

Time

~ T,me Me

- - Condensed Zone (4th -5th Order)

J Fig. 6. Schematic cross-sections of parasequences in both trans- gressive (b) and highstand systems tracts (a). The time lines are drawn based on volcanic ash layers interbedded in the parase- quences. The curves in the figure represent fourth- or fifth-order sea-level changes superimposed on third-order transgression and highstand. Bold parts show the phases in which slope turbidite packets were deposited. PS= parasequence; Sf = slope sand packets.

nearshore to inner shelf environments where upward-shallowing successions were formed. We in- fer that these sandy sediments were trapped at the outer edge of the broad shelf of the HST.

5. Conclusions

Slope sand turbidite packets are recognized in the transgressive and highstand systems tracts. They are characterized by turbidite sand beds with both con- vex-up and concave-up geometry, which show lobe, and small channel and levee features, respectively.

T. Sakai, F. Masuda/Sedimentary Geology 104 (1996) 89-98 97

The sand-dominated turbidite intervals consist of small charmel and lobe complexes.

The sand turbidite packets are intercalated in lower slope deposits in the TST parasequences, and in outer shelf to upper slope deposits in the HST parasequences. They developed during fourth- or fifth-order transgressions in the TST, and during fourth- or fifth-order highstands to regressions in the HST. The sand turbidite packets in the TST are larger than those in the HST.

Ibaraki, M., 1986b. Neogene Planktonic foraminiferal biostratigra- phy of the Kakegawa Area on the Pacific coast of central Japan. Rep. Fat. Sci. Shizuoka Univ., 20: 39- 173.

Iseya and Masuda, F., 1985. ‘Inverse grading’: facies indicator of fluvial flood deposits. Environmental Study of Tsukuba, 9: 63-69.

These differences can be explained by asymmetry of fourth- or fifth-order sea-level fluctuations when they were superimposed on a third-order transgres- sion or highstand. Other contributing factors are the rate of sediment supply to the slope and the width of the shelf.

Ishibashi, M., 1988. Shallow marine processes in the Plio- Pleistocene Kakegawa Group, central Honshu, Japan. Master thesis, University of Tsukuba, 109 pp,

Ishibashi, M., 1985. Shallow marine processes in the Plio- Pleistocene Kakegawa Group, central Honshu, Japan. Gradua- tion Thesis, Shizuoka University, 154 pp. (in Japanese with English abstract).

Ishibashi, M., 1989. Sea-level controlled shallow-marine systems in the Plio-Pleistocene Kakegawa Group, Shizuoka, central Honshu, Japan: comparison of transgressive and regressive phases. In: A. Taira and F. Masuda (Editors), Sedimentary Facies in the Active Plate Margin. Terra Publication Com- pany, Tokyo, pp. 345-363.

Acknowledgements

Ishida, S., Makinouchi, T. Nishimura, A. Takemura, K. Danhara, T., Nishiyama, K. and Hayashida, A., 1980. Middle Pleis- tocene of Kakegawa district, central Japan. Quat. Res. (Daiyonki Kenkyu), 19: 133- 143 (in Japanese with English abstract).

We would like to thank Drs. Yoshiki Saito, Ken Ikehara (Geological Survey, Japan) and Richard N. Hiscott (Memorial University of Newfoundland, Canada) for their critical comments and suggestions. We are grateful to the editor, Dr. Tsunemasa Shiki for his encouragement publishing the manuscript. We wish to thank Prof. Robert M. Carter (James Cook University, Australia) for his help with im- provement of the manuscript.

Ito, M., 1992. High-frequency depositional sequences of the upper part of the Kazusa Group, middle Pleistocene forearc basin fill in Boso Peninsula, Japan. Sediment. Geol., 76: 155-175.

Ito, M. and Katsura, Y., 1993. Depositional sequences in turbidite successions of the lower Kazusa Group, the Plio-Pleistocene forearc basin Ii11 in the Boso Peninsula, Japan. J. Geol. Sot. Jpn., 99: 813-829.

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