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The relationship between glacio-eustatic parasequences and a third-order sequence in the Kakegawa Group, central Japan
ELSEVIER
Sedimentary Geology 122 (1998) 95–107
The relationship between glacio-eustatic parasequences and a third-order sequence in the Kakegawa Group, central Japan Tetsuya Sakai a,Ł , Fujio Masuda b b
a Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan Department of Geology and Mineralogy, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
Received 9 October 1996; accepted 26 September 1997
Abstract The Plio–Pleistocene Kakegawa Group, central Japan, consists of a third-order depositional sequence (2.6–1.0 Ma). The northwestern part of the Kakegawa sequence consists of up to 500 m of alluvial, shoreface, shelf, slope and submarine-channel facies. It contains at least sixteen upward-shallowing cycles (parasequences), the deposition of which was affected by high-frequency eustatic sea-level cycles. The lower part of the sequence is characterized by a retrogradational parasequence set, which formed a transgressive systems tract (2.2–1.75 Ma) followed by a progradational parasequence set comprising a highstand systems tract (1.75–1.4 Ma). Subsidence analysis and evaluation of changes in the shelf sedimentation rate estimated from cross-sections, suggest that formation of the third-order sequence was controlled by tectonic subsidence and variation in the sedimentation rate. Rapid subsidence and a high rate of sedimentation during 2.2–2.0 Ma resulted in deposition of the lower part of the transgressive systems tract, characterized by thick backstepping successions. The rate of subsidence decreased in the period 2.0–1.75 Ma. The sedimentation rate also decreased due to a high rate of sediment bypassing. However, subsidence was still the dominant factor, leading to the formation of thin backstepping successions. The 1.75–1.4 Ma progradational succession resulted from a combination of a low rate of subsidence and moderate sedimentation. The progradational units become thicker basinward owing to faster subsidence in the basin center. The maximum flooding surface was formed around 1.75 Ma even though subsidence was slow at this time. 1998 Elsevier Science B.V. All rights reserved. Keywords: forearc basin; Plio–Pleistocene; sediment supply; sediment transport; tectonic subsidence; third-order sequence
1. Introduction Since their introduction by Payton (1977), sequence stratigraphic concepts have evolved rapidly and numerous theoretical and case studies have been Ł Corresponding
author. Present address: Department of Geology and Mineralogy, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan. Tel.: C81 (75) 753-4158; Fax: C81 (75) 753-4189; E-mail: [email protected]
published (e.g. Haq et al., 1987; Wilgus et al., 1988; Galloway, 1989a,b; Vail et al., 1991; Swift et al., 1991; Posamentier and Allen, 1993). These studies suggest that sequence formation is controlled by changes in accommodation and sedimentation rate (e.g. Thorne and Swift, 1991; Schlager, 1993). Recently, many stratigraphers have focused their interest on the causes of formation of depositional sequences and their bounding surfaces (e.g. Thorne and Swift, 1991; Devlin et al., 1993; Meckel and Galloway, 1996).
c 1998 Elsevier Science B.V. All rights reserved. 0037-0738/98/$ – see front matter PII: S 0 0 3 7 - 0 7 3 8 ( 9 8 ) 0 0 1 0 0 - 6
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Fig. 1. Location map of the Kakegawa Group. Enclosed part is study area. NP D North American Plate.
The Plio–Pleistocene Kakegawa Group in central Japan (Fig. 1), represents the fill of a forearc basin (Ishibashi, 1989), and forms a third-order sequence (Masuda and Ishibashi, 1991; Sakai and Masuda, 1996). The part of the group exposed on the northwestern side of the basin is an ideal succession to assess the factors which control sequence formation, i.e. tectonic subsidence, eustatic sea-level change, sediment supply, and sediment transport (Jervey, 1988; Galloway, 1989a), because of the availability of well constrained age data, facies modeling and
sequence stratigraphy. The objective of this paper is to identify the factors which influenced the formation of a third-order sequence in a forearc basin, using geohistory analysis.
2. Plio–Pleistocene forearc basin stratigraphy The Kakegawa Group consists of nonmarine and shallow to deep marine clastic sediments and is up to 3500 m thick. The group represents the fill of
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a Plio–Pleistocene forearc basin (Kakegawa Basin, Sugiyama et al., 1988) that probably formed as a result of oblique subduction of the Philippine Sea Plate beneath the eastern margin of the Eurasia Plate (Sugiyama, 1989). The Kakegawa Group unconformably overlies pre-Pliocene rocks which comprise Miocene forearc basin fill and Cretaceous–Paleogene deep-sea sediments (Makiyama, 1947; Kano and Matsushima, 1988). The group is overlain by coastal alluvial fan and submarine channel fill deposits of the Pleistocene Ogasa Group (Muto, 1985). The Kakegawa Group itself comprises a depositional sequence consisting of alluvial fan, incised valley fill, shoreface, shelf, slope, submarine-fan, slope-apron and submarine channel fill deposits (Ishibashi, 1989; Sakai and Masuda, 1996) (Fig. 2). Paleocurrent from cross-stratifications indicates that dominant sediment transport was toward the south to southeast (Sakai and Masuda, 1996). The sediment facies distribution suggests that the Kakegawa Basin was similar to the ‘Senoumi Basin’, a recent forearc basin that has a coastal alluvial fan, a narrow shelf (up to 10 km and 30 km wide in the shore-normal and shore-parallel directions, respectively) and a modern marine depocenter (Fig. 1). In the Senoumi Basin (Ishii and Nemoto, 1995) and in the other Japanese forearc basins (e.g. Hyuga and Tosa basins; Okamura and Blum, 1993), maximum thickness of the progradational wedges occurs at the center of subsidence where the sediments mainly accumulate, suggesting that the lateral shift of depositional systems is confined to a very narrow area. Thus the problem of three-dimensional variability of depositional systems (Martinsen and Helland-Hansen, 1994) may be less important in these basins. We believe that the Kakegawa Basin also had a subsidence pattern similar to the Senoumi Basin. 2.1. Sequence stratigraphic architecture Individual systems tracts and bounding surfaces of the Kakegawa sequence have been identified by means of the onlap and downlap geometries shown by interbedded tuff beds (Masuda and Ishibashi, 1991; Masuda, 1994). The lower sequence boundary is a prominent surface between pre-Pliocene rocks and the Kakegawa Group. The lowstand sys-
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tems tract (LST) fills a small depression (ca. 20 km wide and 2 km deep; Fig. 2) and consists of slope, submarine-fan and slope-apron deposits. The transgressive systems tract (TST; ca. 150 m thick on the shelf and 500 m thick on the slope) is characterized by coastal onlap patterns of tuff beds onto the lower sequence boundary. The TST consists of retrogradational alluvial, shoreface and shelf facies and progradational slope facies. The highstand systems tract (HST; ca. 150 m thick on the shelf and 400 m thick on the slope) is characterized by downlap patterns of tuff beds onto the expected maximum flooding surface that might be contained in a condensed zone of massive siltstone (ca. 5–7 m thick on the shelf and more than 10 m thick on the slope). The HST consists mainly of a progradational shelf and slope facies. The upper sequence boundary is an erosion surface which is covered by Pleistocene coastal alluvial fan and submarine channel fill deposits (Ogasa Group). In the northwestern part of the basin, eight formations of the Kakegawa sequence represent alluvial to slope facies (Fig. 2, Table 1), which comprise part of the TST and the HST. The Nobe Formation and the lower part of the Soga Formation consist of trough and tabular cross-stratified and horizontally stratified conglomerates and sandstones of braided channel fill, and sandstones and siltstones of interchannel and flood-plain origin. A small valley (Fig. 3) incised into the basement rock was probably cut by streams and is filled with bioturbated sandy siltstones and laminated sandstones (Dainichi Formation). The Dainichi and the Aburayama formations and the upper part of the Soga Formation consist mainly of hummocky cross-stratified (HCS) sandstones of lower-shoreface to inner-shelf origin, and trough and tabular cross-stratified conglomerates and sandstones of upper-shoreface origin. These deposits grade laterally into bioturbated sandy siltstones of outer-shelf origin (Ukari Formation) toward the southeast. The Hijikata Formation consists of bioturbated siltstones and interbedded sandstones of upper-slope origin. Slumped beds and small channel fill conglomerate, sandstone and shell beds (up to 10 m thick and wide) are common in the siltstones. Poorly sorted conglomerates and sandstones of submarine channel fill, and sandstones and siltstones (turbidites) of attached levee deposits (Soga Formation) are recog-
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Fig. 2. Schematic cross-section of the Kakegawa Group (after Sakai and Masuda, 1996). AR D Arigaya Tuff; SH D Shiraiwa Tuff; IO D Iozumi Tuff; NI D Nishihirao Tuff; HO D Hosoya Tuff; MO D Moridaira Tuff; OK D O’kubo Tuff; KO D Ko’gosyo Tuff; SO D Soga Tuff; HA D Haruoka Tuff; SB D sequence boundary; TS D transgressive surface; MFS D maximum flooding surface; LST D lowstand systems tract; TST D transgressive systems tract; HST D highstand systems tract.
nized near the top of the group. The channel fill deposits are probably a lateral equivalent of the alluvial-fan deposits of the Soga Formation, judging from basinward tracing of the tuff beds interbedded just below and above the alluvial-fan and submarine channel fill deposits (Fig. 2). Several tuff beds (white or pink tuff and pumice beds) are widely traceable in the study area (O’kubo, Hosoya, Soga and Haruoka tuffs, Fig. 3). The Hosoya and the Soga tuffs are dated about 1.9 and 1.6 Ma, respectively, by fission-track dating and biostratigraphic analysis (e.g. Nishimura, 1977; Ibaraki, 1986).
The northwestern part of the Kakegawa Group contains a number of upward-shallowing successions, referred to here as parasequences in the sense of Van Wagoner et al. (1988). Parasequences (5– 40 m) from alluvial to shelf environments consist of upward-shallowing facies successions of tempestite and alluvial conglomerate beds, bounded at their bases and tops by ravinement surfaces. At least ten parasequences (PS1–PS10) have been recognized in the TST and six parasequences (PS11–PS16) in the HST (Fig. 3). The TST parasequences show a retrogradational stacking pattern and those of the HST show a progradational stacking pattern. The internal
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Table 1 Brief facies description of each formation
Water depths are based on paleontologic data (e.g. Chinzei, 1980; Ishibashi, 1985; Nobuhara, 1993). FA D facies association; IV D incised valley fill.
architecture of parasequences and their boundaries are described in an other paper (Sakai and Masuda, 1996). The parasequences are interpreted to result from fifth- or sixth-order (Fulthorpe, 1991) eustatic sea-
level changes, because the frequency represented by global oxygen isotope curves and the number of parasequences between dated tuff beds (1.9 to 1.6 Ma) appear to correspond (Fig. 3) (Sakai and Masuda, 1996).
100 T. Sakai, F. Masuda / Sedimentary Geology 122 (1998) 95–107 Fig. 3. (a) Columnar cross-sections of the northwestern part of the Kakegawa Group. Solid lines are parasequence boundaries correlated based on tuff beds. (b) Oxygen isotope curves (Williams, 1990) from deep-sea foraminifera tests. Dots represent sea-level falls that are interpreted to have contributed to parasequence deposition.
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3. Methods Tectonic and eustatic effects on sequence development of the Kakegawa Group were estimated using the method of geohistory analysis (Van Hinte, 1978), which allows us to understand basin development by producing subsidence and accumulation curves through time. Here we followed the method compiled by Allen and Allen (1990), to provide tectonic and total subsidence, and accommodation curves during deposition of the northwestern part of the Kakegawa Group. The analysis requires the following information: (1) depth–porosity curve; (2) water depth; (3) age; and (4) eustasy. These quantities were estimated for the base and the top of each parasequence for a geohistory plot. Analysis was applied to successions from sections 2 and 5 of Fig. 3, where almost complete successions have been obtained, in order to estimate the effects of differential subsidence on sequence architecture as well as to identify the factors that contributed to sequence development. The rate of sediment supply and the sedimentation rate on the shelf were estimated from the cross-sections of the Kakegawa Group, as discussed later. 3.1. Sections 2 and 5 Sections 2 and 5 contain twelve parasequences (PS5–PS16; Fig. 3) and fourteen parasequences (PS3–PS16), respectively. PS3–PS10 are contained in the TST. PS3 and PS4 can be recognized near section 5, where they consist mainly of amalgamated HCS sandstones. Trough and tabular cross-stratified conglomerate and shell beds of upper-shoreface deposits overlie HCS sandstones in PS3. PS5 consists of poorly sorted conglomerates and sandstones of alluvial origin near section 2, and poorly sorted sandstones of transgressive lag deposits, sandy siltstones of outer-shelf deposits and HCS sandstones of inner-shelf deposits near section 5. PS6–PS9 consist mainly of upward-coarsening HCS successions near section 2 and upward-thickening alternations of sandstones and sandy siltstones near section 5, with intervening HCS sandstones at the tops of PS6 and PS7. Outer-shelf siltstones (1–2 m) overlie lower parasequence boundaries of PS7 and PS9 near section 2. Alluvial conglomerate beds truncate HCS
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sandstones at the top of PS7 in section 2. PS10 is characterized by an upward-thickening alternation of sandstones and siltstones (sandstone < siltstone) at both locations. In the TST parasequences, PS3– PS6 are thicker than other parasequences (Fig. 3). PS3–PS10 indicate an overall upward-fining pattern. However, alluvial conglomerate beds of PS7 prograded to a more basinward position than the underlying alluvial deposits. This can be interpreted as a short period of relative sea-level fall in a third-order transgressive phase. PS11–PS16 show an overall upward-coarsening pattern and together comprise the HST. PS11 consists, from base to top, of a massive siltstone and an alternation of sandstones and siltstones (sandstone < siltstone), followed by an alternation of HCS sandstones and siltstones near section 2. Basal massive siltstone is interpreted as the condensed zone of a third-order sequence, and perhaps marks the maximum flooding surface (Sakai and Masuda, 1996). PS12–PS14 consist, from base to top, of sandy siltstones, alternations of HCS sandstones and sandy siltstones, and amalgamated HCS sandstones in both sections. The siltstones of section 5 parasequences are thicker than those of section 2. PS15 consists of basal conglomerates derived by shoreface erosion and an alternation of HCS sandstones and bioturbated sandy siltstones near section 2, and alluvial conglomerates near section 5. Laterally equivalent shallow marine sediments recognized near section 2 may be truncated by the conglomerates near section 5. The conglomerates also truncate underlying HCS sandstones of PS14 near section 5 and taper laterally toward the northwest into HCS sandstones between sections 2 and 3. PS16 consists of amalgamated HCS sandstones and an alternation of HCS pumiceous sandstones and tuff beds, and is bounded at its top by overlying alluvial conglomerates of the Ogasa Group. Parasequences in the HST tend to thicken basinward. 3.2. Depth–porosity curve The porosity of sandstones and siltstones was measured. 43 samples were collected from the southeastern part of the group as well as the northwestern part. The Pleistocene Ogasa Group of 20–700 m thickness overlies the Kakegawa Group; reduction in
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Fig. 4. Result in measurement of porosity of sandstones and siltstones from the Kakegawa Group. The curves are fit by using the method of least squares.
porosity by accumulation of the Ogasa Group must be taken into account in the depth–porosity curve. Therefore the burial depth of the sampling points is measured from the top of the Ogasa Group. The resultant porosity–depth curves of sandstone and siltstone (Fig. 4) are as follows: s D 0:411 exp. 0:000089H / m D 0:423 exp. 0:00019H / where s and m are porosity of the sandstone and siltstone, respectively, and H is the present thickness of sediment from the top of the overlying Ogasa Group. 3.3. Water depth Changes in paleowater depth were evaluated from the cross-sections of Fig. 3. The average gradients of the shelf were evaluated for the base and the top of each parasequence. Data were based upon the positions of depth indicators on the lower parasequence boundaries and just below the upper parasequence boundaries. Depth indicators consist of shoreline position (0 m), inner- and outer-shelf transition (ca. 50 m), and shelf edge (ca. 120 m) (Fig. 3). Average shelf gradients were evaluated for parasequences in which at least two positions of depth indicators can be recognized in the cross-sections, assuming that the shelf had linear morphology in two dimensions. Water depth was then calculated from the average shelf gradient by measuring the
distance between the positions of the depth indicators and adding the estimated water depth. The derived gradients range from 0.004 to 0.009, which are equivalent to those of shelves in depositional forearc and backarc basins around Japan, measured from bathymetric maps (e.g. Ikehara, 1988; Ikehara et al., 1990, 1994). Shelf gradients for PS10, and the base of PS11 and PS12 could not be estimated because the major part of these parasequences consists of outer-shelf deposits; shoreline and inner- and outer-shelf transition points do not occur in outcrops of these parasequences. The shelf gradient at the base of cycles PS3 and PS4 also could not be evaluated, because shoreline and shelf-edge positions are hard to be estimated at the base of these cycles due to covers of Holocene sediments. Water depths of these parasequences are assumed to be evaluated using average shelf gradient of the TST parasequences (0.00745) and that of the HST parasequences (0.00475) determined by averaging shelf gradients of other parasequences in individual systems tracts. In the TST, there are local slumped deposits in outer-shelf deposits as well as in slope deposits (Fig. 3). The facies suggest that the TST shelf might be steeper than that of the HST, and that the averaged values may be appropriate for the use of the calculation. In cycles PS8, PS12 and PS13, sandstone prograded out to the vicinity of the shelf edge. The estimated shelf gradients at the top of these parasequences are steep, and are therefore inappropriate for use in the calculation of water depth. Average gradients of the TST and the HST were also used for the calculation of water depth for these parasequences. Water depth error was estimated based on the shallowest and the deepest points evidenced by depth indicators which do not always coincide with the position of outcrops (especially shelf edge) but are sometimes between adjacent measured sections (i.e. error of their positions equates to the interval between stratigraphic sections, e.g. up to 2 km and the corresponding variation in depth between sections). The depth range of the inner- and outer-shelf transition in the Senoumi Basin has not been published and was, therefore, evaluated from other storm-dominated shelves. Bourgeois (1980) evaluated the maximum depth of HCS beds by distinguishing innershelf deposits of the Kakegawa Group from over-
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lying ripple laminations, which were interpreted to have formed in water depths shallower than 50 m. Other results indicate this depth on modern stormdominated shelves to be 30 to 80 m (e.g. Cacchione et al., 1984; Saito, 1989). From these evaluations, the depth range of the inner- and outer-shelf transition might be between 30 and 80 m. Depth range in shelf edge (80–140 m) is evaluated from the Senoumi Basin (Ishii and Nemoto, 1995). 3.4. Age Ages of the base and the top of each parasequence were determined from dated tuff beds (Hosoya and Soga tuffs), and from correlation with the deep-sea oxygen isotope curve (Williams, 1990; Fig. 2). The age of the top and the base of PS8, including the Hosoya Tuff, were evaluated from the eustatic curve. The sea-level fall that probably controlled PS8 formation was correlated with sea-level fall which occurred between 1.87 and 1.83 Ma (Fig. 3). Ages of the start and the end of PS8 deposition were equated
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with that of the sea-level maximum and minimum of the oxygen isotope curve (i.e. 1.87 and 1.83 Ma, respectively). The ages of other parasequences were correlated with eustatic sea-level falls of the curve based on a time interval of PS8 deposition (Fig. 3). 3.5. Eustasy Eustatic sea-level changes were estimated from the oxygen isotope curve of Williams (1990). The amplitudes of eustatic sea-level changes were calculated, assuming that the variation in isotope ratio during the past 20,000 years represents 100 m of sea-level rise (e.g. Saito, 1994), i.e. a 0.09‰ change in isotope ratio equates to a 10 m change in sea level. Evaluated eustatic sea-level changes are shown in Fig. 5a. 3.6. Sediment supply and sedimentation rate on the shelf Variation in the sedimentation rate on the shelf was evaluated by measuring the area of each parase-
Fig. 5. (a) Eustatic sea-level changes evaluated from oxygen isotope curve. (b, c) Accommodation, total subsidence, and tectonic subsidence curves from sections 2 and 5, respectively. Water depth errors are shown by bars in the total subsidence curves and by the shaded area in the accommodation and tectonic subsidence curves. (d) Change in sediment supply and sedimentation rate on the shelf evaluated from the cross-sections of the entire Kakegawa Group and Fig. 3. MFS D maximum flooding surface; TST D transgressive systems tract; HST D highstand systems tract.
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quence on the cross-sections (Fig. 3). The rate of sediment supply was also evaluated from the crosssection of the entire Kakegawa Group (Sakai and Masuda, 1996; Fig. 5) to assess basinward sediment transport. Loss of the sediments by shore-parallel transport beyond the basin was probably less important, because the shelf of the Kakegawa Basin may have been similar to that of the Senoumi Basin where submarine channels provide transport paths for sediment to bypass (Uda et al., 1988) and both the southern and the northern ends of the shelf are bounded by capes preventing sediment transport beyond the basin.
4. Result Results of the analysis are shown in Fig. 5. The errors of these curves in terms of water depth are up to 70 m (Fig. 5). The total subsidence curve (isostatic subsidence C tectonic subsidence) for each section corresponds approximately with broad-scale accommodation curve. The plots of tectonic subsidence show two episodes of subsidence. The first episode is represented by rapid subsidence between 2.2 and 2.0 Ma. The rate of tectonic subsidence decreased about 2.0 Ma. Slow subsidence continued until the end of deposition of the Kakegawa Group. A short period of uplift probably took place about 1.6 Ma. The plots also show a slightly faster rate of subsidence around section 5 during 2.0–1.4 Ma. The estimated rate of sediment supply ranges from 0:8 ð 104 to 2:8 ð 104 m2 =ka (Fig. 5d). Because the rate was evaluated from the cross-sections, the unit is shown as m2 =ka. The inferred rates are therefore equivalent to those of sediment discharge from major modern rivers in Japan (on the order of 105 to 106 m3 =year; Saito and Ikehara, 1992), presuming that the lateral extent of the Kakegawa Basin was similar to the Senoumi Basin (ca. 30 km), and that the lateral change in the sedimentation rate was small in the basin. Sediment supply increased between 2.2 and 2.0 Ma, temporarily decreased around 2.0 Ma. Its rate increased again until 1.5 Ma and then finally decreased between 1.65 and 1.6 Ma. Sediment supply between 1.6 and 1.4 Ma cannot be evaluated, because parasequence boundaries are
unclear in slope deposits. The sedimentation rate on the shelf was not in phase with that of the overall sediment supply. The rate was high between 2.2 and 2.0 Ma, and decreased between 2.0 and 1.75 Ma and then increased between 1.75 and 1.4 Ma.
5. Discussion The pattern of low-frequency (third-order) Late Pliocene–Early Pleistocene eustatic sea-level change recorded in oxygen isotope curves indicates sea-level lowstand around 2.4 Ma followed by sea-level rise– highstand between 2.4 and 0.8 Ma (e.g. Williams, 1990; Feeley et al., 1990). The transgressive– regressive cycle of the Kakegawa Group is in phase with this third-order eustatic cycle. Similar third-order transgressive–regressive cycles have also been identified from other forearc and backarc basins in Japan (e.g. Kazusa and Uonuma basins, Ito, 1995; Urabe et al., 1995), and passive margin basins (e.g. Louisiana offshore, Lowrie and McDaniel-Lowrie, 1985). Therefore, we interpret the third-order Kakegawa sequence as controlled by a third-order eustasy (Masuda and Ishibashi, 1991; Sakai and Masuda, 1992, 1996). The broad transgressive and regressive cycle of the Kakegawa Group is interpreted as controlled mainly by both tectonic subsidence and sedimentation rate for the following reasons: (1) the general correspondence of the accommodation curves and the total subsidence curves (Fig. 5) over this time interval suggests that accommodation was generated mainly by total subsidence, that is, the sum of tectonic and isostatic subsidence (function of sediment accumulation); (2) transgression during 2.2–2.0 Ma cannot be explained by third-order eustasy which falls in this period; and (3) regression starts when the sedimentation rate outstrips on the rate of formation of accommodation (e.g. Jervey, 1988). The rate of accommodation formation was almost constant around 1.75 Ma in broad scale (Fig. 5); increase in the sedimentation rate at 1.75 Ma was important to change the accommodation-dominant phase into the sediment-supply-dominant phase. Geohistory analysis and our evaluation of sedimentation rate on the shelf delineate three separate phases of subsidence and sedimentation (Table 2).
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Table 2 Summary table showing temporal changes of factors influencing sequence development
PS3–PS6 PS7–PS10 PS11–PS16
Systems tract
Tectonic subsidence
Sediment supply
Sedimentation rate
TST TST HST
High Slow Slow
Small Moderate High
High Small Moderate
The PS3–PS6 interval, deposited during the first phase (2.2–2.0 Ma), is characterized by a thick retrogradational parasequence set. Tectonic subsidence was rapid and the sedimentation rate was high in this phase (Fig. 5). Tectonic subsidence was the dominant factor, leading to a retrogradational parasequence set. PS7–PS10, which accumulated during the second phase (2.0–1.75 Ma), are thin but also compose a retrogradational parasequence set. The rate of tectonic subsidence decreased in this phase. Sediment supply was moderate, but the sedimentation rate on the shelf was low (Fig. 5). This suggests that much sediment may have bypassed the shelf through the small channels developed on the upper slope and perhaps also by slumping (described in Sakai and Masuda, 1996). Therefore, the PS7–PS10 retrogradational parasequence set is interpreted as formed during slow tectonic subsidence with a low rate of sedimentation induced by a high rate of basinward sediment bypassing. Cycles PS11–PS16, deposited during the third phase (1.75–1.4 Ma), are thin and constitute a progradational parasequence set. Slow tectonic subsidence continued, and sediment supply was high (Fig. 5). The rate of sediment supply was much larger than the shelf sedimentation rate in this phase. This indicates that most of the sediment bypassed the shelf and was deposited on the slope, which consequently prograded rapidly. The slightly increased sedimentation rate (Fig. 5) was enough to initiate a progradational parasequence set in this phase. A short period of uplift possibly occurred around 1.6 Ma, which may have controlled the stacking pattern of parasequences in which positions of shelf edge prograde further basinward (Fig. 3). Subsidence increased towards the basin center, resulting in deposition of thicker parasequences in the basin and preservation of alluvial deposits of PS15 near section 5. The maximum flooding surface of the third-order sequence was developed when the rate of accommo-
dation became less important than the sedimentation rate around 1.75 Ma.
6. Conclusions Geohistory analysis applied to the Plio–Pleistocene Kakegawa Group, together with an evaluation of the sedimentation rate, suggest that an observed thirdorder sequence was controlled mainly by changes in tectonic subsidence and the sedimentation rate on the shelf. The lower part of the TST (PS3–PS6), composed of thick retrogradational parasequences, was formed when accommodation created by a high rate of tectonic subsidence exceeded a high rate of sedimentation. The upper TST (PS7–PS10) consists of thin parasequences and forms a retrogradational set. This parasequence set probably formed under a combination of more slowly increasing accommodation driven by tectonic subsidence and a low rate of sedimentation due to an increase in sediment bypassing into the basin. The HST (PS11–PS16) consists of a progradational parasequence set which accumulated when sediment supply increased during continued slow tectonic subsidence. The maximum flooding surface dividing these systems tracts apparently formed at the time of transition between a system which was dominated by tectonic subsidence to a system dominated by sediment supply.
Acknowledgements This paper summarizes a part of the Ph.D. thesis undertaken by T.S. at Osaka University, Japan. T.S. is most grateful to Prof. T. Sunamura of Osaka University for his guidance and variable suggestions. We thank Dr. T. Tsuji, Mr. M. Ueki and Ms. K. Hatano of Japan National Oil Corporation (JNOC) for their help in the measurement of porosity, and Drs. Y.
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Saito of Geological survey, Japan, M. Yokokawa and N. Endo of Osaka University, and Mr. A. Okui of JNOC, for their suggestions and discussions about the method of basin analysis and the concept of sequence stratigraphy. Thanks are also due to Drs. H.E. Clifton of Conoco Inc., USA, and M. Ito of Chiba University, Japan, for their help in improving an early version of the manuscript, and Drs. R.M. Carter and T.R. Naish of James Cook University, Australia, editors of this special issue, and referees of Sedimentary Geology for their critical comments. This work was partially supported by the award of a fellowship by the Japan Society for the Promotion of Science for Japanese Junior Scientists.
References Allen, P.A., Allen, J.R., 1990. Basin Analysis, Principles and Applications. Blackwell, Oxford, 451 pp. Bourgeois, J., 1980. A transgressive shelf sequence exhibiting hummocky cross-stratification: the Cape Sebastian Sandstone (Upper Cretaceous), southwestern Oregon. J. Sediment. Petrol. 50, 681–702. Cacchione, D.A., Drake, D.R., Grant, W.D., Tate, G.B., 1984. Rippled scour depressions on the inner continental shelf off central California. J. Sediment. Petrol. 54, 1280–1291. Chinzei, K., 1980. Molluscan fauna of the Plio-Pleistocene Kakegawa Group; its composition and horizontal distribution (in Japanese with English abstr.) Nat. Sci. Mus. Mem. 13, 15–20. Devlin, W.J., Rudolph, K.W., Shaw, C.A., Ehaman, K.D., 1993. The effect of tectonic and eustatic cycles on accommodation and sequence stratigraphic framework in the Upper Cretaceous foreland basin of the southwestern Wyoming. Spec. Publ. Int. Assoc. Sedimentol. 18, 501–520. Feeley, M.H., Moore Jr., T.C., Loutit, T.S., Bryant, W.R., 1990. Sequence stratigraphy of Mississippi Fan related to oxygen isotope index. Am. Assoc. Pet. Geol. Bull. 74, 407–424. Fulthorpe, C.S., 1991. Geological controls on seismic sequence resolution. Geology 19, 61–65. Galloway, W.E., 1989a. Genetic stratigraphic sequences in basin analysis, I. Architecture and genesis of flooding surface bounded depositional units. Am. Assoc. Pet. Geol. Bull. 73, 125–142. Galloway, W.E., 1989b. Genetic stratigraphic sequences in basin analysis, II. Application to northwest Gulf of Mexico Cenozoic basin. Am. Assoc. Pet. Geol. Bull. 73, 143–154. Haq, B.U., Hardenbol, J., Vail, P.R., 1987. Chronology of fluctuating sea levels since the Triassic. Science 235, 1156–1167. Ibaraki, M., 1986. Neogene planktonic foraminiferal biostratigraphy of the Kakegawa area on the Pacific coast of central Japan. Rep. Fac. Sci. Shizuoka Univ. 20, 39–173. Ikehara, K., 1988. Marine geology map, Series 37. Explanatory
notes of sedimentological map of Tosa wan (1 : 200,000) (in Japanese with English abstr.). Geological Survey of Japan, Tokyo, 29 pp. Ikehara, K., Katayama, H., Sato, M., 1990. Marine geology map, Series 36. Explanatory notes of sedimentological map offshore of Tottori (1 : 200,000) (in Japanese with English abstr.). Geological Survey of Japan, Tokyo, 42 pp. Ikehara, K., Nakajima, T., Katayama, H., 1994. Marine geology map, Series 41. Explanatory notes of sedimentological map west of Akita (1 : 200,000) (in Japanese with English abstr.). Geological Survey of Japan, Tokyo, 58 pp. Ishibashi, M., 1985. Shallow marine processes in the Plio-Pleistocene Kakegawa Group, central Honshu, Japan (in Japanese with English abstr.). The graduation thesis, Shizuoka University, Shizuoka, 154 pp. 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: Taira, A., Masuda, F. (Eds.), Sedimentary Facies in the Active Plate Margin. TERRAPUB, Tokyo, pp. 345–363. Ishii, R., Nemoto, K., 1995. Variation in water depth of erosional terraces on continental shelf edges off western coast of the Suruga Bay, central Japan (in Japanese with English abstr.) J. Fac. Mar. Sci. Technol. Tokai Univ. 39, 159–171. Ito, M., 1995. Volcanic ash layers facilitate high-resolution sequence stratigraphy at convergent plate margin: the Plio-Pleistocene forearc basin fill in Boso Peninsula, Japan. Sediment. Geol. 95, 187–206. Jervey, M.T., 1988. Quantitative geological modeling of siliciclastic rock sequences and their seismic expression. In: Wilgus, C.K., Hastings, B.S., Kendall, C.G.St.C., Posamentier, H.W., Ross, C.A., Van Wagoner, J.C. (Eds.), Sea-Level Changes: An Integrated Approach. Soc. Econ. Paleontol. Mineral., Spec. Publ. 42, 47–69. Kano, K., Matsushima, N., 1988. The Shimanto Belt in the Akaishi Mountains, eastern part of the SW Japan. In: Taira, A., Ogawa, Y. (Eds.), The Cretaceous — Recent Evolution of the SW Japan Active Margin. Modern Geol. 12, 97–126. Lowrie, A., McDaniel-Lowrie, M.L., 1985. Application of Pleistocene climate models to gulf coast stratigraphy. Gulf Coast Assoc. Geol. Soc. Trans. 35, 201–208. Makiyama, J., 1947. Two stages of Middle Miocene, Japan. Kyoto Univ. Coll. Sci. Mem. Ser. B 19, 33–36. Martinsen, O.J., Helland-Hansen, W., 1994. Sequence stratigraphy and facies model of an incised valley fill: the Gironde Estuary, France — discussion. J. Sediment. Res. B 64, 78–80. Masuda, F., 1994. Onlap and downlap patterns in Plio-Pleistocene forearc and backarc basin-fill successions, Japan. Sediment. Geol. 93, 237–246. Masuda, F., Ishibashi, M., 1991. Onlap and downlap patterns discovered in a depositional sequence of the Plio-Pleistocene Kakegawa Group, Japan. J. Sedimentol. Soc. Jpn. 34, 75–78. Meckel III, L.D., Galloway, W.E., 1996. Formation of high-frequency sequences and their bounding surfaces: case study of the Eocene Yegua Formation, Texas Gulf Coast, USA. Sediment. Geol. 102, 155–186. Muto, T., 1985. Fossil submarine channels found from the Pleis-
T. Sakai, F. Masuda / Sedimentary Geology 122 (1998) 95–107 tocene strata in the Kakegawa district (in Japanese with English abstr.) J. Geol. Soc. Jpn. 91, 439–452. Nishimura, S., 1977. Re-examination of age determination of tephra layers by fission track method (in Japanese). Geol. Soc. Jpn., Rep. Kansai Region, 8. Nobuhara, T., 1993. The relationship between bathymetric depth and climatic change and its effect on molluscan faunas of the Kakegawa Group, central Japan. Trans. Proc. Paleontol. Soc. Jpn. N.S. 170, 159–185. Okamura, Y., Blum, P., 1993. Seismic stratigraphy of Quaternary stacked progradational sequences in the southwest Japan forearc: an example of fourth-order sequences in an active margin. Spec. Publ. Int. Assoc. Sedimentol. 18, 213–232. Payton, C.E., 1977. Seismic Stratigraphy — Applications to Hydrocarbon Exploration. Am. Assoc. Pet. Geol. Mem. 26, 516 pp. Posamentier, H.W., Allen, G.P., 1993. Variability of the sequence stratigraphic model: effects of local basin factor. In: Cloetingh, S., Sassi, W., Horvath, F., Puigdefabregas, C. (Eds.), Basin Analysis and Dynamics of Sedimentary Basin Evolution. Sediment. Geol. 86, 91–109. Saito, Y., 1989. Modern storm deposits in the inner shelf and their recurrence intervals, Sendai Bay, northeast Japan. In: Taira, A., Masuda, F. (Eds.), Sedimentary Facies in the Active Plate Margin. TERRAPUB, Tokyo, pp. 331–344. Saito, Y., 1994. Shelf sequence and characteristics bounding surfaces in a wave-dominated setting: latest Pleistocene– Holocene examples from northeast Japan. Mar. Geol. 120, 105–127. Saito, Y., Ikehara, K., 1992. Sediment discharge of Japanese rivers, and sedimentation rate and carbon content of marine sediments around Japanese Islands (in Japanese). Chishitsu News 452, 59–64. Sakai, T., Masuda, F., 1992. Parasequence sets in the Plio-Pleistocene Kakegawa Group, Shizuoka, Japan (in Japanese with English abstr.) J. Sedimentol. Soc. Jpn. 36, 19–24. Sakai, T., Masuda, F., 1996. Sequence stratigraphy of the upper part of the Plio-Pleistocene Kakegawa Group, Shizuoka, Japan. J. Sediment. Res. B 66, 778–787. Schlager, W., 1993. Accommodation and sediment supply — a dual control on stratigraphic sequences. In: Cloetingh, S., Sassi, W., Horvath, F., Puigdefabregas, C. (Eds.), Basin Analysis and Dynamics of Sedimentary Basin Evolution. Sediment. Geol. 86, 111–136. Sugiyama, Y., 1989. Right-lateral strike slip basins in the Second Paleo-Seto Island Sea — a model of basin development
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associated with the migration of active domain of a large-scale strike-slip fault (in Japanese with English abstr.). Struct. Geol. (J. Technol. Res. Group Jpn.) 36, 99–108. Sugiyama, Y., Sangawa, A., Shimokawa, K., Mizuno, K., 1988. Geology of the Omaezaki district, with geological sheet map at 1 : 50,000 (in Japanese with English abstr.). Geological Survey of Japan, Tokyo, 153 pp. Swift, D.J.P., Oertel, G.F., Tillman, R.W., Thorne, J.A. (Eds.), 1991. Shelf Sand and Sandstone Bodies: Geometry, Facies and Sequence Stratigraphy. Spec. Publ. Int. Assoc. Sedimentol. 14, 532 pp. Thorne, J.A., Swift, D.J.P., 1991. Sedimentation on continental margins, II. Application of the regime concept. In: Swift, D.J.P., Oertel, G.F., Tillman, R.W., Thorne, J.A. (Eds.), Shelf Sand and Sandstone Bodies: Geometry, Facies and Sequence Stratigraphy. Spec. Publ. Int. Assoc. Sedimentol. 14, 33–58. Uda, T., Tsutsumi, H., Omata, A., 1988. Field investigation of sediment movement into a submarine canyon off the Suruga Coast. Coastal Eng. Jpn. 31, 289–303. Urabe, A., Tateishi, M., Kazaoka, O., 1995. Depositional cycle of marine beds and relative sea-level changes of the Plio-Pleistocene Uonuma Group, Niigata, central Japan (in Japanese with English abstr.) Mem. Geol. Soc. Jpn. 45, 140–153. Vail, P.R., Audemard, F., Bowman, S.A., Einser, P.N., PerezCruz, C., 1991. The stratigraphic signatures of tectonics, eustacy and sedimentology — an overview. In: Einsele, G., Ricken, W.X., Seilacher, A. (Eds.), Cycles and Events in Stratigraphy. Springer, Berlin, pp. 617–659. Van Hinte, J.E., 1978. Geohistory analysis — application of micropaleontology in exploration geology. Am. Assoc. Pet. Geol. 62, 210–222. Van Wagoner, J.C., Posamentier, H.W., Mitchum, R.M., Vail, P.R., Sarg, J.F., Loutit, T.S., Hardenbol, J., 1988. An overview of the fundamentals of sequence stratigraphy and key definitions. In: Wilgus, C.K., Hastings, B.S., Kendall, C.G.St.C., Posamentier, H.W., Ross, C.A., Van Wagoner, J.C. (Eds.), SeaLevel Changes: An Integrated Approach. Soc. Econ. Paleontol. Mineral., Spec. Publ. 42, 39–45. Wilgus, C.K., Hastings, B.S., Kendall, C.G.St.C., Posamentier, H.W., Ross, C.A., Van Wagoner, J.C. (Eds.), 1988. Sea-Level Changes — an Integrated Approach. Soc. Econ. Paleontol. Mineral., Spec. Publ. 42, Spec. Publ. 42, 407 pp. Williams, D.F., 1990. Selected approaches of chemical stratigraphy to time-scale resolution and quantitative dynamic stratigraphy. In: Cross, T. (Ed.), Quantitative Dynamic Stratigraphy. Prentice Hall, NJ, pp. 543–565.
Sedimentary Geology 122 (1998) 95–107
The relationship between glacio-eustatic parasequences and a third-order sequence in the Kakegawa Group, central Japan Tetsuya Sakai a,Ł , Fujio Masuda b b
a Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan Department of Geology and Mineralogy, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
Received 9 October 1996; accepted 26 September 1997
Abstract The Plio–Pleistocene Kakegawa Group, central Japan, consists of a third-order depositional sequence (2.6–1.0 Ma). The northwestern part of the Kakegawa sequence consists of up to 500 m of alluvial, shoreface, shelf, slope and submarine-channel facies. It contains at least sixteen upward-shallowing cycles (parasequences), the deposition of which was affected by high-frequency eustatic sea-level cycles. The lower part of the sequence is characterized by a retrogradational parasequence set, which formed a transgressive systems tract (2.2–1.75 Ma) followed by a progradational parasequence set comprising a highstand systems tract (1.75–1.4 Ma). Subsidence analysis and evaluation of changes in the shelf sedimentation rate estimated from cross-sections, suggest that formation of the third-order sequence was controlled by tectonic subsidence and variation in the sedimentation rate. Rapid subsidence and a high rate of sedimentation during 2.2–2.0 Ma resulted in deposition of the lower part of the transgressive systems tract, characterized by thick backstepping successions. The rate of subsidence decreased in the period 2.0–1.75 Ma. The sedimentation rate also decreased due to a high rate of sediment bypassing. However, subsidence was still the dominant factor, leading to the formation of thin backstepping successions. The 1.75–1.4 Ma progradational succession resulted from a combination of a low rate of subsidence and moderate sedimentation. The progradational units become thicker basinward owing to faster subsidence in the basin center. The maximum flooding surface was formed around 1.75 Ma even though subsidence was slow at this time. 1998 Elsevier Science B.V. All rights reserved. Keywords: forearc basin; Plio–Pleistocene; sediment supply; sediment transport; tectonic subsidence; third-order sequence
1. Introduction Since their introduction by Payton (1977), sequence stratigraphic concepts have evolved rapidly and numerous theoretical and case studies have been Ł Corresponding
author. Present address: Department of Geology and Mineralogy, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan. Tel.: C81 (75) 753-4158; Fax: C81 (75) 753-4189; E-mail: [email protected]
published (e.g. Haq et al., 1987; Wilgus et al., 1988; Galloway, 1989a,b; Vail et al., 1991; Swift et al., 1991; Posamentier and Allen, 1993). These studies suggest that sequence formation is controlled by changes in accommodation and sedimentation rate (e.g. Thorne and Swift, 1991; Schlager, 1993). Recently, many stratigraphers have focused their interest on the causes of formation of depositional sequences and their bounding surfaces (e.g. Thorne and Swift, 1991; Devlin et al., 1993; Meckel and Galloway, 1996).
c 1998 Elsevier Science B.V. All rights reserved. 0037-0738/98/$ – see front matter PII: S 0 0 3 7 - 0 7 3 8 ( 9 8 ) 0 0 1 0 0 - 6
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Fig. 1. Location map of the Kakegawa Group. Enclosed part is study area. NP D North American Plate.
The Plio–Pleistocene Kakegawa Group in central Japan (Fig. 1), represents the fill of a forearc basin (Ishibashi, 1989), and forms a third-order sequence (Masuda and Ishibashi, 1991; Sakai and Masuda, 1996). The part of the group exposed on the northwestern side of the basin is an ideal succession to assess the factors which control sequence formation, i.e. tectonic subsidence, eustatic sea-level change, sediment supply, and sediment transport (Jervey, 1988; Galloway, 1989a), because of the availability of well constrained age data, facies modeling and
sequence stratigraphy. The objective of this paper is to identify the factors which influenced the formation of a third-order sequence in a forearc basin, using geohistory analysis.
2. Plio–Pleistocene forearc basin stratigraphy The Kakegawa Group consists of nonmarine and shallow to deep marine clastic sediments and is up to 3500 m thick. The group represents the fill of
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a Plio–Pleistocene forearc basin (Kakegawa Basin, Sugiyama et al., 1988) that probably formed as a result of oblique subduction of the Philippine Sea Plate beneath the eastern margin of the Eurasia Plate (Sugiyama, 1989). The Kakegawa Group unconformably overlies pre-Pliocene rocks which comprise Miocene forearc basin fill and Cretaceous–Paleogene deep-sea sediments (Makiyama, 1947; Kano and Matsushima, 1988). The group is overlain by coastal alluvial fan and submarine channel fill deposits of the Pleistocene Ogasa Group (Muto, 1985). The Kakegawa Group itself comprises a depositional sequence consisting of alluvial fan, incised valley fill, shoreface, shelf, slope, submarine-fan, slope-apron and submarine channel fill deposits (Ishibashi, 1989; Sakai and Masuda, 1996) (Fig. 2). Paleocurrent from cross-stratifications indicates that dominant sediment transport was toward the south to southeast (Sakai and Masuda, 1996). The sediment facies distribution suggests that the Kakegawa Basin was similar to the ‘Senoumi Basin’, a recent forearc basin that has a coastal alluvial fan, a narrow shelf (up to 10 km and 30 km wide in the shore-normal and shore-parallel directions, respectively) and a modern marine depocenter (Fig. 1). In the Senoumi Basin (Ishii and Nemoto, 1995) and in the other Japanese forearc basins (e.g. Hyuga and Tosa basins; Okamura and Blum, 1993), maximum thickness of the progradational wedges occurs at the center of subsidence where the sediments mainly accumulate, suggesting that the lateral shift of depositional systems is confined to a very narrow area. Thus the problem of three-dimensional variability of depositional systems (Martinsen and Helland-Hansen, 1994) may be less important in these basins. We believe that the Kakegawa Basin also had a subsidence pattern similar to the Senoumi Basin. 2.1. Sequence stratigraphic architecture Individual systems tracts and bounding surfaces of the Kakegawa sequence have been identified by means of the onlap and downlap geometries shown by interbedded tuff beds (Masuda and Ishibashi, 1991; Masuda, 1994). The lower sequence boundary is a prominent surface between pre-Pliocene rocks and the Kakegawa Group. The lowstand sys-
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tems tract (LST) fills a small depression (ca. 20 km wide and 2 km deep; Fig. 2) and consists of slope, submarine-fan and slope-apron deposits. The transgressive systems tract (TST; ca. 150 m thick on the shelf and 500 m thick on the slope) is characterized by coastal onlap patterns of tuff beds onto the lower sequence boundary. The TST consists of retrogradational alluvial, shoreface and shelf facies and progradational slope facies. The highstand systems tract (HST; ca. 150 m thick on the shelf and 400 m thick on the slope) is characterized by downlap patterns of tuff beds onto the expected maximum flooding surface that might be contained in a condensed zone of massive siltstone (ca. 5–7 m thick on the shelf and more than 10 m thick on the slope). The HST consists mainly of a progradational shelf and slope facies. The upper sequence boundary is an erosion surface which is covered by Pleistocene coastal alluvial fan and submarine channel fill deposits (Ogasa Group). In the northwestern part of the basin, eight formations of the Kakegawa sequence represent alluvial to slope facies (Fig. 2, Table 1), which comprise part of the TST and the HST. The Nobe Formation and the lower part of the Soga Formation consist of trough and tabular cross-stratified and horizontally stratified conglomerates and sandstones of braided channel fill, and sandstones and siltstones of interchannel and flood-plain origin. A small valley (Fig. 3) incised into the basement rock was probably cut by streams and is filled with bioturbated sandy siltstones and laminated sandstones (Dainichi Formation). The Dainichi and the Aburayama formations and the upper part of the Soga Formation consist mainly of hummocky cross-stratified (HCS) sandstones of lower-shoreface to inner-shelf origin, and trough and tabular cross-stratified conglomerates and sandstones of upper-shoreface origin. These deposits grade laterally into bioturbated sandy siltstones of outer-shelf origin (Ukari Formation) toward the southeast. The Hijikata Formation consists of bioturbated siltstones and interbedded sandstones of upper-slope origin. Slumped beds and small channel fill conglomerate, sandstone and shell beds (up to 10 m thick and wide) are common in the siltstones. Poorly sorted conglomerates and sandstones of submarine channel fill, and sandstones and siltstones (turbidites) of attached levee deposits (Soga Formation) are recog-
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Fig. 2. Schematic cross-section of the Kakegawa Group (after Sakai and Masuda, 1996). AR D Arigaya Tuff; SH D Shiraiwa Tuff; IO D Iozumi Tuff; NI D Nishihirao Tuff; HO D Hosoya Tuff; MO D Moridaira Tuff; OK D O’kubo Tuff; KO D Ko’gosyo Tuff; SO D Soga Tuff; HA D Haruoka Tuff; SB D sequence boundary; TS D transgressive surface; MFS D maximum flooding surface; LST D lowstand systems tract; TST D transgressive systems tract; HST D highstand systems tract.
nized near the top of the group. The channel fill deposits are probably a lateral equivalent of the alluvial-fan deposits of the Soga Formation, judging from basinward tracing of the tuff beds interbedded just below and above the alluvial-fan and submarine channel fill deposits (Fig. 2). Several tuff beds (white or pink tuff and pumice beds) are widely traceable in the study area (O’kubo, Hosoya, Soga and Haruoka tuffs, Fig. 3). The Hosoya and the Soga tuffs are dated about 1.9 and 1.6 Ma, respectively, by fission-track dating and biostratigraphic analysis (e.g. Nishimura, 1977; Ibaraki, 1986).
The northwestern part of the Kakegawa Group contains a number of upward-shallowing successions, referred to here as parasequences in the sense of Van Wagoner et al. (1988). Parasequences (5– 40 m) from alluvial to shelf environments consist of upward-shallowing facies successions of tempestite and alluvial conglomerate beds, bounded at their bases and tops by ravinement surfaces. At least ten parasequences (PS1–PS10) have been recognized in the TST and six parasequences (PS11–PS16) in the HST (Fig. 3). The TST parasequences show a retrogradational stacking pattern and those of the HST show a progradational stacking pattern. The internal
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Table 1 Brief facies description of each formation
Water depths are based on paleontologic data (e.g. Chinzei, 1980; Ishibashi, 1985; Nobuhara, 1993). FA D facies association; IV D incised valley fill.
architecture of parasequences and their boundaries are described in an other paper (Sakai and Masuda, 1996). The parasequences are interpreted to result from fifth- or sixth-order (Fulthorpe, 1991) eustatic sea-
level changes, because the frequency represented by global oxygen isotope curves and the number of parasequences between dated tuff beds (1.9 to 1.6 Ma) appear to correspond (Fig. 3) (Sakai and Masuda, 1996).
100 T. Sakai, F. Masuda / Sedimentary Geology 122 (1998) 95–107 Fig. 3. (a) Columnar cross-sections of the northwestern part of the Kakegawa Group. Solid lines are parasequence boundaries correlated based on tuff beds. (b) Oxygen isotope curves (Williams, 1990) from deep-sea foraminifera tests. Dots represent sea-level falls that are interpreted to have contributed to parasequence deposition.
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3. Methods Tectonic and eustatic effects on sequence development of the Kakegawa Group were estimated using the method of geohistory analysis (Van Hinte, 1978), which allows us to understand basin development by producing subsidence and accumulation curves through time. Here we followed the method compiled by Allen and Allen (1990), to provide tectonic and total subsidence, and accommodation curves during deposition of the northwestern part of the Kakegawa Group. The analysis requires the following information: (1) depth–porosity curve; (2) water depth; (3) age; and (4) eustasy. These quantities were estimated for the base and the top of each parasequence for a geohistory plot. Analysis was applied to successions from sections 2 and 5 of Fig. 3, where almost complete successions have been obtained, in order to estimate the effects of differential subsidence on sequence architecture as well as to identify the factors that contributed to sequence development. The rate of sediment supply and the sedimentation rate on the shelf were estimated from the cross-sections of the Kakegawa Group, as discussed later. 3.1. Sections 2 and 5 Sections 2 and 5 contain twelve parasequences (PS5–PS16; Fig. 3) and fourteen parasequences (PS3–PS16), respectively. PS3–PS10 are contained in the TST. PS3 and PS4 can be recognized near section 5, where they consist mainly of amalgamated HCS sandstones. Trough and tabular cross-stratified conglomerate and shell beds of upper-shoreface deposits overlie HCS sandstones in PS3. PS5 consists of poorly sorted conglomerates and sandstones of alluvial origin near section 2, and poorly sorted sandstones of transgressive lag deposits, sandy siltstones of outer-shelf deposits and HCS sandstones of inner-shelf deposits near section 5. PS6–PS9 consist mainly of upward-coarsening HCS successions near section 2 and upward-thickening alternations of sandstones and sandy siltstones near section 5, with intervening HCS sandstones at the tops of PS6 and PS7. Outer-shelf siltstones (1–2 m) overlie lower parasequence boundaries of PS7 and PS9 near section 2. Alluvial conglomerate beds truncate HCS
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sandstones at the top of PS7 in section 2. PS10 is characterized by an upward-thickening alternation of sandstones and siltstones (sandstone < siltstone) at both locations. In the TST parasequences, PS3– PS6 are thicker than other parasequences (Fig. 3). PS3–PS10 indicate an overall upward-fining pattern. However, alluvial conglomerate beds of PS7 prograded to a more basinward position than the underlying alluvial deposits. This can be interpreted as a short period of relative sea-level fall in a third-order transgressive phase. PS11–PS16 show an overall upward-coarsening pattern and together comprise the HST. PS11 consists, from base to top, of a massive siltstone and an alternation of sandstones and siltstones (sandstone < siltstone), followed by an alternation of HCS sandstones and siltstones near section 2. Basal massive siltstone is interpreted as the condensed zone of a third-order sequence, and perhaps marks the maximum flooding surface (Sakai and Masuda, 1996). PS12–PS14 consist, from base to top, of sandy siltstones, alternations of HCS sandstones and sandy siltstones, and amalgamated HCS sandstones in both sections. The siltstones of section 5 parasequences are thicker than those of section 2. PS15 consists of basal conglomerates derived by shoreface erosion and an alternation of HCS sandstones and bioturbated sandy siltstones near section 2, and alluvial conglomerates near section 5. Laterally equivalent shallow marine sediments recognized near section 2 may be truncated by the conglomerates near section 5. The conglomerates also truncate underlying HCS sandstones of PS14 near section 5 and taper laterally toward the northwest into HCS sandstones between sections 2 and 3. PS16 consists of amalgamated HCS sandstones and an alternation of HCS pumiceous sandstones and tuff beds, and is bounded at its top by overlying alluvial conglomerates of the Ogasa Group. Parasequences in the HST tend to thicken basinward. 3.2. Depth–porosity curve The porosity of sandstones and siltstones was measured. 43 samples were collected from the southeastern part of the group as well as the northwestern part. The Pleistocene Ogasa Group of 20–700 m thickness overlies the Kakegawa Group; reduction in
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Fig. 4. Result in measurement of porosity of sandstones and siltstones from the Kakegawa Group. The curves are fit by using the method of least squares.
porosity by accumulation of the Ogasa Group must be taken into account in the depth–porosity curve. Therefore the burial depth of the sampling points is measured from the top of the Ogasa Group. The resultant porosity–depth curves of sandstone and siltstone (Fig. 4) are as follows: s D 0:411 exp. 0:000089H / m D 0:423 exp. 0:00019H / where s and m are porosity of the sandstone and siltstone, respectively, and H is the present thickness of sediment from the top of the overlying Ogasa Group. 3.3. Water depth Changes in paleowater depth were evaluated from the cross-sections of Fig. 3. The average gradients of the shelf were evaluated for the base and the top of each parasequence. Data were based upon the positions of depth indicators on the lower parasequence boundaries and just below the upper parasequence boundaries. Depth indicators consist of shoreline position (0 m), inner- and outer-shelf transition (ca. 50 m), and shelf edge (ca. 120 m) (Fig. 3). Average shelf gradients were evaluated for parasequences in which at least two positions of depth indicators can be recognized in the cross-sections, assuming that the shelf had linear morphology in two dimensions. Water depth was then calculated from the average shelf gradient by measuring the
distance between the positions of the depth indicators and adding the estimated water depth. The derived gradients range from 0.004 to 0.009, which are equivalent to those of shelves in depositional forearc and backarc basins around Japan, measured from bathymetric maps (e.g. Ikehara, 1988; Ikehara et al., 1990, 1994). Shelf gradients for PS10, and the base of PS11 and PS12 could not be estimated because the major part of these parasequences consists of outer-shelf deposits; shoreline and inner- and outer-shelf transition points do not occur in outcrops of these parasequences. The shelf gradient at the base of cycles PS3 and PS4 also could not be evaluated, because shoreline and shelf-edge positions are hard to be estimated at the base of these cycles due to covers of Holocene sediments. Water depths of these parasequences are assumed to be evaluated using average shelf gradient of the TST parasequences (0.00745) and that of the HST parasequences (0.00475) determined by averaging shelf gradients of other parasequences in individual systems tracts. In the TST, there are local slumped deposits in outer-shelf deposits as well as in slope deposits (Fig. 3). The facies suggest that the TST shelf might be steeper than that of the HST, and that the averaged values may be appropriate for the use of the calculation. In cycles PS8, PS12 and PS13, sandstone prograded out to the vicinity of the shelf edge. The estimated shelf gradients at the top of these parasequences are steep, and are therefore inappropriate for use in the calculation of water depth. Average gradients of the TST and the HST were also used for the calculation of water depth for these parasequences. Water depth error was estimated based on the shallowest and the deepest points evidenced by depth indicators which do not always coincide with the position of outcrops (especially shelf edge) but are sometimes between adjacent measured sections (i.e. error of their positions equates to the interval between stratigraphic sections, e.g. up to 2 km and the corresponding variation in depth between sections). The depth range of the inner- and outer-shelf transition in the Senoumi Basin has not been published and was, therefore, evaluated from other storm-dominated shelves. Bourgeois (1980) evaluated the maximum depth of HCS beds by distinguishing innershelf deposits of the Kakegawa Group from over-
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lying ripple laminations, which were interpreted to have formed in water depths shallower than 50 m. Other results indicate this depth on modern stormdominated shelves to be 30 to 80 m (e.g. Cacchione et al., 1984; Saito, 1989). From these evaluations, the depth range of the inner- and outer-shelf transition might be between 30 and 80 m. Depth range in shelf edge (80–140 m) is evaluated from the Senoumi Basin (Ishii and Nemoto, 1995). 3.4. Age Ages of the base and the top of each parasequence were determined from dated tuff beds (Hosoya and Soga tuffs), and from correlation with the deep-sea oxygen isotope curve (Williams, 1990; Fig. 2). The age of the top and the base of PS8, including the Hosoya Tuff, were evaluated from the eustatic curve. The sea-level fall that probably controlled PS8 formation was correlated with sea-level fall which occurred between 1.87 and 1.83 Ma (Fig. 3). Ages of the start and the end of PS8 deposition were equated
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with that of the sea-level maximum and minimum of the oxygen isotope curve (i.e. 1.87 and 1.83 Ma, respectively). The ages of other parasequences were correlated with eustatic sea-level falls of the curve based on a time interval of PS8 deposition (Fig. 3). 3.5. Eustasy Eustatic sea-level changes were estimated from the oxygen isotope curve of Williams (1990). The amplitudes of eustatic sea-level changes were calculated, assuming that the variation in isotope ratio during the past 20,000 years represents 100 m of sea-level rise (e.g. Saito, 1994), i.e. a 0.09‰ change in isotope ratio equates to a 10 m change in sea level. Evaluated eustatic sea-level changes are shown in Fig. 5a. 3.6. Sediment supply and sedimentation rate on the shelf Variation in the sedimentation rate on the shelf was evaluated by measuring the area of each parase-
Fig. 5. (a) Eustatic sea-level changes evaluated from oxygen isotope curve. (b, c) Accommodation, total subsidence, and tectonic subsidence curves from sections 2 and 5, respectively. Water depth errors are shown by bars in the total subsidence curves and by the shaded area in the accommodation and tectonic subsidence curves. (d) Change in sediment supply and sedimentation rate on the shelf evaluated from the cross-sections of the entire Kakegawa Group and Fig. 3. MFS D maximum flooding surface; TST D transgressive systems tract; HST D highstand systems tract.
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quence on the cross-sections (Fig. 3). The rate of sediment supply was also evaluated from the crosssection of the entire Kakegawa Group (Sakai and Masuda, 1996; Fig. 5) to assess basinward sediment transport. Loss of the sediments by shore-parallel transport beyond the basin was probably less important, because the shelf of the Kakegawa Basin may have been similar to that of the Senoumi Basin where submarine channels provide transport paths for sediment to bypass (Uda et al., 1988) and both the southern and the northern ends of the shelf are bounded by capes preventing sediment transport beyond the basin.
4. Result Results of the analysis are shown in Fig. 5. The errors of these curves in terms of water depth are up to 70 m (Fig. 5). The total subsidence curve (isostatic subsidence C tectonic subsidence) for each section corresponds approximately with broad-scale accommodation curve. The plots of tectonic subsidence show two episodes of subsidence. The first episode is represented by rapid subsidence between 2.2 and 2.0 Ma. The rate of tectonic subsidence decreased about 2.0 Ma. Slow subsidence continued until the end of deposition of the Kakegawa Group. A short period of uplift probably took place about 1.6 Ma. The plots also show a slightly faster rate of subsidence around section 5 during 2.0–1.4 Ma. The estimated rate of sediment supply ranges from 0:8 ð 104 to 2:8 ð 104 m2 =ka (Fig. 5d). Because the rate was evaluated from the cross-sections, the unit is shown as m2 =ka. The inferred rates are therefore equivalent to those of sediment discharge from major modern rivers in Japan (on the order of 105 to 106 m3 =year; Saito and Ikehara, 1992), presuming that the lateral extent of the Kakegawa Basin was similar to the Senoumi Basin (ca. 30 km), and that the lateral change in the sedimentation rate was small in the basin. Sediment supply increased between 2.2 and 2.0 Ma, temporarily decreased around 2.0 Ma. Its rate increased again until 1.5 Ma and then finally decreased between 1.65 and 1.6 Ma. Sediment supply between 1.6 and 1.4 Ma cannot be evaluated, because parasequence boundaries are
unclear in slope deposits. The sedimentation rate on the shelf was not in phase with that of the overall sediment supply. The rate was high between 2.2 and 2.0 Ma, and decreased between 2.0 and 1.75 Ma and then increased between 1.75 and 1.4 Ma.
5. Discussion The pattern of low-frequency (third-order) Late Pliocene–Early Pleistocene eustatic sea-level change recorded in oxygen isotope curves indicates sea-level lowstand around 2.4 Ma followed by sea-level rise– highstand between 2.4 and 0.8 Ma (e.g. Williams, 1990; Feeley et al., 1990). The transgressive– regressive cycle of the Kakegawa Group is in phase with this third-order eustatic cycle. Similar third-order transgressive–regressive cycles have also been identified from other forearc and backarc basins in Japan (e.g. Kazusa and Uonuma basins, Ito, 1995; Urabe et al., 1995), and passive margin basins (e.g. Louisiana offshore, Lowrie and McDaniel-Lowrie, 1985). Therefore, we interpret the third-order Kakegawa sequence as controlled by a third-order eustasy (Masuda and Ishibashi, 1991; Sakai and Masuda, 1992, 1996). The broad transgressive and regressive cycle of the Kakegawa Group is interpreted as controlled mainly by both tectonic subsidence and sedimentation rate for the following reasons: (1) the general correspondence of the accommodation curves and the total subsidence curves (Fig. 5) over this time interval suggests that accommodation was generated mainly by total subsidence, that is, the sum of tectonic and isostatic subsidence (function of sediment accumulation); (2) transgression during 2.2–2.0 Ma cannot be explained by third-order eustasy which falls in this period; and (3) regression starts when the sedimentation rate outstrips on the rate of formation of accommodation (e.g. Jervey, 1988). The rate of accommodation formation was almost constant around 1.75 Ma in broad scale (Fig. 5); increase in the sedimentation rate at 1.75 Ma was important to change the accommodation-dominant phase into the sediment-supply-dominant phase. Geohistory analysis and our evaluation of sedimentation rate on the shelf delineate three separate phases of subsidence and sedimentation (Table 2).
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Table 2 Summary table showing temporal changes of factors influencing sequence development
PS3–PS6 PS7–PS10 PS11–PS16
Systems tract
Tectonic subsidence
Sediment supply
Sedimentation rate
TST TST HST
High Slow Slow
Small Moderate High
High Small Moderate
The PS3–PS6 interval, deposited during the first phase (2.2–2.0 Ma), is characterized by a thick retrogradational parasequence set. Tectonic subsidence was rapid and the sedimentation rate was high in this phase (Fig. 5). Tectonic subsidence was the dominant factor, leading to a retrogradational parasequence set. PS7–PS10, which accumulated during the second phase (2.0–1.75 Ma), are thin but also compose a retrogradational parasequence set. The rate of tectonic subsidence decreased in this phase. Sediment supply was moderate, but the sedimentation rate on the shelf was low (Fig. 5). This suggests that much sediment may have bypassed the shelf through the small channels developed on the upper slope and perhaps also by slumping (described in Sakai and Masuda, 1996). Therefore, the PS7–PS10 retrogradational parasequence set is interpreted as formed during slow tectonic subsidence with a low rate of sedimentation induced by a high rate of basinward sediment bypassing. Cycles PS11–PS16, deposited during the third phase (1.75–1.4 Ma), are thin and constitute a progradational parasequence set. Slow tectonic subsidence continued, and sediment supply was high (Fig. 5). The rate of sediment supply was much larger than the shelf sedimentation rate in this phase. This indicates that most of the sediment bypassed the shelf and was deposited on the slope, which consequently prograded rapidly. The slightly increased sedimentation rate (Fig. 5) was enough to initiate a progradational parasequence set in this phase. A short period of uplift possibly occurred around 1.6 Ma, which may have controlled the stacking pattern of parasequences in which positions of shelf edge prograde further basinward (Fig. 3). Subsidence increased towards the basin center, resulting in deposition of thicker parasequences in the basin and preservation of alluvial deposits of PS15 near section 5. The maximum flooding surface of the third-order sequence was developed when the rate of accommo-
dation became less important than the sedimentation rate around 1.75 Ma.
6. Conclusions Geohistory analysis applied to the Plio–Pleistocene Kakegawa Group, together with an evaluation of the sedimentation rate, suggest that an observed thirdorder sequence was controlled mainly by changes in tectonic subsidence and the sedimentation rate on the shelf. The lower part of the TST (PS3–PS6), composed of thick retrogradational parasequences, was formed when accommodation created by a high rate of tectonic subsidence exceeded a high rate of sedimentation. The upper TST (PS7–PS10) consists of thin parasequences and forms a retrogradational set. This parasequence set probably formed under a combination of more slowly increasing accommodation driven by tectonic subsidence and a low rate of sedimentation due to an increase in sediment bypassing into the basin. The HST (PS11–PS16) consists of a progradational parasequence set which accumulated when sediment supply increased during continued slow tectonic subsidence. The maximum flooding surface dividing these systems tracts apparently formed at the time of transition between a system which was dominated by tectonic subsidence to a system dominated by sediment supply.
Acknowledgements This paper summarizes a part of the Ph.D. thesis undertaken by T.S. at Osaka University, Japan. T.S. is most grateful to Prof. T. Sunamura of Osaka University for his guidance and variable suggestions. We thank Dr. T. Tsuji, Mr. M. Ueki and Ms. K. Hatano of Japan National Oil Corporation (JNOC) for their help in the measurement of porosity, and Drs. Y.
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Saito of Geological survey, Japan, M. Yokokawa and N. Endo of Osaka University, and Mr. A. Okui of JNOC, for their suggestions and discussions about the method of basin analysis and the concept of sequence stratigraphy. Thanks are also due to Drs. H.E. Clifton of Conoco Inc., USA, and M. Ito of Chiba University, Japan, for their help in improving an early version of the manuscript, and Drs. R.M. Carter and T.R. Naish of James Cook University, Australia, editors of this special issue, and referees of Sedimentary Geology for their critical comments. This work was partially supported by the award of a fellowship by the Japan Society for the Promotion of Science for Japanese Junior Scientists.
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