the relationship between glacio-eustatic parasequences and a third-order sequence in the kakegawa...

ELSEVIER Sedimentary Geology 122 (1998) 95–107

The relationship between glacio-eustatic parasequences and athird-order sequence in the Kakegawa Group, central Japan

Tetsuya Sakai a,Ł, Fujio Masuda b

a Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japanb 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 (2.6–1.0Ma). The northwestern part of the Kakegawa sequence consists of up to 500 m of alluvial, shoreface, shelf, slopeand submarine-channel facies. It contains at least sixteen upward-shallowing cycles (parasequences), the deposition ofwhich was affected by high-frequency eustatic sea-level cycles. The lower part of the sequence is characterized by aretrogradational parasequence set, which formed a transgressive systems tract (2.2–1.75 Ma) followed by a progradationalparasequence set comprising a highstand systems tract (1.75–1.4 Ma). Subsidence analysis and evaluation of changes in theshelf sedimentation rate estimated from cross-sections, suggest that formation of the third-order sequence was controlledby tectonic subsidence and variation in the sedimentation rate. Rapid subsidence and a high rate of sedimentation during2.2–2.0 Ma resulted in deposition of the lower part of the transgressive systems tract, characterized by thick backsteppingsuccessions. The rate of subsidence decreased in the period 2.0–1.75 Ma. The sedimentation rate also decreased dueto a high rate of sediment bypassing. However, subsidence was still the dominant factor, leading to the formation ofthin backstepping successions. The 1.75–1.4 Ma progradational succession resulted from a combination of a low rate ofsubsidence and moderate sedimentation. The progradational units become thicker basinward owing to faster subsidence inthe basin center. The maximum flooding surface was formed around 1.75 Ma even though subsidence was slow at thistime. 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), se-quence stratigraphic concepts have evolved rapidlyand numerous theoretical and case studies have been

Ł Corresponding author. Present address: Department of Geologyand 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 suggestthat sequence formation is controlled by changes inaccommodation and sedimentation rate (e.g. Thorneand Swift, 1991; Schlager, 1993). Recently, manystratigraphers have focused their interest on the causesof formation of depositional sequences and theirbounding surfaces (e.g. Thorne and Swift, 1991; De-vlin et al., 1993; Meckel and Galloway, 1996).

0037-0738/98/$ – see front matter c 1998 Elsevier Science B.V. All rights reserved.PII: S 0 0 3 7 - 0 7 3 8 ( 9 8 ) 0 0 1 0 0 - 6

96 T. Sakai, F. Masuda / Sedimentary Geology 122 (1998) 95–107

Fig. 1. Location map of the Kakegawa Group. Enclosed part is study area. NP D North American Plate.

The Plio–Pleistocene Kakegawa Group in centralJapan (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 north-western side of the basin is an ideal succession toassess 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 availabil-ity of well constrained age data, facies modeling and

sequence stratigraphy. The objective of this paper isto identify the factors which influenced the formationof a third-order sequence in a forearc basin, usinggeohistory analysis.

2. Plio–Pleistocene forearc basin stratigraphy

The Kakegawa Group consists of nonmarine andshallow to deep marine clastic sediments and is upto 3500 m thick. The group represents the fill of

T. Sakai, F. Masuda / Sedimentary Geology 122 (1998) 95–107 97

a Plio–Pleistocene forearc basin (Kakegawa Basin,Sugiyama et al., 1988) that probably formed as aresult of oblique subduction of the Philippine SeaPlate beneath the eastern margin of the Eurasia Plate(Sugiyama, 1989).

The Kakegawa Group unconformably overliespre-Pliocene rocks which comprise Miocene fore-arc basin fill and Cretaceous–Paleogene deep-seasediments (Makiyama, 1947; Kano and Matsushima,1988). The group is overlain by coastal alluvial fanand submarine channel fill deposits of the Pleis-tocene Ogasa Group (Muto, 1985).

The Kakegawa Group itself comprises a depo-sitional sequence consisting of alluvial fan, incisedvalley 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 thatdominant sediment transport was toward the south tosoutheast (Sakai and Masuda, 1996). The sedimentfacies distribution suggests that the Kakegawa Basinwas similar to the ‘Senoumi Basin’, a recent forearcbasin that has a coastal alluvial fan, a narrow shelf(up to 10 km and 30 km wide in the shore-nor-mal and shore-parallel directions, respectively) and amodern marine depocenter (Fig. 1).

In the Senoumi Basin (Ishii and Nemoto, 1995)and in the other Japanese forearc basins (e.g. Hyugaand Tosa basins; Okamura and Blum, 1993), maxi-mum thickness of the progradational wedges occurs atthe center of subsidence where the sediments mainlyaccumulate, suggesting that the lateral shift of deposi-tional systems is confined to a very narrow area. Thusthe problem of three-dimensional variability of de-positional systems (Martinsen and Helland-Hansen,1994) may be less important in these basins. We be-lieve that the Kakegawa Basin also had a subsidencepattern similar to the Senoumi Basin.

2.1. Sequence stratigraphic architecture

Individual systems tracts and bounding surfacesof the Kakegawa sequence have been identified bymeans of the onlap and downlap geometries shownby interbedded tuff beds (Masuda and Ishibashi,1991; Masuda, 1994). The lower sequence bound-ary is a prominent surface between pre-Pliocenerocks and the Kakegawa Group. The lowstand sys-

tems tract (LST) fills a small depression (ca. 20 kmwide and 2 km deep; Fig. 2) and consists of slope,submarine-fan and slope-apron deposits. The trans-gressive systems tract (TST; ca. 150 m thick on theshelf and 500 m thick on the slope) is character-ized by coastal onlap patterns of tuff beds onto thelower sequence boundary. The TST consists of ret-rogradational alluvial, shoreface and shelf facies andprogradational slope facies. The highstand systemstract (HST; ca. 150 m thick on the shelf and 400m thick on the slope) is characterized by downlappatterns of tuff beds onto the expected maximumflooding surface that might be contained in a con-densed zone of massive siltstone (ca. 5–7 m thick onthe shelf and more than 10 m thick on the slope).The HST consists mainly of a progradational shelfand slope facies. The upper sequence boundary isan erosion surface which is covered by Pleistocenecoastal alluvial fan and submarine channel fill de-posits (Ogasa Group).

In the northwestern part of the basin, eight forma-tions of the Kakegawa sequence represent alluvial toslope facies (Fig. 2, Table 1), which comprise part ofthe TST and the HST. The Nobe Formation and thelower part of the Soga Formation consist of troughand tabular cross-stratified and horizontally stratifiedconglomerates and sandstones of braided channelfill, and sandstones and siltstones of interchanneland flood-plain origin. A small valley (Fig. 3) in-cised into the basement rock was probably cut bystreams and is filled with bioturbated sandy siltstonesand laminated sandstones (Dainichi Formation). TheDainichi and the Aburayama formations and theupper part of the Soga Formation consist mainlyof hummocky cross-stratified (HCS) sandstones oflower-shoreface to inner-shelf origin, and trough andtabular cross-stratified conglomerates and sandstonesof upper-shoreface origin. These deposits grade lat-erally into bioturbated sandy siltstones of outer-shelforigin (Ukari Formation) toward the southeast. TheHijikata Formation consists of bioturbated siltstonesand interbedded sandstones of upper-slope origin.Slumped beds and small channel fill conglomerate,sandstone and shell beds (up to 10 m thick andwide) are common in the siltstones. Poorly sortedconglomerates and sandstones of submarine chan-nel fill, and sandstones and siltstones (turbidites) ofattached levee deposits (Soga Formation) are recog-


Fig. 2. Schematic cross-section of the Kakegawa Group (after Sakai and Masuda, 1996). AR D Arigaya Tuff; SH D Shiraiwa Tuff; IO DIozumi 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 SogaTuff; HA D Haruoka Tuff; SB D sequence boundary; TS D transgressive surface; MFS D maximum flooding surface; LST D lowstandsystems tract; TST D transgressive systems tract; HST D highstand systems tract.

nized near the top of the group. The channel filldeposits are probably a lateral equivalent of the al-luvial-fan deposits of the Soga Formation, judgingfrom basinward tracing of the tuff beds interbeddedjust below and above the alluvial-fan and submarinechannel fill deposits (Fig. 2).

Several tuff beds (white or pink tuff and pumicebeds) are widely traceable in the study area (O’kubo,Hosoya, Soga and Haruoka tuffs, Fig. 3). TheHosoya and the Soga tuffs are dated about 1.9and 1.6 Ma, respectively, by fission-track datingand biostratigraphic analysis (e.g. Nishimura, 1977;Ibaraki, 1986).

The northwestern part of the Kakegawa Groupcontains a number of upward-shallowing succes-sions, referred to here as parasequences in the senseof Van Wagoner et al. (1988). Parasequences (5–40 m) from alluvial to shelf environments consist ofupward-shallowing facies successions of tempestiteand alluvial conglomerate beds, bounded at theirbases and tops by ravinement surfaces. At least tenparasequences (PS1–PS10) have been recognized inthe TST and six parasequences (PS11–PS16) in theHST (Fig. 3). The TST parasequences show a ret-rogradational stacking pattern and those of the HSTshow a progradational stacking pattern. The internal


Table 1Brief 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 Dincised valley fill.

architecture of parasequences and their boundariesare described in an other paper (Sakai and Masuda,1996).

The parasequences are interpreted to result fromfifth- or sixth-order (Fulthorpe, 1991) eustatic sea-

level changes, because the frequency represented byglobal oxygen isotope curves and the number ofparasequences between dated tuff beds (1.9 to 1.6Ma) appear to correspond (Fig. 3) (Sakai and Ma-suda, 1996).

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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) Oxygenisotope curves (Williams, 1990) from deep-sea foraminifera tests. Dots represent sea-level falls that are interpreted to have contributed to parasequence deposition.


3. Methods

Tectonic and eustatic effects on sequence de-velopment of the Kakegawa Group were estimatedusing the method of geohistory analysis (Van Hinte,1978), which allows us to understand basin devel-opment by producing subsidence and accumulationcurves through time. Here we followed the methodcompiled by Allen and Allen (1990), to providetectonic and total subsidence, and accommodationcurves during deposition of the northwestern partof the Kakegawa Group. The analysis requires thefollowing information: (1) depth–porosity curve; (2)water depth; (3) age; and (4) eustasy. These quan-tities were estimated for the base and the top ofeach parasequence for a geohistory plot. Analysiswas applied to successions from sections 2 and 5 ofFig. 3, where almost complete successions have beenobtained, in order to estimate the effects of differen-tial subsidence on sequence architecture as well asto identify the factors that contributed to sequencedevelopment. The rate of sediment supply and thesedimentation rate on the shelf were estimated fromthe cross-sections of the Kakegawa Group, as dis-cussed 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 containedin the TST. PS3 and PS4 can be recognized nearsection 5, where they consist mainly of amalgamatedHCS sandstones. Trough and tabular cross-stratifiedconglomerate and shell beds of upper-shoreface de-posits overlie HCS sandstones in PS3. PS5 consistsof poorly sorted conglomerates and sandstones ofalluvial origin near section 2, and poorly sortedsandstones of transgressive lag deposits, sandy silt-stones of outer-shelf deposits and HCS sandstonesof inner-shelf deposits near section 5. PS6–PS9 con-sist mainly of upward-coarsening HCS successionsnear section 2 and upward-thickening alternations ofsandstones and sandy siltstones near section 5, withintervening HCS sandstones at the tops of PS6 andPS7. Outer-shelf siltstones (1–2 m) overlie lowerparasequence boundaries of PS7 and PS9 near sec-tion 2. Alluvial conglomerate beds truncate HCS

sandstones at the top of PS7 in section 2. PS10is characterized by an upward-thickening alternationof 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 pro-graded to a more basinward position than the under-lying alluvial deposits. This can be interpreted as ashort period of relative sea-level fall in a third-ordertransgressive phase.

PS11–PS16 show an overall upward-coarseningpattern and together comprise the HST. PS11 con-sists, from base to top, of a massive siltstone and analternation of sandstones and siltstones (sandstone <siltstone), followed by an alternation of HCS sand-stones and siltstones near section 2. Basal massivesiltstone is interpreted as the condensed zone of athird-order sequence, and perhaps marks the max-imum flooding surface (Sakai and Masuda, 1996).PS12–PS14 consist, from base to top, of sandy silt-stones, alternations of HCS sandstones and sandysiltstones, and amalgamated HCS sandstones in bothsections. The siltstones of section 5 parasequencesare thicker than those of section 2. PS15 consistsof basal conglomerates derived by shoreface erosionand an alternation of HCS sandstones and biotur-bated sandy siltstones near section 2, and alluvialconglomerates near section 5. Laterally equivalentshallow marine sediments recognized near section2 may be truncated by the conglomerates near sec-tion 5. The conglomerates also truncate underlyingHCS sandstones of PS14 near section 5 and taperlaterally toward the northwest into HCS sandstonesbetween sections 2 and 3. PS16 consists of amalga-mated HCS sandstones and an alternation of HCSpumiceous sandstones and tuff beds, and is boundedat its top by overlying alluvial conglomerates of theOgasa Group. Parasequences in the HST tend tothicken basinward.

3.2. Depth–porosity curve

The porosity of sandstones and siltstones wasmeasured. 43 samples were collected from the south-eastern part of the group as well as the northwesternpart. The Pleistocene Ogasa Group of 20–700 mthickness overlies the Kakegawa Group; reduction in


Fig. 4. Result in measurement of porosity of sandstones andsiltstones from the Kakegawa Group. The curves are fit by usingthe method of least squares.

porosity by accumulation of the Ogasa Group mustbe taken into account in the depth–porosity curve.Therefore the burial depth of the sampling pointsis measured from the top of the Ogasa Group. Theresultant porosity–depth curves of sandstone andsiltstone (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 andsiltstone, respectively, and H is the present thicknessof sediment from the top of the overlying OgasaGroup.

3.3. Water depth

Changes in paleowater depth were evaluated fromthe cross-sections of Fig. 3. The average gradients ofthe shelf were evaluated for the base and the top ofeach parasequence. Data were based upon the posi-tions of depth indicators on the lower parasequenceboundaries and just below the upper parasequenceboundaries. Depth indicators consist of shoreline po-sition (0 m), inner- and outer-shelf transition (ca. 50m), and shelf edge (ca. 120 m) (Fig. 3).

Average shelf gradients were evaluated for parase-quences in which at least two positions of depthindicators can be recognized in the cross-sections,assuming that the shelf had linear morphology intwo dimensions. Water depth was then calculatedfrom the average shelf gradient by measuring the

distance between the positions of the depth indi-cators and adding the estimated water depth. Thederived gradients range from 0.004 to 0.009, whichare equivalent to those of shelves in depositionalforearc and backarc basins around Japan, measuredfrom bathymetric maps (e.g. Ikehara, 1988; Ikeharaet al., 1990, 1994).

Shelf gradients for PS10, and the base of PS11and PS12 could not be estimated because the majorpart of these parasequences consists of outer-shelfdeposits; shoreline and inner- and outer-shelf transi-tion points do not occur in outcrops of these parase-quences. The shelf gradient at the base of cycles PS3and PS4 also could not be evaluated, because shore-line and shelf-edge positions are hard to be estimatedat the base of these cycles due to covers of Holocenesediments. Water depths of these parasequences areassumed to be evaluated using average shelf gra-dient of the TST parasequences (0.00745) and thatof the HST parasequences (0.00475) determined byaveraging shelf gradients of other parasequences inindividual systems tracts. In the TST, there are localslumped deposits in outer-shelf deposits as well asin slope deposits (Fig. 3). The facies suggest that theTST shelf might be steeper than that of the HST, andthat the averaged values may be appropriate for theuse of the calculation.

In cycles PS8, PS12 and PS13, sandstone pro-graded out to the vicinity of the shelf edge. Theestimated shelf gradients at the top of these parase-quences are steep, and are therefore inappropriate foruse in the calculation of water depth. Average gradi-ents of the TST and the HST were also used for thecalculation of water depth for these parasequences.

Water depth error was estimated based on theshallowest and the deepest points evidenced by depthindicators which do not always coincide with the po-sition of outcrops (especially shelf edge) but aresometimes between adjacent measured sections (i.e.error of their positions equates to the interval be-tween stratigraphic sections, e.g. up to 2 km and thecorresponding variation in depth between sections).The depth range of the inner- and outer-shelf tran-sition in the Senoumi Basin has not been publishedand was, therefore, evaluated from other storm-dom-inated shelves. Bourgeois (1980) evaluated the max-imum depth of HCS beds by distinguishing inner-shelf deposits of the Kakegawa Group from over-


lying ripple laminations, which were interpreted tohave formed in water depths shallower than 50 m.Other results indicate this depth on modern storm-dominated shelves to be 30 to 80 m (e.g. Cacchioneet al., 1984; Saito, 1989). From these evaluations, thedepth range of the inner- and outer-shelf transitionmight be between 30 and 80 m. Depth range in shelfedge (80–140 m) is evaluated from the SenoumiBasin (Ishii and Nemoto, 1995).

3.4. Age

Ages of the base and the top of each parasequencewere determined from dated tuff beds (Hosoya andSoga tuffs), and from correlation with the deep-seaoxygen isotope curve (Williams, 1990; Fig. 2). Theage of the top and the base of PS8, including theHosoya Tuff, were evaluated from the eustatic curve.The sea-level fall that probably controlled PS8 for-mation was correlated with sea-level fall which oc-curred between 1.87 and 1.83 Ma (Fig. 3). Ages ofthe start and the end of PS8 deposition were equated

Fig. 5. (a) Eustatic sea-level changes evaluated from oxygen isotope curve. (b, c) Accommodation, total subsidence, and tectonicsubsidence curves from sections 2 and 5, respectively. Water depth errors are shown by bars in the total subsidence curves and by theshaded area in the accommodation and tectonic subsidence curves. (d) Change in sediment supply and sedimentation rate on the shelfevaluated from the cross-sections of the entire Kakegawa Group and Fig. 3. MFS D maximum flooding surface; TST D transgressivesystems tract; HST D highstand systems tract.

with that of the sea-level maximum and minimumof the oxygen isotope curve (i.e. 1.87 and 1.83 Ma,respectively). The ages of other parasequences werecorrelated with eustatic sea-level falls of the curvebased on a time interval of PS8 deposition (Fig. 3).

3.5. Eustasy

Eustatic sea-level changes were estimated from theoxygen isotope curve of Williams (1990). The ampli-tudes of eustatic sea-level changes were calculated,assuming that the variation in isotope ratio during thepast 20,000 years represents 100 m of sea-level rise(e.g. Saito, 1994), i.e. a 0.09‰ change in isotope ra-tio equates to a 10 m change in sea level. Evaluatedeustatic sea-level changes are shown in Fig. 5a.

3.6. Sediment supply and sedimentation rate on theshelf

Variation in the sedimentation rate on the shelfwas evaluated by measuring the area of each parase-


quence on the cross-sections (Fig. 3). The rate ofsediment supply was also evaluated from the cross-section of the entire Kakegawa Group (Sakai andMasuda, 1996; Fig. 5) to assess basinward sedimenttransport. Loss of the sediments by shore-paralleltransport beyond the basin was probably less im-portant, because the shelf of the Kakegawa Basinmay have been similar to that of the Senoumi Basinwhere submarine channels provide transport pathsfor sediment to bypass (Uda et al., 1988) and boththe southern and the northern ends of the shelf arebounded by capes preventing sediment transport be-yond the basin.

4. Result

Results of the analysis are shown in Fig. 5. Theerrors of these curves in terms of water depth areup to 70 m (Fig. 5). The total subsidence curve(isostatic subsidence C tectonic subsidence) for eachsection corresponds approximately with broad-scaleaccommodation curve.

The plots of tectonic subsidence show twoepisodes of subsidence. The first episode is repre-sented by rapid subsidence between 2.2 and 2.0 Ma.The rate of tectonic subsidence decreased about 2.0Ma. Slow subsidence continued until the end of de-position of the Kakegawa Group. A short period ofuplift probably took place about 1.6 Ma. The plotsalso show a slightly faster rate of subsidence aroundsection 5 during 2.0–1.4 Ma.

The estimated rate of sediment supply rangesfrom 0:8 ð 104 to 2:8 ð 104 m2=ka (Fig. 5d). Be-cause the rate was evaluated from the cross-sections,the unit is shown as m2=ka. The inferred rates aretherefore equivalent to those of sediment dischargefrom major modern rivers in Japan (on the order of105 to 106 m3=year; Saito and Ikehara, 1992), pre-suming that the lateral extent of the Kakegawa Basinwas similar to the Senoumi Basin (ca. 30 km), andthat the lateral change in the sedimentation rate wassmall in the basin. Sediment supply increased be-tween 2.2 and 2.0 Ma, temporarily decreased around2.0 Ma. Its rate increased again until 1.5 Ma andthen finally decreased between 1.65 and 1.6 Ma.Sediment supply between 1.6 and 1.4 Ma cannotbe evaluated, because parasequence boundaries are

unclear in slope deposits. The sedimentation rate onthe shelf was not in phase with that of the overallsediment supply. The rate was high between 2.2 and2.0 Ma, and decreased between 2.0 and 1.75 Ma andthen increased between 1.75 and 1.4 Ma.

5. Discussion

The pattern of low-frequency (third-order) LatePliocene–Early Pleistocene eustatic sea-level changerecorded in oxygen isotope curves indicates sea-levellowstand 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 inphase with this third-order eustatic cycle. Simi-lar third-order transgressive–regressive cycles havealso been identified from other forearc and backarcbasins in Japan (e.g. Kazusa and Uonuma basins,Ito, 1995; Urabe et al., 1995), and passive marginbasins (e.g. Louisiana offshore, Lowrie and Mc-Daniel-Lowrie, 1985). Therefore, we interpret thethird-order Kakegawa sequence as controlled by athird-order eustasy (Masuda and Ishibashi, 1991;Sakai and Masuda, 1992, 1996).

The broad transgressive and regressive cycle ofthe Kakegawa Group is interpreted as controlledmainly by both tectonic subsidence and sedimenta-tion rate for the following reasons: (1) the generalcorrespondence of the accommodation curves andthe total subsidence curves (Fig. 5) over this timeinterval suggests that accommodation was generatedmainly by total subsidence, that is, the sum of tec-tonic and isostatic subsidence (function of sedimentaccumulation); (2) transgression during 2.2–2.0 Macannot be explained by third-order eustasy whichfalls in this period; and (3) regression starts whenthe sedimentation rate outstrips on the rate of forma-tion of accommodation (e.g. Jervey, 1988). The rateof accommodation formation was almost constantaround 1.75 Ma in broad scale (Fig. 5); increase inthe sedimentation rate at 1.75 Ma was important tochange the accommodation-dominant phase into thesediment-supply-dominant phase.

Geohistory analysis and our evaluation of sedi-mentation rate on the shelf delineate three separatephases of subsidence and sedimentation (Table 2).


Table 2Summary table showing temporal changes of factors influencing sequence development

Systems tract Tectonic subsidence Sediment supply Sedimentation rate

PS3–PS6 TST High Small HighPS7–PS10 TST Slow Moderate SmallPS11–PS16 HST Slow High Moderate

The PS3–PS6 interval, deposited during the firstphase (2.2–2.0 Ma), is characterized by a thick ret-rogradational parasequence set. Tectonic subsidencewas rapid and the sedimentation rate was high in thisphase (Fig. 5). Tectonic subsidence was the dominantfactor, leading to a retrogradational parasequence set.

PS7–PS10, which accumulated during the secondphase (2.0–1.75 Ma), are thin but also compose aretrogradational parasequence set. The rate of tec-tonic subsidence decreased in this phase. Sedimentsupply was moderate, but the sedimentation rate onthe shelf was low (Fig. 5). This suggests that muchsediment may have bypassed the shelf through thesmall channels developed on the upper slope andperhaps also by slumping (described in Sakai andMasuda, 1996). Therefore, the PS7–PS10 retrogra-dational parasequence set is interpreted as formedduring slow tectonic subsidence with a low rate ofsedimentation induced by a high rate of basinwardsediment bypassing.

Cycles PS11–PS16, deposited during the thirdphase (1.75–1.4 Ma), are thin and constitute aprogradational parasequence set. Slow tectonic sub-sidence continued, and sediment supply was high(Fig. 5). The rate of sediment supply was muchlarger than the shelf sedimentation rate in this phase.This indicates that most of the sediment bypassedthe shelf and was deposited on the slope, which con-sequently prograded rapidly. The slightly increasedsedimentation rate (Fig. 5) was enough to initiatea progradational parasequence set in this phase. Ashort period of uplift possibly occurred around 1.6Ma, which may have controlled the stacking pat-tern of parasequences in which positions of shelfedge prograde further basinward (Fig. 3). Subsi-dence increased towards the basin center, resulting indeposition of thicker parasequences in the basin andpreservation of alluvial deposits of PS15 near section5. The maximum flooding surface of the third-ordersequence was developed when the rate of accommo-

dation became less important than the sedimentationrate around 1.75 Ma.

6. Conclusions

Geohistory analysis applied to the Plio–Pleisto-cene Kakegawa Group, together with an evaluation ofthe sedimentation rate, suggest that an observed third-order sequence was controlled mainly by changes intectonic subsidence and the sedimentation rate on theshelf. The lower part of the TST (PS3–PS6), com-posed of thick retrogradational parasequences, wasformed when accommodation created by a high rateof tectonic subsidence exceeded a high rate of sed-imentation. The upper TST (PS7–PS10) consists ofthin parasequences and forms a retrogradational set.This parasequence set probably formed under a com-bination of more slowly increasing accommodationdriven by tectonic subsidence and a low rate of sed-imentation due to an increase in sediment bypassinginto the basin. The HST (PS11–PS16) consists of aprogradational parasequence set which accumulatedwhen sediment supply increased during continuedslow tectonic subsidence. The maximum flooding sur-face dividing these systems tracts apparently formedat the time of transition between a system which wasdominated by tectonic subsidence to a system domi-nated by sediment supply.

Acknowledgements

This paper summarizes a part of the Ph.D. thesisundertaken by T.S. at Osaka University, Japan. T.S.is most grateful to Prof. T. Sunamura of Osaka Uni-versity for his guidance and variable suggestions. Wethank Dr. T. Tsuji, Mr. M. Ueki and Ms. K. Hatanoof Japan National Oil Corporation (JNOC) for theirhelp in the measurement of porosity, and Drs. Y.


Saito of Geological survey, Japan, M. Yokokawa andN. Endo of Osaka University, and Mr. A. Okui ofJNOC, for their suggestions and discussions aboutthe method of basin analysis and the concept ofsequence stratigraphy. Thanks are also due to Drs.H.E. Clifton of Conoco Inc., USA, and M. Ito ofChiba University, Japan, for their help in improvingan 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 refereesof Sedimentary Geology for their critical comments.This work was partially supported by the award of afellowship by the Japan Society for the Promotion ofScience for Japanese Junior Scientists.

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