Backward stacking of submarine channel – fan successions controlled by strike-slip faulting : The Izumi Group ( Cretaceous ) , southwest Japan

Field surveys and numerical simulations were conducted to examine lithostratigraphic cyclicity in strike-slip basins, which is still poorly understood due to its complexity. The basin-fi lling processes in strike-slip basins are closely associated with regional tectonics represented by confi guration of faults and spatial/temporal variations in the slip rate. We attempted to bridge the gap between qualitative sedimentary facies analyses and quantitative numerical models in order to better understand the formation of these sedimentary successions. This paper focuses on the Izumi Group (Upper Cretaceous), southwest Japan, which was deposited in an elongate basin (300 km long by 10–20 km wide) along the Median Tectonic Line, which at the time of deposition was a sinistral strike-slip fault related to oblique subduction along a forearc margin. The depositional environments of the group were deduced from fi ve lithofacies associations (LAs): submarine channel-fi ll facies (LA I), proximal facies of lobes or frontal splays (LA II), distal facies of lobes or frontal splays (LA III), slope-apron facies (LA IV), and basin fl oor facies (LA V). LAs I–III represent point-sourced submarine channel–fan successions in the axial facies, with unidirectional paleocurrent directions from ENE to WSW, and LAs IV–V constitute the marginal facies, the paleoslope of which dipped to the SSW. Two units of submarine channel–fan successions are stacked with ~10 km of eastward (backward) shift. Each unit shows a cyclic lithostratigraphy of rapid upward coarsening and thickening in the lower part (~350 m thick) and gradual upward fi ning and thinning in the upper part (1–3.5 km thick). It is estimated to have taken 5–7 × 105 yr for 10 km offset on each stratigraphic unit based on the depositional ages. Although many processes can control the stratigraphic architecture, such as global and local sea level, climate, and tectonics, the stratigraphic cyclicity observed in the study area is closely related to the depocenter migration, suggesting that fault movement was the primary control on the stratigraphy. On the assumption that the formation and fi lling processes of the Izumi sedimentary basin were basically controlled by strike-slip faults, a numerical simulation suggests that episodic changes in fault-slip rate or sediment-supply rate could control the stratigraphic cyclicity. In this paper, we propose a model where cyclic stratigraphy is ascribed to temporal variations of fault activity controlling accommodation generation, sediment supply, and relative sea level, which could generate cyclic stratigraphy associated with depocenter migration in strike-slip basins. LITHOSPHERE; v. 1; no. 1; p. 41–59. doi: 10.1130/L19.1 *Corresponding author e-mail: a.noda@aist.go.jp. Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/1/1/41/3044315/i1941-8264-1-1-41.pdf by guest on 16 March 2019


INTRODUCTION
Strike-slip faults and related basins are commonly observed at plate boundaries or active plate margins in a setting of convergent or oblique relative plate motion (Fitch, 1972;Jarrard, 1986;Lundberg and Reed, 1991).Such sedimentary basins generally develop in areas of crustal extension related to the geometry and kinematic history of the fault confi guration (e.g., Mann, 2007;Nilsen and Sylvester, 1995).Sedimentary fi ll within many strike-slip basins is classically considered as a repetitive, basinwide, upward-coarsening succession that develops in response to tectonically induced basin migration and deepening, as reported for the Hornelen Basin, Norway, and the Ridge Basin, California (Nilsen and Sylvester, 1995).The Hornelen Basin is a Devonian pull-apart basin made up of axial deposits subdivided into 200 coarsening-upward sequences (Steel, 1976;Steel et al., 1977).The depositional facies shows a shallowing-upward succession, changing from lacustrine mudstone to conglomeratic braidedstream deposits.The repetitive sequences have been interpreted to result from discontinuous strike-slip movement (Steel et al., 1977).Similarly, the Ridge Basin is a fault-bend basin that was initially fi lled by marine mudstone, followed by shallower facies of alluvial conglomerates and sandstone (Crowell and Link, 1982;Crowell, 2003).
In contrast to these studies, recent reports have described fi ning-upward successions in strike-slip basins, refl ecting progressive deepening of the basin (Kim et al., 2003;Lee and Chough, 1999).These reports ascribe the origin of the sequences to a temporal increase in tectonic subsidence due to dip-slip displacement rather than strike-slip offset along the bounding faults.Similar fi ning-up successions have been observed in extensional basins with dip-slip displacements, such as half-graben basins (e.g., Postma and Drinia, 1993;Zecchin, 2005).
The lithostratigraphic variations observed in fault-bounded basins have generally been explained in terms of tectonics as an external (allogenic) force (Blair and Bilodeau, 1988;Gordon and Heller, 1993;Heller et al., 1988), although other processes of glacio-eustatic (e.g., Heckel, 1986) or climatic oscillations (e.g., Cecil, 1990;Zhang et al., 2001) could also control lithology.Stratigraphic variations in foreland basins have been modeled numerically based on interactions among tectonic uplift or subsidence, sediment fl ux, and gravel content of the basinfi ll sediment (e.g., Clevis et al., 2004;Flemings and Jordan, 1989;Marr et al., 2000;Paola et al., 1992;Sinclair et al., 1991).Given that strikeslip basins are also fault-related, it is possible that the basin-fi lling processes are syntectonic, affected by external factors such as fault activity and sediment supply; however, few studies have quantitatively investigated the interactions between tectonics and sediment dynamics in strike-slip basins.The causes of variations and cyclicity in lithology within strike-slip basins therefore remain poorly understood.
The present study focuses on the Izumi Group, southwest Japan, which is considered to be a strike-slip basin related to rapid oblique subduction (~24 cm yr -1 ; Engebretson et al., 1985) of the Izanagi plate during the Late Cretaceous (Maruyama et al., 1997;Taira et al., 1983).The axial deposits of the Izumi Group are considered to represent repeated stacks of submarine turbidites and mudstones, with no evidence of shallowing-upward successions within the basin.The objectives of this paper are to (1) describe and interpret the sedimentological characteristics of cyclic lithostratigraphy within the strike-slip basin, and (2) discuss the cause of the stratigraphic cyclicity by using numerical simulations performed with the aim of reproducing the observed repeated coarsening-and fi ning-upward successions together with basin formation via strike-slip faulting.This multidisciplinary approach was attempted in order to bridge the gap between qualitative sedimentary facies analysis and quantitative tectonic modeling.The results enable a better understanding of the spectrum of different stratigraphic architecture possible in strike-slip basins for which development is controlled by bounding faults.

GEOLOGICAL SETTING
The Izumi Group is dominated by Upper Cretaceous subaqueous deposits within a 300-kmlong, 10-20-km-wide zone along the Median Tectonic Line extending from Shikoku Island to Kinki district (Figs. 1 and 2).The sediments of the Izumi Group show the following characteristics of deposition in a strike-slip basin (Ichikawa et al., 1981;Miyata, 1990;Tanaka, 1989): (1) migration of depocenters away from the direction of strike-slip motion; (2) deep basin in relation to its width; (3) axial infi lling by the dominant basin sedimentation; (4) paleocurrent directions in the main facies largely parallel to the migration direction of the depocenters (i.e., opposite to the younging direction of strata); (5) synclines concentrated in the central part of the basin; and (6) an absence of high-grade metamorphism and sparse igneous activity.
The deposits of the Izumi Group are divided into marginal and main (axial) facies (Ichikawa et al., 1979).The former consists of basal conglomerates and thick mudstones, and the latter consists of turbiditic sandstone alternating with mudstone.The depositional environments are considered to have been a line-sourced slope or fan delta for the marginal facies and a point-sourced submarine fan for the main facies (Nishiura et al., 1993;Tanaka, 1989Tanaka, , 1993;;Tanaka and Maejima, 1995).Radiolarian assemblages (Okamura et al., 1984), based on the abundant occurrence of spongodiscid-form spumellars, indicate that the paleobathymetry of the main facies was "intermediate depth," as defi ned by Empson-Morin (1984).
The depositional age of the Izumi Group ranges from early to middle Campanian (in the western and central parts of Shikoku Island) to late Campanian (eastern Shikoku) and Maastrichtian (Awaji Island and Kinki district), as determined from macrofossils (Bando and Hashimoto, 1984;Morozumi, 1986;Suyari, 1973), radiolarian zones (Hollis and Kimura, 2001;Yamasaki, 1987), and magnetic polarity data (Kodama, 1990(Kodama, , 2003)).These ages indicate a rate of depocenter migration of ~1.4-1.9 cm yr -1 (Fig. 2).Submarine-fan successions controlled by strike-slip faulting | RESEARCH Clasts in conglomerate beds are mainly felsic igneous rocks (rhyolite, dacite, and granite), and sandstones in the main facies are mainly quartzo-feldspathic arenites (Nishimura, 1976;Yokoyama and Goto, 2000).Studies of diagenetic zeolite and mudstone porosity reveal that the maximum burial depth of the Izumi Group shallows to the east, decreasing from 5 to 6 km in the western part of Shikoku Island to 3 km depth in the Kinki district (Nishimura et al., 1980;Nishimura, 1984).

Geology
The Izumi Group in the study area (Niihama City) occurs in a narrow zone between granites of the Ryoke belt and high-pressure (P), lowtemperature (T) metamorphic rocks of the Sambagawa belt (Fig. 3).The bounding fault to the Sambagawa belt is the Median Tectonic Line, which records dextral displacement during the Quaternary (Kaneko, 1966;Mizuno et al., 1993;Okada, 1980) and sinistral displacement during the Cretaceous (Ichikawa, 1980).The strata in this area generally strike ENE-WSW and dip 30°-60° to the south.In the eastern part of Niihama City, strata are gently folded into east-plunging synclines; strata on the northern limbs strike NE-SW and dip to the south, and strata on the southern limbs strike NW-SE and dip to the north.Several ENE-WSW-striking faults are oriented subparallel to the Median Tectonic Line.Small NW-SE-striking faults are present as secondary faults.Strata are locally tightly folded into left-stepping synclines and anticlines oriented parallel or slightly oblique to the ENE-WSW-striking faults.
In the eastern part of the study area, the Ryoke granite is unconformably overlain by thin packages (<50 m) of basal conglomerate and sandstone, followed by thick packages (~350 m) of thin-bedded mudstone (Fig. 4).These lithofacies correspond to the northern marginal facies of eastern Shikoku Island (Matsuura et al., 2002;Suyari, 1973).The main facies is represented in geological maps by fi ve lithological facies (Figs. 3 and 4): (1) a conglomerate-dominated facies (100-300 m thick) distributed in western and eastern areas across a horizontal distance of ~10 km; (2) a sandstone-dominated facies, consisting of thick-to very thick-bedded sandstones, widely distributed throughout the study area; (3) a sandstone-mudstone facies; (4) a mudstone-dominated facies in southern areas, near the Median Tectonic Line; and (5) felsic tuff beds (1-30 m thick) locally intercalated with sediments within the main facies.
Clast-supported conglomerate beds (facies Gc1 and Gc2) are generally thick-to very thickbedded (0.5-3 m) and consist of granule-to boulder-sized clasts within a coarse sand matrix (Figs.6A and 6B).Matrix-supported conglomerate beds (facies Gm) are thick-to very thick-bedded (1-5 m) and consist of granule-to boulder-sized clasts within a muddy sandstone or mudstone matrix (up to 10% of clasts by volume) (Fig. 6C).Clasts are subrounded to well rounded in facies Gc1 and Gc2, and angular to well rounded in facies Gm.Clasts are predominantly felsic volcanics (rhyolite and dacite) and granite, with lesser mudstone and sandstone.Channelization, basal erosion, and amalgamation are commonly observed in facies Gc1 and Gc2.Very thin mudstone beds are intercalated with clast-supported conglomerate beds or are absent altogether.Rare fragments of marine fossils are found in both facies Gm and Gc.
The distribution of LA I corresponds to the conglomerate-dominated facies in the geological map (Fig. 3), and it is divided into western and eastern parts.In addition, small elongate or lenticular bodies are intercalated with the sandstone-dominated facies.LA I is stratigraphically underlain by mudstone-dominated successions (LAs IV and V) and overlain by sandstonedominated successions (LA II).

Interpretation
This coarse-grained lithofacies association is considered to represent deposition in a subaqueous environment, given the lack of evidence for emergence (i.e., absence of coals, parasols, plant roots, and mud cracks), the presence of a deep-water trace-fossil association, and the presence of fragments of marine fossils.Graded (Gc1) and nongraded (Gc2) conglomerates were possibly deposited from gravelly debris fl ows or high-concentration turbidity currents (Lowe, 1982;Mulder and Alexander, 2001;Pickering et al., 1986)  traction carpet material (Walker, 1977), whereas graded parts (Gc1) were deposited from dispersive pressure.Matrix-supported conglomerates (Gm) probably represent gravelly debris fl ows or cohesive fl ows (Mulder and Alexander, 2001;Pickering et al., 1986).Channelized or erosional bases of conglomerate and sandstone beds indicate the occurrence of feeder channels.Clastsupported conglomerates might also be interpreted as residual facies in channels (Mutti and Normark, 1991).Lithological variation in the form of small-scale upward fi ning is interpreted to represent progressive channel abandonment (Walker, 1977).
A high slope gradient is suggested by the occurrence of debris fl ows and high-concentration turbidity currents.Possible depositional environments for this association include a channel fi ll (e.g., Bruhn and Walker, 1997), inner fan (Mutti and Ricci Lucchi, 1972), or upper fan (Normark, 1970(Normark, , 1978;;Walker, 1978).
The gradual transition from conglomeratedominated LA I to sandstone-dominated LA II suggests a lack of fan progradation.One interpretation of the observed upward fi ning is long-term changes in the source area that possibly produced smaller and fi ner-grained fl ows (Walker, 1977).A similar interpretation was made by Bruhn and Walker (1997) for canyonfi lling deposits on the Atlantic margin of Brazil, who ascribed an upward-narrowing, thinning, and fi ning stack of channel complexes to a gradual decrease in tectonic activity during deposition, which infl uenced turbidite sedimentation by reducing the long-range sediment supply.An alternative interpretation of the fi ning-upward sequence is backfi lling of channels by channelmouth lobes (Chen and Hiscott, 1999;Normark et al., 1998).

Description
LA II is characterized by structureless (facies Sm), pebbly (facies Sp), and graded (facies Sg) sandstones (Table 1; Fig. 5), with subordinate thin-bedded mudstone and sandstone couplets (facies Zs), graded conglomerate (facies Gc1), and slump deposits (facies SL1).Sandstone beds show a generally tabular geometry and good lateral continuity (Fig. 6D), although shallow channels and lateral amalgamation of sandstone beds are sometimes observed.Structureless sandstone beds (facies Sm) are thick-to very thick-bedded (0.3-3 m), coarse-to very coarse-grained, and show no internal sedimentary structures.Graded sandstones (facies Sg) are thin-to thick-bedded and medium-to coarse-grained.The bases of beds locally contain granule-to pebble-sized clasts (Fig. 6E).Thick-to very thick-bedded pebbly sandstone beds (facies Sp) contain scattered granule-to pebble-sized clasts and are occasionally weakly laminated.Thick sandstone beds of facies Sm and Sg are occasionally accompanied by parallel or wavy laminations in the uppermost parts (Fig. 6F), comparable with Bouma sequence Ta-b.Sole marks of fl ute, groove, and gutter casts are common at the base of thick sandstone beds, and amalgamation is frequently observed in thick beds.Although body fossils are rare in this association, trace fossils (mainly Nereites ichnofacies) are sometimes observed in thinbedded mudstone beds.
The ratio of sandstone to mudstone ranges from 10:0 to 7:3, and the sandstone beds become thinner with increasing proportion of mudstone.No systematic lithological cycles are recognized in this association.The distribution of the association corresponds to the sandstone-dominated facies on the geological map (Fig. 3).LA II grades upward into the sandstone-mudstone facies of LA III.

Interpretation
Pebbly sandstone (facies Sp) beds probably formed from traction carpets of gravelly high-concentration turbidity currents (Stow and Johansson, 2000).Structureless sandstone (facies Sm) beds are considered to represent rapid deposition from suspension without the infl uence of traction (Hiscott and Middleton, 1979) or sustained (prolonged, quasi-steady) fl ows of high-concentration turbidity currents (Kneller and Branney, 1995).Normally graded sandstone beds associated with parallel laminations, convolute laminations, and ripple crosslaminations are interpreted to represent deposits from waning turbidity currents (Lowe, 1982;Middleton and Hampton, 1973).
A sheet-like geometry and laterally persistent sandstone beds are interpreted to indicate deposition from unconfi ned turbidity currents upon a submarine fan or basin fl oor (e.g., Mutti and Normark, 1991).The lack of recognizable channels provides additional evidence for an extensive depositional environment rather than an erosional setting.The depositional environment could be interpreted as lobes or frontal splays (Lien et al., 2003;Posamentier and Kolla, 2003), suprafan lobes (Normark, 1978;Reading and Richards, 1994;Walker, 1978), or depositional lobes (Mutti and Ricci Lucchi, 1972).The dominance of coarse-grained sandstone beds suggests a sand-rich submarine-fan system (Reading and Richards, 1994;Shanmugam and Moiola, 1991).The occurrence of amalgamated sandstone beds and lack of interbedded mudstone suggest a depositional environment in the proximal part of lobes or frontal splays.A gradual shift to fi ner and thinner facies (LA III) could be explained by a reduction in sediment  supply or the progressive backward-stepping of lobes (Chen and Hiscott, 1999).

Description
LA III is characterized by thin-to mediumbedded sandstones showing grading (facies Sg) and parallel or cross-lamination (facies Sc), along with thin-bedded mudstone and sandstone couplets (facies Zs) (Table 1; Fig. 5).Conglomerate and slump deposits are rare, but several thick-bedded structureless sandstone beds (facies Sm) are recognized.Graded and laminated sandstones are medium-to fi ne-grained and medium-to thick-bedded (~50 cm).Beds are generally laterally continuous.Compared with LA II, channelization, amalgamation, and basal erosion are rarely observed; the grain sizes of sandstones are fi ner (fi ne-to medium-grained); and beds are thinner (thin-to mediumbedded).Trace fossils of Nereites ichnofacies are common in mudstone beds, as in LA II.The ratio of sandstone to mudstone ranges from 7:3 to 3:7.The distribution of this facies corresponds to the sandstone-mudstone facies on the geological map (Fig. 3).

Interpretation
Graded and laminated sandstones are comparable with Ta-d of the Bouma sequence and are therefore considered to represent deposition from low-density turbidity currents (Middleton and Hampton, 1973;Mulder and Alexander, 2001).A reduction in bed thickness, grain size, and ratio of sandstone to mudstone indicates a more distal setting of LA II.Parallel-bedded sandstone and mudstone couplets possibly indicate slow hemipelagic deposition, interrupted periodically by turbidity currents.The deposi-tional environment of this association is regarded as the distal part of lobes or frontal splays (Lien et al., 2003;Posamentier and Kolla, 2003), a lower fan (Normark, 1978;Walker, 1978), or an outer fan (Mutti and Ricci Lucchi, 1972).

Description
LA IV is characterized mainly by thin-bedded mudstone and sandstone couplets (facies Zs), irregularly intercalated with slump deposits (facies SL1 and SL2) (Fig. 5).Facies Zs consists of intercalated thin-bedded mudstone and sandstone, and the ratio of sandstone to mudstone varies from 1:9 to 3:7.Sandstones vary from fi ne-to coarse-grained and show tabular and channel-like geometries.Some coarse-grained sandstone beds contain pebble-to cobble-sized mud clasts; muddy intraclasts are comparable Thin-to medium-bedded (10-30 cm); medium-to coarse-grained; planar-or ripple cross-laminated; moderately to well sorted Waning flow or suspension from low-concentration turbidity currents (Tc-d division in Bouma, 1962) Sw (wavy-laminated sandstone) Thin-to medium-bedded (5-30 cm); very fine-to medium-grained sandstones; interbedded with thin-bedded mudstones; sharply defined upper and lower surfaces; rare grading; amplitude and wavelength of laminations are generally 1-3 cm and 10-50 cm, respectively; laminations are generally symmetrical and lack internal discordances Postdepositional convolute laminations (Allen, 1982); oscillatory or bidirectional oceanic currents; storm sandstones; postdepositional reworking by bottom currents (Stanley, 1993) Zs (thin-bedded graded sandstone and mudstone couplets) Thin-bedded (<15 cm); small trace fossils are occasionally abundant near the top of each mudstone bed Low-density turbidity currents (Bouma, 1962;Carter, 1979;Lowe, 1982;D2 of Pickering et al., 1986); surge-like turbidity currents (Mulder and Alexander, 2001) with hemipelagic mudstone Zm (structureless mudstone) Thick-to very thick-bedded (~3 m); structureless or weakly stratified; medium dark gray to grayish black; silt-sized, partly sandy silt-sized; heavily bioturbated; macrofossils at some localities Hemipelagic settling and mixing by organisms (E1 of Pickering et al., 1986) SL1 (slumps with deformed strata) Mudstone matrix with pulled-apart, rolled-up, or rolled-in sandstone beds; thick to very thick bedded (~8 m) Gravity-induced sediment slumping or sliding (F2.1 of Pickering et al., 1986); levee-overbank slumps (Gregory, 1969;Walker, 1975); collapsed channel wall SL2 (slumps) Mudstone matrix with highly disrupted or brecciated sandstone blocks; occasional angular to rounded felsic volcanic rock and granite clasts; chaotic; thick to very thick bedded (~15 m) Gravity-induced slumping with more pervasive internal deformation than facies SL1 (F2.2 of Pickering et al., 1986); cohesive flows (Mulder and Alexander, 2001) FT (felsic tuff) Structureless to thin-bedded layers; structureless beds are sometimes very thick (>10 m); parallel-or cross-lamination in thin-bedded layers Subaqueous pyroclastic flows or ash turbidites (Fiske and Matsuda, 1964;Wright and Mutti, 1981) A (08012401)  with LAs I-III.Bioturbation is common in the mudstone beds.Slump deposits of facies SL1 contain folded and contorted strata within a mudstone matrix, as also observed in LAs I and II.No exotic blocks or clasts are found in facies SL1.Slump deposits of facies SL2 are polymictic and characterized by the ductiledeformation-related mixing of coarse-grained sandstone and mudstone.Felsic volcanic and granite clasts occur locally in facies SL2.Syndepositional low-angle unconformities are also observed, commonly associated with slump deposits.This association is distributed along the coastline in the northern marginal facies.

Interpretation
The monotonous stacking of tabular, laterally continuous mudstone and thin-bedded sandstone beds suggests pelagic sedimentation and expansive turbulent fl ows on a slope or basin fl oor.Thin tabular sandstone beds suggest deposition from low-concentration turbidity currents upon an unconfi ned slope or basin fl oor.In contrast, the occurrence of coarse-grained, channelized sandstone beds with mud intraclasts indicates high-concentration turbidity currents related to slumps or debris fl ows (Johansson and Stow, 1995).Muddy slump deposits indicate the presence of unconsolidated mud or semiconsolidated mudstone on the slope.The slump deposits are mainly related to gravity-induced sediment sliding and slumping, caused by a range of mechanisms including seismic activity, excess pore-water pressure in response to active sediment loading, and oversteepening of submarine slopes (e.g., Hampton et al., 1996).Local intercalations of slumps and coarser-grained sediments within fi ner-grained sediments indicate a slope-apron facies (Hill, 1984;Richards et al., 1998).

Description
LA V is characterized by thick units of thinbedded mudstone (facies Zs) and interbedded thin-bedded sandstone (facies Sg, Sc, and Sw) (Table 1; Figs. 5 and 6G).The ratio of sandstone to mudstone ranges from 1:9 to 3:7.No thick turbidites or slump deposits are recognized.Structureless mudstone (facies Zm) is thick-bedded, and silt-sized to sandy silt-sized sediments are intensely bioturbated.Marine fossils of Inoceramus sp. and echinoidea are found in this facies.Wavy-laminated sandstone beds (facies Sw) are thin-bedded, medium-to fi negrained, ungraded, and intercalated with thick units of mudstone beds (Fig. 6H).Sand dikes and fl ame structures occur in thin-bedded sandstones within mudstone-dominated facies.

Interpretation
Parallel-laminated sandstone and mudstone beds are intercalated with hemipelagic muds and low-density turbidity currents.Thick-bedded, homogeneous mudstones possibly refl ect prolonged suspension settling of low-concentration turbidity currents, or postdepositional bioturbation.This association is therefore interpreted as a basin-plain deposit.The absence of thick turbidites and slump deposits indicates a depositional environment located distal from a submarine fan or the basin fl oor.

Paleocurrent and Paleoslope
Paleocurrent data were obtained from nearly 50 points in the study area.All measured data were collected relative to structural dip.Sole marks of fl ute and groove casts are well developed at the base of coarse-grained sandstones in LAs I-III.Clast imbrication is poorly developed in conglomerate beds due to the high sphericity of clasts.The average paleocurrent direction determined from sole marks is 241° for fl ute casts and 259° for groove casts, representing NE-ENE to SW-WSW directions (Fig. 7).Clast imbrication indicates a paleocurrent direction of 249°, consistent with the sole marks.The paleocurrent directions are largely subparallel to the strike of bedding, although some N-S directions are recorded in the northeastern part of the study area, where slope-apron facies are distributed.
Slump folds provide clues to paleoslope direction, although some caution is required in making such interpretations (e.g., Lajoie, 1972;Smith, 2000;Strachan and Alsop, 2006;Woodcock, 1979).The limited extent of outcrops means that we adopted the simplest measurement method proposed for slump folds: the mean axis method (Jones, 1939).The results indicate the paleoslope dipped to the SSW (Fig. 8), although there is some variability.The paleoslope direction deduced from slump folds is oblique to that deduced from sole marks and imbrication.

NUMERICAL MODELING
In constructing a depositional model of the Izumi Group, analogue experiments were previously performed in the form of a fault-bend basin along a releasing bend of a northwarddipping, left-lateral (i.e., top to the west) strikeslip fault in a sand box (Iwamoto and Miyata, 1994;Miyata and Iwamoto, 1994).The authors alternately generated displacement along the fault in the sand-fi lled box and fi lled the created basin with sand, thereby revealing stepwise eastward migration of depocenters.The experiments also indicated the occurrence of an extensional strike-slip duplex and the generation of secondary NE-SW-striking normal faults, as observed in the fi eld (Ichikawa et al., 1981;Miyata, 1989Miyata, , 1990)).Yamakita and Ito (1999) conducted numerical simulations to evaluate the deformation of the sedimentary body in a faultbend basin generated by a low-angle strike-slip fault with a releasing bend (i.e., an extensional oblique ramp).The authors reproduced eastward-plunging asymmetric synclinal structures resulting from drag effects along the fault.
Neither of the aforementioned studies considered the depositional environment or the lithological evolution of coarsening-or fi ningupward sequences.In addition, they assumed that the basins were always fi lled by sediment as the fault moved: the accumulation rate was exactly balanced by the subsidence rate.The progressive infi lling of a basin is expected to result in shallowing-and coarsening-upward sedimentation; however, the sediments in the axis of the Izumi Group do not shallow upward, suggesting the basin was underfi lled throughout its sedimentation history.An appropriate numerical simulation, therefore, should be conducted while taking into account the depositional environment and lithostratigraphic variations.

Method
The governing equation of the fi nite difference model is established as where z(x) is the depth of the basin fl oor below base level (bed surface elevation, km), x is the horizontal coordinate (km), A(x) is subsidence relative to base level, S(x) is the accumulation of sediments in the basin, and W(x) is the isostatic response as a function of x.
Recent geophysical surveys reveal that the Median Tectonic Line dips gently to moderately (25°-40°) northward to ~4-6 km depth in Shikoku Island (Ito et al., 1996a(Ito et al., , 1996b;;Sato et al., 2005;Tsutsumi et al., 2007).In this simulation, we assumed a listric fault with a releasing bend as the bounding strike-slip fault (cf.Dinter and Royden, 1993;May et al., 1993).The fault plane is given by where F(x) is the depth of the fault at position x, Z d is the depth to the fault detachment (5 km), and θ is the dip of the fault at the surface (30°) (Fig. 9A).The releasing bend is 30 km long and 10 km wide, based on the Gojo bend in the Kinki district (Fig. 1).The fault is a left-lateral strike-slip fault that moves parallel to the fault strike without any vertical, transpressional, or transtensional components.Tectonic subsidence is ascribed to simple shear deformation in response to crustal fault movements (e.g., Hodgetts et al., 1998;White et al., 1986;Williams and Vann, 1987): where A(x) is crustal thinning at position x, and dx is the amount of strike-slip displacement in the direction x at each time step.The strike-slip displacement generates a sedimentary basin behind the releasing bend (Figs.9B and 9C).
The location of the basin axis depends on the width (10 km) of the releasing bend rather than the dip of the fault.Ductile extension by pure shear in the lower crust and mantle lithosphere is not considered in this model.Sediment accumulation in the basin S(x) depends on sediment supply Q s from emergent land.Sediments accumulate in a deep-sea basin where the sediment fl ux can be regarded as a function of bottom slope.This approach makes sense for gravity-driven systems such as rivers or turbidity currents (cf.Paola, 2000).If the amount of material transported per unit width is proportional to the local topographic slope, we have where Q s (x) is the sediment volume transported per unit time per unit width (km 2 yr -1 ), and k is a transport coeffi cient (km 2 yr -1 ).Assuming the transported sediment volume is conserved and neglecting compaction, we arrive at a simple relationship between the rate of change in bed surface elevation (sediment accumulation S[x]) and the change in transport at position x: Substituting Equation 4into Equation 5, we obtain the diffusion equation given by We take the diffusivity coeffi cient k to be independent of basin depth; therefore, ∂k/∂x = 0, and sediment accumulation S(x) is expressed as

S x k z x x
( ) = ∂ ( ) The diffusion equation is commonly used to describe the transport of sediment down a slope, where the rate of transport is proportional to the local gradient (Culling, 1960;Flemings and Jordan, 1989;Kenyon and Turcotte, 1985).For a given topographic profi le, we can use this relationship to specify sedimentation and thereby calculate the change in accumulation at every point along the x axis.Assuming that erosion of the basement and sediments is negligible in the basin, the condition ∂z/∂x = 0 is used at points where ∂ 2 z/∂x 2 > 0.
The model considers isostatic effects due to crustal thinning by faulting or thickening by sedimentation.Crustal thinning or thickening impose a negative or positive load upon the lithosphere; the resulting isostatic uplift or subsidence W(x) is obtained by where D is the fl exural rigidity of the lithosphere, H is horizontal stress (positive for compression), ρ m is the density of the mantle (3300 kg m -3 ), ρ is the density of the material (1030 kg m -3 for water, 2500 kg m -3 for sediment, 2700 kg m -3 for crust) displaced by the load L(x,t), and g is gravity.Because we possessed no data regarding the fl exural rigidity and horizontal stress at the time of deposition, we simply assumed D and H were equal to zero; the equation refl ects local Airy-type isostatic adjustment.An assumption in the lithosphere response is that the isostatic adjustment upon an increment of sediment loading is completed within each time step.
To minimize topographic change of the basement during fault displacement, the initial slope profi le is assumed to be the same as that of the fault plane (Fig. 9D).This model geometrically yields ~3.2 km 2 of accommodation space for every 1 km of fault displacement.The total displacement is set to 80 km, corresponding to 256 km 2 of accommodation space.This implies the equivalent volume of mountain uplifted at the source area.In the simulations, we assume that half of the volume of the uplifted mountains is eroded and supplied to the basin.
The diffusion equation formulated is independent of grain size; thus, the downstream slope decrease forced by deposition does not directly result in downstream fi ning (cf.Paola, 2000).Therefore, there is no conservation of mass for sediments of each grain size, although the magnitude of the slope is inferred to refl ect the energy of deposition.Sediment of a single grain size would be deposited on progressively diminishing slopes that correspond to diminishing energy conditions.The lithology in this study is assumed to depend on the depositional slope (e.g., Flemings and Jordan, 1989).
Equation 1 was solved numerically using an explicit method.Values used in this model are listed in Table 2 and shown in Figure 10.Calculations ceased when the sediment supply Q s achieved the values shown in Figure 10 at each time step.Five cases were tested, each with varying fault slip (dx) and sediment effl ux (Q s ): (A) constant dx and Q s , (B) constant dx and varying Q s , (C) varying dx and constant Q s , (D) intermittent dx and constant Q s , and (E) varying dx and Q s .

Results
Constant rates of basement retreat and sediment supply in case A mean linear increases in accommodation space (A) and sediment deposition (S).No periodic variability represented by this case resulted in continuous migration of the depocenter and noncyclic lithostratigraphy (Fig. 10A).
In case B, even though the rate of fault slip was constant, sediment fl ux into the basin was variable.A relative increase/decrease of sediment input to fault-slip rate (rate of creation of accommodation space) generated a cyclic stratigraphy composed of fi ning-upward and coarsening-upward successions (Fig. 10B).In this case, a large increase of sediment fl ux in short duration caused the successions to rapidly coarsen and then gradually fi ne upward.
Case C is the opposite to that of case B: the slip rate is variable (cf.Chéry and Vernant, 2006), and sediment supply is constant (Fig. 10C).Variation in the slip rate also resulted in temporal changes in the ratio of accommodation generation, yielding cycles of upward-fi ning and upward-coarsening successions similar to case B.
Case D represents cycles of very rapid and long quiescent phases of fault movement (Fig. 10D).This case resulted in repeated episodes of progradational sedimentation that produced coarsening-upward successions, comparable with typical sedimentary successions found in strike-slip basins such as the Hornelen Basin, Norway (e.g., Steel et al., 1977).Large slip rate caused migration of the depocenter and resulted in cyclic stratigraphy.
In case E, the sediment-supply rate corresponds to the fault-slip rate at a fi xed ratio (Fig. 10E).This case represents a basin-fi ll model with instantaneous response of sediment supply to tectonics.No cyclicity was shown in the simulated lithology due to increase of the accommodation space in proportion to sediment input.
These cases demonstrated that the depocenter gradually shifted toward the sediment source point and that the depositional environment became increasingly distal during the period in which subsidence exceeded sediment accumulation.The period of sediment accumulation in excess of subsidence led to progradation, during which coarse and more proximal sediments were deposited farther from the source.

Depositional Environments
The occurrence of conglomerate-and sandstone-dominated successions (LAs I-III) indicates a likely depositional environment of   a coarse-grained, gravel/sand-rich submarine fan (channel-lobe system).The unidirectional paleocurrent directions recorded throughout these successions suggests an elongate basin and point-sourced submarine fans from a channelized sediment-transfer area (LA I) to a distal depositional zone (LAs II-III) (Fig. 11A).
The basin width is estimated to exceed 3-5 km, although the southern margin of the basin is not exposed in the study area.The submarine channel-fan contains a set of coarse-grained channel-fi ll successions (LA I); thick-bedded, tabular sandstone beds in the proximal part of lobes or frontal splays (LA II); and fi ne-grained sandstone-mudstone couplets in the distal part of the lobes or frontal splays (LA III).LA I is considered to be a transfer zone of fi ne-grained material in channel fl ows, as deduced from the occurrence of channelized erosion, clast-rich conglomerates, and relatively rare mudstone beds.The transition from the transfer to depositional zones was possibly related to changes in the fl ow condition of turbidity currents at a "transition point" (Posamentier and Kolla, 2003).This change from confi ned, high-gradient channels to unconfi ned, gently sloping lobes (depositional) probably resulted in reduced fl ow velocity and deposition of the sediment load from sand-rich turbidity currents, leading to the stacking of thick-bedded sandstone facies (LA II).The basin-marginal facies are slope-apron facies (LA IV) rather than an alluvial fan or coarse-grained fan delta (Fig. 11A), in contrast to other strike-slip basins (Crowell, 2003;Steel et al., 1977).This observation indicates that the basin and boundary faults might have subsided completely beneath the sea.The large number of beds in the slope-apron facies affected by gravity-induced deformation suggests seismicity-related sediment instability on the slope, indicating intense syndepositional fault activity.
The geological map and lithological column (Figs. 3 and 4) show two sets of submarine channel-fan units, with the upper unit stacked on the lower unit.The offset between the two units (~10 km) indicates the distance of backward migration of depocenters associated with displacement along the bounding faults.Although many authors have claimed that submarine-fan lobes generally preserve coarseningupward sequences due to progradation of the fan (Mutti and Ricci Lucchi, 1972;Shanmugam and Moiola, 1988;Walker, 1978), the lithology of each of the fan units in the present study shows upward fi ning of grain size and thinning of beds rather than systematic coarsening or thickening (Fig. 4).One explanation for the lack of thickening-upward cycles is that the lobes were aggradational rather than progradational (e.g., Hiscott and Ghibaudo, 1981;Ricci Lucchi and Valmori, 1980), or that backward fi lling of the fan lobes occurred (Chen and Hiscott, 1999); however, this interpretation does not provide a full explanation of the displacement and rejuvenation of the fans.One possible interpretation is a gradual retreat or subsidence of the basement along a strike-slip fault, resulting in a progressive receding of the source area and deposition of increasingly distal facies at a given site, as shown in Figures 11B  and 11C.Given that the submarine-fan units would have had point sources, we can ignore the fact that they had multiple source points from basin-side areas, which could also have caused lateral offset of source areas and stacking of fans along the bounding faults (e.g., Crowell and Link, 1982;Reading, 1980;Steel, 1988).

Cause of Cyclic Stratigraphy
Many processes of cyclical behavior, such as global and local sea-level changes, climatic oscillations, and temporal and spatial variations of tectonic forces, affect stratigraphical architecture.It is, therefore, diffi cult to constrain a unique control on the stratigraphic cyclicity or to extract the dominant process quantitatively at the present stage.The stratigraphy in the study region is closely associated with migration of the depocenters, although global eustatic or climatic oscillations could be independent to the fault activity.We therefore propose a model where the basin-fi lling processes were primarily controlled by activity of the strike-slip fault before discussion of some causes of cyclic stratigraphy.
The development of the stratigraphic architecture is basically controlled by the ratio of accommodation space (A) generated by tectonic subsidence/retreat to sediment accumulation (S), which depends on sediment-supply rate.If basin subsidence/retreat exceeds sediment infi ll (i.e., high A/S), basement retreat would lead to back-fi lling of the submarine fan, potentially leading to upward-fi ning and upward-thinning successions; thus, the depositional environment would change to a relatively distal facies.In the case that accommodation space and sediment accumulation are balanced (cases A and E), any stratigraphic trend would not be developed.For low values of A/S, the submarine fan would aggrade or prograde; sediments would become coarser and thicker, and shallower successions would be deposited in the basin.Therefore, upward-coarsening and upward-thickening deposits could be explained by a decrease in tectonic activity or increase in sediment supply.The stratigraphy in the study area shows repetition of rapidly coarsening-upward and gently fi ning-upward successions with migration of the depocenter.This scenario is represented by cases B and C in Figure 10, with fl uctuating A/S values.In both cases, tectonics represent the primarily control on progressive migration of the depocenter.
Case B (Fig. 10B) represents periodic changes in the sediment-supply rate to the basin with constant fault-slip rate.The sediment-supply rate into the deep-sea basin is partly controlled by the denudation rate in the source area; indeed, sediment yield shows a correlation with elevation or uplift rate (Ahnert, 1970;Montgomery and Brandon, 2002;Pinet and Souriau, 1988).Elevation or uplift rate may vary as a function of fault-slip rate, which is controlled by tectonics, and thus sediment yield is ultimately dominated by tectonics (e.g., Heller and Paola, 1992).Paleoclimatic changes, including precipitation, temperature, and biogenic activity, so-called Milankovitch mechanisms, can also infl uence the erosion rate with frequency of 10 5 yr (e.g., Zhang et al., 2001).Such climatically driven changes in sediment supply are likely to be independent of tectonism (Koltermann and Gorelick, 1992).The fourth-order (10 5 yr) glacio-eustatic variation is another potentially dominant factor in controlling deposition on submarine fans, despite very limited evidence for glaciation during the Cretaceous.Submarine fans commonly develop in response to sea-level lowering or forced regression (Posamentier et al., 1991;Stow et al., 1985).In the case that rivers are directly connected to the shelf edge or upper slope, frequent deep-water turbidity currents are likely to enter the basin.Whereas coarser sands tend to be deposited behind the shoreline or on the shelf during periods of high sea level, deposition upon the submarine fan by high-concentration turbidity currents would be reduced.
Lithological cyclicity might also result from fl uctuations in subsidence rates arising from episodic changes in fault-slip rates (case C in Fig. 10C).Discontinuous strike-slip movement has been deduced from cyclothems in some pullapart basins (Blair and Bilodeau, 1988;Steel et al., 1977).Especially in the case of a nonmarine basin such as Hornelen Basin, eustatic control on cyclicity should not be considered.Periods of tectonic quiescence (i.e., low A/S) might be expected to result in a relatively small increase of accommodation space to sediment input, thereby enabling the progradation or aggradation of sedimentary successions in the basin.The sense of fault slip (i.e., vertical, transpressional, or transtensional) would affect accommodation generation and sediment supply, and it could be another control on the sedimentation pattern and basin-fi ll architecture of marginal depositional systems (Kim et al., 2003;Lee and Chough, 1999).
The average rate of depocenter migration along the Median Tectonic Line can be estimated to be 1.4-1.9cm yr -1 , indicating that a unit of the stratigraphic cycle with 10 km offset requires a period of 5-7 × 10 5 yr.It is possible that fault-slip rates have temporal variations on geological time scale (10 4 -10 6 years) longer than earthquake elastic cycles (Bennett et al., 2004;Chéry and Vernant, 2006;Friedrich et al., 2003;Kirby et al., 2006).Another example of tectonic deformation with long time scales is coupled lowering and raising of basement in subduction-related active margins (Flint et al., 1991).On the western coast of South America, oblique subduction of multiple aseismic ridges may result in cycles of forearc uplift/subsidence that produce relative sea-level changes at a frequency equal to glacio-eustatic processes (~10 5 yr frequencies).A cyclic ridge subduction (15-35 km wavelength) has also been reported from Japanese forearc (Kodaira et al., 2003), under which the Philippine Sea plate subducts at the rate of 2-4 cm yr -1 (Seno et al., 1993); this implies a frequency of 4-18 × 10 5 yr.Geology during the Late Cretaceous age in the northwest Pacifi c was characterized by rapid oblique subduction with a mid-ocean ridge between the Izanagi-Kula and Pacifi c plates (Maruyama et al., 1997;Uyeda and Miyashiro, 1974).Episodic subduction of ridges or seamounts might induce temporal variability in the fault-slip rate.
We here propose a model where temporal variations of fault-slip rate control the strati-graphic cyclicity.At times when the bounding fault moves fast, the basement rapidly retreats and subsides (Fig. 12A).The basin subsidence makes relative sea level rise, therefore increasing accommodation space.In the initial phase of a large fault slip, rapid retreat/subsidence induces a defi ciency of sediment throughout the basin (cf.Pitman and Andrews, 1985) because a certain response time is needed for fault-induced uplift in the source area and hence a rejuvenation of the supply of detritus by erosion-considered to be the same order of 10 4 -10 6 yr as for large rivers (Castelltort and Van Den Driessche, 2003;Tucker and Slingerland, 1996).Small amounts of fi ne-grained sediments would therefore accumulate in the basin.Thereafter, sediment input gradually increases in response to enhancement of denudation in the source region (Fig. 12B).Larger amounts of coarse-grained detritus supplied to the basin would prograde/aggrade the coastal delta or submarine fan, as indicated by relative sea-level fall and development of coarsening-and thickening-upward successions.After local relief is subdued by progressive erosion, sediment supply into the basin gradually decreases.In addition, continuous receding of the source area and isostatic subsidence in the basin cause relative sea-level rise and development of fi ning-and thinning-upward successions (Fig. 12C).

CONCLUSIONS
Geological survey and numerical simulation have revealed the depositional environment and the lithological cyclicity related to strike-slip faulting in the Izumi Group (Upper Cretaceous), southwest Japan.Sedimentary facies analysis discriminated fi ve lithofacies associations (LAs): conglomerate-dominated successions representing submarine channel-fi ll deposits (LA I); coarse-grained, thick-bedded sandstonedominated sediments indicating proximal part of lobes or frontal splays (LA II); interbedded sandstone and mudstone successions in the distal part of LA II (LA III); slump deposits in mudstone-sandstone sequences of a slope-apron facies (LA IV); and mudstone-dominated facies of the basin fl oor (LA V).Sole marks and clast imbrication indicate unidirectional paleocurrents from ENE to WSW.The orientations of slump folds indicate that the paleoslope dipped to the SSW for the marginal slope.LAs I-III, therefore, would constitute point-sourced submarine channel-fan successions in the axial facies, and LAs IV-V would represent marginal facies of the slope apron and basin fl oor.
Two units of submarine channel-fan successions are stacked with 10 km eastward (backward) offset; these consist of a rapidly upward-coarsening and upward-thickening succession in the lower part, and a gradually upward-fi ning and upward-thinning succession in the upper part.The stratigraphic cyclicity is closely associated with migration of depocenters; it would take 5-7 × 10 5 yr.Numerical simulations of pull-apart basins produced by strike-slip faults and fi lled by sediments have indicated that episodic changes of faultslip rate or sediment-supply rate could explain cyclic stratigraphy.We proposed that temporal variations of the fault-slip rate dominated the cyclic stratigraphy in this case.In the initial phase of active fault movement, rapid retreat/ subsidence of the basement caused migration of the depocenter, resulting in accumulation of fi ne-grained sediments due to defi ciency of sediments.Thereafter, progressive erosion on the source land might have increased sediment fl ux into the basin and prograded/ aggraded submarine fans, indicating development of coarsening-upward successions.In the quiescence of the fault, gradual reduction of sediment input by erosion and incremental basement retreat/subsidence initiated fi ningupward successions.

Figure 1 .
Figure 1.Location of the study area and distribution of the Cretaceous Izumi Group.White lines in the Izumi Group indicate the strike of bedding.Thick, black line represents the Median Tectonic Line (MTL).

Figure 3 .
Figure 3. (A) Geological map for the Niihama district.Thick lines (A-D) indicate the locations of columnar sections in Figure 5. (B) Cross section along the line X-Y in A. (C) Distribution of lower and upper submarine channel-fan units.

Figure 4 .
Figure 4. Generalized lithologic columns for the study area.Lithological patterns are the same as Figure 3.
Figure 7. (A) Areal distribution of paleocurrent directions.Stereographs show paleocurrent data based on fl ute casts (B) and groove casts and clast imbrications (C).

Figure 8 .
Figure 8. Paleoslope directions inferred from slump folds.(A) Photograph of slump deposits of tightly folded thin-bedded mudstone and sandstone couplets (facies SL1); and (B) accompanying line drawing.The slump folds indicate a paleoslope dip direction from left (NE-NNE) to right (SW-SSW).(C) Rose diagram of interpreted paleoslope directions in the marginal facies, based on fold hinge data and using the mean axis method of Jones (1939).
Figure 9. (A) Listric strike-slip fault with a releasing bend, as used in the simulation.(B) Fault-bend basin resulting from 80 km of displacement along the left-lateral strike-slip fault.(C) Plan view of the basin (contour interval, 1 km).(D) Cross section of the fault (solid line) and basin topography (dashed line) along the axis of the basin, as indicated by the dashed line in C.

Figure 10 .
Figure 10.Results of fi ve simulation cases (A-E).A1-E1: Plots showing the values used in the simulations: fault-slip distance (blue) and sediment supply (red) with time.Solid blue circles and open red squares represent fault displacement and sediment supply in each time step (right-hand vertical axis), respectively.Thick lines are cumulative values (left-hand vertical axis).The model parameters for the simulations are given in Table 2. A2-E2: Resulting lithology of basin fi ll.Solid lines represent the deduced present-day erosion surfaces after additional uplift and tilting.A3-E3: Deduced lithological columns along the solid lines in A2-E2.
Figure 11.(A) Depositional environments of the various lithofacies associations.(B-C) Schematic models for strike-slip basins of backward shifts of submarine channel-fan successions with progressive retreat/subsidence of the basement.

Figure 12 .
Figure 12.Schematic models for development of cyclic stratigraphy in strike-slip basins.(A) In the active phase of the strike-slip fault, rapid retreat and subsidence of the basement make the accommodation space large and relative sea-level rise.Sediment input is small in this phase due to a certain response time of sediment supply to tectonic uplift, resulting in fi ning-and thinning-upward successions.(B) Increase of sediment supply in association with enhancement of denudation in the uplifted source region causes progradation/aggradation of delta/ submarine fan and fall of the relative sea level, and, thus, formation of coarsening-and thickening-upward successions.(C) Fining-and thinning-upward successions would be developed by gradual decrease of sediment input due to extensive erosion and retreat/subsidence of the basement, leading to relative sea-level rise.

TABLE 1 .
DESCRIPTION AND INTERPRETATION OF THE LITHOFACIES OF THE IZUMI GROUP