Depositional facies, architecture and environments of the Si

Depositional facies,architecture and environments of the Sihwa Formation (Lower Cretaceous),mid-west Korea with special reference to dinosaur eggs S.B.Kim a ,*,Y.-G.Kim b ,H.R.Jo c ,K.S.Jeong d ,S.K.Chough b ,*

a Korea Polar Research Institute,Korea Ocean Research and Development Institute,Ansan 426-744,Republic of Korea

b

School of Earth and Environmental Sciences,Seoul National University,Seoul 151-747,Republic of Korea

c Department of Earth an

d Environmental Sciences,Andong National University,Andong 760-749,Republic of Korea

d Marin

e Geoenvironment and Resources Research Division,Korea Ocean Research and Development Institute,Ansan 425-600,Republic o

f Korea a r t i c l e i n f o

Article history:

Received 23May 2007

Accepted in revised form 15May 2008

Available online 13June 2008

Keywords:

Alluvial fan

Ephemeral braided stream

Depositional facies and architecture

Dinosaur egg and nest

Sihwa Formation

Cretaceous a b s t r a c t This paper presents detailed facies and architectural analyses and palaeoenvironmental reconstruction of the Sihwa Formation (Lower Cretaceous),mid-west Korea,which comprises an about 3-km-thick non-marine succession containing abundant dinosaur eggshells.Based on constituent facies,bedset geometry,stacking pattern,and bounding surface characteristics,the entire succession can be classi?ed into three architectural elements.Element I occurs along the basin margin and shows a monotonous stacking of tabular or crudely strati?ed conglomeratic units.It represents alluvial-fan deposits of debris-?ow-dominated fan and sheet?ood-dominated fan.Element II is characterized by multi-storey,sheet or upward-widening,conglomeratic channel-?lls whose maximum thickness exceeds 1.5m.Each channel-?ll unit is encased within red–brown silty sandstones of Element III with sharp erosional bases but diffuse gradational upper boundaries.It consists generally of (1)cut-and-?ll deposit (trough cross-strati?ed or openwork conglomerate)in the lower part and (2)composite low-relief bar deposit

(lenticular conglomerate and strati?ed gravelly sandstone)in the upper part.Shallowness of each cut-

and-?ll unit,absence of ?ning-upward lateral accretion package and the predominance of simple-bar

deposit collectively suggest deposition from ephemeral braided streams.Element III typically shows

a ?ning-upward stacking of (1)single-or multi-storey small-scale (<1.5m thick)channel ?lls with

limited lateral extent of less than 15m,(2)poorly sorted,graded and diffusely bounded silty con-

glomerates or gravelly siltstones with discontinuous gravel sheets and pockets and (3)homogeneous or

graded,red–brown ?ne-grained deposits with calcretes and burrows in ascending order.Element III is

interpreted as channel-margin to ?oodplain,including crevasse channel ?ll,crevasse splay and ?ood-

plain ?nes.

The entire sequence of the Sihwa Formation can be pided into the alluvial-fan and terminal-fan

successions.The alluvial-fan succession displays a progradational stacking pattern and indicates a low

rate of basin subsidence in the initial phase of rifting.The upper terminal-fan succession consists of

proximal braided stream and distal ?oodplain deposits in the western and central parts of the basin

and is characterized by an axial dispersal pattern and an aggradational stacking.It suggests rapid

subsidence of the basin ?oor during the main phase of rifting.The asymmetrical cross-basin

distribution of each architectural element re?ects a half-graben structure of the basin with steep-

gradient fault-bounded eastern margin (footwall block)and gently sloped,?exural western margin

(hangingwall block).The predominance of ephemeral braided-stream deposits along with red–

brown ?ne-grained ?oodplain deposits with common calcretes indicates arid to semi-arid

palaeoclimates.

Approximately 140dinosaur eggs (Faveoloolithidae and Dendroolithidae)were identi?ed mainly from

the (gravelly)siltstones and small-scale channel ?lls of Element III deposits and partly from the cut-and-

?ll conglomerates of Element II deposits.The eggs commonly retain their original oval shape but are

invariably breached and stuffed with the substrate of gravels and silt.They are either isolated or

*Corresponding authors.Domestic E&P Department,Korea National Oil Corporation,Anyang 431-711,Republic of Korea (S.B.Kim).

E-mail address:sedlab@snu.ac.kr (S.K.

Chough).

Contents lists available at ScienceDirect Cretaceous Research

journal homepa ge:7b279e2d453610661ed9f40c/loca te/CretRes

0195-6671/$–see front matter ó2008Elsevier Ltd.All rights reserved.

doi:10.1016/j.cretres.2008.05.016Cretaceous Research 30(2009)100–126

clustered,forming a circular concentration in plan view.The abundant yield of eggs,more than20eggs in 5separate nests from a single depositional unit,suggests a dense population of the parental dinosaurs. The repetitive occurrence in many stratigraphic horizons re?ects site preference as a nesting habitat of the near-channel or abandoned channel areas.

ó2008Elsevier Ltd.All rights reserved.

1.Introduction

The Cretaceous deposits of the Korean peninsula have recently been recognized as fossiliferous strata yielding abundant dinosaur footprints,eggs and bones(Lee et al.,2001;Huh et al.,2003)(Fig.1). Detailed bed-by-bed facies analysis and basin-wide architectural analysis of the fossil-bearing deposits are,however,rarely made with regard to fossil preservation and palaeoecological interpretations (Paik et al.,2001a,2001b,2004).Poor knowledge on the depositional processes and environments not only hampers proper palae-oecological understanding but also may lead to the erroneous interpretations from reworked fossils.

The Sihwa Basin,mid-west Korea(Figs.1and2)comprises a fossiliferous non-marine succession of the Lower Cretaceous (Sihwa Formation)that has yielded more than140dinosaur eggs of the families Faveoloolithidae and Dendroolithidae.These eggs commonly occur as circular clusters with3–12eggs in1–2layers and thus are interpreted as nests(Lee et al.,2000).The

eggshells

Fig.1.Distribution of Cretaceous sedimentary basins(shaded areas)and fault patterns in the Korean peninsula(modi?ed after Chough et al.,2000).Locations of dinosaur footprints, eggshells,bones and teeth,and pterosaur footprints are also depicted(complied after Lee et al.,2001;Huh et al.,2003).Rectangle represents the study area.

S.B.Kim et al./Cretaceous Research30(2009)100–126101

are,however,characteristically breached or holed and completely stuffed with substrate sediment (gravels and silt),whereas the original oval shape is largely retained.This indicates the possibility of displacements or disturbance of eggs and requires careful sedi-mentologic examinations for their preservation whether being in intact clutches or scattered out of nests.

For this study we have conducted detailed facies and architec-tural analyses of the Sihwa Formation in order to reconstruct palaeoenvironments and the distribution of depositional systems on a basin-wide scale.Particular attention is paid to the charac-teristics of the deposits containing dinosaur eggs in order to clarify their in situ burial and the dinosaur’s nesting habitus.Criteria for distinguishing displaced eggs from intact preserved nests are put forward.This study indicates the importance of sedimentological analysis as a ?rm ground for the comprehension of the taphonomic processes as well as for advanced palaeoecological interpretations,such as site ?delity or preferential habitat for a certain environment.

2.Geological setting

In the southern part of the Korean peninsula,a number of non-marine basins formed along a series of NE–SW-trending left-lateral strike-slip fault systems during the Cretaceous (Chun and Chough,1992;Chough et al.,2000)(Fig.1).The fault systems resulted from intracontinental transcurrent tectonics caused by oblique north-ward subduction of the Izanagi plate below the Eurasia plate during the Jurassic to Early Cretaceous (Kim et al.,1997;Chough et al.,2000).The associated rhomboidal basins were ?lled with alluvial to lacustrine deposits during the Early and Late Cretaceous (Lee,1999;Chough et al.,2000).

The Sihwa Basin is located in mid-west Korea (Fig.1).It is bounded by N–S trending faults in the eastern and western margins (Park and Kim,1972;Park,2000;Park et al.,2000;Hwasung City,2005)(Figs.2and 3a).Transtensional opening of the basin due to dextral strike-slips on the eastern boarder fault has been suggested,based on the fault bend geometry and the development of NW–SE-trending intrabasindal folds (Hwasung City,2005).The counter-clockwise rotation or drags of the sedimentary strata along the eastern boarder fault also supports the dextral slips of the eastern footwall block relative to the basin ?oor.It is interesting to note that the axis of intrabasinal folds forms an acute (ca.30 )junction to the border fault (Fig.3a),which suggests a deformation in an E–W-direction transpressional regime (Hwasung City,2005;cf.Sander-son and Marchini,1984).Due to the transpression,basin growth was most likely conducted mainly in N–S direction (i.e.

northward

Fig.2.Regional geologic map of study area (modi?ed after Park and Kim,1972;Lee et al.,1999a,1999b;Park,2000).Cretaceous sedimentary rocks are unconformably underlain by the basement of Precambrian metamorphic and Mesozoic plutonic rocks,deposited mainly in the fault-bounded basins (Sihwa and Tando basins).Locations of dinosaur remains and dated volcanic samples are also depicted.Age data are collected from this study (the northern part of Sihwa Basin),Park (2000;the southern part of the Sihwa Basin and the eastern part of Tando Basin),and Choe et al.(2001;the western part of the Tando Basin).

S.B.Kim et al./Cretaceous Research 30(2009)100–126

102

propagation)as indicated by the progressive younging of the basin-?ll sediment in the northward direction.The northward basin ex-tension was presumably accommodated by the formation of a series of southward-dipping NE–SW normal faults and thereby segmentation of basin ?oor into a series of northward-tilted fault blocks (Hwasung City,2005).

The NW-SE-trending fault crossing the medial part of the Sihwa Basin cutting the N-S-trending basin-bounding faults (Fig.3a)may have resulted from the synthetic (sinistral)R’shear during the basin inversion,although its apparent left-lateral sense of move-ment is incompatible with the Riedel shear model.Such a reversal of fault movement can only be accounted for by the sinistral reactivation of the boarder fault.Such a reversal of fault movement or basin inversion is a common feature in the cognate Cretaceous basins in the Korean peninsula (e.g.,Lambiase and Bosworth,1995).

The Sihwa Basin rests on the basement of Precambrian gneiss and schist and Mesozoic granite,and contains a Cretaceous sedi-mentary succession (Sihwa Formation).The Sihwa Formation comprises stratigraphically an about 3-km-thick alluvial deposits of conglomerate,gravelly sandstone,sandstone and red–brown siltstone.Clasts are composed mainly of granitic gneiss,banded gneiss,green or blue schist and quartzite with subordinate amounts of limestone,granite and volcanic rocks.K–Ar whole-rock age of a tuff clast in a conglomerate bed is 85–80Ma (Park,2000;Fig.2),which probably indicates a reset age affected by argon loss during postdepositional hydrothermal alteration in the basin 7b279e2d453610661ed9f40cing the same method,alternative ages of 119.8?2.3Ma (basalt dyke),118.6?2.3Ma (ignimbrite clast)and 125.5?2.4Ma (basalt clast)are obtained in the central part of the basin (Fig.2),which are more comparable with the correlated age of the dinosaur eggs of the Early Cretaceous (Lee et al.,2000).The newly obtained ages of reworked volcanic clasts are believed to represent the depositional ages,because volcanic activites were commonly associated with the basin initiation (Lee,1999;Chough et al.,2000).

More than 140dinosaur eggs have been identi?ed from the Sihwa Formation (Hwasung City,2005).Detailed taxonomic anal-ysis indicates that the eggs can be classi?ed into two types (Lee et al.,2000).Type 1eggs are characterized by a thin shell (0.85–1.23mm)and circular pore canals,whereas type 2eggs consist of a thick shell (3.4–5.0mm)possessing

longitudinal

Fig.3.(a)Geologic map of the Sihwa Basin (modi?ed after Park and Kim,1972;Park,2000;Hwasung City,2005).Locations of measured sections and dinosaur remains are denoted.(b)Detailed map of the northwestern part (Hanyom area)of the basin (modi?ed after Hwasung City,2005).

S.B.Kim et al./Cretaceous Research 30(2009)100–126103

furrows on the outer shell surface.The former can be referred to Youngoolithus xianguanensis(Zhao,1979)of Faveoloolithidae(Zhao and Ding,1976)and the latter to Dendroolithidae(Zhao and Li, 1988).The eggs commonly occur as clustered horizons of1–2layers and reveal circular concentration(ca.50–60cm in diameter)of3–9 eggs in plan view,which appear to represent intact preserved clutch-type nests(Lee et al.,2000).

3.Sedimentary facies and architectural analyses

Based on lithology(grain size),bed geometry and internal structures,eleven sedimentary facies are identi?ed in the Sihwa Formation.Table1details the characteristics and inferred de-positional processes for each facies.The deposits can be grouped into seven facies associations(or subelements)and further into three architectural elements(Table2),based on constituent facies, bedset geometry,stacking pattern,and bounding surface charac-teristics.Fig.4summarizes parts of sedimentary columns in the northern Sihwa Basin as well as horizons of egg occurrence,bur-row,and calcareous nodules.Each element or facies association (FA)records deposition in a distinct depositional environment:(1) alluvial fan(Element I),either debris-?ow-dominated fan(FA IA)or sheet?ood-dominated fan(FA IB),(2)ephemeral braided stream (Element II)comprising either cut-and-?lls(FA IIA)and/or low-relief bars(FA IIB)and(3)channel margin to?oodplain(Element III)constituted by crevasse channels(FA IIIA),crevasse splays(FA IIIB)and?oodplain?nes(FA IIIC)(Table2).

3.1.Element I:alluvial fan

Element I is represented by monotonous stacking of tabular or crudely strati?ed conglomeratic units(facies Gt or GSs)with oc-casional intervening?ne-grained deposits(Figs.5and6).Each depositional unit generally ranges in thickness from30to100cm. Clasts are mostly pebble to cobble grade with subordinate amounts of boulders(up to70cm long)and are generally angular to sub-angular.Element I deposits occur mainly along the eastern basin margin and partly extend to the basin center.It is pided into two types,according to the dominant constituent facies:FA IA domi-nated by tabular conglomerates(facies Gt)and FA IB largely by crudely strati?ed conglomerate/sandstone couplets(facies GSs). 3.1.1.FA IA:debris-?ow-dominated fan

Description:FA IA occurs mainly along the eastern basin margin in the southern basin sector(Sections B1–B5,G and H2)and partly extends to the basin center,where it sharply overlies FA IIIA de-posit.It consists entirely of tabular conglomerates(facies Gt)with thin intercalation of graded sandstone or red–brown siltstone (Fig.5).Each conglomerate unit ranges in thickness from25cm to more than1.5m and is laterally persistent over the entire outcrop (>10m in lateral extent).Each bed is sharply based with planar surface but the upper boundary is usually undulatory or irregular with common protruding clasts.Clasts are predominantly angular and range in size mostly from pebble to cobble grade.They are inversely or inverse-to-normally graded and are either matrix-or clast-supported,particularly in the clast-rich upper 7b279e2d453610661ed9f40crge boulder-size clasts generally occur?oating in the middle part of the bed or protruding along the upper boundaries.Elongate clasts commonly reveal an orientation parallel to that of the bedding surface.Matrix material comprises poorly sorted,coarse-grained sandstone and siltstone locally along the upper boundaries.The sandstone beds are generally planar,whereas the siltstones are discontinuous and form lenses or pockets.Both are less than10cm thick and commonly contain pebble clasts(Fig.5c).

Some beds of tabular conglomerate include laterally over-lapping wedges(Fig.5a,b).The wedges are inversely graded and become thinner and tangentially pinch out toward the downdip margins where largest clasts are concentrated.Superposition of the ?ner-grained rear part of the overlying wedge on the coarse-grained front of the underlying one produces a‘‘false’’normal grading(Fig.5a,b).It is also notable that other conglomerate beds show tightly interlocked,locally openwork gravel concentrations along the planar-topped upper boundaries(Fig.5d).In this case, there occurs an abrupt increase in clast size and abundance ac-companied with a pronounced decrease in grain size of the matrix material across the gravel horizon.

Interpretation:The tabular-bedded nature,common?oating or protruding outsized clasts,preferred?at-lying clast orientation and the undulatory upper surfaces of the conglomerate units(facies Gt) indicate deposition from laminar plastic?ows.The common in-verse grading of gravel clasts with framework support in sandstone matrix speci?cally suggests cohesionless debris?ow in which in-ertial grain interaction is dominant(Nemec and Steel,1984;Kim et al.,1995;Sohn et al.,1999).Although similar sand-matrix grav-elly deposits have been interpreted as hyperconcentrated?ows (e.g.,Mulder and Alexander,2001),the present deposits lack reli-able evidence for?ow turbulence,such as scours,traction struc-tures and normal grading,and thus are regarded as resulting from laminar?ows.

The good lateral continuity of each sedimentation unit and the lack of channel incision suggest deposition from uncon?ned?ows. The relative thinness of the depositional unit,compared to the constituent clast size,however,indicates low?ow mobility.The deposits are envisaged as debris lobes at the termini of self-con?ned or leveed debris?ows(Sharp and Nobles,1953;Wells and Harvey, 1987;Kim and Lowe,2004).Pulsatory surges in the feeding debris ?ows are further postulated based on the overlapping wedge-like subunits in a single sedimentation unit.Each wedge may represent inpidual surges that are self-organized into the coarse-grained head and trailing?ner-grained tail.A tangentially pinching-out snout is a characteristic depositional feature of water-saturated debris?ows(Major,1997).The intervening sandstone and siltstone intercalations probably resulted from the trailing tails that com-prised some bedload pebbles.The overlapping of wedge-shaped subunits(Fig.5a,b)may have formed when a succeeding surge took up the deposit of a preceding one.Similar overlapping wedges were also observed from submarine debris-?ow deposits,showing a re-verse downdip organization,i.e.,younger surge stacked up behind the older surge(Sohn et al.,1997).The occasional tightly packed or armoured gravel concentrations along the upper bedding surface (Fig.5d)may have resulted from reworking and winnowing by the following watery?ows(Major,1997;Blair,1999a).

The basin-marginal accumulation of bouldery debris-?ow de-posits is reminiscent of alluvial-fan environment(Nilsen,1982). Because of the limit in available outcrops,radial dispersal pattern or facies changes cannot be discerned;nevertheless,the above infer-red rheodynamics of the debris?ows is strongly indicative of a steep-gradient alluvial setting rather than low-gradient?uvial plains.The predominance of angular clasts also supports this in-ference in that minimum abrasion occurred during the course of basinward transport from the hinterland drainage areas.De-velopment of surging debris?ows has been widely observed in alluvial fans and can be accounted for by the generation of‘‘roll waves’’due to the steep gradient(Davies,1986,1990)or?uctuating bedload discharge at the valley mouth.

3.1.2.FA IB:sheet?ood-dominated fan

Description:FA IB occurs in the northern basin sector(Sections WG and I3),inter?ngering with FA IIIA deposit.It comprises a ca.30-m-thick succession of crudely strati?ed conglomerate/sandstone couplets(facies GSs)with minor amounts of hollow?lling,open-work lenticular conglomerate(facies Glo)and red–brown siltstone

S.B.Kim et al./Cretaceous Research30(2009)100–126 104

intercalations (Fig.6).The conglomerate/sandstone couplets range in thickness from 20to 50cm and are generally laterally continuous for more than a few metres both in strike and dip sections (Fig.6a,f).Each facies unit is mostly planar bedded,although the bases are very irregular with common downtruding clasts and local scoop-shaped scours (Fig.6b,e,f).Some units are,however,partly cross strati?ed,forming either backset-strati?ed pockets (<28cm thick and <3m wide)(Fig.6b,c)or stacked foreset-bedded complexes (each foreset <1m high and total lateral extent >5m)(Fig.6d).Backsets involve a few reactivation surfaces and show an increase in dip angle (11to 15 )and amplitude (18to 28cm)in updip direction,crossing the reactivation surfaces (Fig.6b,c).Clasts are mostly of pebble to cobble grade with minor amounts of scattered boulders and are mostly angular to subangular.On the other hand,the lenticular conglomerates (facies Glo)usually occupy chute-like scour hollows of decimetres to metres wide and decimetres deep (Fig.6b,c).It consists mainly of cobble-grade clasts with minor amounts of pebbles and occasional boulders that are tightly interlocked with open framework or with interstitial material of poorly sorted,?ne-to coarse-grained sandstone.Occasionally,the hollow ?lls comprise crudely trough cross-strati?ed units conforming to the hollow ge-ometry.The siltstone intercalations occur as erosional remnants of thin discontinuous wisps in the lower sections (Fig.6b,c)but be-come thicker,laterally more persistent and more frequent toward the upper sections.

The conglomerate/sandstone couplets (facies GSs)consist of lower pebble–cobble conglomerate pision and an upper pision of pebbly,coarse-grained sandstone that are occasionally transi-tionally topped by laminated or unlaminated medium-to ?ne-grained sandstone (Fig.6e,f).Each couplet generally forms a single sedimentation unit but in places,vertically superposed,two or more couplets constitute a single bed,revealing an overall ?n-ing-and thinning-upward trend (Fig.6e,f).The lower conglomer-ate pision is generally a-few-clast thick but the actual thickness is laterally variable due to local accumulations of coarsest clast frac-tions and lateral lensing.It is usually thickest (up to 20cm)above scooped scours (Fig.6b,e,f).It is either laterally persistent or dis-continuous,comprising a few separate lenses that are generally more than 1m wide and a few centimetres to decimetres apart (Fig.6e,f).Sharp erosional bases are characteristic.Clasts are densely packed or clast supported with small amounts of matrix material that are almost identical in grain size and texture to those of the overlying sandstone pision.Updip clast imbrication is common.The sharply overlying pebbly sandstone pision is usu-ally thicker than the conglomerate pision unless truncated by the overlying bed.It consists of moderately sorted,granular sandstone and is either coarse-tail or distribution-type graded or ungraded (Fig.6f).The associated pebble-size clasts are oriented either ?ow-parallel or ?ow-transverse and are commonly imbricated.They are either uniform,normally,or inverse-to-normally distributed.The occasional medium-grained sandstone top may comprise well-laminated,(sub-)parallel alternating layers of yellow to variegated medium sandstone and red–brown ?ne sandstone.Each bedset consisting of concordantly superposed beds of conglomerate/sandstone couplets commonly displays a ?ning-and thinning-upward trend (Fig.6a,b).

Interpretation:The overall characteristics of the crudely strati-?ed conglomerate/sandstone couplets (facies GSs)are very similar to those of the sheet?ood deposits described by Blair (1987,1999b,2000),particularly in terms of the mode of strati?cation,clast size and packing,thickness and lateral extent of inpidual sedimenta-tion units,development of erosional scours at the base,and the presence of scattered boulders.Blair (1999b,2000)suggested that the planar couplets of pebble–cobble conglomerate and pebbly sandstone can be explained by successive bedload and suspension fallout deposition during the washout stage of sheet?ood

following

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the breakage of supercritical standing waves.The clast-supported fabric,common clast imbrication and the thickening above scour surfaces of the conglomerate pision con?rm the bedload de-position and the common normal grading with no internal strati-?cation in the sandstone pision also corroborates the suspension deposition.However,intermittent bedload transport of gravel clasts is envisaged during the deposition of the sandstone pision based on the fabrics of the associated pebble clasts.The occasion-ally laminated topmost part of the sandstone pision suggests further bedload segregation during the late stage of suspension fallout.The occasional backset-strati?ed pockets indicate antidune deposition during the stage of standing wave development and/or subsequent wave propagation (Blair,1999b,2000).The updip in-creases in height and dip angle of backsets (Fig.6b,c)suggest de-position by growing antidunes during an upslope propagation of standing waves with amplitude enlargement.The backset height of 18–28cm may indicate ?ow depths of 27–56cm,considering the ?ume observations of Simons and Richardson (1966)suggesting that water depth is 1.5–2times the antidune height.The ?ning-upward bedsets of concordantly superimposed couplets may rep-resent a successive cyclic deposition by repetitive formation and breakage of standing waves and subsequent washout ?ows in a single-event sheet?ooding (Blair,1987).On the other hand,the scour-hollow-?lling lenticular conglomerates (facies Glo)can be interpreted as gully-?ll sediment formed during recessional ?ood or by secondary overland ?ows (Blair,1999b,2000).

It should be noted that the term ‘‘sheet?ood’’is used here for referring to a speci?c type of ?ood,i.e.,uncon?ned and rapidly expanding,high-discharge,supercritical gravelly ?ood ?ow in which bedload and suspension sedimentation alternate.It was originally coined by geomorphologists to simply describe wide-spread ?oods that inundate almost all fan surfaces or tremendously large areas of ?oodplains,with little concerns on their hydraulics.Consequently,a wide range of ?ood processes have been included within this term (Hogg,1982;Miall,1985,1996).Laterally extensive,centimetres to decimetres thick,ungraded or graded,laminated (partly rippled)or unlaminated,sand,sandy mud,or mud deposits (e.g.,Mack and Rasmussen,1984;Arguden and Rodolfo,1986;Rhee

and Chough,1993a;Go

′mez-Villar and Garc?′a-Ruiz,2000)may represent an endmember-type sheet?ood facies in the opposite end of the present deposit.These deposits can be viewed as formed by low-magnitude subcritical sheet?oods in which

suspension

Fig.5.Photographs of FA IA deposits of debris-?ow-dominated fan.(a)FA IA consists of stacks of tabular conglomerates (facies Gt)with thin discontinuous red siltstone in-tercalations (from section B4).Note that a conglomerate bed consists of two overlapping wedges (see text for explanation).Upward arrows indicate inverse grading and downward arrows represent apparent but false normal grading.Hammer (30cm long)for scale.(b)Enlarged photograph of part of (a),showing a superposition of the wedges.A large clast (arrowed)is not a ?oating clast in the middle part of the bed but a protruding clast above the lower wedge (wedge 1).(c)Enlarged photograph of part of (a),showing the in-tercalated siltstone lens.Note the irregular and diffuse lower boundary and the incorporation of pebbles.Pencil (14.5cm long)for scale.(d)Inversely graded tabular conglomerates (facies Gt)with tightly interlocked clast-rich upper part (indicated by bidirectional arrows)(from section H2).Note that the interstices of clast-rich part remain void (openwork frame)or ?lled with ?ner-grained sediment,compared to matrix martial of the other parts of the bed.Hammer for scale.

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fallout sedimentation is prevalent with minor or no bedload trac-tion.An intermediate-type deposit,e.g.,massive to supercritically rippled(Boering and North,1993a,b)or horizontal to low-angle cross-bedded(Tunbridge,1981;Deluca and Eriksson,1989;Mar-shall,2000)sand sheets,may be formed by intermediate-magni-tude,supercritical sandy sheet?ood.

The overall predominance of gravelly sheet?ood deposit(facies GSs)in FA IB suggests formation in a moderately sloping alluvial-fan environment(Blair and McPherson,1994;Blair,1999b,2000). Based on the dominant depositional process,such a fan can be termed as‘‘sheet?ood-dominated fan’’,differentiating from‘‘de-bris-?ow-dominated fan’’of FA IA.The term‘‘waterlaid fan’’that is sometimes used for similar fan type(e.g.,Blair,1999b)seems in-appropriate because it obscures the distinction from those fans largely constructed by braided streams(i.e.,‘‘stream?ow-domi-nated fan’’)(e.g.,Harvey,1984;Kochel,1990;Ritter et al.,1993; Nemec and Postma,1993).In the present example,the fan suc-cession is almost exclusively composed of facies GSs units with scarce backset-strati?ed deposits that have been commonly iden-ti?ed as an essential constituent of supercritical sheet?ood de-posits.This suggests that the FA IB deposit was most probably formed in the distal fan where intense washout?ow erased almost all antidune remnants(e.g.,Blair,1999b,2000).The relative thin-ness(<2m)of the composite bedset units also supports this in-ference.The foreset-bedded stacks(Fig.6d)may indicate construction of micro-deltas,?lling topographic lows at the fan margins by recessional?oods or by overland?ows during inter-?ood stages.

3.2.Element II:ephemeral braided stream

Element II is characterized by multi-storey,sheet-or upward-widening-type,conglomeratic channel-?lls whose maximum thickness exceeds 1.5m(Figs.7–10).It occurs mainly in the northwestern part of the Sihwa Basin(Sections D,I2,I3,H,SH,JH and HH),invariably encased within Element III deposits.Sheet-type channel?lls are more common than the upward-widening types;the latter occur preferentially in the proximal sections (Sections D and SH).Each channel?ll reaches up to a few metres in thickness and is laterally persistent over the entire outcrop (decametres in lateral extent).The erosional base commonly in-volves numerous scours of more than1m wide and decimetres deep(Figs.8and9).The upper boundary is generally transitional with an increase in thickness and frequency of the intercalated siltstone layers(Figs.8,9and10).Siltstone plugged scours are locally present along the upper boundary(Fig.9).Lateral margins are rarely preserved in outcrops but interdigitating features with the encasing siltstones of Element III are observed at some places (Fig.9).The channel bodies consist generally of singular or laterally and/or vertically superposed hollow?lls(FA IIA)of openwork lenticular conglomerate(facies Glo)and trough cross-strati?ed conglomerate(facies Gtx)in the lower part and a com-posite deposit(FA IIB)of clast-supported lenticular conglomerate (facies Glc)and inclined or horizontally strati?ed gravelly sand-stone(facies GySix and GySh)in the upper part(Figs.8and9). Relative proportions of the former(FA IIA)to the latter(FA IIB) decrease downstream;the latter constitutes the whole channel body in the distal sections(Fig.10).Gravel clasts are mostly pebble to cobble grade with minor amounts of boulders and are generally angular to subrounded.

3.2.1.FA IIA:cut and?ll

Description:FA IIA is characterized by hollow-?ll units with sharp erosional scoop-shaped bases.Each hollow is decimetres to metres wide(occasionally up to3m wide)and decimetres(up to 80cm)deep.It forms a single isolated unit or more commonly a laterally and/or vertically superposed composite unit.Solitary units display a narrow,scoop-shaped geometry with low width/ height ratios in the range of1–4in strike sections.A unique outcrop section providing a horizontal view of the solitary hollow?ll re-veals the long and narrow chute-like3D geometry with low-sinuosity planform.Hollow walls are mostly asymmetrical with the deepest axis near the steeper(up to110 )?anks.Each unit is completely?lled up by tightly interlocked,openwork lenticular conglomerate(facies Glo)or less commonly by low-angle trough cross-strati?ed gravelly sandstone or conglomerate(facies Gtx) above a basal veneer of one-to several-clast-thick facies Glo unit. Upward decrease in grain size is frequent in each case.Solitary hollow-?ll units occur at the base and lateral margins of the channel body of Element II.

Composite units involving groups of superposed hollow?lls are characterized by broader and shallower scour bases(width/height ratios of4–10)than those of the solitary units.Symmetrical hollows are more common than asymmetrical ones;the latter are invariably laterally superposed at the steeper walls.The levels of deepest part of each hollow may ascend or descend in the next superposing one. Inpidual hollow?lls are mostly composed of single sets of trough cross-strati?ed gravelly sandstones or conglomerates(facies Gtx) locally with minor amounts of openwork lenticular conglomerate (facies Glo)at the base.Cosets of facies Gtx with criss-crossing set boundaries are very scarce.Each hollow?ll usually shows an up-ward-?ning trend.The facies Gtx units consist generally of alter-nating layers of one-to several-clast-thick pebble–cobble conglomerate and(pebbly)coarse sandstone.Each layer is centi-metres to15cm thick and inclined,conforming to the topography of a side of the basal scour surface with a maximum dip angle less than15 .The conglomerate layer comprises tightly packed clasts with small amounts of interstitial material and frequently thickens toward the trough center or?anks.The sandstone layer involves moderately to well sorted,either massive or low-angle trough cross-strati?ed,granular coarse-grained 7b279e2d453610661ed9f40cterally su-perposed composite hollow?lls occur ubiquitous within the channel body of Element II,but concentrate in the lower part of the channel body,particularly at the deepest section.Such a complex of laterally and vertically superposed hollow?lls is up to2m thick and is laterally persistent a few metres to decametres.Each hollow tends to become broader and shallower upward but the clast size is relatively constant.

Interpretation:The openwork lenticular conglomerate(facies Glo)is interpreted as channel-lag deposits(Nemec and Postma, 1993),based on its occurrence above concave-up scour surface and the dense packing with scarce interstitial material.The trough cross-strati?ed conglomerate(facies Gtx)is,on the other hand,envisaged as resulting from successive pulses of bedload sheets(Whiting et al.,1988;Khadkikar,1999).The low-angle (<15 )foreset dips and the scarcity of coset units negate de-position from three-dimensional(3D)dunes(Miall,1977;Rust, 1978;Khadkikar,1999).The alternate layering of conglomerate and sandstone can be explained by vertical accretion of longitu-dinally segregated bedload sheets,i.e.,superposition of?ner-grained tail over the arrested coarser-grained frontal part.The interstrati?cation of the sandstone layer suggests development of low-amplitude bedforms after the removal(deposition)of coarse particles from the bedload.The general?ning-upward trend in each trough set indicates a decrease in?ow velocity at shallower topographic levels within a channel(Hubert and Forlenza,1988) or waning?oods(Stear,1985).

Hollow?lls of decimetres to metres scale have been interpreted as in?lls of minor channels or small scour pools(Miall,1977;Rust, 1978;Jo and Chough,2001).The common asymmetrical geometry of the solitary hollow?lls with an oversteepened or overhanging wall is reminiscent of a cut-bank-like margin(Rhee et al.,1993),

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Fig.6.Photographs of FA IB deposits of sheet?ood-dominated fan at section WG (for location,see Fig.3b).(a)A dip-section view.FA IB consists of progradational stacks of ?ning-upward bedsets of crudely strati?ed conglomerate/sandstone couplets (facies GSs)with minor amounts of hollow-?ll units of openwork lenticular conglomerate (facies Glo),

particularly along the tops of each bedset.Bedsets &thicken downdip but bedset thins downdip.Note the foresets (f)at the downdip margin of bedset and the backset (b)strati?cations in parts of bedsets &.Downward arrows indicate ?ning-upward trends.(b)Enlarged view of part of (a),showing the ?ning-and thinning-upward stacks of facies GSs units in bedset and the laterally superposed hollow ?lls of facies Glo along the top of bedset .Note the thin discontinuous erosional remnants of siltstone (z)at the bedset boundary.The backset-strati?ed unit involves a few reactivation (r)surfaces,marked by the change in mode of clast size and the dip angle.Hammer (circled)is 30cm long.S.B.Kim et al./Cretaceous Research 30(2009)100–126

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whose development necessitates channelized ?ows.The observed 3D appearance of a solitary hollow ?ll (Fig.11f,g)clearly illustrates the elongated,chute-like geometry with a steep-sided wall along the outer curvature.The laterally superposed hollow ?lls are in-dicative of shifts in ?ow passages in discrete manner,which appeals to channel migration or switching.FA IIA is therefore interpreted as channel in?lls and their variations in dimension (width and depth)and stacking pattern most likely re?ect the natural variability in the geometry and dynamics of channels,although the two-dimensional expression on outcrop surface is strongly dependent on the section orientation (i.e.,depositional dip vs.strike faces).The solitary hollow ?lls may have resulted from instantaneous channel incision and subsequent rapid in?lling,whereas the laterally and/or vertically stacked hollow ?lls would have formed by prolonged and multiple episodes of channel incisions with changing channel courses and gradual in?lling through vertical and/or lateral accre-tions (e.g.,Bridge,1993;Leddy et al.,1993;Ashmore,1993).

The complex units of FA IIA with vertically and laterally super-posed hollow ?lls mimic cosets of trough cross-strati?ed deposits of 3D-dunes (Harms et al.,1982).Careful tracing of each erosional concave-up surface,however,indicates preferential lateral trunca-tion rather than downcutting and the partial disconnection be-tween hollow sets.These features deviate from typical con?guration of set boundaries in 3D-dune deposits and can be

more satisfactorily explained by vertical stacking of laterally su-perposed channel ?lls.The upward transition into shallower and broader hollow sets also negates the 3D-dune origin because the height and width of dunes are genetically linked and show a strong co-variation (i.e.,shallower the height,narrower the width).Khadkikar (1999)suggested empirical geometric criteria dis-tinguishing troughs of channel and dune origin.The measured high width/height ratios (4–10)support the channel-?ll origin,since dune troughs are characterized by lower ratios of 2–6(Khadkikar,1999).The vertical superposition of channel ?lls may re?ect an aggradational river bed.In these respects,FA IIA is comparable to the ‘‘small-scale hollow ?ll’’element of Jo and Chough (2001)and distinguished from their ‘‘trough cross-strati?ed set’’element in case of the vertically and laterally superposed,complex hollow ?lls.Geometric con?gurations (width/height ratio and dip values)are suggested by Khadkikar (1999)and the descriptive details provided here may help this distinction.

3.2.2.FA IIB:low-relief bar

Description:FA IIB occurs in the upper part of the channel-?ll bodies of Element II,sharply overlying the FA IIA deposits with planar erosional surface or occupies the entire channel-?ll bodies in the distal sections.It comprises composite deposits of clast-sup-ported lenticular conglomerate (facies Glc)and inclined or

(c)Enlarged view of part of (a).Note that the backsets increase in dip angle (11to 15 )and amplitude (18to 28cm)in updip direction,and are draped by foreset (f)laminations.(d)Enlarged view of part of (a),showing a complex of foresets.Each foreset is based and truncated by concave-up surfaces,suggesting successive incisions and ?llings of scour pools.Note the minor remnants of topsets (t)and bottomsets (b).(e)Enlarged view of part of (b),showing details of facies GSs units.Each facies unit is bounded by distinct erosional surfaces and consists of a lower conglomerate (G)pision and an abruptly overlying upper pision of pebbly sandstone (S).The conglomerate pision is laterally variable in thickness,being thicker above scoop-shaped scours (arrow 1)and locally discontinuous (arrow 2).In some cases,a single bed comprises superposed subunits (SU1and SU 2)of facies GSs,showing an overall ?ning-and thinning-upward trend.Pencil (14.5cm long)for scale.(f)A strike-section view.Planar con?gurations of each bed of facies GSs,likewise in dip-section view,suggest a tabular 3D geometry of inpidual depositional units.The tangential pinch-out (arrow 1)or lateral lensing (arrow 2)of stacked subunits in composite beds,however,indicate a genuine limit in lateral extensions during

sedimentation.

Fig.7.Photographs of large-scale channel-?ll bodies (Element II)of braided streams.(a)Stacked channel-?ll bodies intervened by crevasse splay and channel deposits (Element III)at Section D.Channel-?ll bodies show either upward-widening (CB 1)or sheet-type (CB 2and CB 3)geometry.(b)An upward-widening channel-?ll body at Section SH reveals a dominance of cut-and-?ll units (FA IIA)in the lower part and an abrupt transition into low-relief bar deposits (FA IIB)in the upper part.(c)Enlarged view of part of (b),showing the interdigitating feature in the right wing.

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horizontally strati?ed gravelly sandstone (facies GySix and GySh)(Figs.7–10).These units are either laterally or vertically stacked,showing complex arrangements and are locally bounded by sharp erosional surface with low-angle (<10 )inclination and limited lateral extent of a few metres.Units of facies Glc and GySh are the most prevalent,whereas the units of facies GySix occur only in relative paucity.

The clast-supported lenticular conglomerate (facies Glc)is characterized by planar-convex,concave-planar or biconvex ge-ometries and consists dominantly of pebble-size clasts with sub-ordinate amounts of cobbles and boulders (Figs.8–10).It ranges in thickness from 10to 50cm and is laterally continuous for a few metres,occasionally up to 20m.In the lateral margins,beds are frequently transitional into inclined or horizontally strati?ed gravelly sandstones (facies GySix and GySh).Clasts are often con-centrated in the thicker central part,forming a convex-up hum-mock in which clasts are disorganized and tightly interlocked.In relatively clast-de?cient and ?ner-grained parts,pebbles are fre-quently imbricated and are locally normally graded or more com-monly inversely graded.Matrix materials are composed generally of coarse-grained sandstone but in the central part,silty ?ne sandstone matrix is more prevalent.The inclined strati?ed gravelly sandstone (facies GySix)is rep-resented by a single set of low-angle cross-strati?cation in which each stratum generally conforms to the lower topographic relief (Figs.9and 10).Strata are subparallel and commonly show a di-vergent fanning or of?apping pattern in association with a downdip thickening of inpidual layers.Each layer angularly or tangentially downlaps onto the set boundary.The facies GySix unit occurs typically aside the clast-supported lenticular conglomerate (facies Glc),and is commonly laterally transitional into horizontally strati?ed gravelly sandstone (facies GySh).Accompanied with the facies transitions,a gradual decrease in clast size from the facies Glc through facies GySix to facies GySh units also frequently occurs.Each facies unit is a few decimetres thick (Figs.9and 10).

The facies GySh units are very similar in lithology to facies GySix in that both comprise alternating layers (ca.1–10cm thick)of pebbly coarse-grained sandstone and medium to ?ne sandstone.Each layer is (sub-)parallel,slightly undulatory or low-amplitude wavy and commonly perges into separate 7b279e2d453610661ed9f40cteral conver-gence into a single layer is also common.Low-angle (<15 )internal scour surfaces are locally developed.Inpidual layers are either ungraded,normally graded or inversely graded and in places are interstrati?ed.Pebbles are aligned parallel to the lamination

and

Fig.8.(a)Line drawing of a sheet-type channel body (Element II)at the lowermost part of Section SH.The channel body consists of cut-and-?ll units (FA IIA)in the lower part and low-relief bar deposits of FA IIB in the upper part.Note a siltstone-plugged (ZP)channel ?ll at the upper boundary.(b)(c)Photographs showing the parts of (a).

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are oriented either ?ow-parallel or ?ow-transverse,and commonly

imbricated.Outsized cobbles and boulders are often associated.

Each facies unit is a few decimetres thick (Figs.9and 10).Some

units show an overall ?ning-upward trend.

Interpretation:The clast-supported lenticular conglomerate (facies

Glc)is interpreted as low-relief longitudinal gravel-bar deposits

(Boothroyd and Ashley,1975;Miall,1985;Karpeta,1993;Jo et al.,1997),on the basis of their common convex-up geometry,clast-sup-ported fabric and generally disorganized nature.The lack of distinctive internal cross-strati?cation,the common up-?ow clast imbrication and local inverse grading are representative of bedload transport by diffuse gravel sheets (Nemec and Postma,1993).The laterally ?anking,conforming strata of inclined cross-strati?ed gravelly sandstone (fa-cies GySix)may indicate accretion in bar fringes.The

horizontally Fig.9.(a)Line drawing of a sheet-type channel body (Element II)on the ?ow-transverse exposure at the upper part of Section SH.The channel body consists largely of cut-and-?ll units (FA IIA)in the left part but low-angle inclined clinoforms or vertically stacked low-relief bar deposits of FA IIB in the right part.Note the interdigitating lateral margins (arrows).(b)(c)(d)(e)(f)Photographs showing detailed features of parts of

(a).

Fig.10.Line drawing of a sheet-type channel body (Element II)on the ?ow-parallel exposure in the middle part of Section SH.The channel body (CB 3)consists mainly of vertical stacks of low-relief bar deposits of FA IIB with minor amounts of cut-and-?ll units of FA IIA at the basal asymmetrical hollow in the right part.The bar deposit in the central part shows a downstream facies changes from unstrati?ed conglomerate (facies Glc)to crudely strati?ed gravelly sandstone (facies GySh),while it displays a lateral lensing both in upstream and downstream direction.The underlying small-scale channel-?ll units of FA IIIA (CB1and CB2)indicates oblique to transverse palaeo?ows,compared with that of the main channel body (CB 3).S.B.Kim et al./Cretaceous Research 30(2009)100–126113

Fig.11.Photographs of a 3D exposure of sheet-like channel body (FA IIB)with a narrow chute-?ll (from an islet between Sections JH and HH).(a )Horizontal view,revealing juxtaposition of channel body and low-sinousity chute-?ll.(b )Enlarged view of part of (a ),showing ?ow-parallel gravel clusters.(c )Vertical Section,showing the planar base of channel body above the ?oodplain ?nes.(d )Enlarged view of part of (c ),showing dominance of clast-supported conglomerates (Facies Glc)and horizontally strati?ed gravelly sandstone (Facies GySh)in the channel body.(e )Note the clast imbrication indicating a westward ?ow direction.(f )Oblique view of chute ?ll.(g )Vertical section,showing the overlapping channel body above the chute-?ll deposit.(h )Vertical section of the chute ?ll,showing the asymmetrical walls.S.B.Kim et al./Cretaceous Research 30(2009)100–126

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strati?ed gravelly sandstone(facies GySh)suggests a broad,low-relief or plane-bedded bar(Allen,1983),sand shoal or interbar plain.The (sub-)parallel to undulatory strati?cation with common layer con-vergence and pergence and local internal scour surfaces indicates development and migration of low-amplitude bedforms on the upper plane bed(Paola et al.,1989;Bennett and Bridge,1995;Bridge and Best,1997).Sedimentation of facies GySh units was probably coeval with the deposition of facies GySix units,given the common lateral juxtaposition of the two facies units.The overall characters of this facies association are very similar to those of the‘‘stacked conglom-erate and sandstone sheets’’element of Jo and Chough(2001),except for the absence of trough cross-strati?ed sandstone.

The dominant vertical stacking of inpidual gravel-bar deposits with rare lateral superposition and scarce large-scale erosional truncations suggests an aggradational river bed with little changes in channel courses and consequently little bar migration.Each bar may have developed according to the cyclic impingement and dissipation of large-scale turbulence or surges of the?ood?ow.At the early stage of turbulence impingement and increasing shear velocity,all available clasts,regardless of their size,would have been actively entrained as bedload or bedload sheets(Whiting et al.,1988)by the?ood?ow.With the decrease in shear velocity due to the declining?ow turbulence,bedload clasts begin to se-lectively deposit from the coarsest fraction that would

form

Fig.11(Continued

).

Fig.12.(a)(b)(c)Photographs of a typical succession of Element III deposits,showing a?ning-upward stacking of FA IIIA,FA IIIB and FA IIIC in ascending order(from Section EG). Arrows indicate concentrated horizons of calcareous nodules.

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clusters and in turn grow as longitudinal bars by gathering ?ner clasts around them through blocking and attracting into the behind wake zone.The low relief of the bar deposit (facies Glc)can be explained by the delimited growth due to the subsequently low-ering water level in the waning stage of the ?ood surges.The lateral sediment segregation from cobbly core to pebbly margins suggests selective entrainment of ?ner clasts into the renewed or continued,diffuse gravel sheets over the longitudinal bars.The laterally su-perposed inclined cross-strati?ed pebbly deposit (facies GySix)may represent reworked bedload deposition in the bar fringes at the lowering-water stage.The neighboring horizontally strati?ed sandstone (facies GySh)suggests coeval deposition of plane-bed-ded bars or shoals at the low-water stage.On the other hand,the cobbly hummocks in the central part of the bar deposit (facies Glc)are interpreted as the remnants of the initial bar cores,in which clasts are densely packed with a paucity of matrix material due to the selective deposition and continued stripping and condensing by the turbulent shears (Kim et al.,2003).

3.2.3.Discussion

The shallowness of each cut-and-?ll unit,the absence of ?ning-upward lateral accretion packages and the development of mul-tiple separate barforms in a ?ow-transverse section collectively indicate deposition in braided streams (Miall,1977;Rust,1978).The low-relief gravel and sand bars of FA IIB are products of peak ?ood ?ows with high water level,whereas the cut-and-?ll con-glomerates of FA IIA represent multiple erosion and ?lling pro-cesses in the channels between high-stage bars (FA IIB),probably after peak ?ood ?ows and during small discharge events.The

predominance of vertical stacking of low-relief bar deposits in-dicate repetitive deposition of the bars probably due to the cyclic ?uctuations in ?ow velocity and water level in association with large-scale turbulence or surges in quasi-steady stage of ?ood.The gradual upward transition of Element II deposit into the Element III deposit with an increase in ?ne-grained intercalations indicates gradual abandonment of channels.

The downstream change of channel-?ll geometry from upward-widening type to sheet type suggests that the distal part was less channelized,reminiscent of terminal fans (e.g.,Sadler and Kelly,1993).The simple organization of channel-?ll architecture domi-nated with cut-and-?ll and simple bar-resulting depositional packages as discerned in Element II deposit,together with the common ?ne-grained intercalations,is diagnostic of ephemeral braided streams formed by ?ash ?ows with high sediment dis-charge (Stear,1983;Bromley,1991;Jo,2003).By contrast,perennial braided streams are generally characterized by complex channel-?ll architecture consisting mainly of large-scale compound bar-forms with multiple accretion packages demarcated by erosional bounding surfaces,re?ecting complicated history of intermittent growth,migration,superposition and coalescing of inpidual bars through multiple episodes of ?ood in a longer time span (Crowley,1983;Luttrell,1993;Jo,2003).

3.3.Element III:channel margin to ?oodplain

This element is characterized by ?ning-upward stacks (up to 30m thick)of (1)single-or multi-storey small-scale (<1.5m thick)channel ?lls with limited lateral extent less than 15m (FA

IIIA,

Fig.13.(a )(b )(c )Photograph of a strongly asymmetrical,isolated,small-scale,single-storey channel-?ll body of FA IIIA embedded in a FA IIIB succession comprising alternations of facies Gs and Fh units (from Section H).The corrugated features on the surface of ?ne-grained deposits can be ascribed to the differential weathering between silt and sand populations.

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comprising facies Glc,Glo,GySi and GySh),(2)extremely poorly sorted,graded conglomerate/siltstone couplets (facies GZg)with diffuse gravel sheets or shallow hollow ?lls (facies Gs)(FA IIIB),and (3)poorly sorted,homogeneous or diffusely graded,red–brown ?ne-grained deposit (facies Fh)with gravel pockets or lenses (facies Gp)(FA IIIC)in ascending order.The deposits occur in most of western and central parts of the Sihwa Basin (Sections EG,H,SH,JH,HH,I4–I9,Y,S1and S2),partly extending to the eastern margin (Section H2).In the northern basin,they are closely associated with Element II deposits in the transitional sectors between the FA IB deposits,whereas in the southern basin,they contact rather abruptly with the FA IA deposits,lacking intervening sections of Element II deposits.FA IIIA occurs dominantly in the northern transitional sectors (Sections EG,H,SH,JH,HH and I4–I9)but be-comes absent toward the southern part (Sections G and S1–S3),where FA IIIB and FA IIIC are more prevalent.Thick-bedded units of FA IIIC develop in the central part of the basin (Sections Y,S1and S2).3.3.1.FA IIIA:crevasse channel

Description:FA IIIA is represented by single-or multi-storey small-scale channel ?lls that sharply scour the underlying ?ne-grained substrate (Figs.12–14).The channel ?lls are generally more than 3m wide with a thickness of less than 1.5m,showing an overall bowl-or pan-shaped,symmetrical or asymmetricalal ge-ometry with smooth or slightly irregular bases and ?at,rather distinct tops.In the proximal sections (Sections I1–I3,EG,and H),each channel ?ll is single-storeyed with no internal bounding surfaces and shows a bowl-shaped geometry with a concave-up or broadly planar base and high-angle (60–90 )walls commonly with well-developed channel wings decimetres thick (Figs.12and 13).Its aspect ratio (width/depth)is between 2and 4.These channel ?lls consist mainly of horizontally strati?ed gravelly sandstones (facies GySh)with minor amounts of openwork lenticular con-glomerate (facies Glo).

In the distal sections (Sections H,I4,SH,JH,HH and I5),multi-storey channel ?lls (aspect ratio of 6–12)are more common with planar but locally scooped scour bases and steepened (30–60 ),commonly stepped walls that are distinct in one side but diffuse on the other side (Figs.13and 14).Channel wings are absent.Internal bounding surfaces are prominent and preferentially inclined to-ward the distinctly bounded walls.Impingement of background ?ne-grained deposits along the internal bounding surfaces is common.Each channel ?ll is mostly isolated;in one place,verti-cally stacked channel ?lls show an upward decrease in dimension and grain size (Fig.15).Internal channel-?ll facies and architecture are very similar to those of the FA IIB deposit.That is,trough cross-strati?ed conglomerates (facies Gtx)commonly occupy the basal part and lenticular conglomerates (facies Glc and Glo)and strati?ed gravelly sandstones (facies GySh and GySix)occur above,showing a complex stratal arrangement.Inpidual channel-?lls show,however,very different appearances;for example,one is domi-nated by units only of facies Gtx,whereas another consists

merely

Fig.14.Photograph (a)(b)(c)and detailed sketch (d )of a small-scale but multi-storey channel ?ll of FA IIIA embedded in FA IIIC deposits (from an islet between Sections JH and HH).CF:channel ?ll.

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of units of facies Glo and GySix.The scarce internal chute-like scours and the common ?ning-upward internal grading distinguish themselves from those of FAIIB.

Interpretation:The close proximity of the channel-?ll units of FA IIIA to the large-scale channel deposits of Element II and the characteristics of the constituent facies suggest rapid bedload de-position in channel margins or in the outlet of a channel branch.The distinct channel tops and the lack of vertically stacked internal units,however,preclude distally splitting distributary channels whose deposits are characteristically sheet-like with smooth,pla-nar bases (Rhee and Chough,1993a ).The scour incisions of metre-scale depths with common overhanging and/or stepped walls preclude deposition by simple over?ows and rather suggest cre-vasse channel incision (Rhee and Chough,1993a;Rhee et al.,1993).FA IIIA is therefore interpreted as crevasse channels (Bridge,1984;Rhee and Chough,1993a;Rhee et al.,1993),whose dimension should be markedly smaller than the main trunk channels of Ele-ment II.

The single-storey units in the proximal sections with relatively low aspect ratio and the high-angle to overhanging channel wall indicate rapid channel ?lling by ephemeral ?ows during episodic events (Rhee et al.,1993).The predominance of horizontally strati?ed gravelly sandstones (facies GySh)indicates bedload de-position under upper ?ow regime.The well-developed channel wings further indicate an abrupt decrease in ?ow strength and thereby cessation of furthered channel incision and subsequent formation of over?ows.On the other hand,the multi-storey units in the distal sections with relatively high aspect ratio and the preferentially inclined,shingled internal units suggest gradually migrating sinuous channels with cutbank-like margins at the distinctly bounded side (Rhee et al.,1993).The basal cut-and-?ll units (facies Gtx)indicate initial channel incision and ?lling,whereas the overlying low-relief bar units (facies Glc and GySix)suggest sparse development of longitudinal bars during high discharge.The lack of channel wings suggests relatively well-con?ned channel ?ows with ?ow strengths enough to maintain continued channel incision.

The variation of overall geometry and architecture of inpidual crevasse channels can be accounted for by the proximity to the trunk channel;single-storey units in the proximal part to the main channel and multi-storey units in the relatively distal sectors.Crevasse channels are formed by bank failures along a trunk channel due to localized intense turbulent shear of ?ood ?ows.Outpouring,torrential ?ows would have undercut the substrate as deep as the level of the main channel and as wide as the breached gap in the bank.Subsequent channel ?lling was probably com-menced with lag deposition and/or longitudinal gravel bars and then followed by sand deposition in the upper-plane-bed condi-tion,as discerned from the vertical organization of constituent fa-cies.In the proximal part with relatively high gradient,crevasse channels are quickly ?lled up and perge in a discrete manner,

whereas in the distal part with lower gradient,channels are maintained for a longer period and gradually migrate with sus-tained channel ?ows (e.g.,Rhee et al.,1993).The crevasse-channel origin is further supported by the palaeo?ows measured from the channel walls that are nearly perpendicular to the palaeo?ows estimated from the basal grooves,scour walls and the trough-cross strati?cation of the large-scale channel ?lls of Element II (discussed later).

3.3.2.FA IIIB:crevasse splay

Description:FA IIIB commonly occurs on top of the crevasse channel ?lls (FA IIIA)(Fig.12).It consists of alternation of discon-tinuous gravel sheets (facies Gs)and ?ne-grained deposit (facies Fh)in the proximal part and stacked facies GZg units (coarse-tail graded conglomerate/sandstone/siltstone triplets and conglomer-ate/siltstone couplets)overlain by facies Fh unit in the distal part.The gravel sheet (facies Gs)is characterized by densely packed,tabular or wedged gravel patch,a few clast to decimetres thick and laterally continuous in outcrop scale (>10m in lateral extent)(Fig.8b).Its lower boundary is relatively sharp and ?at or concave-up in some places.The upper boundary is slightly convex upward or planar.Clasts range in size from pebble to cobble grade and are predominantly oriented parallel to bedding plane.Clast size fre-quently decreases toward the wedged margins.Beyond the wedged terminations,discontinuous pebble–cobble trains are commonly present.Matrix material is scarce in the central part and comprises moderately sorted,?ne-to medium-grained sandstone.In the lateral margins,matrix proportions increase and become poorly sorted.Each gravel sheet commonly alternates with red–brown ?ne-grained deposit (facies Fh)decimetres thick,showing an overall thinning-and ?ning-upward trend.The composite units mimic the conglomerate/siltstone couplets of facies GZg but the densely packed,matrix-de?cient fabric and the abrupt grain-size change to the overlying siltstone are distinctive.

Facies GZg units are diffusely or indistinctly bounded and are tabular bedded with a thickness of decimetres to more than 1m (Fig.13c).Each unit consists of lower pebble–cobble silty con-glomerate pision and an upper pision of homogeneous or coarse-tail graded sandy siltstone,intervened by a middle pision of silty sandstone.The conglomerate pision is several clast thick,matrix-to clast-supported and partially strati?ed with local clast clusters.It is in places deformed and contorted in association with ?ame structures.Clasts are loosely packed and commonly imbri-cated.The matrix comprises sandy siltstone.The sandstone pision is a few centimetres to more than 10cm thick and consists of massive (ungraded and unstrati?ed)or normally graded,moder-ately sorted,granular coarse-grained sandstone with matrix pop-ulation of silt to ?ne-sand grade.The siltstone pision is a few decimetres thick and comprises either coarse-tail or distribution-type normally graded,poorly sorted,non-?ssile,red–brown sandy siltstone.Calcareous nodules or burrows are scarce.Inpidual

beds

Fig.15.Sketch of vertically stacked,small-scale,multi-storey channel bodies of FA IIIA (from Section JH).Each channel ?ll consists mainly of hollow ?lls of facies Glc and Glo in the lower part and transitionally overlying horizontal units of facies GySh.Bidirectional arrows indicate the axial directions of the scours at the channel base.

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are usually stacked repeatedly,showing a?ning-and thinning-upward trend.They are transitional upward into the succession of ?ne-grained deposit(facies Fh).

Interpretation:The close association of this facies association with the crevasse channel deposit of FA IIIA and the incorporation of large amounts of gravel clasts as coarse as those in the Element II deposits at numerous stratigraphic intervals suggest voluminous and repeated bedload supply from the main trunk channel.FA IIIB is therefore interpreted as crevasse splays(Stear,1983;Mj?s et al., 1993;Rhee and Chough,1993a;Bristow et al.,1999),which refer to a semi-conical deposit formed by outpouring?oods through a cre-vasse in the breached levee or bank along the main trunk channel.

The facies Gs units formed by the pulsatory surges of the out-pouring?ood?ows through the breached gap in the banks.The sharp erosional base indicates strong turbulence in the surge head. The dense packing,bedding-parallel clast orientation and the lat-eral continuation to discrete pebble–cobble trains indicate bedload deposition.The slightly convex-up upper boundary,common lat-eral pinching-out and the clast-size decrease toward the wedged margins suggest a low-relief splay-like form.This gravel facies unit is therefore analogous to the sand splays or sheets on the channel margins or?oodplains(Mj?s et al.,1993;Rhee and Chough,1993a; Jorgensen and Fielding,1996;Bristow et al.,1999;Jo and Chough, 2001).The overlying?ne-grained deposit(facies Fh)may have resulted from suspension sedimentation during the waning stage of each?ood pulse.Absence of intervening deposits of intermediate-size sediment population can be accounted for by sharp sediment segregation within the?ood?ow due to strong?uid turbulence and bypassing toward the distal areas before?ow collapses.Re-peated alternation of facies Gs and Fh units showing a?ning-and thinning-up trend suggests relatively frequent spillover probably due to the decrease in storage capacity of the main channel by aggradational?lling and/or by repulsive?ood surges with re-curring intervals of?ow waxing and waning.

The facies GZg units may have resulted from intermittent over?ows or weakened?ood?ows in the distal areas because they are based with nonerosional surface and comprise relatively small amounts of gravel clasts.The relative paucity of gravel clasts, compared to gravel sheets(facies Gs)can be explained by the previous sedimentation in the proximal areas or by localized sed-iment supply from re-breached levees or river beds by episodic strong?ood pulses after the original levee breaches were almost blocked.The basal conglomerate pision with common imbricated clasts indicates bedload deposition,whereas the overlying nor-mally graded sandy siltstone pision represents subsequent sus-pension sedimentation.The?ning-and thinning-up stacking pattern suggests a progressive decline in?ow power in succeeding ?ood surges.

3.3.3.FA IIIC:?oodplain?nes

Description:FA IIIC is represented by red–brown?ne-grained deposit(facies Fh),transitionally overlying the crevasse splay de-posit of FA IIIB(Figs.12–14).Each unit is commonly more than 50cm thick and is laterally continuous over the entire outcrop (>decametres wide)with uniform thickness.It is commonly dis-tribution-type graded with poorly sorted sandy siltstone in the lower part and the progressively well-sorted and weakly?ssile siltstone in the upper part.Discontinuous granule-pebble lenses, trains or pockets(facies Gp)are common in the lower part and are either horinzontal or slightly contorted and deformed.Some gravel pockets form tiny scour?lls of decimetre-scale in width and depth. Outsized pebbles and cobbles are occasionally incorporated.Cal-careous nodules or mottles are common in the upper part(Fig.12c). Nodules are as large as15cm in diameter and in places concen-trated in multiple horizons in a single bed.Vertical burrows(less than5cm in diameter and up to20cm long)are also common and usually?lled with sediment penetrated from the overlying bed. Each facies unit is usually stacked repeatedly,bounded by subtle scour surfaces that can only be discerned by sharp truncations of the tops of vertical burrows.This facies succession commonly shows an overall?ning-and thinning-upward trend.

Interpretation:This facies association is interpreted as a?ood-plain deposit,based on its occurrence on tops of and distal sections away from the crevasse splays of FA IIIB and the predominance of ?ne-grained deposits.Each unit of facies Fh was probably formed by the residual over?ows of a single or several episodes of?ood.The lower poorly sorted,sandy siltstone part indicates relatively rapid suspension sedimentation with minor tractive transport,whereas the upper part of more well sorted,?ssile siltstone suggests re-tarded rate of suspension settling,probably from quasi-stagnant, slowly draining waters.The associated calcareous nodules and burrows indicate post-depositional modi?cations by pedogenic process and bioturbation,respectively.The red–brown colour re-veals subaerial exposure and subsequent sediment reddening in an oxidizing condition(Friend,1966;Turner,1980;Retallack,1997). Generally,red siltstone is formed in well-drained proximal?ood-plains,whereas gray siltstone is produced in the poorly drained, commonly waterlogged,distal?oodplains(Jo,2003).

3.3.

4.Discussion

The spatial distribution and difference in structure and archi-tectural arrangement of each component(FA IIIA,FA IIIB and FA IIIC)of Element III can be explained by the processes in the channel margins according to the difference in magnitude of ensuing?ood pulse and/or the proximity to the main channel.In the proximal sections(Sections EG,I3and H),low-aspect,single-storey crevasse channel-?lls(FA IIIA)are overlain by or laterally juxtaposed with the crevasse-splay deposit(FA IIIB)dominated by interlayered gravel sheet(facies Gs)and thin?ne-grained deposit(facies Fh) with rare poorly sorted gravelly siltstone deposits of facies GZg.The deeper channel incision,frequency of gravel sheets and the sharp segregation of grain size in splay deposits may indicate strong?ood ?ows that were issued out of the main channel and thus were strongly turbulent enough to deeply scour and carry the bulk of sediment load far away,leaving only the coarsest bedload fractions. The interlayered?ne-grained deposits indicate dumped suspended load during the waning stage of?ood?ows,before which in-termediate-size sediment populations were already transported down?ow.In the medial sections(Sections I4,SH,JH,HH and I5), high-aspect-ratio,multi-storey crevasse channel?lls are common and associated with splay deposits dominated by poorly sorted, gravelly siltstones of facies GZg with less frequent and thinner gravel sheets(facies Gs)and thicker?oodplain?ne-grained deposit (facies Fh).The shallow and laterally migrating crevasse channels may have resulted from weakened?ood?ows in the distal areas from the main channel.These?ood?ows would have retained low erosional strength but continued longer period through multiple surges in the recessional stage of?oods.Quick collapse of each surge as soon as it debouched and spread onto the?oodplain resulted in poorly segregated splay deposits.Successive arriving of each surges would have partially reworked,deformed and buried the previous deposits,resulting in amalgamated units of facies GZg with sparse intercalations of diffuse and irregular gravel sheets, lenses and pockets(facies Gs and Gp).In the distal sections(Sec-tions Y,H2and S1–S3),metre-thick?ne-grained deposits of facies Fh are dominant,occasionally associated with small-scale muti-storey channel?lls(FA IIIA)and minor gravel sheets and pockets (facis Gs and Gp).The prevalence of relatively well-sorted?ne-grained deposits indicates retarded suspension sedimentation in the?oodplains during the slow draining of?oodwaters.Occasional channel?lls may suggest infrequent incision by extended crevasse channels or distributary channels.Associated gravel sheets or

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pockets are probably analogous to those in crevasse-splay deposits in process.These coarse-grained deposits embedded in?ne-grained?oodplain deposits have been referred to as gravel(or sand)sheets or splays(Jorgensen and Fielding,1996;Rhee and Chough,1993a;Jo,2003)with no critical evaluation on their dif-ference in channel-margin crevasse splays.

4.Depositional environments and model

4.1.Palaeo?ows and depositional environments

Fig.16summaries the palaeo?ow pattern and the distribution of each architectural element and the constituent facies associations in the Sihwa Basin.Alluvial-fan deposits(Element I)occur mainly along the eastern basin margin,forming three discrete sedimentary bodies.Debris-?ow-dominated fans(FA IA)can be grouped into three distinct bodies:one is located at the southeastern basin margin(Sections B1–B5and G)and the other two occur in the medial parts on the eastern margin(for example,Section H1).A partially radial palaeo?ow pattern can be discerned from the for-mer,based on the dip directions and clast orientation.One sheet?ood-dominated fan(FA IB)body is identi?ed at the northern sector of the basin(Sections I2,WG and I3)and shows a north-to northwestward palaeo?ow.These alluvial fans are regarded as basin marginal or basin transverse depositional systems issued from the eastern basin margin.

The braided-stream channel?lls(Element II)occur in the northwestern part of the basin(Sections D,I1,H,SH and JH), showing interdigitating features with the encasing gravelly siltstones of channel-margin and?oodplain deposits(Element III). These channel?lls display a southward palaeo?ow and exhibit a downstream change in channel form from the upward-widening to sheet type,along with a decrease in relative pro-portions of the basal cut-and-?ll deposit(FA IIA)to the overlying low-relief bar deposit(FA IIB).The associated channel-margin deposits also show a southward change in geometry of crevasse channel?lls(FA IIIA)from a single-storey,bowl-shaped form with distinct wings to a multi-storey,asymmetrically shingled form with stepped margins.Scour axes of crevasse channels indicate a rather perse palaeo?ow,toward NW–SW or NE–SE direction,roughly perpendicular to those of the main channel deposits of Element II. The superposed splay deposit(FA IIIB)also shows a

downstream

Fig.16.Schematic map distribution of each element and facies association in the entire Sihwa Basin(a)and for the Hanyom area(b).Palaeo?ows(measured from channel axis orientation and/or clast imbrications)and the maximum clast size(average of ten largest clasts)are also depicted by the arrows and the circles.Circle diameter is proportional to the clast size.Open circles represent the clast size in gravelly deposits,whereas the closed circles indicate the size of scattered clasts in silty deposits(facies GZg and Fh).

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