Palaeogeography, Palaeoclimatology, Palaeoecology, 105 ( 1993): 139-156 139 Elsevier Science Publishers B.V., Amsterdam

The Cretaceous Pyonghae sequence, southeast Korea: Terminal fan facies

C.W. Rhee and S.K. C h o u g h

Department of Oceanography, Seoul National University, Seoul 151-742, South Korea

(Received January 16, 1992; revised and accepted May 27, 1992)

ABSTRACT

Rhee, C.W. and Chough, S.K., 1993. The Cretaceous Pyonghae sequence, southeast Korea: Terminal fan facies. Palaeogeogr., Palaeoclimatol., Palaeoecol., 105:139 156.

The Cretaceous Pyonghae sequence, southeast Korea, comprises a nonmarine clastic sequence of dominant purplish siltstone with subordinate gravelstone and sandstone. The sequence consists of eight sedimentary facies which can be organized into three facies associations. Facies Association I occurs along the basin margins and is represented by disorganized gravelstone (Facies G l b and G2) and stratified gravelly sandstone (Facies GS). The gravelstone and sandstone facies are interpreted as debris flow deposits and braided stream deposits, respectively, formed in alluvial fans. Facies Association I1 mainly occurs in the central part of the basin and comprises sheet-like sandstone bodies (Facies GS, S1 and $2) intercalated with purplish siltstone beds (Facies MI). The sheet-like sandstone beds show a fining upward trend with sharp base and lateral margins and are often horizontally stratified. These features most likely result from frequent diversion of ephemeral, shallow stream channels on floodplain and subsequent infilling with coarse-grained sediments. Facies Association III is characterized by thick siltstone beds (Facies M1) with lenticular sandstone and gravelstone beds. The siltstone beds contain calcareous nodules. The Facies Association 11I is well-developed in the southern part of the basin, where it represents floodplain environments. Alluvial fans (Facies Association I) and adjacent floodplain (Facies Association III), which are connected by ephemeral stream channel networks (Facies Association II), can be referred to as "terminal fan" system. Autocyclic processes such as spasmodic floods under semi-arid climatic conditions are important for the development of terminal fan systems.

Introduction

Cretaceous nonmarine sequences (Kyongsang Basin) in Korea are well exposed in the southeast- ern part (Fig. 1). Earlier studies were primarily concerned with an establishment of stratigraphy (Tateiwa, 1929; Chang, 1975, 1985; Um et al., 1978. 1983; Choi, 1985). Recent sedimentological studies show that the Kyongsang Basin was in fluviolacustrine environments juxtaposed by allu- vial fans during the Early Cretaceous (Choi, 1981, 1986b; Choi et al., 1981, 1982; Lee, 1985; Son, 1989). The Pyonghae area, a northeastern part of the Kyongsang Basin, was filled with a sequence of gravelstone, (gravelly) sandstone and thick, purplish siltstone (Fig. 2). The nonmarine sequence is characterized by an overall fining upward trend, local occurrences of thick (tens of meters) gray-

elstone beds of debris flow origin, and alternation of (gravelly) sandstone beds (2 3 m thick) with thick siltstone beds (up to a few meters thick).

Recent efforts to distinguish between tectonic controls and other influences on alluvial fan con- struction suggest that the occurrence of coarse- grained deposits in alluvial fan sequences may be a simple response to spasmodic floods or climatic changes rather than fluvial gradient changes brought about by tectonism (Blair, 1987; Schumm et al., 1987; Frostick and Reid, 1989; North et al., 1989; Hill, 1989). It has become clear that auto- cyclic processes such as channel migration and fanhead entrenchment play an important role in alluvial fan development. Alluvial fan sequences with considerable amounts of fine-grained deposits allow a recognition of such effects (e.g., Bown and Kraus, 1987; Kraus, 1987; Wright and Alonso

0031-0182/93/$06 00 2"~ 1993 Elsevier Science Publishers B.V. All rights reserved.

140

Fig. 1. Distribution of Cretaceous sedimentary sequence in southern Korea (after Lee, 1987).

C.W. RHEE AND S.K. CHOUGH

Zarza, 1990). In this study, we attempt to explain channel formation and filling with background deposition of thick, purplish siltstone beds in terms of autocyclic processes rather than recourse to external controls such as tectonics. We emphasize that catastrophic stream-channel response to floods plays a major role for the deposition in some nonmarine basins (Stewart and LaMarche, 1967; Burkham, 1972; Baker, 1977).

Geologic setting

The Pyonghae sequence represents a north- eastern extension of Kyongsang Basin (Fig. 1), a postorogenic nonmarine basin formed in the south- eastern part of the Korean Peninsula during the Early Cretaceous (Hauterivian-Albian) (Chang, 1975; Um et al., 1983; Choi, D.K., 1985; Choi, H.I., 1987). The Kyongsang Basin succession (up to 9 km thick) can be divided into three groups (Chang, 1975): the Sindong Group (mainly silic-

EAST SEA

Fig. 2. Geological map of Pyonghae area. Thick line with teeth indicates thrust fault.

L E G E N D ALLUVIUM TERTIARY ROCK

~ ONJUNGRI GRANITE i ~ SILTSTONE/SANDSTONE : m GRAVELSTONE/ ; ~ GRAVELLY SANDSTONE ! ~ GRAVELSTONE i F ' ~ GRANITIC ROCKS I~HUPOR, FM. :. JT~ GNEISS CPLX.

CRETACEOUS P Y O N G H A E SEQUENCE. SE KOREA: T E R M I N A L FAN FACIES 141

iclastic), the Hayang Group (siliciclastic and vol- caniclastic), and the Yuchon Group (volcanic and epiclastic) in ascending order. In the Kyongsang Basin, clastic sediments were largely derived from the west (Chang and Kim, 1968) and accumulated in fluviolacustrine environments with marginal alluvial fans (Choi, 1981, 1986a; Choi et al., 1981; Lee, 1985). Sedimentary sequence of the Pyonghae area comprises various facies of gravelstone, grav- elly sandstone, and sandstone and purplish silt- stone beds (Fig. 2). On the northern margin, the basin is in a thrust fault contact with a gneiss complex (pre-Cretaceous) (Kim et al., 1963). On the southern margin, it was intruded by granite in the Late Cretaceous; on the southeastern margin, it is partly underlain by Jurassic granite; on the northeastern margin, it is overlain by a Miocene succession (Kim et al., 1963; Kim, 1988) (Fig. 2).

Facies analysis

Entire outcrop sections (Fig. 3) were measured in detail using 1:10 scale columnar sections (total 320 m thick) and sketches. Based on a two-tier system of grain size and primary sedimentary structures, eight sedimentary facies were identified (Table 1). Each facies is described and interpreted in the following sections.

Facies G l a." Discontinuous, disorganized, clast- supported gravelstone

Description This facies ranges in thickness from 15 cm up

to 2 m and laterally discontinuous (a few meters long). Subangular, pebble- to cobble-size clasts are in close contact with one another (matrix < 5%) and are randomly oriented (Fig. 4). This facies rests on scoured surface (up to 1 m deep) of purplish siltstone beds (Facies M1) and occasion- ally forms inclined strata. It is frequently overlain by indistinctly stratified gravelly sandstone (Facies GS) or massive sandstone beds (Facies S1) with relatively flat, distinct boundaries (Fig. 4). Mudstone clasts are occasionally found in the lower part. This facies occurs in sections of Dm-I and -2, Sc-3 and -6, Og-1, Kp-1 and -3, An-3, and Nc-I through -4 (Fig. 3).

N

4 m~.~ ,1 c

• ;.o,o.< • 3 o k _ ~

• . \ . .

.> :

*4 \ t

.~NCHON m?',, --~C---

D O N G M A K / . ~ ~

i "71

i / /

O 2 K m .

,1 A N N U B P U M ,~2An 4 rn :,4.D~

B Y U T N A E

rn:~ i

LEGEND : I - - R O A D - - - S M A L L R O A D

- - S T R E A M . . . . G E O L O G I C B O U N D A R Y

129 2 5 ' 0 0 " i

Fig. 3. Location map of measured sections. Small m's indicatc occurrences of dark mudstone (Facies M2).

TABLE 1

Facies classification scheme

Class G

Class GS

Class S

Class M

Gravelstone ( > 20% gravels) G I Disorganized, clast-supported gravelstone Gla Discontinuous disorganized, clast-supported

gravelstone Glb Continuous disorganized, clast-supported

gravelstone G2 Disorganized, matrix-rich/supported

gravelstone

Gravelly sandstone (5 20% gravels, > 80% sands) GS Stratified/cross-stratified gravelly sandstone

Sandstone (< 5% gravels, > 80% sands) SI Massive sandstone $2 Laminated/stratified sandstone

Mudstone ( > 80% muds) M 1 Purplish mudstone M2 Dark mudstone

142 C . W . R H E E A N D S .K. C H O U G H

A . . . . . i i ~ i i i ¸ i : . . . . : ' " . ~ . . . . .

s i l l : , " i ~ ; - : . : . ; . ~ i . " . " i ~. " . ' . . " ; : ; i . ) i i ' . : ; I : 7 .

~ ~: ~'~ :- ~ °!~~ ~ ~ • ~:o : " ",~ °~.'-:~.i.: .: : ~ " ~ ~" "~ : v;-.~. :-~ ~ ;'. ,~., ",..: ~ .- . ' : , . z...~.~ ~.-~

.-v ,.:~ i ~ :! : : : : " :" _[~ °.~ ~ ~?i~'~I.~?:i#'.: : .:.:.....::~~"7~ ~ - ~- ~..:.: :...::.: .: !::: • i-.:. :i :". !.. -:~.J.~

• ~.,: ~ : . : . , . oo . ' 9 . . ~ .C3 ° ~ ~ u . < 3 ~ . ' , ~ . . . , , , , - , -- ,

Gla ~ ~ , ~ ~ ~ . •. ~0~.~~ ~ " ,=. ~ - o.~. ~ '~

o T . o- o

0 1 M

Fig. 4. Disorganized, clast-supported gravelstone (Facies Gla) overlain by stratified gravelly sandstone (Facies GS) which is, in turn, overlain by massive sandstone (Facies S1), forming a fining upward unit. Scale in (B) is 10 cm long. Location (Kp-l) is shown in Fig. 3.

Interpretation Scoured base and presence of mudstone clasts

are indicative of scour and subsequent infill. Random orientation of poorly-sorted clasts and almost matrix-free texture suggest entrapment of coarse-grained sediments in the depressions and winnowing of fine-grained sediments. The corra- sive motion (sandblasting or local vortices) of sediment-laden turbulent flows (floods) scours rela- tively loose bottom on the floodplain, producing channels and local hollows (Allen, 1962). During and after scouring the channel floor, velocity fluc- tuations of near-bed flows result in entrapment of bedload gravels (Simons et al., 1965). Filling of scoured hollows may produce discontinuous, dis- organized/inclined gravelstone beds. Sand size fractions are either entrapped fortuitously, trans- ported or winnowed continuously, leaving pebble clusters as lag deposits. Similar pebble clusters can be formed by falling floods (Blacknell, 1981). As scoured hollows are filled and the channel floor becomes relatively smooth, deposition from some- what steady flows would be continued through

traction or avalanching processes, forming the inclined strata of gravels. With a waning of flows, finer fractions of their load would be successively deposited.

Facies G l b." Continuous disorganized, clast- supported gravelstone

Description This facies is characterized by coarse-tail,

inversely-graded units that are amalgamated. It ranges in thickness from 80 cm to 2 m (Fig. 5). Clasts are mostly rounded granitic gneisses of cobble size; elongate clasts are subparallel to the bedding plane. Unlike facies Gla , it contains matrix of poorly-sorted coarse sands and granules. Lower and upper boundaries are relatively flat and non-erosive. This facies mainly occurs in section of Dm-1 (Fig. 3).

Interpretation The facies G lb resembles clast-rich debris flow

deposits (Type II) of Shultz (1984) in that it iS

C R E T A C E ( ) U S P Y O N G H A E S E Q U E N C E , SE KOREA: T E R M I N A L FAN FACIES 143

Fig. 5. Amalgamated debris flow deposits. Disorganized, clast- supported gravelstone (Facies Glb) is inversely graded. Clasts of matrix-supported gravelstone (Facies G2) are randomly oriented. Stratigraphic top is right. Location (Dm-2) is shown in Fig. 3.

characterized by lack o f stratification, non-erosive lower boundary and poor sorting. Diagnost ic inverse grading may indicate dispersive pressure (Lowe, 1976; Shultz, 1984) arising f rom clast colli- sion (Bagnold, 1954). Relatively small amounts of cohesion-less matrix and high concentra t ion o f clasts are likely to cause inertial clast interactions. In this respect, facies G l a invokes "density- modified grain flows" o f Lowe (1976).

Facies G2." Disorganized, matrix-rich gravelstone

Description The disorganized, matrix-rich gravelstone facies

consists mainly o f granule- to cobble-size clasts and poorly-sorted, very coarse to coarse sand matrix. This facies ranges in thickness f rom 20 to 250cm. In sections o f D o n g m a k (Din) and Dongpa l (Dp) (Fig. 3), it forms thick beds (up to tens o f meters) which are amalgamated or interbed- ded with massive/stratified sandstone (Facies S1/$2) or purplish muds tone (Facies M1) beds. Clasts are dispersed or form aggregates. In some cases, elongate clasts are either vertical or parallel to bedding plane, showing crude stratification (Fig. 6A). Occasionally, large clasts are concen- trated in the upper par t o f each unit and pro t ruded into the overlying bed. Muds tone chips are also contained in the lower part o f the gravelstone bed (Fig. 6B). The lower facies boundary is erosive.

Fig. 6. (A) Matrix-rich, disorganized gravelstone (Facies G2) in the upper part of section Dongpal (Dp). Clasts are more rounded and smaller than those of lower part of the section. Elongate clasts are generally parallel to bedding plane. Scale is I0 cm long. Location (Dp-3) is shown in Fig. 3. (B) Matrix- rich (clast-supported) gravelstone (Facies G2) rests on relatively fiat base scoured into purplish homogeneous mudstone which is graded. Mudstone chips (arrow) of irregular shape are contained. Scale is 20cm long. Location (Dp-3) is shown in Fig. 3.

Smaller units (20-50 cm thick) commonly occur as lenticles within purplish siltstone beds (Facies M1) and are overlain by massive sandstone (Facies S1) or embedded in purplish siltstone (Facies M 1). This facies occurs in sections o f D m - l , Sc-1, -4, -5 and -6 (Fig. 3).

Interpretation Poorly-sor ted matrix and clasts, absence o f strat-

ification, and presence o f outsized clasts are indica- tive o f debris flows. The coarse sand (almost mud- free) matrix and horizontal clast alignment are

144 C.W. RHEE A N D S.K. C H O U G H

suggestive of cohesionless sandy debris flows (Hampton, 1975) in which clasts undergo free movement in medium of relatively low viscosity (Bull, 1977; Larsen and Steel, 1978). Isolated, outsized clasts protruding in the upper part can be explained by buoyancy due to little density differ- ence between clasts and matrix (Rodine and Johnson, 1976). They are partly supported by debris strength or dispersive pressure.

Facies GS: Stratified/cross-stratified, gravelly sandstone

Description The stratified (gravelly) sandstone (Facies GS)

is characterized by an alternation of gravelstone and sandstone layers. It ranges in thickness from 15 to 230 cm (mostly 30-100 cm thick). The gravel (or gravel-rich) layers consist of poorly-sorted granule- to pebble-size clasts, forming discontinu- ous (3-4 m long) gravel stringers or patches (Fig. 4 and 7). Elongate clasts are generally parallel to bedding plane or occasionally imbricated. The sandstone layers consist of moderately to well sorted, fine to very coarse sand (up to 10-20 cm thick). They are crudely stratified or low-angle cross-stratified, and in some cases form trough cross-stratified units (Fig. 7). This facies is fre- quently underlain by disorganized, clast-supported gravelstone (Facies Gla) and is gradational to the overlying sandstone (Facies S1 or $2) or purplish siltstone (Facies M1). It occurs in sections of Dm- 1 and -3, Sc-1, -2, -3, -4, -6, Og-1, -2, -3, Kp-1, -2, -3, -5, and An-1 (Fig. 3).

Interpretation The indistinct alternation of gravelstone and

sandstone layers that are transitional to each other may result from flow fluctuation or intermittent supply of gravels during transport and deposition. Locally distinct stratification accentuated by difference in grain size (gravel versus sand), sorting and thickness reflects effective sorting of hetero- geneous bed materials under unsteady flow condi- tions. As there is little difference in velocities between flows transporting sand grains as sus- pended load and those transporting gravels as bed load, insignificant change in flow velocities can

induce a considerable shear stress fluctuation, resulting in nearly contemporaneous deposition of sands and gravels (Walker, 1975a).

According to recent studies on gravel-bed streams (Laronne and Carson, 1976; Parker and Klingeman, 1982; Iseya and Ikeda, 1987; Whiting et al., 1988), streams with mixed load (gravels and sand grains) tend to develop thin, relatively long, migrating accumulations of gravels called either "pavement" (Parker et al., 1982), "congested gravel reach" (Iseya and Ikeda, 1987), or "bedload sheets" (Whiting et al., 1988). Such armored beds result from longitudinal sorting (Iseya and Ikeda, 1987; Whiting et al., 1988) and act as buffer layer for gravel transport, that is, reduce the difference of mobility between gravels and sand grains (estab- lishment of equal mobility) (Parker et al., 1982). In gravel-bed streams with mixed load, gravels are transported intermittently as bedload under unsteady flow conditions (Reid and Frostick, 1984). This may be a plausible explanation (e.g., particle over-passing: Allen, 1983; Carling and Glaister, 1987) for the simultaneous deposition of gravels and sands without either a concomitant change in flow regime or in conditions of sediment supply. Thus stratified sand layers and discontinu- ous gravel layers might have formed at the same time on a stream bed with mixed load without any significant flow fluctuations under upper flow regime (Wasson, 1977). Variable dip directions of cross-stratification, vertical and lateral facies changes may result from variable discharge or rapid shifting of stream channels, possibly of ephemeral braided streams (Ore, 1963; Walker, 1975b; Nemec and Steel, 1984).

Facies SI: Massive sandstone

Description The massive sandstone facies (Facies S1) shows

large variations in both thickness (10-120 cm) and sorting (well to very poorly sorted). The topmost part of each unit is commonly graded or crudely stratified; gravels (mostly granule- and pebble-size) or very coarse sand grains commonly occur in the lower part (Fig. 8). The upper facies boundary is generally gradational and relatively flat, whereas the lower boundary is either distinct, erosive or

C R E T A C E O U S P Y O N G H A E S E Q U E N C E , S E K O R E A : T E R M I N A L F A N F A C I E S 1 4 5

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• . ~ -: . . . . . . . . . ~ • ~, o " ~ .. .~:£~.-., ~. . . . . . . . ~.~o.~ ~... . . . . . .-... ,~ . .~?~:. , . , - . ~ . ~ , , ~ ~ ' ~ , % - . . ~ . . . .

Fig. 7. Photograph (A) and sketch (B) of cross-stratified gravelly coarse sandstone beds (Nd-1). Dips of cross-strata are 060 ~' and 210 °. Disorganised clast-supported gravelstone (Facies Gla) is overlain by cross-stratified gravelly sandstone (Facies GS). Scale is 20 cm long. For location, see Fig. 3.

diffusive. Pseudonodu le s are occas iona l ly found near the lower b o u n d a r y (Fig. 8). This facies fre- quent ly occurs as lenticles ( 3 - 4 m wide, 20 30 cm

thick) within purpl i sh s i l ts tone beds (Facies M1) or is under la in by strat if ied (gravelly) s ands tone

beds (Facies G S and $2) (Figs. 4 and 8). I t is

found in all measured sect ions (Fig. 3).

Interpretation Both g rad ing in the u p p e r m o s t pa r t and lack o f

organized fabric are indicat ive o f r ap id fa l lout o f suspended loads wi thou t t rac t ion due to a b r u p t loss of competence . Pseudonodu le s and dis t inct

(flat) lower b o u n d a r y also indicate r ap id depos- i t ion. In add i t ion , concen t ra t ion o f coarse gra ins in the lower par t and divers i ty in texture and

thickness col lect ively indicate tha t this facies was fo rmed by tu rbu len t s t ream flows o f var iable

magni tude .

Facies $2. Laminated/strat!l~ed sandstone

Description The l amina ted / s t ra t i f i ed sands tone facies (Facies

$2) is charac te r ized by an a l t e rna t ion o f coarse sand layers (a few mil l imeters to cent imeters thick)

1 4 6 C.W. R H E E A N D S.K. C H O U G H

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:.-.~): .::?:...; :'.. ~ ; : ~ ~

~,.~ • .~. ; . . .-. . :~ . : . .

Fig. 8. Sketch and photograph, part of subsection Dm-2. Boundaries between purple siltstone beds and sandstone beds are sharp and locally accompany pseudonodules (tip of arrows). Arrows indicate clast-supported, disorganised gravelstone (Facies Gla). For location, see Fig. 3. Hammer in photograph is 33 cm long.

and fine to medium sand/silt laminae (approxi- mately 1 mm thick) (Fig. 9A). Each facies unit ranges in thickness from 10 to 80 cm and is moder- ately to well sorted. The layer boundaries are generally gradational; occasionally distinct due to purplish silt laminae which are laterally discontinu- ous (up to 2-3 m long). Locally this facies is low- angle cross-laminated. Each unit is commonly massive just above the erosive, lower boundary (a few centimeters) and graded in the upper part. Gravels occur as lags (Fig. 9B). This facies is gradational to either massive sandstone (Facies S1) or purplish mudstone (facies M1) (Fig. 10A and B). In some cases, this unit occurs as a wedge in disorganized, matrix-rich gravelstone beds (Facies G2). This facies occurs in sections of Bn-I and -2, An-2 and -3, Nd, Kp-2, Og-1, -2, -3, and Sc-2 and -6 (Fig. 3).

Interpretation It is similar to stratified sandstone of Picard and

High (1973) which is characterized by laterally discontinuous laminae (micro-lensing) and erosive lower boundary. The discontinuous stratified sand-

stone can be formed under lower part of upper flow regime and is a feature characteristic of ephemeral stream deposits (Picard and High, 1973; Tunbridge, 1981, 1984). This facies may also form by migrations of bedforms of very low relief (Bridge and Best, 1988; Paola et al., 1989). On the other hand, migration of low-amplitude bedforms under lower flow regime produces alternating coarse and fine laminae by longitudinal sorting processes at the foreset of low-amplitude bedforms which are developed at shallow depths (< 5 cm) (Smith, 1971; McBride et al., 1975).

Facies MI : Purplish mudstone

Description The purplish (5R 6/2) mudstone facies (Facies

M1) comprises mainly homogeneous siltstone. It is a few centimeters to several meters thick and contains dispersed sand grains and gravel clasts (Fig. 11). Few calcareous nodules are dispersed in some siltstone beds. This facies is ubiquitous in the entire basin, especially in the southern part (Fig. 3).

C R E T A C E O U S P Y O N G H A E S E Q U E N C E , SE KOREA: T E R M I N A L FAN FACIES 147

to 5 m) and occurs in some parts of the basin (Fig. 3).

Interpretation Lack of organized fabric and presence of mica

flakes indicate rapid suspension settling. Dark color and local patchy distribution (Fig. 3) appear to indicate deposition from episodic events, i.e. floods, in a poorly drained, wet area such as swamps and flood basins.

Facies associations and depositional environments

Fig. 9. (A) Laminated/stratified sandstone (Facies $2). Cap as a scale is 5 cm in diameter. (B) Laminated sand beds are stacked, which are separated by lag gravels (thin arrow) or silt layer (thick arrow), or are amalgamated. Hammer is 33 cm long.

Interpretation The purplish mudstone with calcareous nodules

is indicative of oxidizing conditions (Walker, 1967) and subaerial exposure (Esteban and Klappa, 1983). This facies formed most likely by suspension settling on floodplains during floods.

Facies M2: Homogeneous, dark mudstone

Description The facies M2 comprises dark gray, homogen-

eous siltstone and contains mica flakes and mud- stone chips. This facies is several meters thick (up

Frequent facies changes of diverse scale in fluvial deposits force to delineate regional facies varia- tions on a broader scale rather than individual facies changes. In the Pyonghae succession, the northern margin (sections of Dongmak, Sinchon and Ogokri) is dominated by gravelstone and (gravelly) sandstone facies, whereas the southern part (Byutnae section) is dominated by the sand- stone and purplish siltstone facies. Occurrence and interpretation of each facies help derive three facies associations (Table2). The coarse member is largely interpreted as channel deposit and the fine member is interpreted as interchannel or overbank deposit. Further specification of channel deposits was possible based on the shape of sand body, sand/mud ratio (alluvial architecture), internal facies relationships and paleocurrent distribution (Collinson, 1986). The three facies associations represent alluvial fan deposits (Facies Association I), ephemeral stream channels (Facies Association II) and floodplain deposits (Facies Association III). Depositional environments of three facies associations are discussed in the following sections.

Facies Association I: Alluvial fan deposits

Occurrence and interpretation The facies Association I is characterized by

poorly-sorted, coarse-grained deposits (Facies Glb , G2 and GS) which are laterally discontinu- ous. This association occurs mainly in the margins of the basin; in the northern part (sections of Sinchon and Ogokri) it is characterized by water- laid deposits (Facies GS and $2) and subordinate debris flow deposits (Facies G l b and G2), whereas

148 c.w. RHEE AND S.K. CHOUGH

B

A

Fig. 10. Sketch (A) and columnar section (B) representing braided channel deposits. Note gravels resting on the scoured bases, which are transitional to stratified/cross-stratified gravelly sandstone (Facies GS). Location, Sc-1, is shown in Fig. 3.

Fig. 11. Part of purplish mudstone facies (M1) containing gravels and sand grains. Sand layer shows mottled-like feature (arrow). Scale is 5 cm in diameter. Location, Nd-5, is shown in Fig. 3.

in the eastern (Dongpal section) and western (Dongmak section) margins it is dominated by debris flow deposits. Presence of debris flow depos- its (Facies Glb and G2) along the basin margins

is suggestive of alluvial fan environments (Nilsen, 1982; Rust and Koster, 1984).

Thick (tens of meters), disorganized gravelstone beds (Facies Glb) occur in the sections of Dongmak (Dm) and Dongpal (Dp) where the beds are laterally transitional to purplish siltstone (Facies M1). Poorly-sorted sand matrix, non- erosive base, and lack of internal scour surface and stratification indicate that these deposits were formed by unchannelized debris flows (Hampton, 1975; Middleton and Hampton, 1976). Clasts in the section of Dongpal (Dp) are mostly rounded quartzite, whereas those of the section of Dongmak (Dm) are mostly subrounded granitic gneiss. The contrast in clast composition represents two different local sources.

Gravelstone beds (Facies Gla/G2) in sections of Sinchon (Sc) and Ogokri (Og) are overlain by sandstone (Facies GS/S2) and in turn by purplish siltstone, forming a fining upward unit (Fig. 10). Although channel geometry is indistinct due to limited exposure, some erosive stepped bases

CREIACI-.OUS PYONGHAE SEQUENCE, SE KOREA: TERMINAL FAN FACIES 149

TABLE 2

Facies Associations of the Pyonghae sequence

Facies Association Facies Interpretation Occurrences

I Glb and G2; Debris flow (Inner fan) GS, SI and and

$2 braided stream

GS, S1 and $2 Ephemeral II (braided) stream (Middle fan) M1 and M2 and interchannel

deposits

III M1 and M2 Floodplain (Distal fan)

Northern part (Din, Sc, Og and Dp)

Central part (Kp, Nd and An)

Southern part (Bn)

(Fig. 10B) indicate that this sequence rests on a scoured bed f'ormed by turbulent streams (floods) on fan surface (Rachoki, 1981). Lateral textural variation, presence of internal scour surfaces, fining upward trend, and segregation of gravel from sandy sediment can be explained by rewor- king/deposition of braided streams or sheetfloods preceded by debris flows (Wasson, 1977; Nemec and Steel, 1984). These deposits can be classified into water-laid deposits of alluvial fan sequences (Bull, 1977).

Discussion Limited lateral extent, occurrence of debris flow

deposits, and wide range of variation in sediment size and sorting are consistent with the presence of randomly shifting ephemeral streams. These have been described as prominent features of "dry" alluvial fans (Schumm, 1977; Fraser and Suttner, 1986).

Considerable amounts of interbedded purplish siltstone (Facies M1) in sections of Sinchon (Sc) and Ogokri (Og) may be related to periodic sheet- wash along the sloping alluvial fan surface. In semi-arid environments, there is sufficient sedi- ments available for transport in suspension during large discharge events such as storms or floods (Sutherland and Bryan, 1989). The sheetwash sus- pended load entirely (98%) consists of silt and clay size material, which may contribute to the formation of purplish siltstone beds between depo- sitional periods of gravelstone or (gravelly) sand- stone beds.

Facies Association II: Ephemeral channel fills

Occurrence and interpretation The Facies Association II consists mainly of

massive (Facies S1)/stratified (Facies GS and $2) sandstones and is significantly developed in the central part of the basin (sections of Kwangpum, Annubpum and Namdaechun). The sandstone bodies are bounded by scoured base and grada- tional (indistinct) flat upper boundary with pur- plish siltstone (Facies M1). Although accurate width/thickness ratios of sandstone bodies cannot be measured due to the limited exposure, they generally show sheet-like geometries (Friend et al., 1979) (Fig. 12). Each sheet sandstone body fines upward; the discontinuous, clast-supported grav- elstone (Facies Gla ) in the lower part is overlain by indistinctly stratified/cross-stratified gravelly sandstone (Facies GS) which is in turn overlain by massive sandstone (Facies SI) (Figs. 4 and 12). Upper parts of the sheet sandstone bodies are less well organized, reflecting that channel becomes wider and shallower through aggradation until it is abandoned (Moody-Stuart, 1966; Tunbridge, 1984).

Such fining-upward sequence is similar to "tran- sitional sequence II" of Steel and Aasheim (1978) which is supposed to form in shallow channels during flood periods. Miall (1973) interpreted two fining-upward cycles (conglomerate~coarse to medium sandstone--*silty sandstone, and pebbly sandstone--*silty sandstone) as flood deposits. Presence of vertically overhanging (and interdigi-

150 C.W. RHEE AND S.K. CHOUGH

" . .......... ~i~fi:.

Fig. 12. Sketch and photograph of sheet-like stacked sandstone beds (Facies $1 and GS) embedded in purplish siltstone bed (Facies M1). For location (Kp-4), see Fig. 3. Hammer is 33 cm long.

tating) lateral margins and dominance of stratified units indicate repeated erosion and successive fill- ing. Allen (1962) interpreted sequential deposits (conglomerate resting on an erosional surface~ sandstone~siltstone) to be formed by torrential flows in distributaries swept into floodplains after the crevassing. They may represent a major diver- sion of flow, avulsion during flooding (Bridge, 1984).

Discussion

Vertical accretion deposits, which are repre- sented by relatively thin (up to 2-3 m thick) sandstone bodies with a fining-up trend, indicate that Facies Association II was mainly deposited in ephemeral channels (braided type) formed by cata- strophic flood events (Fig. 13). Spore and pollen from the Kyongsang Basin indicate that a semi- arid, warm climate was prevalent in Korea during the Early Cretaceous (Choi, 1985). On the other hand, the Pyonghae area with ubiquitous siltstone beds was prone to overland flows during rainfalls. Relatively high relief can be inferred from the

presence of local but thick debris flow deposits. Combined with dominant overland flow due to poor permeability and high relief, flash floods in small basins in semi-arid regions lead to cata- strophic response of stream channels (Baker, 1977). Catastrophic responses to large floods frequently result in shift or formation of channel networks (Stewart and LaMarche, 1967; Costa, 1974; Baker, 1977; Gupta, 1983; Nanson, 1986; Zawada and Smith, 1991). During subsequent periods of less discharge and less abundant sediment supply, the channels were mostly filled with stratified sand- stones (Facies $2 and GS). Each channel might have extended to braided channels upstream and diminish or split distally due to downstream shoal- ing and decline in flow strength. Such channels are apt to be shifted toward local depressions by a successive large flooding. These channel-fills are analogous to "type B channel fill" of Schumm (1960) which represents successive deposition of sands downstream, progressively increasing the concentration of finer sediment. On the other hand, the sandy channel fills indicate that sand-size sedi-

C R E T A C E O U S P Y O N G H A E S E Q U E N C E , SE KOREA: T E R M I N A L FAN FACIES 151

J o I

~ ~ : '~<z.L I I I M

. . . . . . . . ly I ,' '~: "

....... 7" /~" ~" ~!'.,

Fig. 13. Sediment transport and deposition in ephemeral (braided) channels during normal and low to intermediate floods. Sediments (sands and gravels) are mainly transported through confined tributary channels (thick arrow) which are initiated by preceding floods (Fig. 15). Other channels represent diversions of flows. Frequent overflows during flashy floods (small arrows) significantly contribute to the deposition of fine-grained sediments outside of the active channel floor.

ments were consistently supplied during aggrada- tion, perhaps as sandy washes upstream (Love, 1983).

Facies Association III: Floodplain deposits

Occurrence and interpretation The Facies Association III is dominanted by

purplish siltstone (Facies M1) and lenticular sand- stone beds (Facies S! and $2; Fig. 13). The facies M1 commonly contains sand grains, gravels and occasional calcareous nodules (Fig. 11). Internal depositional boundaries are indistinct except where lenticles of sandstone (or gravelstone) are inter- vened. Locally dark mudstones (Facies M2) (a few decimeters thick) are intercalated. The sandstone and gravelstone lenticles are laterally discontinu- ous (< 10 m long) and range in thickness from 20 to 100 cm. This association is dominant in the southern part of the basin, most likely representing floodplain environments.

Isolated, clast-supported gravelstone/massive sandstone facies embedded in purplish siltstone probably represent vertical overbank accretion (Fig. 14) (Costa, 1974; Ritter, 1975; Stene, 1980). The gravel lenses in fine-grained deposits indicate

Fig. 14. Purplish siltstone bed (Facies MI) contains dispersed gravels and sand grains as well as sandstone beds similar to gutter casts (lower and upper sandstone beds; Facies S1). Scale is 20 cm long. Location (Nc-5) is shown in Fig. 3.

the removal of loose sediments from the active channels due to increased discharge and stream power. These interpretations suggest that depos- ition of Facies Association III can be related to exceptional fluvial events with large discharge, e.g., floodings (Schumm and Lichty, 1963: Stene, 1980; Love, 1983) (Fig. 15).

152 C.W. R H E E A N D S.K. C H O U G H

Fig. 15. Sediment transport and deposition during large floods. Some sediments from floods drape over the former topography (stipple area: former channels) and headwater floods (large open arrows) spread out into the downstream area to form flood plain deposits. Confluent waning floods (small arrows) incise floodplain surfaces to initiate new channels. Further undercuttings would result in a progressive extension of channels upstream, whereby ephemeral channels in the floodplain are connected to upstream channels on alluvial fan surfaces (establishment of new channels).

Discussion Incision of the floodplain surface during large

(major) floods results in the formation of unstable, low-sinuosity (somewhat straight and steeper gra- dient) channels in which coarse sediments upstream (sands and gravels) are transported and deposited during recurrence interval of large floodings (Fig. 13). If the wide, shallow channels on an alluvial fan surface cannot effectively drain flashy, large discharge during storms or floods, overland flows and overflows tend to develop into sheet floods under conditions of poor permeability and high gradient (Hogg, 1982). Resultant flashy sheet- floods were most likely effective for the transport and deposition of considerable amounts of fine- grained sediments (Sutherland and Bryan, 1989). In the Pyonghae area, such widespreading, flashy flood events were most likely effective for the floodplain development (Schumm and Lichty, 1963; Burkham, 1972; Rachoki, 1981; Love, 1983; Wells and Dohrenwend, 1985) and contribute to the formation of thick siltstone beds downstream. Peak flows of flashy floods may remove coarse- grained sediments from active channels on surfaces of alluvial fan/floodplain and deposit them as

gravel patches/stringers or sandy lenticles on the floodplain surfaces downstream (Costa, 1974; Ritter, 1975; Stenel, 1980; Wells and Dohrenwend, 1985).

Conclusions

The Pyonghae sequence was formed in alluvial fans and adjacent floodplain environments. Alluvial fan deposits (Facies Association I) are represented by localized, poorly sorted, coarse- grained deposits along the basin margins, mainly formed by either debris flows (disorganized matrix- rich gravelstone: Facies G2) or channelized water- laid deposits (Facies GS). Toward the south (basin center), the alluvial fan deposits are replaced by extensive ephemeral channel deposits (sheet-like gravelly sandstones; Facies GS, S1 and $2) and thick floodplain deposits (siltstones; Facies MI and M2). These can be attributed to depositional features of a terminal fan system (Mukerji, 1976; Friend, 1978; Parkash et al., 1983; Tunbridge, 1984); shallow ephemeral channels (Facies Association II) of alluvial fans (Facies Association

C R E T A C E O U S P Y O N G H A E S E Q U E N C E , SE K O R E A : T E R M I N A L F A N F A C I E S 153

/ J

/ /

/ i /

+ .~

+ -I- + ~ ¢ ,

. - - : . . - . : : . - i : . - : ! : . . . . . . . . c . . , : . . . . . :,..< • .

o.. 5 -

~ . . • • ".ca.' . ' . i -

Fig. 16. Simplified depositional model of the Pyonghae sequence.

I) eventually terminated in adjacent floodplain (Facies Association III) (Fig. 16).

The floodplain deposits commonly contain sand grains and few calcareous nodules, and sometimes include sandstone/clast-supported gravelstone len- ticles, which are suggestive of flooding events and subaerial exposure. Thick purplish siltstone beds were deposited from sheetfloods. Large floods form ephemeral channel networks across the floodplain surface. Common occurrence of stratified units bounded by sharp bases and lateral margins and paucity of cross-stratified units indicate that the sandstone bodies (Facies Association II) were deposited in ephemeral streams that were undercut during floods. These processes can be explained by an alternation of low to intermediate floods (sheet floods) and large floods. Autocyclic events such as spasmodic floods in semi-arid climatic conditions are important for the development of terminal fan systems.

Acknowledgements

The study was supported through grants to Chough by the Seoul National University Development Foundation and partly by the private fund of Mr. S.Y. Hong.

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