Vol. 7, No. 3, p. 263275, September 2003
Contrasting basin fills in a strike-slip setting, Eumsung Basin (Cretaceous), Korea
ABSTRACT: In a strike-slip setting, depositional history of mar- ginal and central basins depends on the basin formation and basi- nal fault movements. The Eumsung Basin contains contrasting basin fills according to marginal settings. The southeastern part of the basin forms sequential development of alluvial/lacustrine sys- tems along transform margin, and the southwestern part consti- tutes synchronous development of alluvial-to-lacustrine systems in pull-apart margin. Along the basin margins and toward the basin center, both sequential and synchronous developments of the allu- vial and lacustrine systems have filled basinal accommodation spaces created by pull-apart opening. The formative processes of the basin were caused by the strike-slip fault movements and the accompanying changes in drainage network along the basin mar- gin. The overall development patterns of the depositional systems conform to the sinistral strike-slip fault activation during the Early Cretaceous.
Key words: basin analysis, strike-slip setting, pull-apart basin, basin fills 1. INTRODUCTION
Basin analysis comprises the processes of basin forma- tion, sedimentary basin filling, and the evolution of basin fill (Allen and Allen, 1990). In ancient basins, understand- ing of sedimentary fills is crucial for unraveling the formative and depositional history because the resultant successions conceal in situ dynamic evolutionary processes. Strike-slip basins including pull-apart basins, in particular, have invoked a significant attention for the recognition of development characters of the basin-filling sequences (Crowell, 1982;
Steel, 1976, 1988). Strike-slip basins can be differentiated by characteristic sedimentation patterns such as lateral off- set of source areas and the subsequent change in clast composi- tion, abrupt lateral facies variations, and cyclic accumulation of skewed alluvial deposits along the strike-slip faults (Read- ing, 1980; Christie-Blick and Biddle, 1985). Most sedimen- tary basins of strike-slip origin have, however, experienced complex function of formative processes (Crowell, 1987;
Ingersoll and Busby, 1995). On a basin scale, the problems stem from the fact that strike-slip basins constitute the most complicated of basin types (Nilsen and Sylvester, 1995).
In a strike-slip setting, synthetic and detailed studies of both surface and subsurface geology and sedimentology are mandatory for unraveling the formative and basin-fill pro- cesses of sedimentary sequences. In this respect, the Eum-
sung Basin provides an unusual opportunity to study depositional systems responding to pull-apart basin forma- tion, distribution of sedimentary facies, and characteristic depositional processes. In order to constrain controlling fac- tors for the sequential changes of alluvial and lacustrine systems, a detailed analysis of sedimentary architecture was instigated for facies distribution, paleoflow directions, clast compositions, and downstream changes in clast size. For subsurface configuration of the basin, high-resolution mag- netotelluric data were acquired across the northern and mid- dle parts of the basin. The profiles are integrately interpreted in terms of basin geometry, development history, and deposi- tional processes.
2. TECTONIC AND GEOLOGICAL SETTING
In the southwestern Korean Peninsula, Cretaceous non- marine basins were formed along a series of strike-slip faults trending NE−SW (inset in Fig. 1; Chun and Chough, 1992). The Kongju and Kwangju fault systems are charac- teristic of normal and en echelon patterns with high-angle transtensional faults. The Kongju faults were activated by sinistral strike-slip movements in the Late Jurassic to the Cretaceous, forming small-scale rhomboidal basins (Chun and Chough, 1992; Cluzel, 1992). These elongated basins were filled with alluvial to lacustrine sediments during the Early to Late Cretaceous, controlled by left-lateral strike- slip faults (Lee and Paik, 1990; Kim, 1996).
The rhomb-shaped Eumsung Basin (~7×33 km2 in area), located within the Kongju fault system (Fig. 1), is bounded by two left-stepping sinistral master faults which are in con- tact with foliated cataclasite, microbreccia, and mylonite in the basin margin (Precambrian gneiss and Jurassic granite) (Fig. 1; Chun et al., 1994; Choi, 1996). An analysis of mylonitic foliation and minor fault orientations suggests that the major faults have attitudes of N13oE/82oW in the eastern part and N38oE/75oE in the western part (Cheong, 1987), and indicates strike-slip or high-angle normal faults border- ing the basin margin. The Eumsung Basin contains the Cre- taceous Chopyong Formation (>8 km thick), consisting of seven lithologic units: volcanics (including andesite, basalt, and tuffaceous sediments), conglomerate, conglomerate/purple mudstone, purple mudstone, green mudstone, green-gray mudstone/gray sandstone, and dark-gray mudstone (Fig. 1).
Ryang, W.H.* Division of Science Education, Chonbuk National University, Jeonju 561-756, Korea
*Corresponding author: [email protected]
These nonmarine deposits were formed in alluvial-fan, allu- vial-plain, and lacustrine environments (Ryang, 1998). Flood- plain in alluvial plain was developed in the vicinity of the
basin margins where crevasse splay and avulsion processes were operative.
The coarse-grained deposits contain clasts of granite, gra- Fig. 1. Geologic map of the Eumsung Basin with magnetotelluric recording sites (solid squares), paleoflow direc- tion data (rose diagrams), and coring site (solid circle C1, ~250 m deep).
Magnetotelluric profiles ES1 and ES2 comprise 18 and 40 sites, respectively.
Inset represents distribution of Creta- ceous basins and fault patterns in the Korean Peninsula (modified after Chun and Chough, 1992; Kim et al, 1994; Korea Institute of Energy and Resources, 1995; Choi, 1996; Ryang and Chough, 1997).
Table 1. Description of sedimentary facies and inferred depositional processes.
Facies Type Description Interpretation
Disorganized conglomerate (C1)
Discontinuous beds (decimeters to a few meters thick); amalgamated with interbedded pebbly sandstone; clast-supported or matrix-rich; poorly sorted, very coarse to coarse sand matrix or occasionally purple (pebbly) siltstone matrix bearing cobble-size clasts; randomly oriented clasts; angular to subangular pebble- to cobble-size clasts
Deposition from high-concentration flow and reworking by stream flows; lag or entrapment on channel bottom or margin by high-magnitude flood flows
Matrix-supported disorganized conglomerate (C1a)
Decimeter thick; poorly defined beds showing partly inverse or inverse-to- normal grading; pebble- to cobble-grade and very angular to subrounded clasts;
sand to mud matrix; non-erosional base; present mainly in the basin margin
Deposition from visco-plastic debris flows
Clast-supported disorganized conglomerate (C1b)
Discontinuous beds of decimeter thick; amalgamated with interbedded pebbly sandstone; partly matrix-rich; poorly sorted, very coarse to coarse sand matrix or occasionally purple (pebbly) siltstone matrix bearing pebble-size clasts; randomly oriented clasts; angular to subangular pebble- to cobble-size clasts
Deposition from high-concentration flow and reworking by stream flows
Stratified conglomerate (C2)
Amalgamated; decimeter- to decameter-thick; indistinctly stratified or fre- quently low-angle cross-stratified; gravel patches with partly open-work fabric (up to 20 cm thick) are common; very coarse to coarse sand matrix; angular to subrounded, pebble- to cobble-size clasts; parallel-oriented elongate clasts [a(t)], occasionally imbricated [a(t), b(i)]
Bedload transport, traction of gravels and winnowed by stream flows
Conglomerate encased in siltstone (CE)
Present in hollows of purple or green (pebbly) siltstone; generally thick (up to several meters); ribbon-shaped or sheetlike geometry; sharp, erosional base and relatively diffuse upper boundary; amalgamated with pebbly sandstone;
disorganized, clast-supported in the lower part of hollows; crudely stratified or low-angle cross-stratified, matrix-rich in the upper part of hollows; very coarse to coarse sandstone matrix; occasionally inversely-to-normally graded; partly open-work fabric; pebble- to cobble-size clasts; parallel-oriented elongate clasts [a(t)]; partly imbricated [a(t), b(i)]
Water flood and subsequent stream flow;
coarse-grained deposits - channel lag or entrapment in scoured hollows; fine grains - rapid settling from overflow
Stratified pebbly sandstone encased in siltstone (PSE2)
Present in hollows of purple siltstone; sheetlike geometry; variable in thickness (up to 2 m); indistinctly stratified or cross-stratified pebbly sandstone; poorly to well sorted very coarse to medium sand matrix; conglomerate or pebbly sandstone layers commonly forms discontinuous stringers or patches
Unconfined high-concentration flow and subsequent stream flow
Pebbly siltstone (PZ) Variable in thickness (decimeters to several meters); poorly sorted;
disorganized and partly stratified; randomly dispersed clasts; includes sandstone stringers, isolated pebbly sandstone lenticles; gravel patches and trains
Muddy debris flow; rapid fall-out deposition from high-concentration flow
Purple sandy siltstone (Zp)
Purple (5R 3/4); ubiquitous facies; generally thick (a few meters); poorly sorted;
common inclusion of granules and sand grains; some calcareous nodules and sand-filled desiccation cracks
Rapid fall-out deposition of fine grains from tractive overflow on floodplain;
oxidizing condition during and after deposition
Green sandy siltstone (Zg)
Green (10G 4/2); thin to very thick (decimeters to a few meters); common inclusion of dispersed granules and sand grains
Rapid settling of suspended grains below water level in pond or lake; reduction condition during and after deposition Purple mudstone (Mp) Purple (5R 3/4); major facies in the depocenter; generally thick (tens meters);
occasionally alternated with green mudstone facies; homogeneous; some calcareous nodules
Gradual settling of fine grains on lacus- trine margin, controlled by fluctuating lake level; oxidizing condition during and after deposition
Green mudstone (Mg) Green (10G 4/2); major facies in the depocenter; generally thick (tens meters);
occasionally alternated with purple and dark-gray mudstone facies;
homogeneous; some calcareous nodules; some pyrite crystals
Gradual settling of fine grains below water-level in pond or lake; reduction condition during and after deposition Dark-gray
mudstone (Md)
Dark gray (N3); very thick (tens meters); occasionally alternated with green mudstone facies; homogeneous; some calcareous nodules
Gradual settling of fine grains and organic materials below water level in lacustrine depocenter where lake level was perennial; reduction condition during and after deposition
nitic gneiss, and banded gneiss derived from the basement, and volcaniclasts from the calc-alkaline volcanic rocks along the basin margin (Fig. 1; Lee et al., 1992). The conglom- erate facies are transitional to pebbly sandstone, pebbly silt- stone, and mudstone facies toward the basin center, showing a lateral facies change (Ryang and Chough, 1997, 1999;
Fig. 1). Ripple marks and calcareous nodules are present in the dark-gray mudstone, and desiccation cracks and calcar- eous nodules are present in the purple siltstone. According to Song et al. (1990) and Chun et al. (1994), the various fossil assemblages of plants (Conifers and Ginkgoales), invertebrates (estherids) and microfossils (charophyta) are suggestive of a temperate climate and a fresh-water lacus- trine environment. In the southern part of the basin, charo- phyta fossils in the green mudstone were dated as Hauterivian-Aptian age (Choi et al., 1995). The main char- acteristics of each facies are briefly described and inter- preted in Table 1.
3. SOUTHEASTERN PART: SEQUENTIAL DEVEL- OPMENT OF ALLUVIAL/ LACUSTRINE SYSTEM 3.1. Facies Associations and Depositional Environments
In the southeastern part of the basin, the conglomerate, conglomerate/purple mudstone, and purple mudstone mem- bers (Fig. 1) are divisible into six lithofacies units: con- glomerate, conglomerate/pebbly siltstone, conglomerate/purple siltstone, conglomerate/green siltstone, pebbly sandstone/
purple siltstone, and purple mudstone. Conglomerate facies are transitional to pebbly sandstone, pebbly siltstone, and mudstone facies toward the basin center, showing a lateral facies change (Fig. 2). On the basis of spatial change in facies and bed geometry, the deposits can be grouped into five facies associations representing five depositional envi- ronments: (1) Facies Association (F.A.) I (debris-flow-dom- inated alluvial fan), (2) F.A. II (stream-dominated alluvial fan), (3) F.A. III (alluvial-fan fringe), (4) F.A. IV (alluvial plain), and (5) F.A. V (floodplain/lake). Channel-fill depos- its in the alluvial-fan fringe or the alluvial plain are divisible into two types on the basis of channel geometry and internal organization (e.g., Jones, 1992; Mjos et al., 1993): (1) trunk (or main) channels (>3 m thick), and (2) crevasse/distrib- utary channels (<2 m thick).
3.2. Basin-fill Sequences
In the southeastern part of the Eumsung Basin, the con- glomeratic sequences are present along the basin margin, successively overlapping toward the northeast and north (Ryang and Chough, 1997). These sequences are differen- tiated on the basis of facies associations and stratigraphic architecture of channel-fill and floodplain deposits (Fig. 2).
Each sequence shows a basinward spatial change in the
presence of each facies association. The lower Dootasan Sequence is characterized by a basinward change from debris-flow-dominated alluvial-fan deposits (F.A. I) to channel fills in the alluvial plain (F.A. IV), whereas the upper Berjae Sequence is dominated by stream-dominated alluvial-fan (F.A. II) to alluvial-fan-fringe (F.A. III) and alluvial-plain deposits (F.A. IV).
According to the counting data of clasts in each section, the Dootasan Sequence largely contains about 50% basalt/
andesite clasts and 45% granite/gneiss clasts, whereas the Berjae Sequence contains about 40% basalt/andesite clasts and 55% granite/gneiss clasts. Between the two sequences, green siltstone and pebbly siltstone beds are intercalated (Fig. 2). An abrupt vertical facies change from conglomer- ates encased in purple siltstone (F.A. IV) to amalgamated decameter-thick conglomerate bodies (F.A. II) is observed.
3.3. Basin-fill Model
In small-scale strike-slip basins of a transtensional regime, depositional styles of alluvial/lacustrine systems are gener- ally in response to fault movement (Mann et al., 1983;
Dunne and Hempton, 1984). The fault movement results in cyclic skewed alluvial-fan bodies in the direction opposite to that of the basin-floor displacement (Steel, 1988). The noncyclic features and changes of depositional architecture in alluvial suites are poorly known in terms of the relation- ship between lateral and vertical displacements caused by the fault-slip movements. In the southeastern margin of the Eumsung Basin, the development of two skewed alluvial systems was most likely due to the strike-slip fault move- ments and the accompanying changes in drainage network along the basin margin (Ryang and Chough, 1997). The consistent northeastward/northward paleocurrents also sug- gest a northeastward and northward basin tilting along the fault-bounded margin. On the other hand, the deposition of the green siltstones in the upper part of the Dootasan Sequence reflects the formation of a small-scale lake (lake- level rise), which is suggestive of back faulting in the basin margin (e.g., Heward, 1978). The overlapping patterns of the basin-fill sequences along the southeastern margin (Fig.
2) are also indicative of northward migration of the depo- center. The migration of depocenter was due to the sinistral strike-slip fault movement and facilitated shift in mountain valley front and catchment area along the basin margin.
A three-stage model is constructed for the sequential dep- osition in response to the sinistral fault displacements (Ryang and Chough, 1997; Fig. 3). Deposition in Stage 1 is represented by the Dootasan Sequence, in which dominant matrix-supported conglomerate bodies along the basin mar- gin are interpreted as representing deposit of a debris-flow- dominated alluvial-fan environment. In the alluvial-plain envi- ronment, conglomeratic channel fills in a purple siltstone matrix reflect episodic channel shifting of braided streams
Fig. 2. Measured stratigraphic section (Z, siltstone; C, conglomerate) in the southeastern part of the Eumsung Basin. Thickness is in meters. Paleof- low directions are synthesized in rose diagrams of Fig. 1, and north is towards the top of the figure. Note lat- eral facies transition from Section Dj to Section Yc and from Section Jc-4 to Section Jc-5. Note stratigraphic transi- tion from Dootasan Sequence (con- glomeratic channel fills encased in purple siltstone beds: Facies CE, Zp) to Berjae Sequence (mainly thick amal- gamated conglomerates: Facies C1, C2). Homogeneous or faintly laminated green siltstone beds (Facies Zg) are intercalated between the two sequences.
Facies codes are given in Table 1.
and well developed and/or unconfined floodplain environ- ments in a setting of rapid aggradation (e.g., Bentham et al., 1993). The high aggradation rate may have been caused by rapid fault displacement in the basin margin, related to intense fault activity in the early stage of strike-slip fault movements (e.g., Schubert, 1982). The faulting involved dominantly lateral displacement with relatively high rate of fault slip during the early stage of fault movements. In Stage 2, deposition of green siltstone with conglomerate beds in the upper part of the Dootasan Sequence is sugges- tive of the formation of a small-scale lake or of abrupt lake- level rise due to the intensified fault activity, i.e., back fault- ing. It is indicative of the onset of a new depositional
regime (e.g., Blair, 1987; Steel, 1988; Frostick and Reid, 1989). The overall scour surfaces in the upper part of the Dootasan Sequence can be interpreted as incision surfaces (sensu Salter, 1993), judging by the abrupt change of strati- graphic facies and architecture (Fig. 2). In Stage 3, the marked decrease in abundance of purple siltstone beds in the Berjae Sequence probably reflects intense reworking by recurrent channel shifting within stream-dominated alluvial systems under relatively low aggradation rate. The fault activity became relatively stable and resulted in dom- inant vertical displacement with relatively low rate of fault slip. This is probably the late stage of basin-margin fault activation (e.g., Royden, 1985).
Fig. 3. Basin-filling model for the development of sedimentary facies and stratigraphic architecture and corresponding plan view in the southeastern part of the Eumsung Basin (Stages 1, 2, 3) (Ryang and Chough, 1997). Alluvial depocenter migrates northeastward or northward with time (1, 2, 3), and successively stacked alluvial systems form the Dootasan and Berjae sequences. Alluvial systems are younger north- ward. Solid arrows indicate transtensional faulting with left-slip motion, and relatively little solid arrows in Stage 3 indicate decrease of displacement on sinistral faults. Open arrows show the dominant directions of sediment transport.
4. SOUTHWESTERN PART: ALLUVIAL-TO-LACUS- TRINE SYSTEMS IN PULL-APART MARGIN
4.1. Pull-apart Margin
The southern part of the basin is characterized by irreg- ular marginal boundary and volcaniclastic sediments, unlike those of the northern part. According to the sinistral angular
relationship of basinal faulting in a transtensional regime (e.g., Wilcox et al., 1973; Christie-Blick and Biddle, 1985), the southeastern part was under compressional fault regime, whereas the southwestern part experienced fracturing in an extension-dominated regime. Within the obviously rhomb- shaped basin, coarse-grained systems along the irregular- bounded southwestern margin developed relatively trans- verse to the master fault (Fig. 1). This is indicative of sed- Fig. 4. Measured stratigraphic sections in the southeastern part of the Eumsung Basin, representing alluvial-to-lacustrine systems from the basin margin to the depocenter (M, mudstone; C, conglom- erate). Paleoflow directions are synthe- sized in rose diagrams of Fig. 1, and north is towards the top of the figure.
Facies codes are given in Table 1.
imentation in a pull-apart margin.
4.2. Facies Associations and Depositional Environments In the southwestern part of the basin, the deposits are divisible into six lithofacies units: conglomerate, conglom- erate/purple siltstone, sandstone/green siltstone, purple mud- stone, purple/green mudstone, and green mudstone (Fig. 1).
Conglomerate facies are transitional to sandstone, purple mudstone, and green mudstone facies toward the basin center, forming alluvial-to-lacustrine systems (Figs. 1 and 4). Based on the spatial changes in sedimentary facies and bed geome- try, the deposits can be grouped into three facies associations representing three distinct depositional environments (Ryang and Chough 1999): (1) Facies Association I (volcaniclastics- dominated alluvial fan), (2) Facies Association II: (alluvial plain), (3) Facies Association III (floodplain/lake).
4.3. Alluvial-to-Lacustrine Systems
The paleoflow directions and the facies transition from conglomerate and conglomerate/purple siltstone to pebbly sandstone/siltstone and purple/green mudstone indicate that the alluvial systems developed from the west to the north- east (060o) and southeast (135o), showing a fan-shaped pat- tern (Figs. 1 and 4). These conglomerate-to-mudstone units (~3 km thick) sequentially filled the basin margin, the dep- ocenter migrating slightly northeastward with time. Based on basinal structure, facies associations, changes in clast composition, and stratigraphic architecture of the channel- fill and floodplain deposits, the succession is divisible into three units: 1) Ochang, 2) Baeti, and 3) Googokri (Ryang and Chough, 1999; Fig. 4).
4.4. Basin-fill Model
The rhomb-shaped Eumsung Basin is one of the well-pre- served basins in a discontinuous fault trace without severe structural and stratigraphic deformations (inset in Fig. 1). A sedimentological study on the southwestern part of the basin provides insight into the sequence development and changes in characteristics of alluvial-to-lacustrine systems that were caused by fault slip in a pull-apart margin (Ryang and Chough, 1999). The entire succession (~ 3 km thick) is divisible into three large-scale vertical and lateral units: the lowermost Ochang (~1.5 km thick), the medial Baeti (~1 km thick), and the uppermost Googokri sequences (~0.5 km thick), on the basis of the changes in clast composition and stratigraphic architecture (Fig. 4).
In the southernmost part, the Ochang Sequence was prob- ably formed in the early stage of the basin development accompanied with volcanism in the fault-controlled pull-apart margin. The Baeti Sequence was then followed, filling the adjacent basinal sliver in the pull-apart margin (Fig. 5). The
distinction between the two sequences is based on the inter- vening thick purple siltstones and basinal structure (Fig. 4).
The Ochang-Baeti transition with time is attributable to simul- taneous division of the basement block in the pull-apart mar- gin (e.g., Aydin and Nur, 1982). The initial fault geometry could constrain the development of depositional systems as well as drainage networks. The Googokri Sequence formed on another basement sliver of relatively small scale (Fig. 5), which implies a partial shift in depocenter from a pull-apart to a transform margin. The overlapping patterns on the Baeti Sequence along the margin are suggestive of northward tran- sition of the basin-margin faulting (Fig. 5). The Baeti-Goo- gokri transition was due to the sinistral strike-slip fault movement, which probably caused shifts in catchment area along the basin margin. This transition is evinced by the changes in clast composition and facies associations.
In the three alluvial-to-lacustrine systems, the consistent bas- inward paleocurrents (Fig. 1) suggest a basinward tilting along the fault-bounded pull-apart margin. In addition, downstream changes in clast size and facies represent a lateral transition from a coarse-grained marginal system to a fine-grained dep- ocenter system. The paleocurrent data and overall facies tran- sitions indicate that the three systems (Figs. 1 and 5A) converge to the southwestern depocenter (Fig. 5B). In the pull- apart margin, tectono-sedimentary model largely comprises simultaneous and/or sequential developments of the alluvial- to-lacustrine systems in response to the normal fault slip Fig. 5. (A) Plan view of basin-margin development in the south- western part of the Eumsung Basin (Ryang and Chough, 1999).
Note three basinal slivers developed in the pull-apart margin. (B) Basin-filling model for the development of sedimentary facies and stratigraphic architecture in the southwestern part. Note three allu- vial-to-lacustrine systems developed in the pull-apart margin (1, Ochang; 2, Baeti; 3, Googokri systems). Open arrows show the dominant directions of sediment transport. Solid arrows indicate transtensional faulting with left-slip motion.
including synthetic faults (Ryang and Chough, 1999). Sedi- mentation patterns are characterized by transverse-to-margin channel network and relatively synchronous development of depositional systems. In addition, the clast composition records drainage lithology in the pull-apart margin as well.
5. CENTRAL−−−−NORTHERN PART: THREE-DIMEN- SIONAL CONFIGURATION OF A PULL-APART BASIN
Since Crowell’s (1974, 1982) models for the formation and filling of strike-slip basins, the basin analyses have been based on the present map-view geometry and surface geology (Aydin and Nur, 1982; Mann et al., 1983; Christie- Blick and Biddle, 1985; Nilsen and Sylvester, 1995). The formative and filling processes, however, demand 3-dimen- sional data of deeper parts of the basin combined with detailed data of surface geology. In this case, the new high- resolution magnetotelluric (MT) technique can help analyze basinal scale architecture without costly burden of seismic reflection survey. According to the surface sedimentological study, the pull-apart basin was filled with alluvial to lacus- trine sediments, controlled by left-lateral strike-slip faults (Ryang and Chough, 1997, 1999). In this study, we illustrate two profiles of high-resolution magnetotelluric data acquired across the northern and middle parts of the basin and inter- pret integrately in terms of basin geometry, development his- tory, and depositional processes (Ryang et al., 1999; Fig. 1).
5.1. Interpretation
In the Eumsung Basin, the high electrical resistivity (>5,000 Ohm-m in Fig. 6) indicates unweathered basement rocks of granite (Jurassic) and gneiss (Precambrian), whereas the low resistivity (<5,000 Ohm-m) largely represents sedimentary rocks of conglomerate, sandstone, and mudstone (e.g., Palacky, 1987; Fig. 6). This interpretation conforms to the results of direct-current resistivity method (resolvable depth, ~250 m) (Kim et al., 1998).
In profile ES1 (Figs. 1 and 6), the maximum basin depth is about 700 m. The basement in the western part of the basin shows a gentle gradient, whereas that in the eastern part is relatively steep. The surface sedimentological data (Fig.
1) suggest that axial channel systems were formed in the western part of the basin, whereas the coarse-grained deposits filled the eastern part. The channel systems drained to the lacustrine depocenter to the south where lake level was relatively perennial (Fig. 7).
In profile ES2 (Figs. 1 and 6), two basins are present: a main basin in the central-western part and a subbasin in the east. In the main basin, the basement is about 4 km deep and shows a high slope gradient in the western part, whereas low in the eastern part. The low-resistivity areas in the western part (open arrows in Fig. 6) most likely repre- sent detachment zones of the eastern basinal faults. The
blocks of high resistivity in the upper part (solid arrows in Fig. 6) are interpreted as an extension of the northern base- ment (in profile ES1) that formed during the basin subsid- ence. The subbasin in the eastern part occurs as a graben- type depression at the depth of 1 km, as an extension of the southeast basinal fault (Fig. 7). The basement high (thin arrow in profile ES2) probably acted as a barrier for the transport of coarse-grained sediment toward the west. The sys- tem developed to the north along the basin boundary, as ver- ified by the surface sedimentological data (Figs. 1, 6 and 7).
An abrupt change in basement depth (~3 km deep) at a distance of 4 km (between profiles ES1 and ES2) is typical of rapidly subsiding (central) blocks in pull-apart basins (e.g., Aydin and Nur, 1982; Mann et al., 1983). These sec- tions also show an asymmetric cross-basinal form, which probably affected the southward development of the axial channel systems in the northern part (Figs. 1 and 7).
5.2. Basin-fill Model
The two MT survey lines in the Eumsung Basin (Fig. 1) show a distinct difference (~3 km deep) in basin depth Fig. 6. Two-dimensional resistivity cross-sections, based on observed MT data (cross-basin lines ES1 and ES2 in Fig. 1). Maximum depth is ~700 m in ES1, whereas it is ~4 km in ES2. Note depth changes (~3 km deep) between the two lines (~4 km distance). Pseudosec- tions of the recording cells in sections ES1 and ES2 calculated using the function of inverse distance to a power. Note two basins in section ES2: main basin in the west to center and subbasin in the east. In the main basin, the basement shows a high slope gra- dient in the western part, whereas a relatively low gradient in the eastern part. Thin arrow indicates basement high in the central- eastern part. The subbasin in the east occurs as a graben-type depression at the depth of 1 km. Open arrows indicate low-resis- tivity areas in the western part, and solid arrows indicate circled blocks of high resistivity in the upper part.
between the northern (ES1) and middle (ES2) parts of the basin (Fig. 6). They represent a 3−D basin configuration reflecting downfaulting of the central blocks. This change in depth along with the faults or stratigraphic unconformi- ties has not been recognized in the surface geologic data (Fig. 1). On the contrary, some syn-depositional faults along the basin margin in the northern and southern parts were thought to represent dynamic sedimentation during the basin evolution (Choi, 1996; Ryang and Chough, 1997). The two profiles combined with these geologic characteristics sug- gest that the central blocks (profile ES2) synthetically sub- sided due to the pull-apart opening (Fig. 7), not that the northern blocks (profile ES1) were uplifted at the post-dep- ositional stage. These fault movements also conform to the large-scale longitudinal faults in the central part (profile ES2 in Fig. 6) which seem to have accommodated the prin- cipal offset during the basin formation (Fig. 7). Using sand- box analogue models of pull-apart basins, McClay and Dooley (1995) suggested that the longitudinal cross-basin faults could accommodate the main offset of basinal fault displacement. The small-scale subbasin in the southeastern part probably formed as a consequence of an extension of the basinal faults (Fig. 7).
The new high-resolution MT survey seems to effectively configure 3−D architecture of basin fills and is very useful for integrate modeling of dynamic processes of sedimenta- tion in strike-slip basins. The high-resolution MT imaging
enables to configure 3−D internal architecture of the pull- apart Eumsung Basin (Cretaceous) (Fig. 7). The data sug- gest that the basin experienced a pull-apart opening with rapid subsidence of the central blocks, an asymmetric cross- basinal extension, and a formation of a subbasin in the southeast. Combined with the outcrop data on asymmetric lithofacies distribution, facies transitions, and paleoflow directions of the alluvio-lacustrine systems, the data help explain basin-fill processes during the basin formation.
6. BASIN FORMATION AND FILLS
In the mid-southern part of the Korean Peninsula, Creta- ceous nonmarine basins were formed along the strike-slip faults trending NE−SW (Kongju and Kwangju fault systems) (inset in Fig. 1). The strike-slip movements were mostly sinistral and activated in the Late Jurassic to Cretaceous, forming small-scale rhomboidal basins (Chun and Chough, 1992; Cluzel, 1992). The rhomb-shaped Eumsung Basin is also bounded by two left-stepping sinistral master faults which are in contact with the foliated cataclasite, microbreccia, and mylonite in the basin margin (Precambrian gneiss and Jurassic granite) (Fig. 1). The Eumsung Basin within the Kongju fault system (inset in Fig. 1) experienced pull-apart opening with rapid subsidence of the central blocks, asym- metric cross-basinal extension, and formation of a subbasin in the southeast.
Fig. 7. Schematic basinal form and filling model for the development of basin architecture and depositional systems (Ryang et al., 1999). Coarse- grained alluvial systems in the north- east develop westward and converge into an axial channel system in the northwest basin. The axial channel system develops southward, alternat- ing with dark-gray mudstone beds of lacustrine origin. Solid arrows indicate transtensional faulting with left-slip motion, and open arrows show domi- nant directions of sediment transport.
The sinistral pull-apart opening of the Eumsung Basin caused the northward sequential development of the alluvial and lacustrine systems in the southern part of the basin and the southward development of the axial channel systems in the northern part. In the south central part of the basin, two subbasins were developed: a main basin in the central-west- ern part and a subbasin in the east. The large-scale intra- basinal faults in the central-eastern part are interpreted as longitudinal cross-basin faults accommodating the principal offset during the basin formation. The subbasin in the east- ern part was probably formed as a graben-type depression, which may have resulted from an extension of the southeast basinal faults. The resultant basement high probably acted as a barrier for the transport of coarse-grained sediment toward the west. Instead, the system developed to the north along the basin boundary.
The large-scale sequences in the basin are interpretative of sequential development of alluvial/lacustrine systems along the transform margin (southeastern part) and of syn- chronous development of alluvial-to-lacustrine systems in the pull-apart margin (southwestern part). In the southeast- ern part of the basin, the alluvial systems comprise alluvial fans at the margin and parallel-to-margin channel systems (006o−067o) in alluvial-plain environments. The succession (~2.2 km thick) is divisible into two large-scale vertical units: the lower Dootasan (~1.2 km thick) and upper Berjae (~1 km thick) sequences, based on the stratigraphic archi- tecture and facies association of channel, sheetflood, flood- plain, and lacustrine deposits. In the southwestern part of the basin, the succession (~3 km thick) is divided into three vertical/lateral units on the basis of the changes in clast composition and stratigraphic architecture: Ochang (~1.5 km thick), Baeti (~1 km thick), and Googokri (~0.5 km thick) sequences. The three alluvial-to-lacustrine systems show transverse-to-margin channel network (060o−135o). The northern part of the basin is characterized by conglomerate units in both margins (east and west) and alternating units of pebbly sandstone and dark-gray mudstone beds in the northern basin center. An axial channel system shows a south- westward paleoflow direction (~242o) along the basin-margin fault. In the northern part of the basin center, paleoflow direc- tions are largely biaxial (east-to-west), where the channel sys- tems probably originated from the northeastern coarse-grained systems. These sequences also include characteristic sedimen- tary structures and bed geometry.
The presence of cut-and-fill structures with erosional bases, poorly to moderately sorted sandstone matrix, and domi- nant stratification indicates that these deposits were formed by flooding with high discharge and channelized stream flows. The lower part of the channel fill is commonly char- acterized by disorganized conglomerate bodies, which are usually overlain by parallel- and cross-stratified conglom- erates. These channel fills were probably formed by multi- stage deposition such as scour fill or gravel lag by flooding
and subsequent infilling by stream flows. The lateral grain- size variation, the coarse-grained openwork fabric, and the upward fining of the hollow fill can be explained by dep- osition from waning floods and subsequent reworking/win- nowing by stream flows. The sheetlike geometry is indicative of unconfined ephemeral flows. The discontinuous coarse- grained beds may have been emplaced by overflows with- out significant scour, probably because of the lack of chan- nel confinement. Ribbon or sheet conglomerate bodies with irregular bases are indicative of scouring by turbulent flows and subsequent filling. The single-story units show a fining- upward trend, which suggests deposition from episodic flows (e.g., storm or flood). The fining-upward sequences of con- glomerate bodies (>3 m thick) encased in the purple silt- stone can be interpreted as shifting and abandonment of trunk channels. Some symmetrically scoured hollows (<2 m thick) with sheetlike wings suggest that they acted as dis- tributary or crevasse-splay channel systems. The sheetlike crevasse splays with irregular and sharp bases were formed by sudden influxes of unconfined sediment-laden floodwater.
The isolated small-scale pebbly sandstone lenticles are inter- preted as overbank deposits by flash flooding. Away from the trunk channel, the distributary flow became semi-confined or unconfined, forming sheetlike coarse-grained beds. These bas- inward facies changes represent deposition from basin-margin trunk stream to basinward distributary stream.
The depositional processes include debris flow, high-con- centration flow/sheetflood, stream flow, and fine-grained settling. The matrix-supported inversely graded textures are suggestive of debris-flow origin, and the presence of the inter- vening stratified conglomerates is indicative of reworking by stream/flood flows. The poorly sorted sandy matrix, the ran- domly oriented clasts, and the distinct or partly scoured bounding surface are suggestive of high-concentration flow/
sheetflood during sporadic flooding events. The stratified, openwork fabric and the textural segregation of gravels and sands indicate deposition in gravel-bed streams during waning stages of floods. The sharp bases, distinct upper boundaries, poor sorting, and the limited extent of the beds are suggestive of erosion, short-distance transportation, and deposition by ephemeral flows rather than perennial flows. The poorly sorted purple sandy siltstone includes some calcareous nod- ules and desiccation cracks, which are indicative of subaerial exposure. The coarse-grained layers in the green and dark- gray mudstones with sharp and/or diffuse boundaries were deposited in a fluctuating lake of subaqueous and subaerial environments. In the basin center, the development of green or dark-gray mudstone facies is indicative of settling deposition in poorly drained floodplain and lake environments.
7. CONCLUSIONS
The Eumsung Basin contains contrasting basin fills according to the marginal settings. The southeastern part of
the basin forms sequential development of alluvial/lacus- trine systems along the transform margin, and the southwestern part constitutes synchronous development of alluvial-to-lacus- trine systems in the pull-apart margin. The sinistral pull- apart opening of the Eumsung Basin caused the northward sequential development of the alluvial and lacustrine systems in the southern part of the basin and the southward develop- ment of the axial channel systems in the northern part.
The development of alluvial and lacustrine systems in the basin reflects the formative processes of the strike-slip fault movement and pull-apart opening and the accompanying changes in drainage network along the basin margin. The consistent paleocurrent directions suggest that basin tilting occurred along the fault-bounded margin. The overlapping patterns of the large-scale sequences along the basin margin are also indicative of continuous migration of the depo- center. The depositional migration was caused by the sinistral strike-slip fault movement and facilitated shift in mountain val- ley front and catchment area along the basin margin.
ACKNOWLEDGMENTS: I am very grateful to Drs. S.K. Chough (Seoul National University), D.K. Cheong (Kangwon National Uni- versity), C.W. Rhee, J.S. Kim (Chungbuk National University), and H.
Shon (Paichai University) for useful discussion and comments.
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Manuscript received June 30, 2003 Manuscript accepted August 5, 2003
