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. Chough 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 Glb 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.