The Cretaceous Pyonghae sequence, southeast Korea: Terminal fan facies

Abstract

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 G1b 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.n Facies Association II mainly occurs in the central part of the basin and comprises sheet-like sandstone bodies (Facies GS, S1 and S2) intercalated with purplish siltstone beds (Facies M1). 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 III 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.