Contrasting development patterns of crevasse channel deposits in cretaceous alluvial successions, Korea

The Eumsung and Pyonghae basins (both of Cretaceous age) in Korea comprise a thick alluvial sequence of conglomerate, sandstone, and purplish siltstone. Laterally impersistent, gravelly sandstone beds can be classified into two types based on bed geometry, internal structure and occurrence: (a) vertically stacked, sheetlike bodies (facies 1), and (b) migratory, ribbon-shaped bodies (facies 2). Facies 1 is characterized by symmetrical channel geometry with a large ratio of width to thickness of sandstone beds which are mostly stratified and bounded by relatively distinct lower and upper boundaries. They are frequently found in aggradational style. Facies 2 shows asymmetric geometry with a relatively small ratio of width to thickness of sandstone beds. It comprises massive, cross-stratified units bounded by sharp basal contacts and gradational upper boundaries, which resulted from laterally shifting mode. Both types represent crevasse/distributary channels developed in floodplain environments. Facies 1 is suggestive of gradual migration of crevasse/distributary channels under the conditions of overall aggradation, whereas facies 2 represents discrete, abrupt diversion of crevasse channels. These features are due to differences in floodplain gradients, paleohydrology and trunk channel behaviour.