Fluvial plain/lacustrine facies transition in the Cretaceous Sindong Group, south coast of Korea

Abstract

Along the south coast of Korea, a 250 m stratigraphic interval corresponding to the uppermost Hasandong and lowermost Jinju Formations of the Cretaceous Sindong Group has been studied in terms of facies transition. The interval displays a passage from a fluvial plain facies association consisting of floodplain, levee, crevasse splay and channel facies to a lacustrine facies association composed of crevasse splay, channel, overbank sheet splay, marginal lake, open lake and stream mouth bar facies. Between these two associations a transitional facies association occurs in which some facies of both associations alternate. In the lacustrine association, shallow marginal lake deposits including channel facies are gradually replaced upwards by deeper open lake deposits made up of open lake and stream mouth bar facies. During the Hasandong/Jinju period, small channels, which originated in the uplifted areas beyond the fault(s)-bounded western margin of the Gyeongsang Basin, flowed through alluvial fans developed along the fault(s) and a fluvial plain to a lake which occupied the central part of the basin. As a result, the lake shoreline was non-coincident with the basin margin. Sinuosity of the streams increased downstream, reaching a maximum near the shoreline, but decreased in the marginal lake areas. The fluvial plain was progressively submerged by the transgression of a non-deltaic shoreline, resulting in a deepening of the depositional sites from a fluvial plain through a marginal lake to an open lake setting. Intermittent channel overflows deposited levee and crevasse splay facies in the fluvial plain, and crevasse splay and overbank sheet splay facies in the marginal lake. Stream mouth bars were developed at the ends of the sublacustrine channels by hyperpycnal density flows operated mainly during floods. The non-deltaic shoreline was formed due to: (1) maintenance of shallow and wide marginal lake areas; (2) dispersal of sediments by small channels; (3) continuous shifting of the channels and their mouths; and (4) direct transport of coarser sediments to the open lake and their subsequent wide dispersions.