Sequential Development of Alluvial/Lacustrine System: Southeastern Eumsung Basin (Cretaceous), Korea

Abstract

ABSTRACT In a strike-slip setting, depositional history of marginal basins depends on the fault movements. In order to constrain controlling factors for the sequence changes of alluvial/lacustrine systems in the southeastern part of Cretaceous Eumsung Basin, a pull-apart basin, southwestern Korea, we made a detailed analysis of sedimentary facies, paleoflow directions, clast compositions, and downstream changes in clast size. The conglomeratic sequence ( 2.2 km thick) in the study area can be organized into five facies associations representing distinctive depositional environments: (1) facies association (FA) I (debris-flow-dominated alluvial fan), (2) FA II (stream-dominated alluvial fan), (3) FA III (alluvial-fan fringe), (4) FA IV (alluvial plain), and (5) FA V (floodplain/lake). The entire succession can be divided into two stratigraphic units, the Dootasan sequence below and the upper Berjae above, on the basis of facies associations and stratigraphic architecture of channel-fill and floodplain deposits. The Dootasan sequence ( 1.2 km thick) is characterized by a basinward change from matrix-supported onglomerate (lower part) to ribbon-shaped channel fills and sheetlike beds encased in purple siltstone with a thinning-upward trend (upper part). The matrix-supported conglomerate facies is indicative of deposition in a debris-flow-dominated alluvial fan. The hollow-fill or sheetlike deposits in purple siltstone are suggestive of channel or sheetflood deposition on an alluvial plain. The Berjae sequence ( 1 km thick) is transitional from superimposed channel-fill/sheetflood deposits with infrequent intercalation of purple siltstone wedges to conglomerate beds encased in siltstone. The stacked channel-fill or sheetflood deposits with rare siltstone beds are indicative of deposition in stream-dominated alluvial fan. Each sequence shows a basinward spatial change in facies. Between the two sequences, green siltstone beds (max. 100 m thick) with conglomerates are intercalated, suggesting deposition in a lacustrine environment. The sequence transition from the Dootasan to the Berjae sequence along the basin-margin fault was most likely due to the changes in fault movements. The Dootasan sequence was formed during sinistral-fault movement with a relatively high rate of fault slip (early stage of fault activation), whereas the Berjae sequence was deposited in the late stage with a relatively low rate of fault slip.