Development of a streamflow-dominated alluvial-fan system in the southwestern margin of Gyeongsang Basin (Lower Cretaceous): implications for initial basin-fill history

Abstract

This paper focuses on the initial basin-fill processes in an extensional nonmarine back-arc basin. Sedimentary facies analysis in the southwestern part of the Gyeongsang Basin (Jinju Subbasin), Hapcheon area reveals that the succession consists of four facies assemblages. The assemblage I mainly consists of disorganized conglomerate deposited by flash floods in a streamflow-dominated alluvial fan. The assemblage II is dominated by stratified conglomerates and massive, stratified, and cross-stratified sandstones, showing architecture of stacked channels and bars. The assemblage III largely comprises massive, stratified to laminated, and trough cross-stratified sandstones, which are organized into channel-fill and bar-accretion structures. It most likely formed in sandy sinuous rivers. The assemblage IV comprises gray mudstone interlayered with fine sandstone, representing water-logged floodplains. Conglomerate and sandstone bodies formed in the fluvial systems appear to be randomly distributed within floodplain fines, displaying a great lateral and vertical lithologic variation. This study helps understand the complexity of basin-fill history, i.e., switching of fluvial networks in time and space, in contrast to the simple notion of margin-parallel stacking of lithostratigraphic units.