Distinctive characteristics of a streamflow-dominated alluvial fan deposit: Sanghori area, Kyongsang Basin (Early Cretaceous), southeastern Korea

Abstract

In the northwestern part of the non-marine Kyongsang Basin (Early Cretaceous), conglomeratic deposits occur at intervals along the northern basin margin and grade basinward into interbedded sandstone and mudstone. One of these conglomeratic deposits, referred to as the Sanghori deposit, is 150 m thick and characterized by amalgamated sheets of conglomerate and sandstone (facies assemblages 1 and 2) alternating with interbedded sandstone and purple siltstone (facies assemblage 3). Facies assemblage 1 comprises clast-supported, disorganized and organized sheet conglomerates interbedded with massive to stratified sandstones. This assemblage is interpreted as the product of proximal gravel-bed stream deposition. Facies assemblage 2 consists of trough and planar cross-stratified and horizontally stratified sandstones. It represents sandy braided-stream deposits in which sand bars and dunes were common. Sedimentary facies and architecture of the coarse-grained facies assemblages suggest that the gravel-bed and sandy braided streams were wide and shallow, and rapidly aggraded during flash floods. Facies assemblage 3 consists of interbedded purple siltstone and tabular sandstone which show evidence of weak pedogenic modification and bioturbation. These characteristics are interpreted as the result of floodplain processes. Sediments were supplied from the north and dispersed radially, forming a streamflow-dominated alluvial-fan system. The Sanghori alluvial system was most probably fed from a large drainage basin, resulting in the predominance of streamflow deposits and rounded clasts. The alternation of the gravel-bed stream and floodplain deposits was due to autocyclic switching of active fan sectors under the conditions of relatively slow and steady basin subsidence.