Non-marine successions in the northwestern part of Kyongsang Basin (Early Cretaceous): Fluvial styles and stratigraphic architecture

Abstract

Non-marine successions in the northwestern part of Kyongsang Basin (Early Cretaceous) are divided into successive stratigraphic units based on facies assemblages and architecture of sandstone bodies. In the present study, two stratigraphic units (Sinpyong-Anpyong and Jotap units) are documented in detail in terms of fluvial architecture. The Sinpyong-Anpyong unit is divided into thick sandstone, thin sandstone, and mudstone-dominated bodies, representing major channels, sand splays, and floodplains, respectively. The channel sandstone bodies are characterized by superposition of numerous bars and channels deposits formed in braided rivers. The mudstone-dominated bodies largely consist of purple siltstone in the northern (proximal) part and gray mudstone in the southern (distal) part, reflecting well-drained floodplains and poorly drained floodplains with local shallow lakes, respectively. The Jotap unit is divided into thick sandstone, interbedded sandstone/siltstone, and siltstone-dominated bodies, respresenting major channels, proximal floodplains, and distal floodplains, respectively. The thick sandstone bodies consist of trough cross-stratified sets and horizontally stratified sets with common internal scour surfaces and lenticular beds of purple siltstone, indicative of ephemeral channels dominated by small-scale bedforms. The siltstone-dominated bodies consist of purple siltstone and suggest well-drained floodplains. Along with the marked change in fluvial styles, the successions show systematic variations in the stacking patterns of channel bodies and the distribution of floodplain/lake facies. The basal and uppermost parts of the Sinpyong-Anpyong unit are characterized by sourceward expansion of distal, poorly drained floodplain/shallow lake facies and the low proportion and connectedness of channel bodies. Overlying these gray mudstone-dominated sequences, the middle part of Sinpyong-Anpyong unit and Jotap unit show increases in the proportion, connectedness, and grain size of channel bodies and basinward progradation of proximal, well-drained floodplain facies. Such a stratigraphic architecture can be attributed to the fluctuations in the ratio of accommodation space/sediment supply, regulated by repeated basin subsidence.