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, representing 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.