Depositional environments, architecture, and controls of Early Cretaceous non-marine successions in the northwestern part of Kyongsang Basin, Korea

Abstract

In the northwestern part of Kyongsang Basin, southeast Korea, non-marine successions of interbedded sandstone and mudstone are divided into successive stratigraphic units on the basis of facies assemblages and sandstone architecture. Parts of the successions (Sinpyong–Anpyong and Jotap units), formed in southward- or southeastward-draining fluvial systems, are documented in detail in terms of macroform-scale to stratigraphic-level architecture for the assessment of fluvial styles and evolution of non-marine depositional environments. The Sinpyong–Anpyong unit consists of thick sandstone bodies (2–47 m thick), thin sandstone bodies (generally <2 m thick), and mudstone-dominated bodies. The thick sandstone bodies are characterized by superposition of numerous bars and channels, indicative of braided-channel systems. The channels are estimated at a few to 10 m deep and 9 m to tens of meters wide. The thin sandstone bodies comprise planar beds of massive, horizontally stratified, and trough cross-stratified sandstones, interpreted as sand sheets and splays. The mudstone-dominated bodies largely consist of purple siltstone in the northern (proximal) part and gray mudstone in the southern (distal) part, representing well-drained floodplains and poorly drained floodplains with local shallow lakes, respectively. In the basal and uppermost parts of Sinpyong–Anpyong unit, the distal poorly drained facies (gray mudstone) expands to the northern basin margin. The Jotap unit, overlying the Sinpyong–Anpyong unit, comprises thick sandstone bodies (2–10 m thick), interbedded sandstone/siltstone bodies, and siltstone-dominated bodies. The thick sandstone bodies are dominated by the deposits of small bars and dunes with common lenticular beds of purple siltstone and internal scour surfaces, suggestive of bedform-dominated channels with ephemeral discharges. The channels are estimated at generally a few meters in depth and a few tens to hundreds of meters in width. The interbedded sandstone/siltstone bodies comprise thin, sheet-like and lenticular sandstone beds (<2 m thick) interbedded with purple siltstone, interpreted as channel-proximal floodplains where sand was frequently deposited as sand sheets and splays, levees, and crevasse-channel fills. The siltstone-dominated bodies consist mostly of purple siltstone with subordinate thin sandstone beds, representing channel-distal, well-drained floodplains. Along with the marked change in fluvial styles, the successions show systematic variations in the proportion and connectedness of channel sandstone bodies and the distribution of floodplain/lake facies. The basal and uppermost parts of the Sinpyong–Anpyong unit are characterized by the sourceward expansion of distal, poorly drained floodplain/shallow lake facies and the low proportion and connectedness of channel bodies. Following 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 expansion 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.