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. (C) 2003 Elsevier Science B.V. All rights reserved.