Architectural analysis of fluvial sequences in the northwestern part of Kyongsang Basin (Early Cretaceous), SE Korea

Abstract

The northwestern part of Kyongsang Basin (Early Cretaceous) largely comprises interbedded sandstones and mudstones of fluvial depositional environments with local basin-margin conglomeratic deposits of alluvial fan systems. These non-marine successions are divided into 5 successive stratigraphic units, each of which is unique in facies assemblages and architecture of sandstone bodies. One stratigraphic unit (Sinpyong-Anpyong unit) is documented in this paper with detailed architectural analysis of sandstone bodies. The deposits of the Sinpyon-Anpyong unit are divided into three major components: thick sandstone, thin sandstone, and mudstone-dominated bodies. The thick sandstone bodies (2-47 in thick) comprise stacked element groups bounded by laterally persistent erosional surfaces, probably representing superposition of channel belts. Within individual element groups, elements are laterally superposed, indicating growth and superposition of bars and movement of associated channels. The thickness of bar deposits and dip angles of laterally inclined strata suggest that channels were a few to 10 m in depth and 9 m to several tens of meters in width. Paleocurrent data indicate southward- or southeastward-draining channel systems. The thin sandstone bodies (generally <2 m thick) are characterized by planar bed geometry, representing sand sheets and splays on floodplains. The mudstone-dominated bodies mostly consist of massive and laminated, gray mudstone beds. In the northern (proximal) part, the bodies comprise mainly purple siltstone. Laminated limestones and stromatolites are commonly associated with gray mudstone beds. The mudstone-dominated bodies represented well-drained floodplains in the proximal part and poorly-drained (water-logged) floodplains with shallow lakes in the distal part. The thick sandstone bodies, formed in major channels and channel belts, are characterized by lateral superposition of numerous bar and channel deposits, limited extent of individual lateral-accretion sets, and the presence of mid-channel bar deposits, suggesting deposition in braided rivers. Detailed reconstruction of bars and channels reveals a range of processes including growth, coalescence, and erosion of bars and migration, switching, and filling of channels. Lateral accretion of bars and the associated channel migration might have been principal processes in the Sinpyon-Anpyong river systems. Bars were commonly coalesced by lateral superposition, forming large bar complexes. (C) 2001 Elsevier Science B.V. All rights reserved.