Controls and evolution of fan delta systems in the Miocene Pohang Basin, SE Korea

Abstract

The Miocene Pohang Basin, southeastern Korea, occurs in the eastern part of the NE-SW-trending Yangsan Fault which has experienced right-lateral strike-slip deformation since the Eocene. During the Miocene, however, the strike-slip movement was negligible and instead was under extensional regime. Large-scale drainage systems were developed at the junction between the strike-slip zone and the oblique transfer faults, acting as major sources for coarse-grained sediments. High rate of sediment supply and rapid subsidence of the hanging wall resulted in the progradation of large-scale Gilbert-type fan deltas (Doumsan, Duksung and Gohyun systems) which all show a radial distributional pattern. Along the steeply inclined footwall scarps, created by NE-SW-trending faults, the drainage systems were relatively small. Here, low rate of sediment supply formed scree-apron-type (Malgol and Yugye systems) and steep-faced slope-type (Maesan system) fan deltas.Based on variations in sedimentary facies and distribution, architecture and overlapping patterns, the fan delta systems in the Pohang Basin can be divided into four stages of evolution. During the early stage (Early Miocene), alluvial fans prograded into shallow-marine setting. In the Doumsan and Duksung systems, high rate of sediment supply and rapid rise of relative sea level resulted in the transformation of shoal-water-type to Gilbert-type foresets. In the Malgol and Maesan systems, rapid sea-level rise accompanied with low rate of sediment supply caused transformation of alluvial fans to scree-apron-type and steep-faced slope-type fan deltas, respectively. In the second stage, subsidence of the hanging wall formed large-scale truncation surfaces in the Doumsan and Duksung fan deltas. Coarse-grained sediments were continuously supplied in the Doumsan system, whereas only fine-grained sediments were deposited in the Duksung system. In the Malgol system, sediment supply was almost terminated in this stage. In the third stage (Middle Miocene), sediment supply rate of the Doumsan fan delta decreased abruptly and the system was overlain by fine-grained Gilbert-type deposits. The Duksung fan delta was overlain by the Maesan and Gohyun systems. The Maesan system prograded further basinward. During the last stage (Late Miocene), sediment supply of the Doumsan and Maesan systems was also terminated and the entire basin was draped by fine-grained biogenic materials (mainly diatoms).