Variability of Fan-Shaped Depositional Systems of the 4th Member of Eocene Shahejie Formation in Minfeng Half Graben, Bohai Bay Basin, Eastern China

Li, Zhixin; Yang, Wan; Zhang, Liqiang; Luo, Xiaorong; Liu, Shuhui; Luo, Hongmei

The geometry, thickness, lithology, and depocenter of fan-shaped depositional systems in continental rift basins vary from subaerial proximal to subaqueous distal settings and change during progressive basin filling. Those variations are caused by interplay of topography, provenance, subsidence, transport pathway, and sedimentary dynamics. This study characterizes and compares fan systems in two vertically-adjacent sequences on the steep margin of the Minfeng half graben to understand the processes and controlling factors. The study area covers 434 km2 and is bounded by an E-W and a N-S basement highs to the north and east, respectively. Fan systems in the lower sequence emanate from the NE corner around the structurally-weakened intersection of basement highs. They are elongate and confined to valleys in the upper slope and spread out and coalesce to form an apron downslope. Further downslope, fans from the north turned to the east to join those sourced from the east to form a single fan system. Topographic barriers may have forced this convergence. Those fans are interpreted as alluvial fans. The joined fan system further progrades westward into the basin to deposit a well-defined, elongate, tabular fan system ~10 km long and 2.5 km wide. It is interpreted as a subaqueous fan, filling a pre-existing fluvial valley on the underlying uniformity. The winding transport pathways, change of fan type, geometry, and location are determined by basin paleogeography, which were controlled by syndepositional rift tectonics. Fan systems in the upper sequence differ from those in the lower sequence in geometry, locality, type, and depocenter. They are not fully stacked on top of the lower fans, but step upslope due to a transgression after deposition of the lower sequence. Laterally-persistent lacustrine deposits, showing high-amplitude seismic reflection patterns, indicate this transgression. The basin-floor fan systems are absent probably because basin topography had been subdued by the transgression. As a result, upper fan systems mainly developed in front of basement highs, switched to the west along the E-W basement high, and are generally broader than the lower fans, indicating an expansion of the catchment basin. The migration of fan systems indicates a shift of depocenter. Our results demonstrate the significant roles of catchment evolution and pre-existing topographic controls on fan development and evolution.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013