SPATIAL DISTRIBUTION AND TEMPORAL EVOLUTION OF FAN-SHAPED DEPOSITIONAL SYSTEMS IN THE EOCENE FOURTH MEMBER OF SHAHEJIE FORMATION, MINFENG HALF GRABEN, BOHAI BAY BASIN, EASTERN CHINA

Spatial distribution and temporal evolution of fan-shaped depositional systems (FSDS) in rift basins are critical to understand basin filling architecture and history. Various FSDS were identified in the Minfeng half graben to investigate the filling history of a continental rift basin.

FSDS within four vertically-adjacent sequences of the study interval were interpreted from termination patterns, seismic facies, and isochron mapping of 3-D seismic data. They can be divided to 3 types. Type 1 is fan delta, characterized by a wedge-geometry along the fan axis, thickening to the border fault, distributed along basement uplift, and coalesces laterally. It shows mediate amplitude seismic reflections with distinct sigmoidal prograding clinoforms downlapping to basal boundary. Type 2 is delta, which incises into fault escapement with a confined front and gradually extends basinward. The delta lobe can be narrow or flaring. This geometric disparity may have arisen from: destruction process, such as wave modification, or underlying topography. Seismic reflections of delta are contorted, chaotic, variable in amplitude, and show downlap termination patterns. Type 3 is elongate subaqueous fan, located in the basin center, parallel to the basin axis, and sourced from contiguous fan delta, showing low-mediate sub-parallel reflections in seismic sections. The FSDS vary in type and geometry, formed by different depositional processes, and distributed in diverse localities.

Stratigraphically, the occurrence of fans changes systematically from lower to upper sequences. In the basal sequence, FSDS dominate in the eastern graben with development of fan delta and subaqueous axial fan. However, FSDS are poorly-developed in the second lower sequence. Strata thicken to the NW-SE and gradually thin toward the NE and SW margins. In the third sequence, FSDS consist of fan delta and delta. They shift westward and upslope. The absence of axial fan suggests a deficiency in subaqueous sediment supply or a lack of a pre-existing topographic low. In the uppermost sequence, FSDS shift eastward, and extend upslope and basinward. Fan deltas overlap in downslope direction forming bajadas. The different FSDS in the four sequences highlight the catchment evolution and topographic controls on fan spatial distribution and temporal evolution.