Sublacustrine Fan Systems of Bohai Rift Basin, China

Robert R. Remy, Dag Nummedal, Ming Pang, Fuping Zhu, and Yong Mo

Sublacustrine fans are a significant component of the syn-rift Shahejie Formation and post-rift (thermal subsidence stage) Dongying Formation in Liaodong Bay, one of the major sub-basins of Tertiary Bohai (North China) rift basin in northeastern China. Sublacustrine fans are major petroleum reservoirs in other parts of the Bohai Basin. Fans in Liaodong Bay are characterized by a wide variety of grain sizes (mud to pebbles), textures, and sedimentary structures, numerous scour surfaces, rip-up clasts, load features, and other evidence for rapid high-energy sedimentation, and relatively rare traction-current structures. Core analyses indicate that coarse-grained sedimentation was dominated by high- and low-density turbidity currents and frictional and cohesive debris flows. Although sediment gravity flows are capable of transporting coarse-grained sediment long distances, most fans pinch-out into mudstone within a few kilometers.

The fans are characterized by irregular, discontinuous, strong to weak seismic reflectors. Fans on hanging wall and footwall slopes exhibit updip onlap and down-dip downlap, whereas smaller fans in lakefloor settings commonly have a mounded geometry. Relief on deltaic and fan delta clinoforms adjusted for compaction indicate that sublacustrine fans accumulated in water depths that generally equaled or exceeded several hundred meters.

Sublacustrine fans differ from their marine counterparts in several ways: (1) they are significantly smaller than marine fans, (2) underflows are probably a major sediment transport mechanism in deep fresh-water lakes, and (3) sublacustrine fans in Liaodong Bay accumulated during all phases of lake-level fluctuations and may have been most abundant during lake high stands, whereas current sequence stratigraphic models place marine fans in the lowstand systems tract.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California