Paleogene Lacustrine Mass Transport Complexes and Turbidites at the Steep Margin of Minfeng Half Graben, Bohai Bay Basin

Abstract

Sedimentology, stratigraphy, spatial distribution, and temporal evolution of mass transport complexes (MTCs) and turbidites in lacustrine rift basins are complex, because of highly variable accommodation space, frequent lake level fluctuations, and short sediment transport distance in small drainage basins. These gravity deposits are significant components of basin fills at steep margins, but rarely documented in the subsurface. MTCs and turbidites are interpreted in 7 syn-rift sequences in Paleogene lacustrine deposits, Minfeng half graben, Bohai Bay Basin, using core, log, and 3-D seismic data. MTCs are composed of 0.1–10s m thick conglomerate and sandstone units intercalated with dark-gray shales, reflecting parent rocks. They have blocky log patterns of low acoustic and neutron, and high gamma-ray and resistivity values. MTCs are faulted and folded, and show chaotic, discontinuous, and randomly-oriented seismic reflections of variable amplitude, indicating a contorted nature. Individual reflections are 100s m long and 10s m thick, as mass-transport blocks. The MTCs are ~200 m thick, 2.5 km long, and wedge-shaped with a convex top and a fairly flat base in dip sections and a mounded shape in strike sections. They are located at the toe of a concave escarpment. The above features suggest that slumping had occurred along the escarpment and deposited MTCs downslope. The other sediment gravity deposits at the steep margin occur as coarse-grained turbidites. They are upward-fining successions and 0.01–1 m thick; and contain conglomerate and sandstone capped by thin shale. Beds are massive, graded, planar, or cross-stratified with a sharp or erosional base and a gradational top. The turbidites have similar log patterns to MTCs. However, they comprise fan complexes, showing different seismic facies as continuous to discontinuous, progradational clinoforms with intermediate amplitude. Laterally persistent shale and evaporite onlap the fan outer boundaries. The fan complexes occur at the mouths of gullies and valleys, and are separated by ridges, indicating syn-depositional topographic control on fan development. At a sequence scale, turbidite fans are progradational or retrogradational, whereas MTCs do not show a systematic pattern. The results demonstrate the diversity and complexity of gravity-driven deposits in ancient lacustrine basins.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015