--> Abstract: Lacustrine Sequences of the Oligocene Nanpu Depression, Bohai Basin, China, by D. Nummedal, C. Wu, and S. Xue; #91012 (1992).

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ABSTRACT: Lacustrine Sequences of the Oligocene Nanpu Depression, Bohai Basin, China

NUMMEDAL, DAG, and CHANGLIN WU, Louisiana State University, Baton Rouge, LA, and SHUHAO XUE, Research Institute of Petroleum Exploration and Development, Beijing, People's Republic of China

Seismic reflection profiles, well logs, and cores from the Nanpu "depression" of China's Bohai basin provide a complete database for documentation of the lacustrine sequence stratigraphy in one of the world's largest rift systems. The major phase of rifting spanned the late Eocene and Oligocene Epochs. By Miocene time, broad regional postrift subsidence characterized the Bohai basin. The Late Eocene and Oligocene section attains a maximum thickness of about 4000 m in the deepest parts of rift-generated half-graben. The section can be divided into four (locally five) seismic sequences of probable tectonic origin; each one containing fine-grained lacustrine and coarse-grained subaqueous fan, deltaic, and fluvial(?) facies.

Repetitive facies stacking patterns ("sequences") can be recognized at three scales: (1) At the scale of the whole basin fill there is an upward succession of fluvial, through (deep?) lacustrine, deltaic, and back into fluvial facies. This trend is consistent with published general facies trends in many other rift basins. (2) At the scale of each individual seismic sequence (hundreds of meters, a few million years) there is a typical vertical succession of inferred fine-grained lacustrine strata grading laterally and vertically into deltaic and fluvial(?) deposits. (3) At the scale of about 80 m (100,000 years?) there are repetitive alternations of lacustrine, laminated mudrocks and coarse-grained sediment gravity flow deposits interpreted as sublacustrine fans. We hypothesize, but ca not yet prove, that sequence trends on scales 1 and 2 reflect changes in tectonic rates of subsidence, and that patterns on scale 3 (and smaller) reflect changes in lake level and sediment supply in response to (Milankovitch) climate cycles.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)