Syn-sedimentary faulting on a lacustrine delta, northeast Ordos Basin, China
Jana M. Van Alstine
University of Wisconsin, Dept. of Geology & Geophysics
A large, continuous lacustrine delta outcrop is exposed in the western Daqing Shan along the northern margin of the North China Block, Inner Mongolia, on the northeast rim of the Ordos Basin. This unusually well-exposed deposit allows for mapping deltaic geometries in three dimensions and their response to syn-sedimentary faulting. The complicated sedimentary architecture is difficult to characterize using standard geophysical techniques – outcrop data is essential. Detailed mapping using high resolution photos, stratigraphic logs, and sequence stratigraphic concepts were employed to develop an understanding of the deltaic architecture and changing accommodation.
The delta is sand dominated, and exhibits complex transgressive and regressive geometries. Beds are laterally continuous, and the dominant lithologies range between siltstone and medium-grained sandstone. Distinct packages are differentiated between coarser, thicker bedded sandstones, and finer-grained, thinner bedded lithologies. Bedding angularities exist within these packages where coarser-grained units exhibit steep, prograding and aggrading sigmoidal geometries, while the finer-grained suites display subtle angular disparities in bedding between the base and the top. Extensional faults occur at 30 m spacing laterally across the outcrop. The faults cut sub-vertically through thick sandstone units and migrate sub-horizontally through siltstone intervals, creating a step-wise pattern with decreasing displacement up-section. The faults tip-out in sandstones and cause a local collapse within the bed. Faulting is the consequence of increasing sediment load and basin tectonics, and sedimentation responds with prograding geometries or chaotic dumping. This study will create an outcrop-based model that characterizes sedimentary response in a setting where faulting exerts a first-order control on accommodation.
AAPG Search and Discovery Article #90060©2006 AAPG Foundation Grants-in-Aid