Fault Controlled Sandstone Distribution on a Lacustrine Delta: Ordos Basin, North-central China

Van Alstine, Jana M.¹, Alan R. Carroll¹ (1) University of Wisconsin-Madison, Madison, WI

The Ordos Basin of north-central China is rimmed by uplifts on its northern margin that represent multiple inverted Triassic and Jurassic elongate lacustrine basins. A large, continuous delta outcrop is exposed in the western Daqing Shan range on the north-east edge of the Ordos Plateau. This unusually well-exposed delta allows us to examine the details of a feed-back system between sedimentation and syn-depositional faulting. The delta displays response of sedimentation to structural growth in a stratigraphic interval that lies between a transition from deeper basinal lacustrine to fluvial plain and coal with significant interbedded conglomeratic intervals. The delta is sand-dominated, with complex transgressive and regressive geometries. Packages are differentiated between 2 to 4 meter thick coarser, thicker bedded sandstones, and 10 meter thick finer-grained, thinner bedded lithologies. Bedding angularities exist within these packages where coarser-grained units exhibit steep, eastward prograding and aggrading sigmoidal geometries of the delta top. Finer-grained facies of the delta slope display subtle angular disparities in bedding between the base and the top. Syn-sedimentary 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. Faults act jointly as conduits and barriers to sediment flow, controlling the amount of bypass or sediment ponding. Sedimentation in the basin was controlled locally by an evolving slope profile, but was also integrated into an eastward-flowing drainage network on the margin of the Ordos Basin that responded to regional tectonic activity.