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.