LITHOFACIES AND DEPOSITIONAL ENVIRONMENTS OF THE ORDOVICIAN MAJIAGOU GROUP, QISHAN AREA, ORDOS BASIN NORTH CHINA

We studied two sections of the Middle Ordovician Majiagou Group, at Jueshan, Qizhan of the southwest Ordos Basin, to describe and interpret its lithofacies and gamma ray profile in high resolution.

The lower section is composed of mottled facies, with intervals of microbial and planar laminite. The mottled facies is bioturbated dolomudstone, interpreted to have formed in a subtidal restricted marine lagoon. The microbial laminated facies is dolomudstone containing fenestra and mudcracks, interpreted to have formed in a tidal flat. Alternations the facies indicate transgressive-regressive cycles. Scour surfaces, monomict breccia and flat pebble conglomerates indicate episodic storm reworking in the lagoon and tidal flat.

The upper section is comprised of argillaceous lime mudstone to wackestone, skeletal packstone-grainstone, and polymict breccia. Argillaceous mudstone to wackestone contains planar lamination and radiolarians; interpreted to represent pelagic, dysoxic deposition on a deep carbonate slope. Skeletal grainstone-packstones punctate the succession and contains diverse fragmented shallow-marine fossils interpreted to have been transported from platform margin to the slope as turbidites. Polymict breccias are poorly sorted, containing shallow and deep marine granule- to boulder-sized clasts with lime mud matrix interpreted to represent subaqueous debris flows. Fining upward grainstone caps breccia beds.

Using gamma ray data, we interpret that our sections occur in the uppermost Majiagou Group (zones 5 and 6). We interpret the dramatic shift from shallow-marine lagoon to deep-marine slope represents tectonic convergence of a terrane and downwarping of the southwest Ordos Basin as the central basin was uplifted and karstified. The occurrence of volcanic ash supports arc volcanism. Our studies are consistent with previous paleogeographic reconstructions that show volcanism and deepening in this area in the Late Ordovician Pingliang time. However, our data indicates the deepening started earlier and resulted in the drowning of the Majiagou Platform. Petroleum exploration within the Majiagou in the southwest part of the basin should be based on different play concepts (e.g. dolomite intercrystalline rather than karst porosity) than those applied in the central basin.