Characteristics of Debrites, Turbidites, and Contourites in the Upper Ordovician Pingliang Formation along Southwestern Margin of the Ordos Basin, Western China

Deep-sea deposit is one important work in sedimentological field, but work on the ancient sedimentation is relatively rare, especially about relationship between sedimentary record and paleo-environment. Using outcrop, petrography, paleocurrent, trace element, δ13C, δ18O, 87Sr/86Sr, etc., deep-sea deposits had been researched in the Ordovician Pingliang formation along southwestern margin of the Ordos Basin. The Pingliang formation could be divided into five typical lithofacies. Lithofacies 1 is horizontal-bedded micritic limestone, mudstone, chert layer and tuff, which is correspongding to deep-water autochthonous deposits. Lithofacies 2 consisting of calcirudite with massive bedding, result from debris flow deposits. Lithofacies 3 is graded bedded calacrenite with imcomplete Bouma sequence indicating calacentie turbidites. Lithofacies 4 consisting of bioturbated and cross-lamination micrite with fine-coarse-fine sequence, it is due to low energy contour current. And cross-lamination lens-shaped calcarenite (lithofacies 5) being various scale of fine-coarse-fine sequence, is higher energy contour current deposits. The δ13C, δ18O, 87Sr/86Sr and trace elements of these lithofacies show distinctly different features. They are helpful to reconstruct paleo-environment including relative sea-level, paleosalinity, and paleoclimate. The relative sea-level showed rising feature consisting of 3 rising-falling cycles, paleosalinity also display 3 high-low cycles, and the paleoclimate conclude 4 dry-moist cycles. Contour current usually active dramatically when relative sea-level is rising, salinity changes obviously and environment is relatively moist, but turbidity current and debris flow are inverse.