Factors controlling the organic matter accumulation across the Pliensbachian-Toarcian transition in the Qiangtang Basin, Tibetan Plateau

Abstract

The Lower Jurassic oil shales in the Bilong Co area are the most important hydrocarbon source rocks in the southern Qiangtang Basin, Tibetan Plateau. The onset of the deposition of these shales was closely related to the global warming during the Pliensbachian-Toarcian transition; however, the mechanism of organic matter accumulation in these shales remains poorly constrained. In this study, a multiproxy geochemical study was performed on the organic-rich shales spanning the Pliensbachian/Toarcian boundary in the Bilong Co section in order to assess the organic matter sources, detrital influx, paleosalinity, paleoredox conditions, and paleoproductivity during the deposition of these shales. The distribution patterns of the n-alkanes, as well as the H/C and O/C ratios of the kerogen, suggest that the organic matter primarily originated from marine plankton/algae. The detrital proxies (K/Al, Si/Al, Ti/Al, Zr/Al, and Zr/Rb) indicate relatively stationary sea level and stable continental weathering intensity along the section. The paleosalinity proxies indicate that the organic-rich shales were deposited under marine salinity conditions with little freshwater input. The covariation patterns of the redox proxies (MoEF, UEF, ReEF, DOPT, and Corg/Ptot) suggest that the organic-rich shales were deposited under suboxic conditions, while the MoEF-UEF and Mo-TOC plots suggest a weakly hydrographically restricted depositional setting. The productivity proxies (Ptot, CuEF, ZnEF, and CdEF), as well as CdEF/MoE-CoEF*MnEF plot indicate low-to-moderate primary productivity, indicating that the organic matter accumulation was driven by preservation rather than by productivity. The development of the ocean deoxygenation in the Bilong Co area was probably the result of sluggish oceanic circulation and an enhanced thermocline on the shelf between the Qiangtang central uplift and the Bangong-Nujiang Ocean. The climatic changes during the Pliensbachian-Toarcian transition led to the onset of organic matter accumulation in the Bilong Co area, while the hydrographically restricted depositional setting associated with good preservation conditions may have been the primary reason for the massive deposition of the Lower Jurassic oil shales in the southern Qiangtang Basin.