Geochemical, palynological and organic matter characteristics of the Upper Triassic Bagong Formation from the North Qiangtang Basin, Tibetan Plateau
Abstract The Upper Triassic Bagong mudstones are considered to be the most significant source rocks in the eastern part of North Qiangtang Basin, Tibetan Plateau. This study investigates the vertical variation of total organic carbon (TOC) contents as well as mineral compositions, carbon isotope, palynology, major and trace element concentrations of the Upper Triassic sediments from Geladandong area of North Qiangtang Basin to understand the mechanism of organic matter accumulation. The TOC concentrations of Bagong Formation mudstone range from 0.77 to 1.37% (mean 1.04%), while the Boni La Formation micritic limestone displays lower TOC contents (0.18–0.37%, average 0.26%). Terrigenous flux proxies including Al, Si, Ti, Th, and Zr concentrations suggest that relatively high detrital flows originate from aluminosilicate compositions during the Upper Triassic sediment deposition. The paleoredox indicators (MoEF/UEF and Corg/P) reflect an oxic-suboxic condition during deposition of the Bagong mudstones and Boni La micritic limestones. The correlation between TOC and Porg and Babio indicates that the Upper Triassic Bagong mudstone has a higher primary productivity compared with most of the Boni La micritic limestone. The palynological investigation and climate proxy (B/Ga and illite/smectite) reflect warm and humid to hot and sub-humid climate condition during the Bagong mudstone deposition. The warm and humid climate condition is favorable for the survival of organisms, which in turn would enhance the primary productivity of surface water. Relatively high primary productivity is the main factor for the organic carbon accumulation in the Upper Triassic Bagong mudstones. In addition, the relatively fast sedimentation rate resulting in preservation a high TOC content of Bagong mudstone deposit in an oxic-suboxic condition. The low primary productivity and the high carbonate production lead to the dilution of organic matter with respect to the low TOC content of Boni La micritic limestone.