Redox conditions and climate control on organic matter accumulation and depletion during the Toarcian in the Qiangtang Basin, eastern Tethys

Abstract

The Toarcian (Early Jurassic) sediments are regarded as the most significant hydrocarbon source rocks in the Qiangtang Basin, Qinghai–Tibet Plateau. In this study, geochemical and mineralogical data are reported from the Middle to Late Toarcian organic-lean sediments in the Qiangtang Basin, eastern Tethys. Combined with previous studies, the factors that control organic matter (OM) accumulation during the Toarcian are investigated. The Early Toarcian organic-rich sediments were deposited in stratified water column with oxygen-deficient bottom water, which was conducive to OM preservation. A warm and humid paleoclimatic condition favored the growth of organisms, which in turn could enhance the surficial biotic productivity. During the Middle to Late Toarcian, oxic waterbody was widely distributed in the Qiangtang Basin. This condition was unbeneficial to the preservation of OM. During this interval, a cold–arid climate had an adverse effect on organism growth, which in turn would reduce the initial productivity of surface water. The consistently low Ti/Al and Si/Al ratios revealed restricted but rather homogeneous clastic influx conditions during the Toarcian. This may suggest that the dilution of terrigenous material input had a little influence on OM accumulation. In addition, the differential enrichment of OM between shales and marls should attribute to carbonate dilution. These studies emphasize that redox conditions and paleoclimatic conditions were likely responsible for OM accumulation during the Toarcian.