Organic matter accumulation in Barremian marine black shales of the Qiangtang Basin, China

Abstract

Barremian organic-rich black shales are significant source rocks in the eastern Tethyan Qiangtang Basin. Based on petrological, inorganic and organic geochemistry analyses, the black shales are divided into three units from bottom to top. Unit 1 micritic limestones exhibit high total organic carbon (TOC) contents and Type II₂ kerogen, indicating a mixture of marine microalgae and land plants. Unit 2 black shales show the highest TOC contents, predominantly Type II₁ kerogen suggesting marine microalgal source. In contrast, Unit 3 marls have relatively low TOC contents, Type II₂ kerogen, indicating mixed terrestrial and marine OM sources. The black shales show a low organic maturity, and the hydrocarbon generation potential of the black shale and micritic limestone samples is substantially higher than that of the marl samples. Palaeoredox proxies indicate that Unit 1 deposited under dysoxic-anoxic conditions, and Unit 2 formed under anoxic-euxinic conditions, while Unit 3 deposited under oxic-suboxic conditions. Primary productivity proxies reflect high productivity in Units 1 and 2, and low productivity in Unit 3. Rb/K and total sulfur/TOC ratios suggest brackish environment in Units 1 and 2 and brackish or seawater condition in Unit 3. Hydrothermal activity during Unit 2 black shale deposition provided essential nutrients for phytoplankton in the photic zone, leading to high OM production. Upwelling/restriction proxies imply deposition under moderately restricted conditions for Unit 1, strongly restricted conditions for Unit 2, and upwelling/weakly restricted conditions for Unit 3. Palynological analysis indicates a warm, semi-humid to humid temperate climate during deposition of Units 1 and 2, contrasting with a hot, arid to semi-arid climate during Unit 3 marl deposition. OM accumulation of Unit 1 micritic limestone was primarily controlled by stratified dysoxic-anoxic conditions, high primary productivity, warm humid/semi-humid climate, and moderate watermass restriction. For Unit 2 black shale, the main controlling factors were stratified anoxic-euxinic environment, warm humid temperate climate, strongly restricted water condition, and intermittent strong hydrothermal activity. During Unit 3 marl deposition, low primary productivity, an oxygen-rich water environment leading to OM degradation, combined with a hot arid/semi-arid climate, resulted in organic-lean deposition.