PLATFORM EVOLUTION AND MARINE SOURCE ROCK DEPOSITION DURING THE TERMINAL SINIAN TO EARLY CAMBRIAN IN THE MIDDLE YANGTZE REGION

Abstract

Extensive thick black chert-shale successions with high abundance of sapropel-type organic matter occur in the Lower Cambrian in the middle Yangtze area, and hence they are considered as one of the most favourable source rocks in this region. In this paper, detailed crossing-platform-strike facies tracing from the platform margin through marginal slope, and petrological studies of the chert successions, integrated with geochemical approaches, were carried out to unravel the platform evolution and mechanism of source rock deposition. Our data indicate that hydrothermal venting of silica chimneys along the platform margin, likely induced and channeled by the synchronous deep-seated faulting there in the course of accelerated breakup of Rodinian supercontinent, occurred intensely in the Early Cambrian; it was proposed to responsible for the extensive deposition of the chert-black shale successions as that the hydrothermal venting could have provided abundant nutrient materials, anoxic fluids and greenhouse gases to the ocean and/or atmosphere, leading to the oceanic anoxia and increase in primary productivity, possibly with the help of upwelling currents, which were favourable for the accumulation and preservation of organic matter.