Geochemistry And Paleoenvironment Of Black Shales In Mid-Yangtze Region During Cambrian Terreneuvian–Series 2

The Early Cambrian black shales in the middle Yangtze region are good materials to explore the formation mechanism of black shales and the ocean environment in the middle Yangtze region. Based on the research of sedimentology, rare earth elements and trace elements geochemistry of black shales samples from three different sedimentary environments in Mid-Yangtze region during Cambrian Terreneuvian – Series 2, we discussed the environment division of paleocean systems, third-order sea-level changes, paleocean productivity evaluation, major control mechanism of organic-rich black shales formation, and made the following main understandings and conclusions:(1) With reference to the PAAS(the Post-Archean Australian Shale), the whole black shales shamples exhibited significantly low total REE contents(ΣREE), including the silicalite layers of Yanjiahe Formation(Fm) which showed much lower contents than others and were possibly relevant to the dilution effects of the siliceous content. The PAAS-normalized REE distribution patterns of black shales displayed almost flat or slight inclination rightward features.(2) Systematic research on enrichment factors of redoxsensitive elements were made to establish the variation characteristics of forming condition of different Paleogeographic position black shales. This was taken to indicate that the water-column experienced highly stratified, anoxic, sulfide- weakly stratified, suboxic- weakly stratified, anoxic, weakly sulfide- suboxic state transition, which implied the instable physical-chemical characteristics of paleoseawater.(3) Based on UMo and Mo-TOC covariantion research, results showed that the Niutitang Fm, Shuijingtuo Fm, and Yanjiahe Fm deposited under different degree of watermass restriction. The sedimentation during the first and third stage of Niutingtang, the first and second stage of Yanjiahe and early stage of Shuijingtuo period was under strongly restricted conditions.(4) Geochemical features of siliceous rocks from Yanjiahe Fm near the PC/C boundary in the Yangtze Gorges area revaled that they originated in hydrothermal fluids, indicating that western margin of middle Yangtze platform could have not evolved into a passive continental margin.(5) According to the Ce anomaly and reference sea-level, four times main sea-level fluctuation cycles can be identified across the platform, slope and basin facies(6) The study of the paleocean productivity proxies showed that during the Niutitang Fm and lower part of Shuijingtuo Fm deposition, the ocean was in a relatively high level of productivity, while the ocean experienced relatively low productivity during Yanjiahe and upper part of Shuijingtuo period. The organic matter enrichment was mainly controlled by the redox conditions and third-order sea-level changes.(7) Comprehensive analysis suggested that the main controlling factors on organic matter enrichment were redox state of paleocean systems and the third-order sealevel changes in Mid-Yangtze region during Cambrian Terreneuvian – Series 2. When the primary productivity reached to a certain level, the preservation efficiency of organic matter in the sediment became higher.(8) In the reference of comprehensive analyses based on the geochemical characteristics of trace elements and rare earth elements, combined with previous research and the regional geological tectonic pattern, we proposed a new complex genetic model “hydrothermal activity – sea-level changes – tectonic paleogeography – formation and burial environment of organic matter” for the organic-rich black shales in Mid-Yangtze region during Cambrian Terreneuvian – Series 2.