Abstract
Under the promotion of exploration and development for shale oil and gas, marine shale has become a hotspot of fine-grained sedimentary studies in China. Paleo-environment reconstruction has always been an important aim, especially for black shale. Based on a large number of samples collected from Wufeng and Longmaxi Formations of two field sections in western Hubei and eastern Chongqing, this study compares the vertical variations in total organic carbon content, quartz/clay mineral value, and trace element compositions of these two profiles to reflect the paleo-environment changes from the latest Ordovician to the earliest Silurian. The results show that during the later Ordovician period, when Wufeng Formation deposited, the study area experienced a water depth change from relative deep facies to shallow marine, and paleo-redox changed from oxygen rich to anoxic, and even euxinic, followed by the content of oxygen increase. When the Guanyinqiao Bed deposited, the basin suddenly became shallower, and the deposition environment changed to be oxygen enriched for high water energy, circulating oxygen enriched. In the earliest stage Silurian, when the Longmaxi Formation formed, and the paleo-water also suffered several changes of sea level, and the paleo-environment also changed from anoxic to oxic. These changes can correspond to the third-forth order sequences well.
Highlights
Black shale is a kind of special sedimentary rock with enriched organic matter, which may contain shale oil and gas
Douglas (1972), Sliter (1972), Robert and Samuel (1978), and Chen (1997) studied the paleo-environment variations according to the abundance and differentiation of planktonic foraminifera in the shelf marginal marine environment of black shale after Silurian; Zhang and Zhang, (2006) analyzed the paleo-environment of Silurian using the degree of negative Ce anomaly in Tarim basin; Li et al (2005) and He et al (2013) calculated the paleo-water depth by planktonic foraminifera content in Yingqiong Basin and western South China Sea; Qiu, (2015) pointed out that the paleo-water depth of blocky shale and parallel laminar shale were significantly deeper than that of nonparallel laminar shale by studying the shale of Wufeng–Lower Longmaxi Formation in the eastern margin of Sichuan Basin
The total organic carbon content (TOC), quartz, clay minerals, and trace elements’ contents are essentially controlled by sedimentary environment, and their relative changes can be used as indicators to reflect sedimentary environment, especially paleo-water depth
Summary
Black shale is a kind of special sedimentary rock with enriched organic matter, which may contain shale oil and gas. With the outbreak of shale oil and gas revolution in the United States around 2010, black shale has become a hotspot of sedimentary studies in the world, especially in China (Xian et al, 2014). Paleo-environmental reconstruction plays significant role in regional basin evolution research and in global petrology study (Zhong et al, 2015; Li et al, 2017). Both lacustrine and marine shales have been well studied with multiple proxies for paleo-environmental reconstruction. We present a comprehensive study with multiple proxies of the black shales from two O–S boundary sections in western Hubei and eastern Chongqing for paleo-environmental reconstructions
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