Organic Matter Enrichment of Upper Ordovician Ordovician-Lower Silurian Black Shales in Yichang Area

Abstract

Abstract

Upper Ordovician-Lower Silurian black rock series are important source rocks in South China. Several hypotheses, including redox environment, paleo-productivity and sedimentation rate, have been proposed to responsible for these organic-rich sediments. The ultimate cause, however, is still highly controversial. In order to understand the processes of organic accumulation, organic carbon content (TOC), trace element and clay mineral content of Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation are presented here from the Wangjiawan section, Yichang, Hubei Province. The results show that the TOC content vary from 3.47% to 0.74% through Wufeng Formation to Guanyinqiao Member, and then increased markedly to 5.13% in Longmaxi Formation. The V/(V+Ni) ratios are highly variable(from 0.33 to 0.93), but are generally low(avg.0.49) in the Guanyinqiao; The Ni/Co ratios in sediments vary from 4.18 to 15.57 in the Wufeng Formation, and then increased(from 9.27 to 17.48) distinctly in Guanyinqiao Member and the lower Longmaxi Formation, but reduced(9.49) sharply in the upper Longmaxi Formation. The V/Cr ratios vary between 0.32 and 8.01(avg. 3.11) throughout the entire section, but are consistently low (from 0.32 to 1.89) in Guanyinqiao Member. The content of Ba_bio are variable (from 417ppm to 4841ppm), but are high in the Guanyinqiao Member. In addition, the samples in Wufeng and Longmaxi Formation have more clay mineral and those in Guanyinqiao Member are dominated by carbonate minerals. According to the presented data above, combined with previous research, it can be concluded that there is an anoxic Yangtze sea during Wufeng period. Meanwhile, higher paleo-productivity condition, supported by content of Ba_bio, provides numerous organic matter for sedimentary rock this time. The drop of sea level during the Guanyinqiao period, caused by the global glaciation, could have enhanced the circulation of dense oxygen-rich waters onto the seafloor of Yangtze Sea, leading to the oxygenation of seafloor, so that the previous anoxic ocean was disrupted. the drop leads to the increase of terrigenous clast inputs and improves the paleo-oceanic productivity obviously. However, organic matter cannot be preserved effectively due to enormous terrigenous clast inputs and oxic environment druing the Longmaxi period, the swift return to a warm climate and rapid sea level rise could have in turn re-established the anoxic ocean similar to the case of Wufeng period.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016