Marine Redox Conditions and Their Effects on Primary Production and Preservation of Organic Matter During Deposition of the Early Cambrian Black Shale on the Yangtze Platform in Eastern Guizhou

Rumana Yeasmin1,2, Daizhao Chen1, Yong Fu3, Zenghui Guo1, and Chuan Guo1
1Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China 2Department of Geological Sciences, Jahangirnagar University, Savar, Dhaka, Bangladesh 3College of Resources and Environmental Engineering, Guizhou University, Guiyang, China

The widespread organic-rich black shales (Niutitang Formation) were deposited during the Early Cambrian in South China as a result of large-scale marine transgression and subsequent drowning over the carbonate platforms. To elucidate the oceanic changes, geochemical indices and trace metal/Al ratios are applied as redox proxies, whereas TOC, Ba, P, excess Si, Ni and Sr are used as paleoproductivity proxies. These data show that the black shales deposited in previous slope (Well No. ZK102, Daotuo, Songtao County) and basinal (Bahuang section, Tongren) settings experienced vast oceanic changes, which could have significantly influenced the organic matter production and preservation in sediments.

At well ZK102 where it was located in the slope setting, three formations: the Niutitang, Bianmachong and Balang Formations in ascending order, are involved in which the TOC contents, on average, are 8.15, 3.10 and 1.79 wt.%, respectively, showing decrease upwards. The Niutitang Formation has high illite and quartz contents in which the lower part also contains apatite, barite and other clay minerals like chlorite and kaolinite. Geochemically, it is characterized by high concentration of Ba (EFBa avg. 23.07), Ni (EFNi avg. 4.73), V (EFV avg. 23.34), Cr (EFCr avg. 14.26) and excess Si (3.76 - 99.13%), and low concentration of Mn (EFMn avg. 0.37) relative to average shale (AS; Wedepohl, 1971, 1991). This formation also features high concentrations of P, Ca, Ti, Sr and Zr, and high ratios of V/(V+Ni) (avg. 0.93) and V/Cr (avg. 6.39), however, they are higher in the lower part. The Bianmachong Formation is characterized by alternation of limestones and black shales, so mineral calcite dominates limestones, whereas black shales are composed mainly of quartz, plagioclase, illite and chlorite, with minor amount of kaolinite and pyrite. The black shale interbeds, although having Si, Na, K and V concentrations close to AS values, they show fluctuations in Ba (EFBa 3.03 - 46.32), P (EFP 0.39 - 3.1), excess Si (0 - 66.75), Ni (EFNi 0.67 - 2.47) and Mn (EFMn 0.12 - 3.5) contents as well as ratios of V/(V+Ni) (0.55 - 0.83) and V/Cr (0.94 - 2.56). The overlying Balang Formation comprises mainly black shales intercalated with some siltstone bands, and has high contents of clay minerals and quartz. Although it has relatively high V (EFV 1.03 to 2.5) concentrations, this unit has lower Ba (EFBa 2.43 to 3.09), Mn (EFMn 0.38 - 1.21) and P (except in one sample) (EFP 0.34 - 1.88) concentrations than those in Bianmachong shales.

At Bahuang where it was located in the basinal setting, only Niutitang Formation is involved in which TOC contents are high (avg. 6.7 wt.%) as well. Concentrations of Ba (EFBa avg. 9.03), V (EFv avg. 10.81), Cr (EFCr avg. 2.56) and excess Si (14.45 - 69.81%), although very high relative to AS, are lower than those in the Niutitang shale at ZK 102. This formation also characterizes high P, Ca, Ti, Sr and Zr concentrations as well as high ratios of V/(V+Ni) (avg. 0.8) and V/Cr (avg. 6.46), however, they are higher in the lower part.

Positive correlations between TOC and different geochemical proxies such as V, Mn, Cr, Ba, P, Sr, Ni reveal that these elements were enriched in association with organic matter influx. High values of productivity proxies (e.g., TOC, Ba, P/Ti, EFNi, EFSr and Ca/Al) in the Niutitang Formation suggest high primary production during deposition of this formation. In the meantime, high abundances in V and Ni, and high ratios of V/(V+Ni) and V/Cr, especially in the lower part of Niutitang Formation, at the two localities as mentioned above suggest an anoxic bottom water, which may have been enhanced by increased primary productivity as well. Higher TOC, Ba and P concentrations in Niutitang Formation deposited in the slope setting signify relatively high primary production there. However, more restricted, anoxic deep basinal waters could have enhanced organic preservation, reasonably accounting for somewhat still high TOC abundance there.

After the prolonged anoxic condition over the flooded shelf, the depositional system evolved into a shallower mixed carbonate/fine-grained siliciclastic (black shale) system, itself characterizing frequent anoxic-oxic fluctuations, on the previous slope as a result of sea-level fall during the Bianmachongian. This scenario is generally suppoted by the wide variations in V/(V+Ni) and V/Cr ratios and EFMn concentration in Bianmachong shales. An anoxic condition once again took place as a result of recurring sea-level rise during deposition of the Balang Formation. Relatively high V contents and V/(V + Ni) ratios in this formation agree with the anoxic condition. However, the lower Ba and P concentrations imply a decrease in primary production, which may account for the decrease in TOC abundance.

AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013