Preliminary nannofossil and geochemical data from Jurassic black shales from the Qiangtang Basin, northern Tibet

摘要

This paper presents new biostratigraphic and geochemical data from the Biluo Co section in northern Tibet, which exposes Jurassic black organic-rich shales, locally containing abundant coccoliths. Because of a general lack of macrofossils, the stratigraphic ages have been a matter of debate. However, coccoliths suggest an Early Bajocian through Bathonian to possibly Early Callovian age (Middle Jurassic) for the middle-upper part of the section. In this study, a range of trace-metal paleoredox proxies is used to assess how seawater oxygen levels varied both locally and globally during the deposition of these shales. The redox-sensitive elements V, Cr, U, Ni, Cu, Mo, Co, Cd and Zn exhibit relatively high concentrations and element/Al ratios. In particular, the Ni compositions fluctuate between similar to 75 ppm and similar to 106 ppm and Mo between similar to 1 ppm and similar to 7 ppm: values that are higher than those of the post-Archean Average Shale. Palaeoproductivity proxies, such as Zn, P and Cd, which can be fixed in elevated concentrations in sediments deposited under generally reducing conditions, are also relatively enriched. Furthermore, the U-Mo concentrations and Enrichment Factors (EFs) are consistent with deposition under predominantly suboxic to weakly anoxic conditions. Scattered bivalves, however, point to at least intermittent oxic conditions on the sea floor. Based on the redox-sensitive trace-element concentrations, together with ratios (V/(V + Ni), Ni/Co and V/Cr), the formation of the Biluo Co black shales, in Tibet was probably caused by increased productivity and organic-matter flux, leading to enhanced preservation of organic material under low-oxygen conditions. (c) 2015 Elsevier Ltd. All rights reserved.