Paleo-environmental variation and its control on organic enrichment in the black rock series, Cambrian Yuertusi Formation in northwestern Tarim Basin

doi:10.13745/j.esf.sf.2023.2.12

Abstract

Abstract:

Based on the sedimentary and geochemical characteristics of the Lower Cambrian Yuertusi Formation in the northwestern margin of the Tarim Basin, the variation of seawater redox condition and primary productivity during sediment deposition and its control on organic enrichment in the black rock series and source rock distribution in the Yuertusi Formation were investigated. The Yuertusi Formation in the northwestern Tarim Basin can be divided into five members: black phosphorus-bearing siliceous rock at bottom; black mudstone in the lower part; gray dolomite in the middle part; interbedded black shale and gray microcrystalline limestone in the upper part; and gray crystal dolomite near top; this vertical lithofacies sequence incorporates a complete transgression-regression sedimentary cycle. The redox index and hydrothermal tracing parameters revealed the details of paleoenvironmental variation: the bottom siliceous rock is formed in an anoxic environment with strong hydrothermal activity; the lower black mudstone in an euxinic environment with slight hydrothermal activity; and the upper black shale in a sub-anoxic to anoxic environment with weak hydrothermal activity. Comprehensive studies have shown that during the deposition of the black rock series sea level first rises then falls, oxygen fugacity first decreases then increases, and paleo-productivity gradually weakens. The anoxic condition and high productivity influenced by hydrothermal activity jointly control the distribution of ancient, high-quality source rocks represented by the black mudstone in the lower part of the Yuertusi Formation.

Key words: Tarim Basin, Yuertusi Formation, redox conditions, primary productivity, organic enrichment

CLC Number:

CHEN Jianfa, XU Jin, WANG Jie, LIU Peng, CHEN Feiran, LI Maowen. Paleo-environmental variation and its control on organic enrichment in the black rock series, Cambrian Yuertusi Formation in northwestern Tarim Basin[J]. Earth Science Frontiers, 2023, 30(6): 150-161.

Figures/Tables 8

Fig.1 Location of the northwestern margin of the Tarim Basin and the stratigraphic distribution. a and b. tectonic units of the Tarim Basin and its northwestern margin; c. geological setting of the Akes area of the NW Tarim Basin; d. Cambrian lithologic columns in NW Tarim Basin. Modified after [18-19].

Fig.2 Location of the northwestern margin of the Tarim Basin and the stratigraphic distribution. a and b. tectonic units of the Tarim Basin and its northwestern margin; c. geological setting of the Akes area of the NW Tarim Basin; d. Cambrian lithofacies in NW Tarim Basin

Fig.3 Profiles of TOC, Ni/V, V/Cr, Ni/Co and U/Th from the Cambrian Yuertusi Formation black rock series of Kungaikuotan section in NW Tarim Basin

Fig.4 Plots REE versus Ce/Ce* ratios of the Cambrian Yuertusi Formation black rock series of Kungaikuotan section in NW Tarim Basin

Fig.5 Profiles of ∑REEs, LREE/HREE, Ce/Ce* and Eu/Eu* from the Cambrian Yuertusi Formation black rock series of Kungaikuotan section in NW Tarim Basin

Fig.6 Profiles of Rb/Sr, Cu and Ba from the Cambrian Yuertusi Formation black rock series of Kungaikuotan section in NW Tarim Basin

Fig.7 Plots TOC versus redox index and hydrothermal tracing parameters showing organic matter enrichment of black rock series in Cambrian Yuertusi Formation, NW Tarim Basin

Fig.8 Schematic diagram showing Early Cambrian marine redox variation, distribution of organic-rich rock, hydrothermal activities and spatial variation of TOC in NW Tarim Basin

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