The genetic relationship between paleoenvironment, mineral compositions and lithofacies in the Ordovician–Silurian Wufeng–Longmaxi sedimentary succession in the Sichuan Basin, SW China

Abstract

• Five intervals (I–V) showing changes in minerals, lithofacies and paleoenvironment were identified. • High productivity and low detrital influx lead to the enrichment of nondetrital quartz in siliceous shale in Intervals I-II. • The enrichment of carbonate in calcareous shale in Interval III was due to high detrital influx. • High detrital influx and strong chemical weathering lead to an abundance of silt and clay minerals in silty–argillaceous shale and argillaceous shale in Intervals IV-V. Changes of sedimentary environments recorded in the Late Ordovician–Early Silurian sedimentary succession can be reflected in mineral composition and lithofacies of shales. Outcrops and cores from this set of shales in the Sichuan Basin, China, are analyzed. The results show that quartz, clay minerals, and carbonate are the main minerals. Five major lithofacies are developed. Based on the provenance proxies (Al 2 O 3 /TiO 2 , Th/Sc, and Zr/Sc ratios), chemical index of alteration (CIA), paleoproductivity proxies (Cu/Al and Ni/Al ratios), redox condition proxies (U/Th, V/Cr, and Ni/Co ratios), proxies of detrital influx (Ti and Al contents), and sedimentation rate (SR), the paleoenvironment is analyzed. The source rocks are dominated by felsic igneous rock. Comparison between five lithofacies revealed that the relatively low degree of chemical weathering, anoxic conditions, maximum paleoproductivity probably associated with the high frequency of volcanic activities, and minimum detrital influx and SR (0.60–5 m/Ma) led to the enrichment of nondetrital quartz and organic matter, which eventually formed siliceous shale. The moderate degree of chemical weathering, dysoxic–oxic conditions, decreasing paleoproductivity, and increasing detrital influx and SR (10–16.67 m/Ma) resulted in the increase in detrital quartz, clay minerals, and carbonate content, and the development of mixed shale and calcareous shale. Relatively strong chemical weathering, oxic conditions, minimum paleoproductivity, and maximum detrital influx and SR (100.74 m/Ma) favored the enrichment of silt and clay minerals, forming silty–argillaceous shale and argillaceous shale. This study provides an insight into the relationship between the environmental changes and shale mineral composition and lithofacies.