Characteristics and environmental significance of concretions in the Lower Cambrian Qiongzhusi Formation in the Middle–Upper Yangtze region, China

Abstract

Based on the outcrop and drilling data of the Sichuan Basin and its periphery, this paper reveals the concretion development characteristics of the Lower Cambrian Qiongzhusi Formation and its significance to the environment. Three preliminary conclusions are drawn as follows. (1) The exposure layers and lithofacies characteristics of the concretions in the Qiongzhusi Formation are very different from area to area. In the eastern Sichuan-western Hubei-Hunan & Guizhou and southern Sichuan depression areas, the concretionsare developed in the section of SQ1, and are mainly composed of calcareous siliceous mixed shale facies, siliceous shale facies, dolomitic shale facies and dolomite facies. The clay content is generally less than 25% and is significantly less than the surrounding rocks, and the TOC is generally higher in the lower concretion layers and less in the upper concretion layers. In the central Sichuan and northern Sichuan depression areas, the concretions appear in the sections of SQ1 and SQ2, and are dominated by calcareous shale facies and marl facies. The calcic content is obviously higher than that of surrounding rocks, and the clay content is less than that of surrounding rocks, and the TOC is less than 1.50%. Because mineral composition difference is high, most of the GR curves of the concretions show trough response characteristics, reflecting the central region enriched with calcium or siliceous, and the edge enriched with clay and organic matter. (2) The concretion layers generally occur in carbonaceous shale, silty shale and siliceous shale with a TOC over 0.7%, and are especially developed in carbonaceous shale and siliceous shale of SQ1, and occasionally appear in grey silty shale with a TOC over 0.2%, showing that the concretion layers are basically symbiotic with grey shale (especially with high-quality shale). (3) The concretions are a kind of important sedimentary response of rifting activities in Yangtze marine basin. According to the distribution characteristics of concretions and black shale in the Qiongzhusi Formation, the rifting activities of Yangtze marine basin can be divided into three periods including development period (SQ1), adjustment period (SQ2) and abortion period (SQ3). In the period of SQ1, controlled by massive rifting and upwelling ocean currents in Yangtze marine basin, the concretions and organic-rich shale were widely developed in the Lower Qiongzhusi Formation. In the periods of SQ2 and SQ3, while rifting activity of Yangtze marine basin decreased and aborted, and while sea level fell and upwelling ocean currents receded to the southeast, the concretions and organic-rich shale just appeared in the local depression areas in small scale.