Decoding sedimentary environment and synsedimentary tectonic activity information in nodules: a case study of the black rock series in the border area of Guizhou and Hunan

Abstract

The black rock series contains energy resources such as oil and gas, coal, shale gas, and various solid resources such as iron, manganese, phosphorus, and barium. The restoration of sedimentary reservoir formation and sedimentary mineralization conditions is the cornerstone of resource prediction, and the restoration of ancient sedimentary environments and synsedimentary tectonics has therefore received additional attention. There are a large number of nodules in the black rock series in the border area of Guizhou and Hunan, and the nodules have sealing properties on the original records. Therefore, based on the observation of exposed profiles in the field and laboratory microscopic analysis, and through the integrated analysis of sedimentation and tectonic processes, it is found that there are significant differences in mineral composition and textural types among nodules in different layers: (1) The nodules of Upper Sinian Series-Cambrian Terreneuvian Series Liuchapo Formation (Z€l) are an assemblage of multi-components such as barite, calcium, phosphorus, and silicon, with texture of gravel or sand fragments. The cementation temperature of the two nodule layers is different, and the one directly covering the barite ore is relatively higher; (2) The nodules of Cambrian Series 2 Palang Formation(€2p) are mainly composed of pyrite and three stages of growth structure are developed: the core layer shows a framboidal texture, and the outerside two layers show a nephritic shape; (3) The nodule components of Cambrian Series 2 Wuxun Formation (€2w) are calcium and pyrite, of which calcium is dominated, with scattered pyrite. There are four stages of growth structures, with relatively higher content of pyrite in the first and third stages. Different paleose-dimentary environmental conditions and synsedimentary tectonic activity information can be decoded from nodules at different layers: (1) The barite bearing nodules in the Liuchapo Formation belong to low-temperature hydrothermal origin, and the reduction environment is limited near the fault zone; (2) The pyrite nodules in the Palang Formation are formed near the redox interface, and the environment has medium conditions of high concentration of active iron; (3) The calcareous and pyrite nodules of the Wuxun Formation were formed in a periodic reducing environment of low concentration iron, which is speculated to be caused by frequent relative sea level fluctuation caused by episodic subsidence of the basement, leading to frequent changes in the redox properties of the water medium. Therefore, the study of nodules helps to reconstruct the medium conditions of black rock series and is of great significance for resource prediction.

Keywords

• nodule
• black rock series
• medium conditions of environment
• upper sinian-cambrian
• the border area of guizhou and hunan