Enrichment mechanism of organic matter in black rock series of the Niutitang Formation in Bahuang, Tongren, Guizhou

Abstract

Objective A set of black rock series rich in organic matter developed in the Yangtze Plate of China in the Early Cambrian. Methods To explore the main controlling factors affecting the enrichment of organic matter, the samples of the Niutitang Formation in Bahuang, Tongren Guizhou Province were tested for total organic carbon (TOC), major, trace and rare earth elements. Results The results show that the organic carbon content of the black rock series show a trend of high→low→high from bottom→lower middle→upper. The redox indexes Ni/Co, V/Cr, V/(V+Ni) and U/Th and the index Mo/TOC for determining the retention degree of water in the basin indicate that the carbonaceous shale and polymetallic element shale rich in organic matter at the bottom and upper part of the Niutitang Formation are formed in oxygen-poor to anoxic, medium to strong limited retention environments. The lower middle phosphate rock layer is formed in oxygen-poor to oxygen-rich, medium retention sedimentary environment. The δCe shows that the study area experienced a process of transgression regression in the Early Cambrian. Babio reveals the high paleoproductivity at the bottom and upper part of the Niutitang Formation and the medium to high paleoproductivity at the lower-middle part. The Ti/Al indicates weak terrigenous input as a whole. Rare earth distribution mode, δEu anomaly and La/Yb-REE diagram show that the phosphorus ore bed and metal-rich shale are affected by hydrothermal activity, which brings rich nutrients and reducing sedimentary environment for the enrichment of organic matter in metal rich shale. The phosphorus ore bed was formed in the period of sea level rise, and the upwelling of ocean currents led to the oxygenation of the sedimentary environment, which was not conducive to the preservation of organic matter. Therefore, the content of organic matter in the lower middle phosphorus-bearing formation is low. Conclusion The key factor restricting the enrichment of organic matter in the black rock series of the Niutitang Formation is the generally reducing and retained sedimentary environment, and the medium-high paleoproductivity is an important basis for the enrichment of organic matter. Hydrothermal activity creates favorable conditions for the enrichment of organic matter in local strata (polymetallic enrichment layer), and terrigenous clastic input has little impact on the enrichment of organic matter. The rise in sea level is an unfavorable factor for the enrichment of organic matter. It is the "comprehensive model" under the joint action of multiple factors that control the enrichment of organic matter in the black rock series of the Niutitang Formation.

Keywords

• niutitang formation
• organic matter
• sedimentary environment
• enrichment mechanism
• black rock series
• tongren, guizhou