Geochemical Characterization of Organic Rich Black Rocks of the Niutitang Formation to Reconstruct the Paleoenvironmental Settings during Early Cambrian Period from Xiangxi Area, Western Hunan, China

Abstract

The Niutitang Formation in the South China Block might be a source of hydrocarbon as it contains an enormous quantity of organic matter. Black rock of the Early Cambrian Niutitang Formation is widely distributed in the Yangtze region, but detailed geochemical understanding of it is still emerging. This research discusses the detailed geochemical characteristics of the Niutitang Formation to reconstruct the paleoenvironmental conditions, employing total organic carbon (TOC) content, major, trace, and rare earth element data. For this purpose, black rock specimens of the Niutitang Formation from two outcrop sections were utilized for geochemical characterization, and the results compared with another eight sections from the South China Block. The average total organic carbon in these sediments is significantly higher (5.80 wt.%). In the platform region, lower quantities of TOC indicate a poor potential to produce hydrocarbons. At the same time, significantly higher TOC is observed in the deep shelf and slope sediments, indicating a significant potential to produce hydrocarbons. The average Ce, Eu and Y anomalies from both Longbizui and Sancha sections studied are 0.74, 0.86, 1.77, 1.07, and 1.19, 1.30, respectively. The chemical index of alteration (CAI) throughout the Yangtze block is higher (averaging 71.32) than that of Post Archean Australian Shale (PAAS 69), indicating a moderately weathered source of the Niutitang Formation relative to PAAS. As the sediments are moderately weathered, this suggests these rocks might have been derived from felsic rocks, mainly granite-granodiorite. The normalization of REEs in the black rocks reveals a reduction of light REEs with increase in heavy REEs enrichment. Similarly, a positive Eu anomaly, negative Ce anomaly, and a moderate Y/Ho (34.61) are clues to a hybrid depositional mechanism associated with hydrothermal action and terrigenous input. These anomalies are also evidence of upwelling in the paleo-ocean and mixing of organic matter, which created anoxic bottom water during the deposition of the Niutitang Formation in the basin and upper oxic water conditions before deposition. The main controlling factors for the distribution of rare earth elements in these black rocks of the Niutitang Formation are pH, terrigenous input, source rock composition, tectonism, an upwelling mechanism, and hydrothermal activity.