Paleo-environments and organic matter enrichment in the shales of the Cambrian Niutitang and Wunitang Formations, south China: Constraints from depositional environments and geochemistry

Abstract

A comprehensive study of the lithology, mineralogy, major element and trace element geochemistry of the organic-rich shales in the Lower-Middle Cambrian Niutitang and Wunitang Formations in South China was performed to assess the depositional environments and the mechanisms for organic matter enrichment. Results show that the Lower Cambrian Niutitang shales were mainly deposited under anoxic conditions with high paleo-productivity, while the overlying Wunitang shales were deposited under dysoxic conditions with lower paleo-productivity. Both these shales were deposited in a moderately restricted deep-water basin. This paleogeographic setting was conducive to limited water interchange between the outer shelf and open ocean, resulting in a high flux of nutrients from shelf upwelling. These nutrients were produced from upwelling and hydrothermal vents and were carried by oceanic currents, enhancing blooms of primary producers within the surface waters. Organic matter enrichment of the Niutitang and Wunitang shales was mainly controlled by redox conditions and paleo-productivity. The intermittent renewal of connections to open ocean water during sea level rise weakened the restricted conditions in the depositional area, which significantly accelerated the consumption of organic matter.