Geochemical Characteristics of the Upper Permian Shales in the Central Nanpanjiang Basin: Implications for Paleoenvironment Conditions

Abstract

Sedimentary environment can be restored qualitatively or semi-qualitatively by using elements or element combinations that are sensitive to paleoenvironment conditions. By measuring the major elements, trace elements and rare earth elements of 23 shale samples collected from coring wells in the Central Nanpanjiang Basin, we discussed the paleoenvironment conditions, including paleo-water depth, redox conditions, paleoclimate and provenance. La and Co contents indicate that the paleo-water depth in the Central Nanpanjiang Basin gradually deepened during the Late Permian. The ratios of U/Th, U_au, V/Cr, Ni/Co and V/Sc suggest that the Central Nanpanjiang Basin was in an oxic condition in the Late Permian, which was stable during the Permian Longtan and Dalong depositional periods. C-value (Climate index value) and binary diagrams of Sr/Cu and Ga/Rb show that the Central Nanpanjiang Basin was characterized by a warm and arid climate during the depositional of the Permian Longtan Formation, a warm and humid climate in the lower part of the Dalong Formation, and a warm and arid climate again in the upper part of the Dalong Formation. The chemical index of alteration (CIA), plagioclase index of alteration (PIA), index of chemical variability (ICV), and Th/U and K/Rb values can indicate the geological tectonic settings of source regions. From the Longtan period to the Dalong period, the small CIA amplitude and relatively stable ICV indicate that chemical weathering in the source area was constantly slighty weak. However, Th/U increased significantly but PIA increased slightly in the lower Dalong Formation, indicating an obvious climate change in the early deposition of the Dalong Formation. In addition, the geochemical discrimination calculation and plots show that the provenance of the studied shales was related to felsic volcanic rocks and the tectonic settings of the Upper Permian shale source areas in the Central Nanpanjiang Basin were mainly oceanic island arc and continental arc.