Geochemical Characteristics and Their Geological Significance of Intrasalt Mudstones from the Paleogene Qianjiang Formation in the Qianjiang Graben, Jianghan Basin, China
Geochemical characteristics of intrasalt mudstones have great implications for the sedimentary environment of saline source rocks and the conditions for the symbiotic formation of halite ore. In this paper, 15 mudstone samples collected continuously from the lower part of the 3~(rd)member of Qianjiang Formation in the Qianjiang Sag of the Jianghan Basin have been analyzed for major, trace and rare earth elements, in order to obtain a better understanding of the paleosalinity, redox conditions and paleoclimatic background. The high Sr contents and Sr/Ba ratios, and also the presence of glauberite indicate high-salinity conditions with organic-rich mudstones deposited. The redox indexes of V/Cr, V/Ni, V/(V+Ni), deltaCe、Ce/La、deltaU and Ceanom and the alteration of some redox-sensitive trace elements such as V、U、Ni、Zn and Cr suggest low oxygen contents in water and the existence of water delamination with anaerobic conditions. The chemical index of alteration, Mg/Ca ratios and core observation indicate a semi-humid climate for source rocks deposition and semi-arid one for halite formation. The frequent variations of source rocks and halite deposits may indicate a fluctuated climate, probably corresponding well to the late Eocene doubt house climate. The inter-salt mudstones have a great potential to form high-quality source rocks. On the one hand, along with the alternatives of water freshening and salinizing due to climatic fluctuations, short pulses of feast and famine for living beings are conductive to the formation and accumulation of organic matters. On the other hand, deep-water and anoxic conditions during the humid pulse provide an appropriate environment to preserve organic matters.