Evidence of huge evaporite mine clusters under extreme arid climatic conditions of Cretaceous to Paleogene in South China: Example of the Huichang large salt deposit

Abstract

Large amounts of evaporites are distributed throughout all epochs of the Phanerozoic eon, which is closely related to important tectonic activities and extremely arid paleoclimates in specific geological periods. During the Cretaceous-Paleogene period, the Earth's climate was always arid, providing unique conditions for evaporite formation. The large number of evaporites in South China Block is an important part of the puzzle in the study of massive global evaporite deposition. The Huichang typical salt deposit is in the Upper Cretaceous Zhoutian Formation in the Huichang Basin, where the rock salt are interbedded with argillaceous rocks, constituting a rhythmical deposit. Halite was dominant among the saline minerals, followed by gypsum. The homogenization temperatures of halite inclusions ranged from 11.1 ℃ to 24.7 ℃, with a concentration of 15.1–22.2 ℃ and an average of 17.9 ℃. The δEu and δCe of the argillaceous rock studied ranged from 0.84 to 1.14 and 0.96–1.06, respectively, which combined with V/(V + Ni), V/Cr, Ni/Co, and U/Th indicators reflect that the environmental of the sedimentary water body has a weak Redox property. The spore-pollen assemblage is dominated by gymnosperms (Classopollis annulatu and Classopollis minimus) and fern spores (Schizaeoisporites kulandyensis), representing an arid environment in the early stage, and the increase in hygrophilous Taxodiaceaepollenites hiatus in the later stage indicates a shift from an arid to a humid climate. The Milankovitch Earth orbital periods of 401, 93–138, 38–45, 24–27, and 14 kyr were identified by the multiple rhythmic deposition of the rock salt and argillaceous rocks, indicating that the deposition was controlled by climate change, which was driven by the long-and-short eccentricity, obliquity, and precession cycle of the Earth orbit. In addition, the differences in the thickness and alternating frequency of the upper, lower, and middle sediments in the 'Huichang Gypsum-salt Section' also indicated that the intensity and duration of regional tectonic activities and the supply of material sources are important factors that control rock salt deposition. The strong subduction of the paleo-Pacific plate into the Eurasian plate caused the paleogeographic pattern of high in the east and low in the west, in the South China Block during the Cretaceous period, forming a large number of 'high mountain and deep basin' tectonic groups. Under the conditions of an extremely arid climate, combined with the coupling effect of regional tectonic activities and material sources, a large evaporite mineral concentration area was formed in South China, with the Huichang salt basin as a typical representative.