Sedimentary characteristics and palaeoclimatic significance of glauberite in Paleocene lacustrine deposits of the Jiangling Depression, central China

摘要

The Paleocene sedimentary sequence in the Jiangling Depression was deposited in a salt lake environment. Based on detailed core observation of four drillholes in the Jiangling Depression, a study of sedimentary characteristics of glauberite in Paleocene lacustrine deposits was conducted through thin section petrography, X-ray diffraction, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). This study has revealed five types of geological occurrences and six types of profile distribution features of glauberite. On the basis of petrological and mineralogical characteristics, we propose that the glauberite is mainly formed by crystallization from saturated sulfate solution and by ion exchange between sulfate solution and calcium-bearing pore solution. Glauberite mainly occurs in the late stage of Paleocene, when brine is relatively highly concentrated. Replacement of gypsum by glauberite is subordinate. The depositional model of glauberite is also discussed. The sulphur isotopic values (delta S-34) and the strontium ratios (Sr-87/Sr-86) of the sulphate suggest a non-marine brine supply. The profile distribution features of glauberite contribute to a better understanding of the past climate change. The glauberite in Late Paleocene is frequently interbedded with dolomite, anhydrite, halite or thenardite, suggesting fluctuation of lake water salinity and climate. Trace element compositions of the mudstones imply the reducing condition and fluctuating salinity. The Homogenization temperatures and hydrogen and oxygen isotopic values (delta D-SMOW and delta O-18(SMOW)) in halite and thenardite inclusions further indicate that in Paleocene the temperature was relatively high and fluctuant. Moreover, the palaeoclimate was drier in the late stage of Late Paleocene, though the temperature was cooler/lower.