ABSTRACT
Major, trace and rare earth element (REE) geochemistry of the late Cretaceous lower Zhoutian Formation from the Jitai Basin of Southeast China were measured by inductively coupled plasma mass spectrometry (ICP-MS) analysis to infer the provenance of the sediments and to reconstruct the palaeoenvironment and palaeoclimate. The wide range of Sr/Cu ratios point to a fluctuating palaeoclimate, and the negative correlation between the FeO/MnO and Al2O3/MgO ratios and the Sr/Cu ratio indicates that the late Cretaceous climate during the lower Zhoutian Formation in the Jitai Basin can be divided into two parts. The lower part experienced two cooling periods, whilst the upper part was dominated by warm-humid climate. Mostly corresponding trends of the B/Ga, Sr/Ba and Sr/Cu ratios show that the salinity changed consistently with the late Cretaceous climate during the lower Zhoutian Formation in the Jitai Basin. During the lower part, the salinity changed from salt water to fresh/brackish water. In the upper part, water was mainly fresh/brackish, and there were many changes from fresh/brackish water to salt water. The relatively stable Ni/Co, V/Cr, V/(V + Ni) and Ce/Ce* data indicate a long period of oxic conditions. The La-Th-Sc, Th-Sc-Zr/10 and La/Th-Hf data of the silt- and sandstones of the lower Zhoutian Formation show that its provenance was mainly a mixture of felsic upper crust sediments and older sedimentary rocks.
ABSTRACT
Scanning electron microscopy (SEM) was used to analyze and study micron-nanometer evaporite samples collected from Paleocene and Eocene drill cores in the Jiangling Depression. Accordingly, seven beds of potassium-bearing solid rocks were accurately identified. Sylvite, carnallite, syngenite, dolomite, thenardite, anhydrite, glauberite, halite, barite, celestite, and other solid salt minerals were found, and carnallite, syngenite, and thenardite were found for the first time in the Jiangling Depression. Sylvite, syngenite, and carnallite indicate that the Paleogene salt lakes in the Jiangling Depression had evolved to the sylvite stage and that prospecting for solid sylvite would be satisfactory. Micron-nanometer celestite is contained in the evaporites, from which we can infer that strontium may have been provided by deep formation water (or oil-field water). This finding is of great significance to studying the genesis of sylvite sediment in the Jiangling Depression. From the extensive development of primary glauberite beds typical of warm salt minerals in the Shashi Formation, it can be inferred that the late Paleogene paleoclimate in the Jiangling Depression of the Jianghan Basin was dry and hot. Based on the extensive distribution of micron-nanometer pyrite, siderite, iron and Fe2O3/FeO ratios in evaporite sediments and color analysis of mudstones, the evaporites in the study area formed in an underwater anoxic, reducing environment during sedimentation. Therefore, the evaporite sediments in the Paleocene-Eocene interval of the Jiangling Depression are proposed to have formed in a saltwater lake sedimentary environment, and the ancient lake was characterized by a deep-water salt lake sedimentary model.
