Geochemical Characteristics and Paleoenvironmental Significance of Limestone-Marl Alternations: An Example from the First Member of Maokou Formation in Central Sichuan Basin

The limestone-marl alternations are widely developed in the first member of the Maokou Formation in Sichuan Basin, showing the characteristics of an authigenic self-storage unconventional gas reservoir with good exploration prospects. Based on core observation and thin section identification, the marl and limestone samples in the first member of Maokou Formation in central Sichuan Basin were selected for carbon and oxygen isotopes, major elements, trace elements, and rare earth element tests. The geochemical characteristics and paleoenvironmental significance of the limestone-marl alternations were studied. Research shows that: (1) the mud content of limestone are less than 10% and the average values of δ13C and δ18O of limestone are 4.07‰ and −6.15‰, and the average value of ΣREE is 3.42 μg/g; (2) the mud content of marl range from 10% to 45% with an average value of 25%, and the average values of δ13C and δ18O of marl are 4.46‰ and −5.67‰, and the average value of ΣREE is 5.15 μg/g; (3) the average content of CaO of limestone is higher than that of marl, while the average content of SiO2, MgO and Al2O3 are lower than that in marl; (4) the content of trace elements and terrigenous elements in marl are significantly higher than those in limestone indicating that there were differences in the original sedimentary materials of the two lithofacies, and the input of terrigenous material in marl were higher than those in limestone. The palaeoenvironment of limestone and marl were similar overall, both deposited in high salinity seawater, with average palaeowater temperatures of 23.2 ℃ and 21.1 ℃, and the palaeooxic phases are both anoxic environments, whereas paleoclimate differences were most significant, limestone were deposited in a hotter paleoclimate. The difference of paleoclimatic was the main factor for the difference of original sediments of the two lithofacies and the relatively humid climate favors the input of terrestrial-sourced materials and nutrients, forming organic-rich marl. These research results are of great significance to understand the genesis of the limestone-marl alternations.