Influence of differential diagenesis on primary depositional signals in limestone-marl alternations: An example from Middle Permian marine successions, South China

Abstract

The diagenetic alteration of rhythmic successions remains one of the main challenges in paleoclimatology and cyclostratigraphy. To address this very important problem, thin sections and geochemical data of the limestone-marl alternations from Middle Permian carbonate successions in South China are examined to identify differential diagenetic effects in the petrography, organic and inorganic compositions, strontium concentrations and stable isotopic compositions of limestone beds and marl interlayers. In the Lengshuixi outcrop and the JY66-1 drill core, the most conspicuous features are that the limestone beds exhibit dense cementation, high strontium concentrations, negative oxygen isotopic values and positive carbon isotopic values. In contrast, the marl interlayers contain talc and exhibit strongly deformed and oriented bioclasts, low strontium concentrations, positive oxygen isotopic values and negative carbon isotopic values.Although differential diagenesis has occurred, some paleoclimate proxies (e.g., computed gamma-ray values) still preserve their primary signals and thus are suitable for cyclostratigraphic research. Eccentricity rhythm and millennial-scale rhythm are obviously recorded in Middle Permian limestone-marl alternations in South China. The variations in the oxygen and carbon isotopic values of limestone-marl alternations from different basins represent responses to different processes of differential diagenesis. This knowledge assumes that differential diagenesis occurs in diagenetically closed systems under the deep sea-floor. However, this assumption may not be valid in diagenetically open systems, where exotic fluids may add unexpected disturbances. Hence, it is crucial to understand diagenetic systems by determining the variations in strontium concentrations, which are as important as variations in carbon and oxygen isotopic values.