Recognition of diagenetic contribution to the formation of limestone-marl alternations: A case study from Permian of South China

Limestone-marl alternations (LMAs) have been extensively used in studies of sedimentology, cyclostratigraphy, and paleoclimatology. However, it has long been debated whether LMAs are derived from primary sedimentation or from diagenetic alteration, and the markers to distinguish one source from the other remain ambiguous. In this study, we investigate widespread Permian LMAs from various depositional settings in South China using traditional petrographic, mineralogical, and major element (Al2O3 vs.TiO2) methods, as well as emerging trace and rare earth element (REE) analyses performed using solution-based and in situ techniques. The original depositional differences of the limestone and its coupled marl beds can be identified based on (1) the high volumes of siliciclastic minerals in the marls, (2) unremarkable differential compaction, (3) the recognizable signatures of the detrital admixture (i.e., the Al, Zr, and Th characteristics), and (4) flattened shalenormalized REE patterns. Diagenetic LMAs are characterized by the compaction of fossils, rare aragonite skeletons, and abundant diagenetic clays (sepiolite and talc) preserved in the marl beds. We also identified fully diagenetic LMAs originating from primary carbonate sedimentation based on two almost completely consistent seawater-like REE patterns with diagnostic positive La and negative Ce anomalies ((La/La*)(SN) = 1.64 +/- 0.17, (Ce/Ce*)(SN) = 0.73 +/- 0.05), superchondritic Y/Ho (47 +/- 4), and depleted light REEs relative to high REEs ((Pr/Yb)(SN) = 0.39 +/- 0.09). Diagenesis also dominates the formation of LMAs with limited terrigenous input in shallow-water carbonate depositional environments. It is likely that both the active metabolic processes in early diagenesis (especially aerobic sulfide oxidation) and the temporal palaeoceanographic conditions, e.g., high levels of dissolved oxygen and sulfate, play crucial roles in the genesis of the diagenetic LMAs in the Permian strata of South China.