Episodic dolomitizations driven by sea-level fluctuations in the Permian–Triassic boundary interval: a case study from a dolomitized Changhsingian reef, Shitouzhai section, South China

Abstract

The strata exposed in Shitouzhai section, Ziyun region, on the south margin of Yangtze Platform, consists the transaction from Changhsingian reef carbonates (Bed 1) to Griesbachian micritic carbonates (Bed 4). Beds 2–3, which are sandwiched by Bed 1 and Bed 4, are massive layers of fine-crystalline dolostones. The boundary between Bed 1 and Bed 2 is defined by the subaerial exposure related to the end-Permian regression, whereas the Permian–Triassic Boundary (PTB) is emplaced within Bed 3. The dolostones from Bed 1 have 271–493 ppm Sr, 182–1970 ppm Fe and 328–597 ppm Mn. Their dolomites are entirely low-Ca calcian dolomite (LCD, 55 mol% CaCO3) and lesser amounts of LCD, with δ13C of + 2.67‰ to + 3.34‰, δ18O of − 2.53‰ to − 0.66‰. These geochemical attributes point to two phases of dolomitization. Phase 1 is associated with the end-Permian regression and caused partial dolomitization of porous zones in Bed 1. Phase 2 is penecontemporaneous dolomitization associated with the transgression following the end-Permian regression, and responsible for the dolostones of Beds 2–3. This phase stopped when the Early Griesbachian regression exposed Beds 2–3 and led to dedolomitization in these dolostones. The linkage between diagenesis and sea-level changes indicates that dolomitizations and dedolomitization are “regional events”, to which other Permian–Triassic boundary carbonates can refer.