Multi-structural limestone-marl alternations from the Permian of northwestern Yangtze Craton: Petrogenesis and environmental-tectonic implications

Abstract

Limestone-marl alternations (LMAs), a significant type of cyclothem, contain valuable information on paleoclimate and cyclostratigraphy. However, great controversies remain regarding the genesis of LMAs and their validity as environmental or tectonic archives. This study provides a typical example of LMAs from the Permian of northwestern Yangtze Craton, demonstrating a dynamic formation process closely related to environmental and tectonic changes. Integrated petrological, mineralogical, and geochemical data reveal that the LMAs exhibit multiple structures—bedded, nodular, and chaotic—reflecting the combined influence of depositional, diagenetic, and tectonic processes. During the depositional stage, high-frequency fluctuations in climate, terrigenous input, redox conditions and oceanic productivity led to the initial deposition of rhythmic limestone-marl beds. Concurrently, synsedimentary fault-associated slump deposits contributed to the transition from original bedded structure to disordered structures. Subsequently, differential diagenesis, mainly involving burial compaction and dissolution-cementation, further reinforced the mineralogical and structural differences between limestone and marl, ultimately resulting in the formation of multi-type LMAs. The results indicate that LMAs, especially those with well-bedded structure and good lateral continuity, can serve as effective environmental archive. They reflect subtle and frequent environmental variations in the northwestern Yangtze Craton during the icehouse-greenhouse transition period following the late Paleozoic ice age. The widespread chaotic limestone-marl structure, derived from synsedimentary fault-associated slump deposits in the lower Maokou Formation can be interpreted as associated with the pre-eruption uplift of the Emeishan mantle plume. This suggests that the Emeishan mantle plume activity predates the end of the Maokou stage as previously thought.