Abstract

The Middle - Upper Cambrian carbonate sediments in the Đồng Văn region, Hà Giang, record a complex diagenetic history that has contributed to shaping the evolution of this carbonate succession. This study employs petrographic analysis using optical microscopy and cathodoluminescence (CL) to identify diagenetic processes, including micritization, cementation, dissolution, dolomitization, dedolomitization, and compaction. The results indicate that micritization formed thin micrite coatings around bioclastic fragments and ooids in a very shallow marine environment, while cementation produced four types of early calcite cements: (1) radiaxial fibrous (30-900 μm, syndepositional marine or shallow burial), (2) granular (80-300 μm, meteoric-phreatic or burial), (3) blocky (100-400 μm, meteoric and burial), and (4) syntaxial calcite overgrowth (near-surface marine or meteoric-phreatic).

Dissolution is evidenced by fissure filled with micrite and lithoclast, formed due to subaerial exposure during sea-level fall. Dolomitization occurred in two stages: Type 1 (10-30 μm) within fissures, representing early, low-temperature dolomitization (<50-60°C), and Type 2 (50-150 μm) within ooids and microfractures, indicating late-stage, high-temperature dolomitization (>50-60°C), followed by near-surface dedolomitization. Compaction involved two stages, with small stylolites forming in Stage 1 and larger stylolites in Stage 2, following Type 2 dolomitization.

The proposed diagenetic sequence highlights a transition from syndepositional marine to meteoric exposure and deep burial, influenced by sea-level fluctuations and tectonic activity on the South China Block. These findings provide a deeper understanding of the controlling factors of early diagenesis in the development of the Middle to Upper Cambrian carbonate system in the Đồng Văn region, Hà Giang.