Litho-, bio-, and chemostratigraphy of the Middle Triassic carbonate succession in the North-Central Coast Region of Vietnam

Middle Triassic carbonates extend from the North-Central Coast region of Vietnam to northern Laos. We conducted sedimentological, paleontological, and geochemical analyses on the carbonates of the Hoang Mai Formation in Nghe An province, Vietnam. The carbonates consist of the following six lithostratigraphic units (from bottom to top): sandy limestone (unit 1), peloidal packstone with a microbialite interlayer (unit 2), peloidal packstone and oncoidal floatstone (unit 3), peloidal–bioclastic packstone (unit 4), peloidal packstone with variable quantities of bioclasts and microbialite (unit 5), and peloidal packstone with variable quantities of bioclasts (unit 6). The sandy nature of unit 1 and of three of the interlayers in unit 2 indicates a supply of terrigenous material during the early stages of formation of the Hoang Mai carbonate platform. The dominance of carbonates with a fine-grained matrix throughout the overlying succession implies a low-energy depositional environment. Common occurrences of dasycladalean algae in units 4–6 indicate a back-reef lagoon environment. A total of 24 foraminiferal taxa, including Citaella dinarica and C. deformata, were identified. These two age-diagnostic species occur in units 3–6, suggesting that this interval is correlated with the Pelsonian and that the lower units (units 1–2) are potentially correlated with the Bithynian. Thus, the entire core section can be assigned to the middle Anisian (lower Middle Triassic). An assessment of the diagenetic alteration using geochemical parameters indicates that the carbonates of the studied succession have not retained their initial strontium (87Sr/86Sr) and oxygen (δ18O) isotope compositions. In contrast, the carbonates in the lower (units 1–2) and middle (units 2–5) intervals have retained their initial carbon isotope compositions (δ13C), making them suitable for C isotopic analyses. In the lower interval, we observe a slight δ13C-enrichment, followed by a gradual δ13C-depletion upwards into the middle interval. The observed trend in δ13C values from the Hoang Mai Formation can be correlated with similar trends reported from South China and Romania. Samples within the upper interval (units 5–6) of the Vietnamese δ13C profile are relatively δ13C-depleted, which is likely the result of diagenetic overprinting.