Foraminiferal biochronology of the Triassic Hoang Mai Formation, central Vietnam

Foraminiferal biostratigraphy was investigated for the first time in the Triassic Hoang Mai Formation distributed in the southeastern part of the Sam Nua Basin which was developed along the northeastern margin of the Indochina Block during the Permian-Triassic. The formation consists entirely of carbonates and is embedded within the underlying volcano-sedimentary Dong Trau Formation and overlying fine-siliciclastic Quy Lang Formation. We examined an approximately 300 m-long core section drilled in the northeastern part of Nghe An province of north Central Vietnam. Based upon the stratigraphic distributions of 24 foraminiferal taxa, including Citaella dinarica, Citaella? deformata, Endotriada tyrrhenica, Endotriadella wirzi, Endotriadella pentacamerata, Pilamminella grandis, Pilammina cf. densa, and Triadodiscus cf. praecursor, we assigned a Pelsonian age for the main part of the Hoang Mai Formation, with its lower/basal part of the core section probably extending down into the Bithynian. Thus, the Hoang Mai Formation is referred to the middle Anisian (early Middle Triassic). We also attempted taxonomic reexamination of foraminifera reported previously from the formation and confirmed the probable occurrence of Aulotortus eotriasicus. This and other taxonomic revision executed on formerly reported foraminifera resulted in further strengthening a middle Anisian appraisal for this formation. In ascending order, the three Middle Triassic lithostratigraphic units distributed in the Sam Nua Basin are the Dong Trau, Hoang Mai, and Quy Lang formations; they have been considered to overlie each other with simple superposition. Elsewhere in the Sam Nua Basin in north Central Vietnam, however, the Balatonites ammonoid fauna, which is considered to be coeval with the present foraminiferal fauna from the Hoang Mai Formation, is known in the uppermost part of the "underlying" Dong Trau Formation and the lowermost part of the "overlying" Quy Lang Formation. This strongly implies heteropic facies development of these three formations in the Sam Nua Basin during Middle Triassic time.