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.

The Taho Formation in western Shikoku Island, Japan, consists of Triassic carbonates that formed on a seamount in the Panthalassic Ocean. In order to investigate the stratigraphy and paleoceanography of this carbonate succession, we analyzed the biostratigraphy and chemostratigraphy of a 17.6 m-thick section of the upper Taho Formation at the stratotype area in Tahokamigumi, Seiyo City. This section comprises bioclastic limestone containing Triassic bivalves, ammonoids, and conodonts. We recognized six conodont zones (in ascending order): the Novispathodus pingdingshanensis, Novispathodus brevissimus, Triassospathodus symmetricus, Triassospathodus homeri, Chiosella timorensis, and Magnigondolella cf. alexanderi zones. Thus, the studied carbon-

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 (87 Sr/ 86 Sr) and oxygen (δ 18 O) 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 (δ 13 C), making them suitable for C isotopic analyses. In the lower interval, we observe a slight δ 13 Cenrichment, followed by a gradual δ 13 C-depletion upwards into the middle interval. The observed trend in δ 13 C 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 δ 13 C profile are relatively δ 13 C-depleted, which is likely the result of diagenetic overprinting.

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