Middle Triassic foraminifers from northern Laos and their paleobiogeographic significance

Triassic foraminifers are reported for the first time from limestones in the upper part of the Nam Sam Suite distributed in the Sam Neua area of northern Laos. The study area is located in the northern part of the Indochina Block, based on the basic geotectonic subdivision of mainland Southeast Asia. The fauna consists of 17 taxa and includes age-diagnostic Pilammina densa, Pilamminella grandis, Citaella dinarica, C.? deformata, Arenovidalina amylovoluta, and Diplotremina astrofimbriata. It is referable to the middle Anisian (Pelsonian) of the Middle Triassic. The microfacies of the studied samples are subdivided into three types, suggesting low wave-energy uppermost subtidal, moderately wave-agitated uppermost subtidal, and intertidal environments, respectively. Two species are specific to the intertidal microfacies. A notable paleontological feature is the dominance of P. densa, shown by the large number of individuals in the foraminiferal assemblage. This could be related to its opportunistic nature in the stressful conditions of the intertidal environment. We also consider that C.? deformata may be ecologically adapted to a hypersaline environment. The Sam Neua fauna shares several important species, such as C. dinarica, P. densa, P. grandis, and A. amylovoluta, with those found in the Sibumasu Block, Sukhothai Zone, and South China Block, suggesting that these major geotectonic domains in mainland Southeast Asia formed a single paleobiogeographic province during Anisian time. In view of paleogeography, the present results point toward a rather restricted (narrow) Paleo-Tethyan ocean basin allowing faunal interchanges among separated Eastern Tethyan continental blocks during the Middle Triassic.