The first Triassic microfossil bonebed found in Argentina is located 80 meters from the top of the Quebrada del Barro Formation in the Marayes-El Carrizal Basin, in the province of San Juan. It consists of specimens from at least 63 individuals from an anomalously high concentration of fossils distributed laterally and vertically in a meter-thick fine-grained deposit. Two new taxa from the genera Sphenodontia and Eucynodontia had previously not been found in Argentine Triassic quarries. The bonebed is preserved in a mudflow deposit interbedded with calcic-Argillisols in the medial-distal zone of a distributary fluvial system (DFS). The accumulation is characterized by small-sized skeletal fragments (skulls, jaws and vertebra; all less than 50 mm), low degree of articulation, variable degrees of subaerial exposure, tooth marks, surface dissolution, and an alkaline authigenic mineral suite. Detailed paleoenvironmental and taphonomic characterization indicate that this accumulation underwent three stages in its taphonomic history: (1) biogenic extrinsic concentration; (2) local transportation and re-deposition by a mudflow on a swampy floodplain; and (3) drying and pedogenesis. Additionally, the deposit highlights biogenic activity as a way to concentrate a paleofaunal assemblage that likely represents the original community, and mudflow deposits from crevasse splays in DFS as a potential facies for microfossil preservation.