PALEOENVIRONMENTAL CRISIS ACCOMPANIED PALEOFAUNISTIC CHANGES AT TRIASSIC-JURASSIC BOUNDARY, NW ARGENTINA

The Triassic-Jurassic boundary is marked by a significant paleoenvironmental crisis that coincides with one of the five major mass extinctions of the Phanerozoic. Strata and fossils that span the Triassic-Jurassic boundary are rarely reported from SW Pangea. However, the Balde de Leyes locality in northwest Argentina preserves a continental red bed sequence that includes a well-preserved assemblage of fossil vertebrates from this critical interval. The sequence includes the Quebrada del Barro (QBF), Los Riscos (LRF), Balde de Leyes (BLF) and El Jume (EJF) formations. QBF consists of muddy sandstone deposits that accumulated in floodplain settings interbedded with high sinuosity conglomeratic channel deposits. QBF is interpreted as laterally extensive facies of a distributary fluvial system (DFS) and preserves an Upper Triassic (Norian) paleovertebrate assemblage. Two different units cap the QBF. To the south, the upper boundary of QBF is an incised surface capped by the LRF. LRF deposits consist of three nested paleovalleys that preserve an Upper Triassic (Rhaetian) paleovertebrate assemblage. LRF paleovalley consists of conglomeratic alluvial fan deposits that pass upward to sandstones that represent distal playa lake deposits in the depocenter. Along the basin margin eolian deposits are preserved. In the north of the study area the QBF is capped by localized deposits of the BLF. The BLF consist of coarse conglomerate facies and preserves vertebrate fossils of lower Jurassic age. Throughout the entire study area deposits of the LRF, QBF, and BLF pass conformably up section to EJF, which is characterized by mudstones, fine sandstones, and abundant gypsum, consistent with deposition in an extensive playa lake setting. This heterolithic sequence of fossiliferous strata yields a well-preserved history of faunal change spanning the end Triassic to early Jurassic. The rocks hosting the fossils are interpreted to represent an arid piedmont landscape and record at least three events of basin scale incision and progradation (above the QBF and below EJF). This suggests that the Triassic-Jurassic faunal turnover in southern South America was associated with major shifts in paleoenvironment (as has been noted elsewhere in Pangea), possibly related to climatic perturbations.