Tectonosedimentary evolution of a lacustrine-deltaic system in a Triassic rift

MANCUSO, ADRIANA; CASELLI, ALBERTO; ROGERS, RAYMOND; BARREDO, SILVIA; MARSICANO, CLAUDIA

Mendoza

Congreso; Gondwana Conference; 2005

In the Ischigualasto-Villa Unión Basin, the study succession rests on a regionally extensive exposure surface (Milana and Alcober, 1994; Mancuso et al., 2004) which may be associated with tectonic quiescense in the basin after its deposition. Thus, the rift initiation stage is represented by the coarse-grained alluvial fan deposits derived from the footwall (Quebrada de Ischichuca area) and tuffaceous sandstone and siltstone fluvial deposits from the hanging wall along the basin axis (Río Gualo and Agua de la Peña- Aguada de Ischigualasto areas) (Chañares Formation) (Mancuso et al., 2004). The early rift climax stage is represented by basinward shoreline facies translation in both the hanging wall and footwall, and is typically characterized by the deposition of fine-grained sediments of the Los Rastros Formation. The mid-rift climax stage is represented by the developing of retrograding systems in the hanging-wall and the progressive deepening of Los Rastros Lake against the footwall, with main paleocurrents to the southeast in the Quebrada de Ischichuca area and to the north-northwest in the Agua de la Peña - Aguada de Ischigualasto area. The extraordinary thickness of black shales recorded in the Quebrada de Ischichuca area (footwall) clearly represents the late rift climax stage for it suggests that the rate of sediment supply to the basin was evidently low (starved basin). In the immediate post-rift stage, the local drainage systems of the  Ihigualasto-Villa Unión Basin were mostly subaerial and expanded rapidly, leading to an increase in the rate of sediment supply. The development of delta facies on the top of Los Rastros Formation suggests that the coarse-grained material derived from these proximal sources progressively filled the basin in coarsening-upwards successions. In the Ischigualasto-Villa Union Basin, the late post-rift stage is represented by the fluvial system of the base of the Ischigualasto Formation interpreted as the sag phase of the basin.