Anatomy of a mixed marine-non-marine lowstand wedge in a ramp setting. The record of a Barremian-Aptian complex relative sea-level fall in the central Neuquén Basin, Argentina

During the Cretaceous, western Argentina was occupied by the Neuquén Basin, a back-arc-foreland basin that was open through the proto-Andes to the Pacific Ocean in the west. The Neuquén Basin contains a thick succession of sediments that include the offshore marine deposits of the Agrio Formation. These deposits represent a time when the arc was an island chain and the Neuquén Basin was freely connected to the Pacific. This offshore marine succession is punctuated by two intervals of arid continental deposits that represent major, second-order, relative falls in sea level. In both of these cases there is no evidence of tectonic uplift or angular truncation along a basal bounding unconformity. The upper of the two lowstand wedges is characterized by a complex arragement of shallow-marine and continental deposits. Shallow-marine deposits sharply overlying offshore shales and capped by a master sequence boundary are interpreted as falling-stage deposits recording a complex relative sea-level fall. On top of a regional erosion surface, a drying-upwards succession of fluvial-aeolian deposits is developed, recording a fully non-marine stage in the evolution of the basin. These deposits are overlain by a marginal marine evaporite succession. The absence of a return to fully open-marine conditions is attributed to uplift in the Andes and marks the transition of the Neuquén Basin from a back-arc to a foreland system. This succession has important implications for the basin's evolution and in the timing of the uplift of the Andes, is a very spectacular example of a lowstand wedge and is also a major hydrocarbon reservoir.