The limestones of the Calabozo Formation of early Callovian age, crop out in the Neuquén Basin, southwestern Mendoza Province, Argentina. Facies/microfacies analysis at Arroyo El Plomo, west of Malargüe, shows that the limestones of the Calabozo Formation originated during a transgressive /regressive episode over the delta-margin facies of the Lajas Formation (late Bajocian-Callovian), with a change from siliciclastic to carbonate sedimentation. An extensive layer of floatstone marks the beginning of this stage. Within a ramp environment, a well represented mid-ramp subenvironment can be distinguished, consisting of a subtidal facies arrangement (wackestones, packstones, packstones with channels and floatstones) and an inner ramp subenvironment, characterized by a system of ooidal bars (grainstones) with tidal channels (floatstones) and predominantly intertidal facies passing into peritidal, and supratidal facies (boundstones). Three shallowing-upward cycles were recognized, the lower and middle ones being characteristic of the mid- ramp. The upper cycle is the best developed and consists of a middle ramp lower hemicycle and an inner ramp upper hemicycle. The Calabozo Formation is capped by deposits corresponding to a supratidal environment with restricted circulation, on top of which the evaporitic succession of the early Callovian Tábanos Formation is developed