The Santa Cruz Formation in the coastal area of Santa Cruz Province, Austral Patagonia, Argentina, is a sedimentary sequence consisting of mainly fine and tuffaceous sediments in the lower section (Estancia La Costa Member) and coarser and siliciclastic sediments towards the top (Estancia La Angelina Member). This formation rests upon and is transitional with marine sediments of the Monte León Formation. The lower member bears a rich mammalian association of Santacrucian Age (late Early Miocene). Compositional and paleoenvironmental analysis of the lower member allows the differentiation of three sections: lower, middle, and upper. The lower section crops out in the northern part of our study area and comprises a basal part with bioclastic sandstones and bioturbated heterolithic facies of continental-marine transitional environment. This is gradually replaced by fine primary and reworked tuffs and massive silty sandstones from volcanic source, with immature paleosols deposited in a relatively low-energy fluvial system with vegetated floodplains with a high sedimentation rate. The middle section shows an evident increase in the coarse facies and paleosols, and a decrease in pyroclastic materials, which suggests a higher energy for the system compared with the lower section, with a lower sedimentation rate. The upper section is compositionally even coarser and is exclusively siliciclastic in origin, and evinces even more energetic conditions of the fluvial system but with fluctuating flow regimes. During the deposition of the Estancia La Costa Member, the climate changed from warm with somewhat cooler and/or drier intervals (lower and middle sections) to cool and dry conditions towards the top.