The Santa Cruz Formation in the coastal area of Santa Cruz Province, AustralPatagonia, Argentina, is a sedimentary sequence consisting of mainly fine andtuffaceous sediments in the lower section (Estancia La Costa Member) and coarserand siliciclastic sediments towards the top (Estancia La Angelina Member). Thisformation rests upon and is transitional with marine sediments of the MonteLeón Formation. The lower member bears a rich mammalian association ofSantacrucian Age (late Early Miocene). Compositional and paleoenvironmentalanalysis of the lower member allows the differentiation of three sections:lower, middle, and upper. The lower section crops out in the northern part ofour study area and comprises a basal part with bioclastic sandstones andbioturbated heterolithic facies of continental-marine transitional environment.This is gradually replaced by fine primary and reworked tuffs and massive siltysandstones from volcanic source, with immature paleosols deposited in arelatively low-energy fluvial system with vegetated floodplains with a highsedimentation rate. The middle section shows an evident increase in the coarsefacies and paleosols, and a decrease in pyroclastic materials, which suggests ahigher energy for the system compared with the lower section, with a lowersedimentation rate. The upper section is compositionally even coarser and isexclusively siliciclastic in origin, and evinces even more energetic conditionsof the fluvial system but with fluctuating flow regimes. During the depositionof the Estancia La Costa Member, the climate changed from warm with somewhatcooler and/or drier intervals (lower and middle sections) to cool and dryconditions towards the top.