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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.