CONICET | Buscador de Institutos y Recursos Humanos

The stratigraphy of the Oligocene-Miocene southern Patagonia marine deposits hasbeen extensively discussed. However, the Monte Observación Member (MOM) wasvaguely defined and many aspects related to its boundaries, distribution, lithology andpaleoenvironments of deposition remain uncertain. In this paper we present resultsobtained from the field survey of several early Miocene outcrops exposed along theAtlantic coast of southern Santa Cruz Province (Austral-Magallanes Basin). Theycorrespond to the transitional interval between the marine shelf Monte León Formationand the terrestrial Santa Cruz Formation. Sedimentological analysis allowed to definesix Facies Associations (FAs) for the studied beds: FA-1 corresponds to inner shelf tolower shoreface deposits; FA-2 corresponds to prodelta to distal delta-front deposits;FA-3 corresponds to proximal delta-front deposits; FA-4 corresponds to terminaldistributary channels and meandering channels in the delta plain; FA-5 corresponds tointer-distributary bays and tidal flats; and finally FA-6 corresponds to the upper deltaplain deposits. In turn, analysis of fossil concentrations allowed defining three maingenetic types: biogenic, sedimentologic, and mixed biogenic-sedimentologicconcentrations. Biogenic concentrations are represented by beds and lenses of themarginal marine oyster Crassostrea orbignyi (Ihering), which is a significant fossilinvertebrate typical of the transitional deposits and is the last invertebrate showingmarine influence in all the studied sections and beyond. Sedimentologic and mixedbiogenic-sedimentologic concentrations exhibit different attributes and evidence theaction of different process, including from mostly monospecific within-habitatreworked concentrations, with little post-morten disturbance to polyspecificconcentrations bearing the imprint of various taphonomic processes. The distribution ofFAs suggest a regressive system that prograded over shelf deposits in the form ofdeltaic depositional systems with tidal influence. This analysis allowed to define betterthe MOM because it shows: 1) null to very low bioturbation degrees; 2) a reducedautochthonous invertebrate faunal content; 3) a well stratified pattern given by thealternating lithologies; and 4) abundant decimetric intercalations of gray to greenishsandstone beds. The fast accumulation of the MOM, which would be part of theregressive phase of the Monte León Formation, could be the consequence of a peak ofsiliciclastic and volcaniclastic input into the Austral Basin, produced as a result of rapidexhumation in Southern Patagonian Andes during the early Miocene.