Leonardo Legarreta, Miguel A. Uliana


The Maastrichtian through Miocene series in the midplate San Jorge Basin contains rich and comprehensively studied vertebrate and invertebrate fossil assemblages. These strata reflect a depositional seuing featuring a shallow to deeper epicontinental shelf, framed by a paralic tract with shallow neritic to estuarine-lagoonal belts and a variably wide coastal plain dominated by low energy alluviation and aeolian processes. Gradual facies changes and evidences of stratigraphic condensation, both at the proximal subaerial and distal marine domains where paleosoil stacks and glauconitic concentrations have been found, reveal an accumulation regime controlled by mild depositional gradient and meager sediment supply. Observations near the Atlantic coast at Comodoro Rivadavia (46° S) and data from outcrops, oil wells, and seismic sections scattered across the patagonian hinterland, demonstrate massive late Cretaceous epicontinental flooding and episodic Danian through Eocene marine retreat, leading to the development of an aelian-fed (Loessic) coastal plain superimposed on Mesozoic basins and arches that sepárate them. Deposition resumed after a widespread late Eocene-early Oligocene gap, following a pattern that altemated phases of shelfal flooding and coastal aggradation, with periods of shelfal emersion and degradational incision of the coastal plain. By middle and late Miocene the region was finally dominated by easterly directed progradation, possibly after roughly linear coastal accretion of the chenier-plain type. Sequence analysis ofthe 850-450 m thick Maastrichtian-Miocene coastal wedge dernonstrates the presence of, at least, thirteen unconfonnity bound stratal packages with basin-wide persistence. Sequence boundaries occur as complex surfaces irregularly shaped after subaerial incision, coastal abrasion, and marine scouring. The coastal plain members of most of the sequences contain characteristic fossil mammal assemblages (distinct Land Mammal Ages). The present sequence stratigraphic examination supports previous work pointing out that faunal changes take place mostly at stratigraphic breaks, that we now attribute to base level fluctuations. Considering the chronology of these stratal discontinuities and region allocation at the back of a mature passive margin dominated by mild and steady subsidence, we suggest that the discontinuities in the Central Patagonian mammal succesion developed in phase with accomodation changes linked to global eustatism.

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