Physical stratigraphy of the Sarmiento Formation (middle Eocene – lower Miocene) at Gran Barranca, central Patagonia

BELLOSI E. S.

The Paleontology of Gran Barranca: Evolution and Environmental Change through the Middle Cenozoic of Patagonia, eds. R. Madden, A. Carlini, M. Vucetich, and R. Kay

Cambridge University Press

Lugar: Cambridge; Año: 2010; p. 19 - 31

The middle Eocene to early Miocene pyroclastic Sarmiento Formation is a 319-m thick succession accumulated on loessic (eolian) and fluvial plains, and subordinately in shallowlakes. At the type section three memberswere originally recognized. Detailed studies along the continuous extent (7km) of this very well-exposed escarpment, allow therecognition of several stratigraphic discontinuities and lithologic changes, resulting in a new stratigraphic framework with six members. From the base to the top of the section,discontinuities have been numbered from 1 to 10 and classified morphologically and genetically into high-relief erosive unconformities, slightly erosive paraconformities, and nonerosive paleosurfaces. The Gran Barranca Member (middle Eocene) is continuously exposed along the length of the escarpment and includes within it the Discontinuity 1 atthe base of the richly fossiliferous marker Bed Y. The Gran BarrancaMember is separated by the Paraconformity 2 from the new Rosado Member (late middle Eocene), a paleosoldefined in central-eastern profiles. The former Puesto Almendra Member is subdivided by two erosive coplanar unconformities (Discontinuities 5 and 6). Discontinuity 3marks the base of the Lower Puesto Almendra Member (late Eocene). Discontinuity surface 4 subdivides thismember into two units. The new VeraMember (late Eocene to early Oligocene) is delimited at base and top by two deeply erosive unconformities (Discontinuities 5 and 6). As established by Re´ et al. (Chapter 3, this book), it is chronologically (and physically) interposed between the Lower and Upper Puesto Almendra Members. The Upper Puesto Almendra Member (Oligocene) includes three units separate by slightly erosive surfaces (Discontinuities 8 and 9). The Colhue-Huapi Member (early Miocene) is bounded at the base by the highly erosive and temporally important Unconformity 10. Changing paleotopography in central Patagonia is reconstructed by an assessment of these erosional unconformities.