CONICET | Buscador de Institutos y Recursos Humanos

Marine rocks comprising three uncoformably bounded units (Upper Cretaceous, Lower Paleocene and Paleocene) are first described in Sierra de Apen. The Upper Cretaceous unit massive-faintly laminated mudstone with minor cross-stratified sandstone indicating north-northwest directed paleocurrents. Some intervals and calcareous concretions display a variety of trace fossils. Ammonites are rare and dinocysts indicate a Maastrichtian age. The unit is topped by an ersion surface above which lays the sandier Lower Paleocene unit, with a thin basal conglomerate. Clasts are well rounded mudstone and concretions, eroded from the underlying unit, and quartz, indurated mudstone and acidic volcanics. The dinocyst assemblage in this middle unit has an Early Paleocene age. The overlying unit has a very coarse grained and thick polymictic orthoconglomerate resting on an apparently major unconfromity that laterally eliminates all the Lower Paleocene and part of the Upper Cretaceous units within a short distance. The Upper Paleocene unit consists of fining and thining upward lenticular conglomerate encased in thin bedded mudstone-sandstone rhythmites. Conglomerate clasts involve bluish gray sandstone and coquina, presumably derived from the underlaying Lower Paleocene unit. Clast imbrication reflects northeast directed paleoflows. Dinocysts in this unit suggest a Paleocene age. The Upper Cretaceous unit reflects deposition below wave base in the outer shelf. Both Paleogene units form part of a fan-deltaic succession. Though somewhat older, the Paleocene conglomerates correspond to part of the Chilean Ballena Formation and other roughly age equivalent rocks of Argentinean Tierra del Fuego. The unconformities appear coeval with similar horizons in the subsurface of the Austral and Malvinas basins. The Late Cretaceous N-NW directed sediment dispersal pattern probably mirrors early stages of the foreland basin development, whereas the coarser grained Paleogene represents deposition in the already defined foreland basin in front of the rising cordillera.

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