Sistemas deltaicos dominados por avenidas fluviales en el Grupo Curamalal (Paleozoico Inferior). Cuenca Paleozoica de Ventania, Provincia de Buenos Aires, Argentina

ZAVALA, C.; AZÚA, G.; FREIJE, H.; PONCE, J.J.

Revista de la Asociación Geológica Argentina -RAGA-

Año: 2000 vol. 55 p. 165 - 178

0004-4822

The Curamalal Group (Ordovician ?) represents the first depositional episode recorded in the Paleozoic Ventania Basin. Although it seems clear that the origin and evolution of this basin were related to the structural history of the south-western margin of Gondwana during the Paleozoic, its precise genesis continues to generate controversies. The main outcrops are located in the north-western side of the Sierras Australes foldbelt, and comprise an overall fining-upward succession of clastic deposits up to 1000 m thick, unconformably overlying a Precambrian Basament. The Curamalal Group is composed of four formations, named (from base to top) La Lola (conglomerates), Mascota, Trocadero (sandstones) and Hinojo (fine sandstones and mudstones). Field studies, supported by three detailed stratigraphical sections measured in the Cerro Pan de Azúcar area, permit the recosntruction of the depositional processes and geometry of the La Lola and Mascota formations. Seven facies have been identified, and are related here to the downstream dilution of fluvio-derived highly-concentrated sediment dispesions. The proximal facies are composed of sand-rich matrix-supported conglomerates (facies 1) related to hyperconcentrated flows (HCF), which underwent flow transformation downstream, resulting in residual clast-supported conglomerates (facies 2) and a range of fine-grained sandstones (facies 3 to 5) with HCS; and mudstons (facies 6) related to high to low density waning turbidity currents. In some place, these gravity-derived deposits show evidences of tidal reworking (facies 7). The deposits are characterized by an essentially tabular geometry, and may be interpreted as proximal shelfal sandstone lobes partially modified by tides, in a flood-dominated fan-delta system.