Resumen

BUSTOS, Ulises Daniel  y  ASTINI, Ricardo A. Formación Punta Negra: análisis secuencial y evolución de la Cuenca Devónica Precordillerana. Rev. Asoc. Argent. Sedimentol. [online]. 1997, vol.4, n.2, pp.97-109. ISSN 1853-6360.

The Punta Negra Formation (Braccacini, 1949) is a thickening and coarsening-upward succession of sandstones interbedded with mudstones, of aproximately 1000 m thick, that outcrop in the Central Precordillera of San Juan province. The lower boundary is a muddy-shelf succession (Astini, 1991), the Talacasto Formation, some 600 m thick and dated as Lower to Middle Devonian (Herrera, 1991, 1993). It is unconformably overlain by Carboniferous deposits (Paganzo Group) of mostly transitional and continental environments (López Gamundí et al., 1994). The age of The Punta Negra Formation was formerly assigned to the Middle and Upper Devonian (Baldis, 1973; González Bonorino, 1975; Furque & Cuerda, 1979), based on stratigraphic relationships. New fossil findings include it in the Middle Devonian. With sedimentation rates typical of foreland basins (0.044-0.927 m/1000 years, Schwab, 1986), the period of deposition of the Punta Negra Formation could be around 15 million years. The cycles recognized in this unit, constitute aggradational to progradacional parasequence sets, interpreted as parts of a highstand systems tract (HST), linked to fourth order eustatic cycles. This interpretation is also based on the fact that the Punta Negra Formation overlies the Talacasto muddy shelf with which it is transitional. From a sequence stratigraphic viewpoint this can be interpreted as the transition from a mainly aggradational to a aggradational/progradational stage during the HST (Posamentier & Vail, 1988). Their main differences are the sandstone/shale ratios and the abundant plant remains both of which increase in the Punta Negra Formation, whereas the macrofauna drastically decreases. The average grain size increases and the maturity of the sandstones decreases in the Punta Negra Formation, which yields a high percentage of stable and unstable heavy minerals. The sedimentary proccesses would have operated in a ramp-type foreland basin, caracterized by an asymmetrical profile with the deeper zone adjacent to the orogenic belt (to the west), and depositional slopes of very low gradient (1º) adjacent to the foreland, with constant subsidence, high terrigenous influx and polarity of sedimentation east-west. According to its petrography the source area would have been the Sierras Pampeanas Occidentales (Pie de Palo-Valle Fértil-La Huerta). The unconformity which separated the Carboniferous from the Devonian is parallel and represents a sequence boundary, interpreted as a type 1 boundary. It may have been generated by a combination of tectonic and glacio-eustatic mechanisms, although due to the absence of angularity it may have had a strong sea-level control. For the first time we suggest that a probable major glacial-induced sea-level fall is more likely that an origin related to a main collisional event (the Chanic Orogeny). The local angularities between the Carboniferous and Precarboniferous strata in the Precordillera could be instead due to local extensional faulting.

Palabras clave : Punta Negra Formation; Highstand systems tract; Foreland ramp-type basin; Devonian-Carboniferous boundary; Chanic Orogeny.

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