Latin American Journal of Sedimentology and Basin Analysis (Dec 2005)
La formación Puncoviscana y unidades estratigráficas vinculadas en el Neoproterozoico - Cámbrico Temprano del Noroeste Argentino The Puncoviscana Formation and related units in the Neoproterozoic - Early Cambrian transition of Northwest Argentina
Abstract
El lapso Neoproterozoico/Cámbrico temprano en la región noroeste de Argentina se encuentra representado por una potente secuencia sedimentaria que se destaca por su escaso metamorfismo y gran tectonización. Bajo la denominación de Formación Puncoviscana (s.l.) se unifican una serie de unidades rocosas escasamente fosilíferas con esquistos, areniscas, calizas y conglomerados con amplios afloramientos en la Cordillera Oriental de las provincias de Tucumán, Jujuy y Salta. Si bien la secuencia fue originariamente considerada como perteneciente al "basamento Precámbrico" de la región, el hallazgo de trazas fósiles eocámbricas permitieron su reasignación cronológica posteriormente ratificada por medios isotópicos. Análisis petrológicos y geoquímicos corroboran su asignación asociada al margen continental con el desarrollo de series metamórficas de un orógeno reciclado. Paleoambientalmente se reconocen una amplia variedad de contextos sedimentarios con sectores someros dominados por oleaje, secuencias turbidíticas, facies emipelágicas y calizas que denotan variados paleoambientes en el protomargen del Gondwana. La distribución de los elementos icnológicos en las unidades de la cuenca de Puncoviscana se presenta en fajas subparalelas con un sector este caracterizado por Nereites, y un sector oeste por Oldhamia. Esta distribución espacial representaría tanto caracteres morfológicos de la cuenca como niveles temporales distintos en la evolución del mar de Puncoviscana y su biota. La presente contribución provee un análisis actualizado de la Formación Puncoviscana y las unidades vinculadas en la región, aportando nuevas lineas de evidencia sobre las ideas más recientes desarrolladas sobre la transición Neoproterozoico/ Cámbrico temprano del noroeste de Argentina.Neoproterozoic/Early Cambrian rocks are represented in northwest Argentina by a thick and heterogeneous succession of highly deformed and slightly metamorphosed sedimentary rocks. The denomination of Puncoviscana Formation (s.l.) unifies a series of rock units, represented by scarce fossiliferous sligthly metamorphosed slates, schists, carbonates, conglomerates and sandstones cropping out mostly in the Cordillera Oriental of northwest Argentina (Turner, 1960; Turner and Mon, 1979; Aceñolaza et al., 1999) (Fig.1-3, 5). Originally described as the "basal Precambrian shield" of the region, it was reconsidered during the 70's by means of trace fossil associations denoting a Neoproterozoic - Early Cambrian age (Fig. 5). Integration of several lines of research as geochemistry, ichnology and stratigraphy suggest that sedimentation in the Puncoviscana Basin spanned from Neoproterozoic to the Early Cambrian (Fig. 3). The recent usage of the Siberian stages to date some strata of the Puncoviscana Formation has provoqued confusions in the literature, being highly recommended its abandonment. Siberian stages are defined by a precise set of fossils that is lacking in the strata of Northwest Argentina. Petrological and geochemical analysis corroborates a mainly authochthonous Gondwanan continental block origin (peripheral basin), with sedimentary and low grade metamorphic series developed in shallow tectonic stages of a multi-phase orogen. On the basis of general and detailed sedimentological studies, a varied spectrum of environmental settings have been recognized for the Puncoviscana basin since the 60's. Today a complex paleoenvironmental framework is understood, including shallow water wave-influenced strata, submarine fan sedimentation, hemipelagic clays and relatively deep ocean flysch-like shales and sandstones (Fig. 4). Although recent papers deny the presence of body fossils in the unit, the Puncoviscana Formation has provided two remarkable soft body fossils: Selkirkia sp. and Beltanelloides from outcrops of Jujuy and Tucumán provinces (Fig. 8). The ichnological research during the last 30 years contributed with a fairly acceptable knowledge of the paleontological content of the Puncoviscana Formation. Even though this is not a taxonomical contribution, the most obvious equivalences and synonymies are given. An updated review of the trace fossils in the Puncoviscana Formation provides the following list: Archaeonassa fossulata, Asaphoidichnus isp., Cochlichnus anguineus, Didymaulichnus lyelli, Dimorphichnus obliquus, Diplichnites isp., Glockerichnus isp., Helminthoraphe isp., Helminthopsis abeli, Helminthopsis tenuis; Helminthoidichnites tenuis, Monomorphichnus lineatus, Monomorphichnus isp., cf. Multipodichnus, Nereites saltensis (non Psamichnites saltesis, Seilacher et al., 2005), Neonereites uniserialis, N. biserialis, Oldhamia alata, O. antiqua, O. curvata, O. flabellata, O. geniculata, O. radiata, Palaeophycus tubularis, Paleophycus isp. Protichnites isp., Protovirgularia isp., Tasmanadia cachii, Treptichnus isp. Treptichnus cf. aequalternus and T. pollardi (Aceñolaza et al., 1999; Aceñolaza and Alonso, 2001; Aceñolaza, 2004) (Figs. 6-8). The geographical distribution of trace fossil assemblages display a remarkable alignment as belts, with a shallower eastwards Nereites association, and a deeper westwards Oldhamia associacion, that do not represent archetypal ichnofacies (Fig. 4) These are related to the morphology of the basin, a chronological record of the ichnofaunas, and may also represent different temporal levels on the evolution of the Puncoviscana Sea. Within the Oldhamia association most trace fossils are small sized, developed parallel to bedding planes, and bioturbation is essentially restricted to the first millimeters of strata. Deeper burrowers are recognized in the base of sandstone layers of the sequences. This spatial restriction of bioturbation with the frequent presence of wrinkle marks denotes a microbial mat related lifestyles of the fauna as it happens in other Neoproterozoic/Early Cambrian basins (Fig. 8). Within the Nereites association, a more diverse trace fossil set is recognized, with large and mid size grazing and crawling traces developed in sand-mud interfaces. Arthropod traces are common in this association. Integration of sedimentary and paleontological evidence provides remarkable elements to better understand the ecology of Late Proterozoic/Early Paleozoic biota. This contribution gives an updated analysis of the Puncoviscana Formation and related units of the region, offering new lines of evidence for the most recent ideas regarding the Neoproterozoic/Early Cambrian of NW Argentina.
