INSTITUTO SUPERIOR DE CORRELACION GEOLOGICA
Unidad Ejecutora - UE
Insights in the Neoproterozoic-Early Cambrian transition of NW Argentina: facies, environments and fossils in the Proto -margin of Gondwana
ACEÑOLAZA, G. Y ACEÑOLAZA, F.
Geological Society, Special Publication
The Geological Society of London
Año: 2007 vol. 286 p. 1 - 1
In Northwest Argentina, over 3000 meters of highly tectonized and metamorphosed siliciclastics of the Puncoviscana Formation underlies the more fossiliferous Cambro-Ordovician strata of the Mesón and Santa Victoria groups. Historically regarded as the non-fossiliferous basal Precambrian shield of the region, its age was later re-considered in part Phanerozoic, by the discovery of Early Cambrian trace fossils, and more recently by geochronologic data. Widespread siliciclastics characterize this sequence (with sandstones, shales and conglomerates). Limestones, volcanoclastics and lava flows have also been mentioned, emphasizing the complexity of facies and lithologies included in the basin. 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 Psammichnites saltensis in Seilacher et al. 2005), Neonereites uniserialis, N. biserialis, Oldhamia alata, O. antiqua, O. curvata, O. flabellata, O. geniculata, O. radiata, cf. Thalassinoides isp., Palaeophycus tubularis, Palaeophycus isp. Protichnites isp., Protovirgularia isp., Tasmanadia cachii, Treptichnus isp. Treptichnus cf. aequalternus and T. pollardi. 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. 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. 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. In addition, the recent discovery of a new fossiliferous locality, and the reevaluation of early described medusoids, provides the only two records of body fossils in the unit, with Selkirkia sp. and Beltanelloides sp. in the slates of Jujuy and Tucumán provinces. The recent usage of the Siberian stages to date some strata of the Puncoviscana Formation led to mis-interpretations 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 sedimentary and low grade metamorphic series developed in shallow tectonic stages of a multi-phase orogen.This contribution analyzes the Puncoviscana Formation and related units in the Neoproterozoic / Cambrian transition of Northwest Argentina, giving an updated view of problems and tasks. New data and a precise interpretation of facies are used as the only reliable source of information. Thus systematic studies and reevaluations of the strata and fauna in the Puncoviscana basin are the only way that will help to better understand the Neoproterozoic-Cambrian transition in the Andean margin of South America.