Middle East<subtitle>Stratigraphic Evolution and Oil Habitat</subtitle>

In NW Argentina, over 3000 metres of highly tectonized and metamorphosed siliciclastics of the Puncoviscana Formation underlie the more fossiliferous Cambro-Ordovician strata of the Mesó.n and Santa Victoria groups. Historically regarded as the non-fossiliferous 'basal Precambrian shield' of the region, its age was later found to be in part Phanerozoic, with the discovery of Early Cambrian trace fossils, and more recently refined with geochronological data. Widespread siliciclastic deposits characterize this sequence. A review of trace fossils in the Puncoviscana Formation has added new taxa. Trace fossil assemblages denote? geographical belts, indicating shallower marine conditions to the east (Nereites association) and deeper to the west (Oldhamia association). Such assemblages related to the palaeomorphology of the basin also represent different temporal levels, and in so doing reflect the overall evolution of the Puncoviscana Sea through time. The use of taxa that characterize Siberian stages in order to date parts of the Puncoviscana Formation should be abandoned as Siberian stages are defined by a specific set of fossils that are absent in the strata of NW Argentina. Thus, careful systematic studies and reevaluation of the strata and fauna in the Puncoviscana Basin are the only way to improve understanding of the Neoproterozoic-Cambrian transition in the Andean margin of South America.