The Rio Sassito sedimentary succession (Ordovician): a pinpoint in the geodynamic evolution of the Argentine Precordillera

In the Precordillera of western Argentina, an isolated outcrop of Llandeili an siliciclastics and Caradocian limestones (Rio Sassito succession) reveal s a complex interplay between the tectonic and the sedimentary history of t he Precordillera during Middle and Late Ordovician times. The succession is composed of a lower siliciclastic interval and an upper carbonate interval and is bounded below and above by erosional unconformities. Dating of thes e unconformities. which in many places merged to form one single surface, d emonstrates that the most important erosional event took place prior to the deposition of the Rio Sassito succession. This erosional event is correlat ed to extensional tectonics during continental breakup and the separation a ration of the Precordillera from Laurentia. Block faulting with the formati on of horst and graben structures provided the topography for the establish ment of a pelagic carbonate platform during the Caradoc. In our view, there are no indications that these phenomena are related to the accretion of th e Precordillera to Gondwana or to the formation of an Ordovician superconti nent. The carbonate sediments are typical of temperate-water settings, char acterized by the absence of ooids, oncoids, and algae, and by the presence of abundant abraded bioclasts, intraclasts, and peloids. The inference of a temperate-water environment does not, as previously supposed, indicate the accretion of the Precordillera to Gondwana. but is more likely related to global cooling prior to the Ashgillian glaciation.