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

Abstract

In the Precordillera of western Argentina, an isolated outcrop of Llandeilian siliciclastics and Caradocian limestones (Rio Sassito succession) reveals a complex interplay between the tectonic and the sedimentary history of the 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 these unconformities, which in many places merged to form one single surface, demonstrates that the most important erosional event took place prior to the deposition of the Rio Sassito succession. This erosional event is correlated to extensional tectonics during continental breakup and the separation of the Precordillera from Laurentia. Block faulting with the formation of horst and graben structures provided the topography for the establishment of a pelagic carbonate platform during the Caradoc. In our view, there are no indications that these phenomena are related to the accretion of the Precordillera to Gondwana or to the formation of an Ordovician supercontinent. The carbonate sediments are typical of temperate-water settings, characterized 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.

Cryptostomid Bryozoans From The Sassito Formation, Upper Ordovician Cool‐Water Carbonates of the Argentinean Precordillera

“…Conodonts recovered from this upper carbonate interval (Keller and Lehnert 1998) reveal elements from the A. tvaerensis to A. ordovicicus biozones indicating a Katian age for the deposits.…”

“…The Sassito Formation is a 25 m thick Upper Ordovician carbonate unit described in detail by Astini and Cañas (1995) and Keller and Lehnert (1998). Outcrops are located in the central Precordillera, 45 km west of San Juan and along the Rio San Juan valley (Text‐fig.…”

“…The calcarenite facies are richly fossiliferous with brachiopods, crinoids and bryozoans as the main components. Bryozoan fragments have been reported from thin sections in sedimentological analyses of the unit (Astini and Cañas 1995; Keller and Lehnert 1998).…”

Graptolite and conodont faunas of Upper Ordovician (Sandbian) successions of the Argentine Precordillera: Biostratigraphic implications

“…The represented part of the A. tvaerensis Zone, would be the lower interval as occurs at La Cantera and La Pola formations in Villicum (Heredia & Milana, ), unlike the record at Los Azules Formation in the Cerro Viejo de Huaco (upper member; Ottone, Albanesi, Ortega, & Holfeltz, ) and Las Plantas Formation in the Cerro Potrerillo (Albanesi, Hünicken, & Barnes, ), where the middle part is recorded. In the Sierra de la Invernada Formation (Ortega et al, ) and Sassito Formation (Keller & Lehnert, ), the A. tvaerensis Zone is represented with the respective B. variabilis , Baltoniodus gerdae , and Baltoniodus alobatus subzones.…”

“…In North America, it has been identified in the eastern part of the Appalachians and from 1999) and Las Plantas Formation in the Cerro Potrerillo (Albanesi, Hünicken, & Barnes, 1998), where the middle part is recorded. In the Sierra de la Invernada Formation and Sassito Formation (Keller & Lehnert, 1998), the A. tvaerensis Zone is represented with the respective B. variabilis, Baltoniodus gerdae, and Baltoniodus alobatus subzones.…”