Integrated correlation of Neoproterozoic to Cambrian sedimentary successions from the Río de la Plata Craton (Argentina and Uruguay)

GAUCHER, C., POIRÉ, D.G., GÓMEZ PERAL, L. & CHIGLINO, L.

Mendoza

Congreso; Godwana 12; 2005

Academia Nacional de Ciencias

The litho-, bio- and chemostratigraphy of the Neoproterozoic to Cambrian Arroyo del Soldado Group (ASG) of Uruguay and Sierras Bayas Group ¨CCerro Negro Formation (SBG-CNFm) of Tandilia (Argentina) is compared. Both successions are characterized by an alternation of carbonates and siliciclastics, as well as a geotectonic setting coresponding to a passive continental margin. The Yerbal, Polanco and Cerro Espuelitas Formations (ASG) are correlated lithostratigraphically with the Cerro Largo, Loma Negra and Cerro Negro Formations of Tandilia. Whereas the ASG is up to 5 km thick, the SBG-CNFm represents a strongly condensed section showing numerous erosive surfaces and only 350 m in thickness. This difference may be explained by the shallower, epicontinental sedimentary environment of the SBG-CNFm. The siliciclastic Yerbal and Cerro Largo Formations represent the transgression of the Ediacaran sea onto the R¨ªo de la Plata Craton. The overlying Polanco and Loma Negra Formations are characterized by thick, pure carbonate (especially limestone) deposits. An expressive palaeokarst is developed on top of these carbonates, indicating sea-level drop and platform exposure. The lower Cerro Espuelitas Formation and the Cerro Negro Formation are made up of marls at the base, passing into shales and heterolithic facies up section. We report for the first time the occurrence of Cloudina in micritic limestones of the Loma Negra Formation. This supports the correlation with the lower Polanco Formation, where these shelly fossils also occur. Moreover, Cloudina is currently regarded an index fossil of the late Ediacaran. Low diversity acritarch assemblages occur both in the ASG and SBG-CNFm. In the former, two assemblages were distinguished: a Bavlinella faveolata-Soldadophycus assemblage occurring mainly in the siliciclastic units, and a slightly more diverse Leiosphaeridia-Lophosphaeridium assemblage characteristic of the Polanco Formation. In the SBG-CNFm, the acritarch asemblage is sphaeromorph-dominated (mainly Leiosphaeridia), and similar to the Leiosphaeridia-Lophosphaeridium microflora of the ASG. Absence of the open marine Bavlinella-Soldadophycus assemblage in the SBG-CNFm could be due to the shallower, epicontinental setting of this unit. We report acritarchs occurring in greenish pelites of the Villa Mónica Formation, assigned here to Leiosphaeridia minutissima, L. tenuissima Synsphaeridium, which are the oldest fossils so far reported from Argentina. Chuaria olavarriensis, Leiosphaeridia Paleorivularia ontarica were previously reported from the upper Cerro Largo Formation. The former two taxa can be reassigned to Chuaria circularis Leiosphaeridia minutissima respectively. Shales of the Cerro Largo Formation yielded Leiosphaeridia tenuissima as dominant species, and a single vesicle assigned to Leiosphaeridia minutissima. From the Cerro Negro Formation the following acritarch species have been previously described: Leiosphaeridia Trachysphaeridium Synsphaeridium sp. and granular sphaeromorphs grouped under type acritarchs. The application of the currently accepted taxonomic criteria to these acritarchs allows to assign them to