Southernmost Exposures of the Arroyo del Soldado Group (Vendian to Cambrian, Uruguay): Palaeogeographic Implications for the Amalgamation of W-Gondwana

Abstract

The occurrence of the Vendian to lowermost Cambrian Arroyo del Soldado Group (ASG) is reported from an area located to the SW of Minas, which was formerly mapped as part of the Lavalleja Group. The Yerbal, Polanco and Cerro Espuelitas Formations of the ASG occur there as a rather continuous, microfossil-rich sedimentary cover, intruded by granites of probable Cambrian age (Minas Granite). Organic-walled microfossils recovered from the Yerbal and Polanco Formations include Bavlinella faveolata, Leiosphaeridia tenuissima, Soldadophycus bossii, Soldadophycus major, Siphonophycus solidum, Glenobotrydion aenigmatis, and filament mats. The acritarch Dyctiotidium sp. is described for the first time from the Yerbal Formation. Essentially the same microfossil assemblage occurs in five samples of this unit collected immediately to the north of Minas. No identifiable microfossils were found in fifteen samples of organic-rich lithologies of the Lavalleja Group. Based on detailed geological mapping, litho- and biostratigraphic data, we show that the contact between the Arroyo del Soldado and Lavalleja Groups is of tectonic nature (Arroyo La Plata Lineament), the latter being pre-Vendian in age. The Villa Serrana Block is defined, and consists of the Carapé and Lavalleja Groups, Las Ventanas and Playa Hermosa Formations, an unnamed siliciclastic-ultrabasic succession showing low-grade metamorphism, and various granitic intrusions mostly of Neoproterozoic to Cambrian age. Its boundaries are the Sarandí del Yí-Piriápolis shear zone to the W, the Sierra Ballena shear zone to the E, the Arroyo La Plata Lineament to the N, and the Río de la Plata to the S. The Villa Serrana Block could be an allochthonous terrane, or alternatively, a part of the Nico Pérez Terrane that was thrusted onto the ASG. No continuity exists between the Brusque and Porongos groups of southern Brazil and the Lavalleja Group. According to available data these units are probably not coeval, and do not represent the suture between the cratonic areas to the W (Río de la Plata Craton) and the remnants of a Neoproterozoic magmatic arc to the E (Cuchilla Dionisio-Pelotas Terrane). Accretion of these blocks was controlled by sinistral, continental-scale megashears. The geology of the area is far too complex to be explained in terms of a single orogeny following one Wilson cycle.