CONICET | Buscador de Institutos y Recursos Humanos

The Chubut Group constitutes the most widespread sedimentary unit in NE Patagonia, characterized by variable-energy fluvial deposits. UePb analysis of detrital zircons from two sections of the Chubut Group constraint the age of the oldest sedimentary rocks in the northeast of the Somuncura e Ca~nadon AsfaltoBasin. In the Ca~nadon Williams area, at San Jorge section, 20 km NW of Telsen locality, dating of 56 detrital zircons from a medium to coarse sandstone indicated a maximum depositional age of 109 ±1Ma (n ¼ 4). These sandstones were interpreted to represent shallow channels, associated with a lacustrinesystem. In the Telsen locality, a laser ablation analysis of 115 detrital zircons from a medium to coarsegrained sandstone, from fluvial channel facies, yielded a maximum depositional age of ca. 106 ± 1 Ma (n ¼ 8). Both ages are consistent with volcanic events of the Barremian to Albian age in the centralPatagonian Andes Region. Cathodoluminescence images of zircons from the San Jorge sample suggest an igneous origin, which is further supported by Th/U values above 0.5 in most of the grains. The distributionof the statistical modes of the main age populations of detrital zircons for the two samples [182,185 and 189 Ma for Telsen sample (T2S) and 181 ± 1 Ma for San Jorge sample (SJS)] matches the age of the volcanic Marifil Formation. The rocks of the Marifil Formation of these ages are exposed NE to SE ofthe study area. The abundance of zircons of similar Jurassic ages (n ¼ 52 for SJS and n ¼ 105 for T2S) and the external morphology of the zircons in the sample SJS, implies a close proximity of the source area.Suggestion that the Marifil Formation was the main provenance source is also supported by northeast esoutheasterly paleocurrents measured at the San Jorge and Telsen sections.

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