Tectono-sedimentary model for the evolution of the Silves Group (Triassic, Lusitanian basin, Portugal)

At the beginning of the Alpine cycle, the breakup of Pangea lead to the early stages of the North Atlantic opening. In the western Iberian sector of the European margin, the Lusitanian basin starts to evolve bordered eastwards by inherited reliefs from the late episodes of the Variscan orogeny. The base unit, the Silves Group, considered not earlier than the Carnian, is mainly siliciclastic and predominantly formed by arcosarenites to feldspar litarenites, coarse to very coarse-grained (wackes) and pebbly, where the sediment architecture denotes organizations in continental (Conraria. Penela and Castelo Viegas Formations) environments. A first marine episode (Isocyprina Beds of Pereiros Fm.) marks a significant change within the sedimentary record; the uppermost part corresponds to intertidal transitional environments (sabkha). New and detailed field work of sedimentological and structural nature that has been carried out in recent exposures from the type-region of Coimbra-Penela enabled us to make significant observations and to improve data collection. This allowed a full reinterpretation of the paleotectonic and paleogeographical conditions under which the Silves Group and, consequently, the eastern border of the Lusitanian basin, evolved. This study carried out in the type-region also allowed a better understanding of its sequential organization. All units are unconformity bounded by strong influxes of coarse siliciclastics from the Iberian meseta. One of those unconformities (D2b) is an angular unconformity with cartographic expression. Tectonic reconstructions were possible to make after a detailed structural analysis of normal synsedimentary faults. Regional comparisons with Eastern Iberian basins that evolved since Permian times are also discussed. We conclude that the lower red siliciclastic units are older than has been considered until now. Those units were formed in a previous tensional stress pattern of tardi-Variscan affinities, related to megashear dextral kinematics of Permian-Triassic age. We propose that units below D2b unconformity can record a Proto-Lusitanian basin; the Lusita-Man basin is younger and evolved mainly after the Triassic-Jurassic limit (Castelo Viegas Fm.) within an E-W extensional context related to Atlantic type basins.