Abstract
Triassic continental sedimentation in Southern South America is related to the Gondwanides orogenesis directly. There were two distinct phases of sedimentation, (1) the wide alluvial basins during Early Triassic and (2) the restricted extensional basins during Middle and Late Triassic. The main Early Triassic deposits are the Sanga do Cabral Supersequence (Southernmost Brazil and Uruguay) and the Talampaya and Tarjados sequences (Western Argentina). These deposits represent poorly channelized braided rivers with source-areas in the Gondwanides units under compressional tectonic setting. The main Middle and Late Triassic strata are rifts in the back-arc setting such as the Ischigualasto and Cuyo basins (Western Argentina) and in the intracontinental setting (Santa Maria Supersequence, Southernmost Brazil). These basins are mainly composed of alluvial, lacustrine and deltaic deposits with a great pyroclastic contribution in the case of the basins of Argentina. Two rift sequences were identified in the Western Argentina basins composed of syn-rift alluvial deposition followed by lake transgression. The sag phase is marked by the progradation of deltaic and high sinuosity rivers. The Southernmost Brazil deposits were divided into three rift sequences composed of low sinuosity rivers followed by lake transgression. The progradation of deltaic and fluvial systems marks the sag phase. The accommodation space generation within the back-arc rifts was high if compared to the intracontinental coeval basin due to higher base level and higher tectonic subsidence rates. The development of deep organic matter-rich lakes suggests a more humid palaeoclimate in the back-arc region than in the intracontinental setting where playa lakes dominated. The back-arc rifts are related to sinistral transtensional stresses in the western branch of Gondwanides region. The intracontinental rifts (the Southernmost Brazil deposits and coeval rift basins of Southern Africa) were generated by transtensional stresses within a dextral shear zone. The shear is related to stress transmission from the southern branch of the Gonwanides where oblique compression occurred.
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