Spatio-temporal variations in the Mesozoic sedimentary record, Golfo San Jorge Basin (Patagonia, Argentina): Andean vs. cratonic sources

The continental succession of the Cretaceous Chubut Group (Aptian-Maastrichtian) in the Golfo San Jorge Basin (GSJB), central extra-Andean Patagonia (Argentina) contains a high proportion of volcaniclastic input derived from explosive volcanism at the Andes Cordillera. Cutting records obtained from regionally distributed oil-wells distanced by hundreds of kilometers show a significant west-to-east decreasing in the proportion of tuffaceous components. These regional lithologic changes currently difficult the recognition of lithostratigraphic units both in the western Rfo Mayo sub-basin (high tuffaceous content) and in the eastern basin center-offshore (low tuffaceous content). We use seismic sections and cutting records for: a) to study a basement intra-basin paleo-high in the Northern Flank (Rio Chico High) and b) to analyze the marginal basin substrate in offshore areas of the Northern Flank. The sedimentary record adjacent to the igneous-metamorphic Rfo Chico High contains variable amounts of quartz and detrital muscovite, identified in marine, pre-rift sequences of Lower Jurassic age and within the continental units of the Chubut Group. Basement influence in the infill composition decreases both upward and away from the Rfo Chico High, regarding the gradual burial of the paleo-high. Our data reveal that the igneous-metamorphic basement remained as a positive paleorelief compartmentalizing the basin and providing siliciclastic sediments at least during 100 millions of years, from early Jurassic to late Cretaceous, even before the beginning of the extensional phase that originated the basin in the uppermost Jurassic. Thus, the GSJB displays a spatially variable volcaniclastic record derived from the Andes, distorted by the presence of inherited morpho-structures that supply siliciclastic sediments provided from cratonic sources.