Structure of the foothills of the Andes at the southern Chilean-Pampean flat subduction zone: The Cuyana Basin re-evaluated as a not-inverted basin

Abstract

The Cuyana Basin is amongst the most mature hydrocarbon-productive basins of South America. However, there is an ongoing discussion about how basin-forming mechanisms (Triassic-Jurassic) encroached with deformative mechanisms (Paleogene- Neogene) to create the hydrocarbon plays that are productive until now. To find better answers to this dilemma, an unprecedented detailed study describing the structure and distribution of the main depocenters of the Cuyana Basin was carried out. This basin is the main oil-producing Triassic rift basin in Argentina, and its subsidence relates to the break up of southern Gondwana, while the oil reservoirs were generated in association with Andean uplift at these latitudes. For this purpose, we have used 3D seismic, profiles and borehole data from all the different depocenters of this basin, located at the foothills of the Southern Central Andes between 33° and 34°S. From the correlation between bore-hole data and the different reflectors of the 3D seismic data, we disclosed a more complex structure of this rift basin than previously determined. The 3D seismic analysis also served to distinguish synrift geometries, sag, and synorogenic facies, determining the existence of only one synrift stage, and not two synrift stages as proposed previously. Additionally, we have found that the Cuyana Basin has a longer sag evolution than previously described, including the Barrancas Formation in this stage. This finding served to prolong the extensional period of this rift basin from the Early Triassic to the Late Jurassic, hence overlapping its evolution to other neighboring sub-Andean rift depocenters. Pre-Andean E-W structures have accommodated limited strike-slip displacements in the order of 2.5 km, previous to the uplift of the Frontal Cordillera, to the west. This basin is incorporated into the Andean deformational front from at least the latest Miocene to the Pliocene-Quaternary forming the Cerrilladas Pedemontanas Belt, in the low foothills. From this analysis, the NNW Neogene compressional structures are mostly decoupled from the Triassic-Jurassic extensional architecture, suggesting this is not an inverted basin. While we interpret that rifting was controlled by basement Early Paleozoic accretional structures, Neogene ones were not coinciding with them, a fact that we portrayed with our 3D analysis.