The structural and stratigraphic evolution of the La Rioja basin, Argentina

Abstract

The La Rioja sedimentary basin underlies the Llanos de La Rioja valley in the Sierras Pampeanas region of western Argentina. The Sierras Pampeanas is a region of young reverse-fault-bounded ranges of crystalline basement separated by broad valleys. Repsol-YPF allowed use of 280 km of 2D seismic lines registered in the western La Rioja basin, which were processed and interpreted for structural and stratigraphic relationships. Lacking wells within the seismic grid, we relied on refraction data, nearby geological studies, and seismic stratigraphy to constrain the interpretation. The findings indicate a substantial thickness (up to 800 m) of Paleozoic strata thinning to the east and directly covering the crystalline basement. The predominate unit is Cenozoic fill, which dominates the upper 1–3 s of the seismic data and ranges in thickness from 3 km below the city of La Rioja, near the western basin margin, to approximately 1.5 km at the east end of the seismic grid, 60 km east of La Rioja. Between the Paleozoic and Cenozoic strata are two units, one regionally distributed and the other very localized. The regionally extensive unit is a thin horizon with high reflectivity that overlies the more localized unit. This local unit is interpreted as a narrow rift basin in the northern La Rioja basin that is likely of Cretaceous age. Deformation within the La Rioja basin is controlled predominantly by north-trending reverse faults with offsets ranging from 200 to 2000 m. Faults in the north half of the basin are not continuous with those in the south half, which suggests the existence of an east-trending transfer zone, aligned with the east-trending valley separating Sierra de Velasco into northern and southern blocks and aligned with the north end of Sierra Brava. The major north-trending faults were active during accumulation of the upper half of the Cenozoic strata.