Unsteady and spatially variable evolution of the Neogene Andean Bermejo foreland basin, Argentina

The Miocene to Recent Bermejo retroarc foreland basin formed during development of the Andes Mountains and Sierras Pampeanas broken foreland, in west-central Argentina. Three geometrically distinct intervals existed. First, between 20 and approximately 7.3 Ma, a broad wedge-like ‘simple foreland’ basin formed adjacent to the advancing thrusts of the Western and Central Precordillera thrust belt. Second, between 7.3 and 6.5 Ma, a symmetrical foreland basin formed, while the Precordillera was likely in a subcritical state. Third, between 6.5 Ma and today, the Bermejo basin was narrow and asymmetrical, with polarity opposite to that of the simple foreland stage. The third stage accompanied both thin-skinned thrusting in the Central Precordillera and mechanical disruption of the foreland basement into small segments bounded by reverse faults of the Sierras Pampeanas. gh the 20–7.3 Ma interval can be characterized as simple foreland, there was considerable along-strike variability. The Bermejo basin was both wider and thicker in the north (29–30°S latitude) than in the south (near 31°S latitude). A northward increase in sediment supply, lateral variability in the basement, and minor along-strike variability of crustal thickening within the Precordillera thrust belt may have all contributed to the along-strike changes in the basin, but a unique cause of this difference cannot be determined with the available data. tent aridity was the dominant climate state throughout the 20 million year Bermejo basin history. Variability in climate is expressed in the facies, especially demonstrated by the contrast between preservation of extensive lower and middle Miocene dune deposits and their absence in strata younger than about 14 Ma, and more subtly by changes in influx of water to the basin from the high Andes catchment area.