1990/07/01 by Teresa E. Jordan, Patrick M. Rutty, Lee E. McRae +4
Earth and Planetary Sciences · #Evolution and Paleontology Studies #Geological and Geochemical Analysis #Geological and Tectonic Studies in Latin America
paper · doi:10.1086/629422
The chronostratigraphy of synorogenic strata of Río Azul in the Bermejo foreland basin, Argentine Andes, reveals details of Miocene tectonic activity and deposition. Chronostratigraphic interpretations are based on magnetic polarity stratigraphy and zircon fission-track dates of interbedded volcanic ashes. The Río Azul section is located within the Precordillera thrust belt, uplifted and rotated by motion on subjacent faults. The Cenozoic strata are themselves broken by two principal zones of faulting, whose displacements are great enough to affect correlation of the local magnetic stratigraphy to the magnetic polarity time scale. The 3200 m thick section coarsens upward, from sandstone and mudstone (Cuculí Formation) to conglomerate (El Corral Formation); the magnetostratigraphic approach could be used only in the relatively fine-grained strata of the lower part of the section. The radiometric ages indicate the entire section is of middle to late Miocene age, and we correlate the magnetic polarity pattern of each fault block to the magnetic polarity time scale. The lower 500 m of the section apparently ranges from approximately 16 to 12 Ma, although the age of the lowest 100 m is quite uncertain. The fault-bounded block from 500 to 1000 m probably correlates to approximately 13.5-12.5 Ma. The block from 1000 to 2000 m correlates to approximately 12.5-10 Ma. Given the lack of unconformities in the coarse uppermost 1200 m of section and a zircon fission track age within the conglomerates, we speculate that the upper part of the section was deposited very rapidly; the top of the section is probably no younger than about 8 to 9 Ma. These correlations and similarities of facies, paleocurrents, and provenance imply that the Cuculí Formation represents the proximal facies correlative to the Jarillal Formation of Sierra de Huaco, and that the El Corral conglomerates are the up-dip equivalent of the Huachipampa Formation of Huaco. These results are consistent with a paleogeographic model of a major drainage reorganization in the Bermejo foreland basin at about 10 Ma. It is clear, however, that thrusting in the Precordillera began more than 2 m.y. prior to the drainage reorganization.