BASIN EVOLUTION RECORDED IN CRETACEOUS STRATIGRAPHY WITHIN THE MALARGüE FOLD AND THRUST BELT (NORTHERN NEUQUéN BASIN, ARGENTINA)

Timing of the onset of compressional deformation along the Andean margin is poorly constrained. Although most of the early foreland basin record has been eroded or covered with volcanics in the Neuquén basin of Argentina, from 32° S to 40° S latitudes, excellent exposures are still preserved. The Neuquén basin contains a 6000m thick sedimentary sequence ranging in age from late Triassic to late Cenozoic. The Neuquén basin initiated as an east-west trending rift basin in the late Paleozoic-early Mesozoic and then evolved into a foreland basin in the Cretaceous. Previous workers have used changes in detrital zircon signature between the Aptian-Albian Rayoso Group and the Cenomanian Neuquén Group as evidence for the initiation of foreland basin sedimentation. However, apparent growth strata within the Neuquén Group indicate deposition in a wedge-top depozone implying an older timing of foreland basin initiation.

Cretaceous exposures in the Malargüe fold and thrust belt have been correlated with previously studied units to the south and likely record the same transition from post-rift thermal subsidence to flexural subsidence. Although there is a major facies change between the marine evaporites of the Huirtin Formation (within the Rayoso Group) and the fluvial sandstones and conglomerates of the Diamante Formation (equivalent to the Neuquén Group), this transition could be due to a global sea level fall at this time and unrelated to local Neuquén Basin geodynamics. Timing of this critical transition is poorly constrained due to a lack of adequate age data. Detrital zircon and volcaic tuff U-Pb geochronology and apatite fission track thermochronology will be used to determine the age and exhumation of both the basin sediments and their source areas to the west.

Three stratigraphic sections were measured in the Malargüe area to assess thickness and facies variability, and samples were collected at regular intervals for petrographic and provenance analysis. The regional map pattern indicates that either an angular unconformity or an erosive disconformity exists between the Huirtin and Diamante formations, which potentially represents the migration of the flexural forebulge through the area. Our preliminary data suggests an earlier (Aptian-Albian) age for initial shortening at this latitude than previously reported.