PALEOGENE INITIATION OF ANDEAN FORELAND SEDIMENTATION IN THE BERMEJO BASIN AND PRECORDILLERA THRUST BELT OF NW ARGENTINA FROM DETRITAL GEOCHRONOLOGY AND THERMOCHRONOLOGY

The central Andean orogenic belt is composed of multiple tectonic domains with widely variable subducting plate geometries, lithospheric structure, deformational style, sedimentation patterns, physiography, and climate. Using a combination of detrital provenance methods, thermochronology, and sedimentology, this study characterizes the earliest foreland basin record in the Bermejo Basin in NW Argentina. We address the question of whether or not foreland basin development progressed synchronously along the Andean margin - and across these major domains - during early Cenozoic time.

The frontal Precordillera thrusts expose the Paleozoic sedimentary succession, unconformably overlain by 90-130 m of regionally extensive, massive siltstone with interbedded silty paleosols. Disconformably above the paleosol interval, eolian sandstone of the Vallecito Fm. transitions upward into the Miocene fluvio-alluvial strata with dominantly eastward sediment transport direction. Detrital zircon U-Pb ages from the base of the Vallecito Fm. indicate ca. 27 Ma maximum depositional age, suggesting that the underlying condensed paleosol interval is Oligocene and older. The remarkable similarity in timing and depositional facies of the paleosol interval and coarsening-upward trend of the Bermejo Basin section to foreland deposits preserved along-strike in the Eastern Cordillera supports the developing hypothesis of a continuous Paleogene foreland basin and associated retroarc thrusting. We interpret the ~100 m thick paleosol to represent previously unrecognized forebulge deposition in the Bermejo Basin.

Cenozoic detrital samples yield an upwards-younging of Andean magmatic arc zircons (40-10 Ma) and age peaks at 230-270 Ma, 290-320 Ma, 480-650 Ma, and 1.0-1.2 Ga. Similar ages characterize Permian eolian units beneath Cenozoic strata. This combined with eastward paleocurrent indicators suggest reworking of the Permian strata into the Cenozoic Bermejo Basin. The progressive reduction in 230-270 Ma grains from 20-6 Ma suggests isolation of the Frontal Cordillera during uplift of the Precordillera thrust sheets. Detrital (U-Th)/He and fission track thermochronology from the Cenozoic basin and Precordillera characterize spatial patterns and rates of thrust sheet exhumation during deposition.