Provenance record of late Maastrichtian-late Palaeocene Andean Mountain building in the Amazonian retroarc foreland basin (Madre de Dios basin, Peru)

Biostratigraphic, sedimentological and provenance analyses suggest that a proto-Andean Cordillera already existed in southern Peru by late Maastrichtian-late Palaeocene times. A 270-m-thick stratigraphic section shows changes in depositional environments from shallow marine (early Maastrichtian) to non-marine (late Maastrichtian) then back to estuarine (late Palaeocene) conditions. An erosional surface separates lower Maastrichtian from upper Maastrichtian deposits. Above this surface, the late Maastrichtian unit exhibits moderately developed palaeosols and syn-sedimentary normal faults. The sedimentary evolution is accompanied by a decrease in sedimentation rate and by changes in provenance. Shallow marine lower Maastrichtian deposits have a cratonic provenance as shown by their low epsilon Nd(0) values (-15 to -16) and the presence of Precambrian inherited zircon grains. The upper Maastrichtian deposits have a mixed Andean and cratonic origin with epsilon Nd(0) values of similar to 12.6 and yield the first Cretaceous and Permo-Triassic zircon grains. Estuarine to shallow marine upper Palaeocene deposits have an Andean dominant source as attested by higher epsilon Nd(0) values (-6 to -10) and by the presence of Palaeozoic and Late Cretaceous zircon grains. The changes in depositional environments and sedimentation rates, as well as the shift in detrital provenance, are consistent with a late Maastrichtian-late Palaeocene period of Andean mountain building. In agreement with recently published studies, our data suggest that an Andean retroarc foreland basin was active by late Maastrichtian-late Palaeocene times.