Abstract
Biostratigraphic, sedimentological and provenance analyses suggest that a proto-Andean Cordillera already existed in southern Peru by late Maastrichtian–latePalaeocene times. A 270-m-thick stratigraphic section shows changes in depositionalenvironments from shallow marine (early Maastrichtian) to non-marine (late Maastrichtian)then back to estuarine (late Palaeocene) conditions. An erosional surfaceseparates lower Maastrichtian from upper Maastrichtian deposits. Above this surface,the late Maastrichtian unit exhibits moderately developed palaeosols and synsedimentarynormal faults. The sedimentary evolution is accompanied by a decreasein sedimentation rate and by changes in provenance. Shallow marine lower Maastrichtiandeposits have a cratonic provenance as shown by their low eNd(0) values(15 to 16) and the presence of Precambrian inherited zircon grains. The upperMaastrichtian deposits have a mixed Andean and cratonic origin with eNd(0) valuesof ~12.6 and yield the first Cretaceous and Permo-Triassic zircon grains. Estuarineto shallow marine upper Palaeocene deposits have an Andean dominant source asattested by higher eNd(0) values (6 to 10) and by the presence of Palaeozoicand Late Cretaceous zircon grains. The changes in depositional environments andsedimentation rates, as well as the shift in detrital provenance, are consistent with alate Maastrichtian–late Palaeocene period of Andean mountain building. In agreementwith recently published studies, our data suggest that an Andean retroarcforeland basin was active by late Maastrichtian–late Palaeocene times.