Neogene litho and chronostratigraphy in the southwestern margin of the Colorado Basin (Río Negro, Argentina): The Gran Bajo Del Gualicho Formation

Abstract

The stratigraphy and age of the Patagoniense and Entrerriense Miocene marine deposits in northernmost Patagonia are not well established. In the Gran Bajo Del Gualicho and Las Grutas area (Río Negro Province, Argentina), these deposits are included in the Gran Bajo Del Gualicho Formation (GBDGF), which is divided into the lower Saladar Member and the upper Arriola Member. To provide new data on the general stratigraphy, numerical ages and provenance of the GBDGF, five detailed lithostratigraphic sections were measured and described and a tuffaceous sandstone sample was taken from each member for U–Pb zircon geochronology. Detrital zircon U–Pb records three main populations of Cenozoic, Mesozoic, and Paleozoic ages for both members, with two main age peaks at Middle-Late Miocene and at Early Cretaceous for the Arriola Member and a prominent peak in the Permian for the Saladar Member. Estimated maximum depositional ages are 10.94 ± 0.23 Ma and 19.07 ± 0.96 Ma for the Arriola and Saladar members, respectively. The geochronologic data, together with the stratigraphic analysis, allow to differentiate two marine sedimentation cycles separated by two important hiatuses: in the early Paleocene-Early Miocene and in the Early Miocene-Late Miocene. The Lower cycle contains a level of finegrained rocks with abundant fossils, which has been interpreted as a MFZ, indicating a deepening of the basin in the Early Miocene that would be controlled by dynamic subsidence as reported for other basins in Patagonia. Main sources for the zircon grains during the Early Miocene are from the North Patagonian Massif, with a great contribution of Permian rocks. During the Late Miocene, however, the main source would be the exposed rocks of the Neuquén Basin, including Mesozoic aged plutons and tuffs, and the contemporaneous Andean volcanism.