Los depósitos continentales del cretácico inferior tardío-superior en el norte de la cuenca neuquina (34-35°S) y su relación con el inicio del levantamiento andino

Abstract

A multi-proxy provenance approach of the Early Cretaceous-Late Cretaceous succession in the northern Neuquén Basin using sedimentology, petrology, detrital zircon U–Pb geochronology and detrital low-temperature thermochronology provides new constrains for the evolution of the Southern Central Andes. The stratigraphic interval studied represents the transition of the Neuquén Basin between the back-arc and foreland stage as the result of acceleration in the absolute movement of South American tectonic plate after the opening of Atlantic Ocean. This resulted in a compressive tectonic regime in the Andean sector and the eastward migration of the orogenic front. The Upper Cretaceous non-marine deposits of the Neuquén Basin have an important regional exposure and characterized as the first foreland basin associated with the early uplift of the Andean orogen at ca. 100 Ma. These deposits are included in the Neuquén Group, a well-studied unit in both the south and central part of the basin and associated with an important paleontological content and as reservoir rock in some depocenters of the basin. However, the northernmost exposed between the Laguna del Diamante and the Atuel River (34°-35°S)-assigned to the Diamante Formationhad not been studied in detail before this work. In the studied area, the Diamante Formation corresponds to a braided fluvial system evolving through time towards a meandering fluvial system possibly as a part of fluvial fan o Distributive Fluvial System (DFS). Petrographic analyses show a predominance of feldspathic litharenite, lithic feldarenite and litharenite, while the source areas are mainly recycled orogen (QFL diagram), mixed and dissected arc (QmFLt diagram). The presence of limestone clasts and fragments of ammonites in the middle part of the stratigraphic sections together with changes in paleocurrents suggest a regional source shift and provenance from the west of the studied area associated with the onset of an important stage of deformation on the west. Detrital zircons from the base of the Diamante Formation were supplied sources from Upper Jurassic and Lower Cretaceous that were exposed during Late Cretaceous times, whereas in the top, a high content of Permo-Triassic detrital zircons indicates exhumation of the surrounding Permo-Triassic crystalline basement. Two maximum depositional ages were obtained, 107.2 ± 1.4 Ma (1σ; n = 4) from the base and 91.1 ± 2.2 Ma (1σ; n = 3) to the top of the Diamante Formation. In particular, the age of 107.2 Mawould indicate that the sedimentation of the Upper Cretaceous non-marine deposits occurred in the Albian at these latitudes (34°-35°S). The comparison between U-Pb maximum depositional ages documented for nonmarine Upper Cretaceous deposits throughout the Neuquén Basin allowed a re-evaluation of the diachronism between the first foreland deposits of the basin. An apatite fission track analysis (AFT) of a sample from the lower part of the Diamante Formation indicated an Albian central cooling age. The sample also showed a notable presence of angular apatite crystals (21%) and a predominance of rounded and sub-angular grains (56% and 23%, respectively). Likewise, it indicates a predominance of sub-angular zircon crystals (28%), with a significant percentage of angular crystals (23%) and to a lesser extent rounded (19%). A zircon fissiontrack analysis (ZFT) of a sample from the upper part of the Diamante Formation yielded two discrete populations of cooling ages, both reflecting source-cooling during the Late Jurassic (~161 Ma) and the Permian (~265 Ma). Finally, comparisons between the AFT and the U-Pb maximum depositional zircon-ages of the same sample revealed a short lag time (ca. 3 ma), likely related to the rapid magmatic cooling of a coeval volcanic source at ~110 Ma (Albian). The results presented in this work suggest coeval volcanic activity during the Diamante Formation deposition. The new data provided indicates a need to re-evaluate the role of the volcanic arc in the Southern Andes during the late Early-Late Cretaceous and its influence over the foreland deposits; as well as a re-evaluation of the timing of the onset of the foreland basin.