Tectonostratigraphic evolution of the Atuel depocenter during the Late Triassic to Early Jurassic rift stage, Neuquén basin, west-central Argentina

FLORENCIA BECHIS 1, LAURA B. GIAMBIAGI 2, MAISA A. TUNIK 3, JULIETA SURIANO 2, SILVIA LANÉS 4, JOSÉ F. MESCUA 2

Opening and Closure of the Neuquén Basin in the Southern Andes

ELSEVIER SCIENCE BV

Año: 2020; p. 23 - 52

The Neuquén basin presents an almost continuous record from the Late Triassic until the Paleocene, making it an excellent case study that registers the most relevant tectonic stages of southern South America during the Mesozoic. It was initiated in the Late Triassic to Early Jurassic as a continental rift basin in the context of a widespread extensional stage that affected western Gondwana and culminated with the break-up of the supercontinent. The Atuel depocenter is located in the northern sector of the basin. Synrift and sag units are represented by Upper Triassic to Lower Jurassic siliciclastic marine and continental sedimentary rocks, and it bears the oldest marine deposits of the basin, of Upper Triassic age. The depocenter infill has been deformed and exhumed during the Andean orogeny, being presently exposed in the northern sector of the Malargüe fold and thrust belt. In this review, we integrate the large set of stratigraphic, sedimentologic, geochronologic and structural data in order to unravel the tectono-sedimentary evolution of the Atuel depocenter, and to evaluate the main factors that controlled its development during the synrift stage. We analyze data from facies and thickness distribution of the synrift units, provenance studies on sandstone samples, detrital zircon geochronology data, kinematic data from outcrop scale normal faults, angular and progressive unconformities and subsurface information. Reactivation of pre-existing NNW-striking anisotropies under a regional NNE extension resulted in an oblique rift setting, which generated a bimodal distribution of NNW- and WNW-striking major normal faults. Strain and stress tensors obtained from the kinematic and dynamic analysis of structural data show a complex heterogeneity that we interpret as the result of local stress permutations due to the activity of the larger faults, and to strain partitioning inside the Atuel depocenter. Sedimentologic and petrographic data revealed a complex evolution with strong lateral variations of the deposition environments during the synrift phase, which took place from Rhaetian to Pliensbachian times. We identified several stages that were controlled by processes of initiation, propagation, growth, linkage and deactivation of new and reactivated faults along the depocenter evolution, in combination with sea level changes related to global eustatic variations. Provenance data suggest an important basin reorganization by the Toarcian, which could be related to initiation of the sag stage in this depocenter.