TECTONOSTRATIGRAPHIC SIGNIFICANCE OF THE NEOGENE SEDIMENTARY RECORD OF NORTHWESTERN AUSTRAL-MAGALLANES BASIN, ARGENTINEAN PATAGONIA

CUITIÑO, JOSÉ I.; ARAMENDÍA, INÉS; BOUZA, PABLO; GHIGLIONE, MATIAS

Latin American Journal of Sedimentology and Basin Analysis

Asociación Argentina de Sedimentología

Año: 2019 vol. 26 p. 99 - 126

1851-4979

Despite their geodynamic significance, the stratigraphy and paleoenvironments of the lower middle Miocene clastic deposits, genetically related to subduction-driven lithosphericprocesses at the latitude of the Chile Triple Junction are poorly known. These beds,outcropping along the foothills of the Southern Patagonian Andes in the northwestern edge ofthe Austral-Magallanes Basin, include the marine Centinela Formation transitionally coveredby the continental Río Zeballos Group. The aim of this work is to present an integratedstratigraphic section and to discuss its paleoenvironmental evolution, based on faciesanalysis. We inquired into possible tectonic-drivers during continentalization including a shortextensional episode affecting the basal Miocene sequences associated to the initial stages ofthe Andean uplift. We measured a 980 m-thick succession, subdivided into three sedimentaryunits (SU I, SU II and SU III). The lower SU I is dominated by shallow marine, bioturbatedsandstones and heterolithic deposits. SU II is dominated by interbedded medium- to pebblygrained cross-stratified sandstones and mudstones, interpreted as fluvial channels andfloodplains with incipient paleosols. Following, SU II shows a clear upward increase in thesandstone/mudstone ratio. Topping the sequences, SU III is composed of structurless andcross-bedded conglomerates interbedded with coarse-grained sandstones, interpreted aschannel and sheet-flood deposits within an alluvial fan. The whole coarsening upward trend,from shallow marine to fluvial and finally to alluvial fan deposits, suggests that these depositswere influenced by a progressive reduction in accommodation space. The sequence iscovered by basalts, truncated by a regional relictic surface composed of terraceconglomerates of the Rodados Patagónicos , reflecting basin abandonment and generation ofa bypass surface during the late Miocene-Quaternary. Based on our results, published U-Pbdetrital ages, and thermochronological data, we relate the evolution of the Neogenesedimentary systems to a synchronous contraction-related eastward migration of thedeformation front of the adjacent Andean orogenic belt, followed by neutral tectonics.