--> Preliminary Sedimentology and Provenance Analysis From the Magallanes-Austral Retroarc Foreland Basin: Implications for Miocene Evolution of the Patagonian Fold-and-Thrust Belt

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Preliminary Sedimentology and Provenance Analysis From the Magallanes-Austral Retroarc Foreland Basin: Implications for Miocene Evolution of the Patagonian Fold-and-Thrust Belt

Abstract

Production of topographic relief within fold-and-thrust belts can affect sediment dispersal into foreland basins by exposing or isolating upland sources, and by increasing erosional efficiency and sediment supply. Kinematic reconstructions of early Miocene time in the Patagonian Andes document the eastward migration of deformation on reactivated reverse faults, which likely produced significant topographic relief. This study investigates an Oligocene-Miocene sedimentary succession of the Magallanes-Austral foreland basin in Argentina (∼51°S) to determine the extent to which this relief generation controlled basin sedimentation. We report new sedimentology and sandstone petrography of 19 samples taken from ∼350 m of measured section of the Río Guillermo Fm., near Cancha Carrera, Argentina. Locally, the Río Guillermo Fm. rests disconformably on the Eocene-Oligocene shallow marine Río Turbio Fm. and represents the first major phase of fluvial sedimentation. Preliminary analysis of sedimentological data shows distinctive up-section changes in the style of fluvial sedimentation. The base of the section is characterized by cyclic coarse- and fine-grained facies consistent with an avulsion-dominated fluvial environment. This lower interval is overlain by a middle interval of organic-rich, horizontally bedded mudstone and siltstone facies, which is succeeded by an upper interval of channelized, coarse-grained strata. The upper coarse-grained interval lacks the distinct cyclic nature of the lower coarse-grained interval and is consistent with aggradation of a more stable channel configuration. Petrographic analysis of sandstone indicates immature lithic-rich compositions consistent with eastward directed paleocurrent measurements and transverse drainage of the thrust belt. Clast counts of interbedded conglomerate show an up-section covariance between granitoid and shale clasts, as well as an up-section decrease in silicic metavolcanic clasts that are diagnostic of hinterland sources. Additionally, porphyritic andesite appears in clast counts coincident with the change from the middle fine-grained interval to the upper channel-fill interval. These preliminary findings are consistent with the hypothesis that relief generated by renewed tectonic activity in the thrust belt exerted a primary control on basin sedimentation during this time. Ongoing detrital geo-thermochronology work will help differentiate tectonics from other controls on sedimentation history.