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Intendente Alvear, Argentina

Early Miocene shallow marine deposits in the region of Lago Posadas-Meseta Belgrano (Argentina) represent part of the “Patagoniense” transgression, an Atlantic marine incursion that flooded large part of Patagonia, including the Austral (foreland) Basin (southern Patagonia). These deposits, referred as El Chacay (Argentina) or Guadal (Chile) formations, and the transition to the overlying Santa Cruz Formation were divided into six facies: subtidal sand-bars, shallow marine sandy deposits, muddy shelf deposits, estuarine complex deposits, fluvial channels and fluvial floodplains. These are arranged in a general transgressive-regressive cycle, subdivided into two stratigraphic sequences, separated by a major erosional surface.87Sr/86Sr ages from shell carbonate in eight oysters yielded an age range of 20.3 to 18.1 Ma for these “Patagoniense” deposits. Correlation with other dated “Patagoniense” sections in southern Patagonia, like those at Lago Argentino or Comodoro Rivadavia, indicates that they belong to a single transgression that flooded several Patagonian basins approximately at the same time. Eustasy, flexural subsidence created by tectonic loading in the adjacent fold-and-thrust belt, and basin floor paleo-topography controlled the duration of the depositional event and the sedimentation style of these shallow marine deposits. © 2015, Servicio Nacional de Geologia y Mineria. All rights reserved. Source


Cuitino J.I.,University of Buenos Aires | Cuitino J.I.,Institute Geociencias Basicas | Santos R.V.,University of Brasilia | Scasso R.A.,University of Buenos Aires | Scasso R.A.,Institute Geociencias Basicas
Palaios | Year: 2013

The lower Miocene Estancia 25 de Mayo Formation consists of shallowmarine to estuarine deposits that bear numerous oyster shell beds distributed throughout the succession. Facies analysis reveals that oysters grew in the nearshore paleoenvironments of both the lower Quién Sabe and the upper Bandurrias members. Two oyster species were identified: Crassostrea(?) hatcheri, distributed in the lower two thirds of the column, and Crassostrea orbignyi, distributed in the upper third of the column within the transition from marine to the fluvial deposits of the overlying Santa Cruz Formation. Petrographic, cathodoluminiscence, and carbon and oxygen stable isotope analyses of individual growth increments were performed on carbonate from all the oyster beds, together with consecutive time-series analyses for one specimen of each species. The isotopic composition of the shell seems to be associated with the microstructure of the growth increments. Selective diagenetic alteration affected chalky growth increments; whereas translucent (foliated and prismatic) growth increments are well preserved and can be used to infer paleoenvironmental conditions. Isotopic data indicate that C.(?) hatcheri lived in normal marine waters with a range of paleotemperatures from 10.6 to 20.5 °C. Monospecific beds of this oyster are related to opportunistic and quick colonization of the sea bottom. More negative δ13C and δ18O values in C. orbignyi shells likely reflect decreased paleosalinities, and the resultant environmental stress controlled the development of these low-diversity oyster accumulations. The separate stratigraphic distribution of each species was therefore determined by the paleoenvironmental conditions. © 2013, SEPM (Society for Sedimentary Geology). Source


Ozan I.L.,CONICET | Ozan I.L.,Institute Geociencias Basicas | Ozan I.L.,University of Buenos Aires | Orgeira M.J.,CONICET | And 2 more authors.
Revista de la Asociacion Geologica Argentina | Year: 2015

The aim of the present work is to get an insight into the origin of the magnetic signature (natural and/or anthropogenic) of archaeological deposits and its relation to soil micromorphology. For this purpose, two profiles were sampled from the Late Holocene archaeological site Marazzi 2, located in northwestern Isla Grande de Tierra del Fuego (Chile). The P1 profile shows a magnetic signature peak (due to magnetite and/or titanomagnetite) between 30 - 70 cm depth, which coincides with Ab and ABb soil horizons for which intense anthropic activity (including combustion) was recorded. It is states that the anthropogenic activity is the main cause of that enrichment. The P2 profile exhibits a peak in the coercivity of remanence parameter between 20 - 40 cm depth (due to hematite) which coincides with few anthropogenic components and an AB soil horizon. The record of “red pigments” (carried by past-human populations) in a surrounded excavation could explain the increment of this parameter. The study highlights that soil micromorphology can help in the understanding of the processes that affect the magnetic properties of soils and paleosols. © 2015, Asociacion Geologica Argentina. All rights reserved. Source

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