| Summary: | The Eocene La Meseta Formation is the youngest exposed unit of the back-arc James Ross Basin, Antarctic Peninsula, cropping out in Seymour (Marambio) Island. The formation comprises 720 m of clastic sedimentary rocks of deltaic, estuarine and shallow marine origin. It was subdivided into six unconformity-based units (Valle de Las Focas, Acantilados, Campamento, Cucullaea I, Cucullaea II and Submeseta Allomembers) grouped into three main facies associations. Facies association I represents valley-confined deposition in a progradational/aggradational tide-dominated and wave-influenced delta front/delta plain environment. Facies association II includes tidal channels, mixed tidal flats, tidal inlets and deltas, washover and beach environments. Facies association III represents nonconfined tide- and storm-influenced nearshore environments. La Meseta Formation sandstones are quartzofeldspathic with some hybrid arenites (glauconite and carbonate bioclasts-rich). Sandstone detrital modes are subdivided into two distinctive petrofacies: the low quartz petrofacies (petrofacies I, Q < 55% and L > 12%), interpreted to retain the original provenance signal, and the high quartz petrofacies (petrofacies II, Q>55% and L < 12%), representing the reworking product of the former after selective elimination of the more labile components. Petrofacies I sandstone framework grains were mainly derived from a dissected magmatic arc and an associated metamorphic belt. Textural evidence for recycling of some grains (e.g. garnet) from older sedimentary units during valley incision is not conclusive. Changes in the relative participation of source areas during the evolution of the incised-valley system are evaluated from the relative proportions of lithic fragments and monomineralic clasts derived from each rock type. Two lithic assemblages were recognized. The mixed lithic assemblage (Rv/Rm + Rp < 1.4) shows participation of all rock types; it represented valley-confined environments, either during the initial stage of ...
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