A Silurian-Devonian marine platform-deltaic system in the San Geological Society, London, Special Publications

The San Rafael Block is included as a part of the pre-Andean region, in the southern sector of the Argentine Precordillera–Cuyania terrane, within the western Gondwana margin. The Rı́o Seco de los Castaños Formation (Upper Silurian–Lower Devonian) is one of the major marine-siliciclastic pre-Carboniferous units, and is interpreted as a distal to proximal silty platform-deltaic system. The dominant sedimentary processes were wave and storm action and the source areas were located to the east, close to the study area. The rocks are mainly of immature arkosic sandstones showing both recycled orogen and continental block provenances. Sedimentological characteristics of conglomerate-filled channels and an organic-matter-rich bed are described. X-ray diffraction analyses of the clay minerals from the sequences show that very low-grade metamorphic conditions acted during the Early Carboniferous. Geochemical analyses indicate moderate to strong weathering, and potassium metasomatism. Zr/Sc ratios lower than 22, no important enrichments of Zr, Th/Sc ratios, high Sc and Cr concentration and the Eu-anomalies indicate a provenance from a less evolved upper continental crust. TDM ages and 1Nd are within the range of the Mesoproterozoic basement and Palaeozoic supracrustal rocks from the Precordillera–Cuyania terrane. Probable sources, tectonic setting and land–sea interactions are discussed. The southern Pacific South America Gondwana margin (Fig. 1a) is characterized during the Palaeozoic by the presence of orogenic belts orientated approximately north–south (Ramos et al. 1986). They were accreted to the cratonic areas during the Cambrian (Pampean), Mid-Ordovician (Famatinian) and Late Devonian (Gondwanian) tectonic cycles (Fig. 1b). The Argentine Precordillera or Cuyania composite terrane in the sense of Ramos et al. (1986) is related to the Famatinian cycle and lies eastward of the present-day Andes. Four sectors constitute this composite terrane: (a) the Precordillera thin-skinned fold and thrust belt that was generated by shallow east-dipping flat-slab subduction of the Nazca plate; (b) the Pie de Palo area, (c) San Rafael and (d) Las Matras blocks. This terrane had been considered stratigraphically and faunally unique to South America mainly for the Lower Palaeozoic carbonate and siliciclastic deposits overlying an igneous-metamorphic crust of ‘Grenville age’ (Ramos et al. 1998; Sato et al. 2004, and references therein). The Precordillera–Cuyania terrane has been the object of several lines of research during recent years, attempting to constrain its allochthonous or para-autochthonous origin with respect to Gondwana. One of the tectonic interpretations suggests that the Precordillera–Cuyania was detached from Laurentia in Cambrian times, was transferred to western Gondwana during the Early to Middle Ordovician, and was amalgamated to the early proto-Andean margin of Gondwana by the Mid-Late Ordovician (Thomas & Astini 2003 and references therein). Other studies have claimed a para-autochthonous-to-Gondwana origin based on strike-slip displacements from the South Africa–Antarctica regions (Aceñolaza et al. 2002; Finney et al. 2003). A late Middle to Late Ordovician time of docking for the Precordillera– Cuyania is constrained by a variety of geological and palaeontological evidence (Ramos 2004 and references therein). The terrane deformation linked to the collision started in the Ordovician, and continued until the time of approach of the Chilenia terrane during the Late Devonian, against the Pacific side (Fig. 1b). In this tectonic scenario, Silurian–Devonian siliciclastic depocentres of foreland basins were developed in the Precordillera–Cuyania terrane. One of them is preserved in the San Rafael Block, within the southern sector of this terrane (Fig. 2). From: KÖNIGSHOF, P. (ed.) Devonian Change: Case Studies in Palaeogeography and Palaeoecology. The Geological Society, London, Special Publications, 314, 215–240. DOI: 10.1144/SP314.12 0305-8719/09/$15.00 # The Geological Society of London 2009. Fig. 1. (a) Location of the San Rafael Block in the Upper Silurian–Lower Devonian palaeogeographic reconstruction (after Torsvik & Cocks 2004). In Gondwana continent land masses are dark grey, marine transgressions are grey. (b) Regional location map showing the pre-Andean San Rafael Block in the Argentine Precordillera–Cuyania composite terrane and arrangement of the adjacent tectonic terranes. M. J. MANASSERO ET AL. 216