The Late Devonian glaciation in western Argentina: expanding the frontiers of Gondwanan glacial epoch.

EZPELETA M. Y ASTINI, R.A.

Bochum, Alemania

Congreso; SEPM-CES Annual Meeting 2008; 2008

Unquestionable Devonian glacial record has not been documented in Argentina. The absence of Upper Devonian – Lower Carboniferous deposits has been attributed to tectonic causes, linked with the collision of Chilenia Terrane along the Gondwana margin, or to the combined action of tectonism and glaciation during the late Early Carboniferous. In the Las Minitas Ranges, northern Precordillera, a notable record of diamictites within a strongly folded marine sequence (Jagüel Formation) is recorded. Bimodal igneous rocks bracketed Late Devonian-Early Carboniferous (Ar-Ar ages ca. 346-364 Ma, Coughlin 2000), where folded together with these rocks and both have been truncated in angular unconformity before deposition of Late Carboniferous deltaic systems (Río del Peñón Formation). Devonian trilobites recorded elsewhere in the Precordillera and associated brachiopod and bivalve faunas occur below and above this interval and certifie the Devonian age of the interval. Gray-greenish massive and stratified diamictites and pebbly mudstones (~70 m) including a variety of suspended boulders, are sharply overlain by a well stratified interval of bioturbated silty shales and graded beds yielding marine faunas. Diamictites possess a massive and meter scale interval at the base with subtle textural changes and different concentrations of suspended clasts inside the matrix (Dmm). Suspended subrounded to subangular blocks (> 1m) yield polished and striated surfaces. Well rounded boulders with flattened faces (flat-iron) or bullet-shapes are also common. All these features indicate that the sediment was partly involved within the subglacial load . Stratified diamictites (Dms), interbedded with the Dmm develop as interbedded intervals. Dms show fine-grained partings and contain abundant dropstones that commonly exceed bed thickness. The facies may be indicating water-lain processes where thin fine-grained partings are the product of residual settling after more massive rain out. Oversized clasts are clearly dropstones and show impact associated structures. Facies association also contain intervals (<2.5 m) of laminated mudstones with dispersed dropstones and sandy intervals (1.5 m) composed by groupings of tabular graded to massive layers separated by thoroughly bioturbaded mud partitions. Internal features are compatible with gravity flows (turbidity current deposits). Some of the sandy beds show subtle coarser grains on the top or pebble concentrations indicating winowing effects and rework by diluted bottom currents. Diamictites are locally involved within slumped and folded intervals (~17 m) linked with coherent flow processes (flow tills?). Clast composition is dominated by highly compacted sandstones, limestones, and basic rocks derived from underlying Ordovician units cropping in the nearby region. These compositions clearly differ from Nammurian diamictites where high-grade metamorphic rocks and granitic boulders are dominant. Paleontological and geochronological record allow constraining this glaciomarine sequence to Late Devonian and contribute to separate this glacial record from the overlying Río del Peñón Formation recording the Nammurian glaciation, immediately above the angular unconformity. The deformation and the bimodal igneous dykes that affect the Devonian record do not affect the Río del Peñón Formation, indicating an important tectonic event also explaining the angular unconformity between both. Glacial tillite included in the Jagüel Formation allows expanding the area covered by the Late Devonian Glaciation in South America, previously recorded in Brasil and along the Central Andes.