Evidence of glacial abrasion in the Calingasta-Uspallata and western Paganzo basins, mid-Carboniferous of western Argentina

Evidence of glacial abrasion is present in the basal part of the Carboniferous section of the Calingasta-Uspallata and Paganzo (western Argentina), where subglacial and proximal glacial-marine diamictites with pebbly (dropstone) shales constitute the dominant facies association. Intraformational striated pavements are exposed in the Hoyada Verde and Leoncito Formations of the Calingasta-Uspallata basin. Marine invertebrate fossils (Levispustula zone) in associated shales indicate a mid-Carboniferous age for both units. A single layer striated boulder pavement is superbly exposed in the Hoyada Verde Formation. Local a-axis clast imbrication in the underlying bouldery diamictite and strong parallel striae on the flattened upper surfaces of boulders indicate a subglacial origin for the pavement. Paleo-ice flow directions have an NNE-SSW strike. The Leoncito intraformational pavement is shaped on bioturbated, fine-grained sandstones with plant remains. Glacial striations are straight and parallel with subcircular cross-sections. Dome-like features are present in the surface, which is covered in turn by a sandy bouldery diamictite. Four striated surfaces are present in the western Paganzo basin. Three of them (pavements at the San Juan river valley, Loma de Los Piojos and Agua Hedionda) are developed on basement rocks. They contain parallel to subparallel striations, nailhead striations, crescentic gouges, impact marks and furrows. The pavements are overlain by thin (up to 1.5 m), laterally discontinuous diamictite beds with striated clasts, some of them bullet-shaped. Paleo-ice flow directions are N-NE-directed. An intraformational pavement is also present at the San Juan river valley section; it is carved on the top of the basal diamictite and exhibits furrows, probably caused by subglacial soft-sediment grooving. The dispersion of paleo-ice flow directions in the Calingasta-Uspallata and Paganzo basins indicates: (i) regional slopes toward the north into a brackish water embayment and northwest into the open marine conditions; (ii) glacial erosion by ice grounded below sea level and confined to the margins of the basins. (C) 2000 Elsevier Science B.V. All rights reserved.