Upper Paleozoic strata of the Chaco-Paranábasin, Argentina, and the great Gondwana glaciation

Abstract

Late Paleozoic glaciation in the southern Chaco-Paranábasin, Argentina, is recorded in cores from the Ordoñez Formation. The upper Ordoñez Formation consists of diamictite with lesser amounts of sandstone and shale, and is up to 1600 m thick. The diamictite-bearing interval was deposited during the Potonieisporites- Lundbladispora to Cristatisporites palynozones, probably in the latest Carboniferous-Early Permian. Diamictites are red to gray, thickly bedded, very poorly sorted mixtures of clay, silt, and sand with floating pebbles up to 5 cm long. These beds are structureless or show soft-sediment shear banding. An origin of diamictites related to glaciation is interpreted because of the following: the presence of dropstone pebbly mudshale; the angularity of sand grains and pebbles; the mixture of clast types in diamictite; and the freshness of most sand grains and pebbles. Subaqueous mudflow deposition is suggested for some diamictites because of interbedding with dropstone-bearing strata. Medium-scale, moderate to high-angle cross bedding is interbedded with other diamictite. That diamictite is interpreted as having been deposited as subglacial till or from subaerial ice-related mudflows. Pebble and sandstone composition in the Ordoñez Formation indicate a provenance terrane consisting of limestone, granite, gneiss, shale, sandstone, and volcanic units that existed to the west. Ordoñez Formation diamictite also becomes redder in wells in the Chaco-Paranábasin toward the western provenance area. The upper Ordoñez Formation is inferred to be partly contemporaneous with Paleozoic glacial deposits of the Paranábasin of Brazil, but was associated with a different ice lobe. Overlying the Ordoñez Formation is the Permian Victoriano Rodrı́guez Formation, which consists of shale and sandstone that was deposited in fluvial, floodbasin, and shallow to possibly deep-aqueous environments when the basin apparently was free of ice.