Cordao Formation: loess deposits in the southern coastal plain of the state of Rio Grande do Sul, Brazil

Loess consists of silt-dominated sediments that cover similar to 10% of the Earth's surface. In southern South America it occurs in Argentina, Bolivia, Paraguay and Uruguay, and its presence in southern Brazil was never studied in detail. Here is proposed a new lithostratigraphic unit, Cordao Formation, consisting of loess deposits in the southern Brazilian coastal plain. It consists of fine-very fine silt with subordinate sand and clay, found mostly in lowland areas between Pleistocene coastal barriers. These sediments are pale-colored (10YR hue) and forms similar to 1,5-2,0 meter-thick stable vertical walls. The clay minerals include illite, smectite, interstratified illite/smectite and kaolinite, the coarser fraction is mostly quartz and plagioclase. Caliche and iron-manganese nodules are also present. The only fossils found so far are rodent teeth and a tooth of a camelid (Hemiauchenia paradoxa). Luminescence ages indicate that this loess was deposited in the latest Pleistocene, between similar to 30 and 10 kyrs ago, and its upper portion was modified by erosion and accumulation of clay and organic matter in the Holocene. The estimated accumulation rate was similar to 630 g/m(2)/year. The probable source of this loess is the Pampean Aeolian System of Argentina and it would have been deposited by the increased aeolian processes of the last glacial.