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

Renato P Lopes,Sergio R Dillenburg,Cesar L Schultz

doi:10.1590/0001-3765201620150738

Abstract

Loess consists of silt-dominated sediments that cover ~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, Cordão 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 ~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 ~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 ~630 g/m2/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.

Highlights

Loess is a type of sediment dominated by silt-sized particles (0,004-0,062 mm in the scale of Wentworth 1922), transported and accumulated by aeolian activity (Pye 1984, Muhs 2006)
The traditional lithostratigraphic subdivision of the CPRS was replaced by the concept of depositional systems (Villwock and Tomazelli 1995), the proposal of a new lithostratigraphuic unit presented here is based on the recognition that its origin is distinct from the barrier-lagoon systems
Delaney (1962) considered that this silt represents the continuity of loess deposits found to the west, in the Province of Entre Ríos, which extended through Uruguay and the southern Rio Grande do Sul

Summary

Introduction

Loess is a type of sediment dominated by silt-sized particles (0,004-0,062 mm in the scale of Wentworth 1922), transported and accumulated by aeolian activity (Pye 1984, Muhs 2006). SCHULTZ transportation and deposition in floodplains, glacial outwashes or peridesertic areas by ice and rivers, and aeolian deflation and accumulation, which requires a trapping mechanism, such as topographic features or vegetation (Pye 1995, Wright 2001, Iriondo and Kröhling 2007a). Most loess deposits are found close to glaciated areas, formed by particles that were generated by frost weathering and mechanical grinding by glaciers, other deposits occur close to deserts, where processes such as insolation and salt weathering form silt-sized particles (Tsoar and Pye 1987, Iriondo 1999a, Wright 2001)

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