Evidence for Late Quaternary aeolian activity in the Roraima-Guyana region

New evidence is provided for the aeolian origin of the upper unit of the Bo a Vista Formation, found in the eastern part of the Brazilian state of Rora ima and in neighboring Guyana. This unit has two geomorphologically/sedimen tologically distinct domains: a sandy and a silty realm. Located to the eas t and southeast (upwind) of the silty realm, the sandy realm includes very highly eroded, inactive parabolic and linear dunes and sand sheets. An exte nsive field of parabolic dune remnants was detected in an orbital radar ima ge and in low-altitude air photos of the interfluve of the Cauame and Urari coeira rivers, near the town of Boa Vista. Along the downwind periphery of the sandy realm is a thin mantle of silty sediments forming a flat plain an d penetrating into intermontane valleys. The eroded fossil dunes, oriented 50-60 degreesE, and the downwind grain size selection suggest a late Pleist ocene dry period with trade wind direction the same as that of the modem dr y season. A more recent dry period, probably Holocene, is suggested by the presence of many well preserved circular depressions in the upper unit of t he Boa Vista formation, interpreted as circular pans or blowout hollows. (C ) 2001 Elsevier Science B.V. All rights reserved.