Evidence for late quaternary aeolian activity in the Roraima-Guyana region

Abstract

New evidence is provided for the aeolian origin of the upper unit of the Boa Vista Formation, found in the eastern part of the Brazilian state of Roraima and in neighboring Guyana. This unit has two geomorphologically/sedimentologically distinct domains: a sandy and a silty realm. Located to the east and southeast (upwind) of the silty realm, the sandy realm includes very highly eroded, inactive parabolic and linear dunes and sand sheets. An extensive field of parabolic dune remnants was detected in an orbital radar image and in low-altitude air photos of the interfluve of the Cauamé and Uraricoeira 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 and penetrating into intermontane valleys. The eroded fossil dunes, oriented 50-60°E, and the downwind grain size selection suggest a late Pleistocene dry period with trade wind direction the same as that of the modern dry season. A more recent dry period, probably Holocene, is suggested by the presence of many well preserved circular depressions in the upper unit of the Boa Vista formation, interpreted as circular pans or blowout hollows. © 2001 Elsevier Science B.V.