Late cenozoic history of the eastern llanos of venezuela: Geomorphology and stratigraphy of the Mesa Formation

Abstract

The study area corresponds to the Mapire River basin in the southern part of the State of Anzoátegui and constitutes a region of low relief covered by the Pleistocene Mesa Formation. This formation consists of horizontally bedded alluvial sediments, represented by light gray, coarse- to fine-grained quartz sand, with cross-bedding marked by ochre, red and pink alteration, and to a minor extent by marsh deposits, represented by laminated and spotted shale. The typical topography consists of mesas north of the left margin of the Orinoco River (the Musinacio and Mapire Mesas). To the north of the mesas there is a topography of low hills covered by blocks of ferruginous crusts. A typical tropical savanna climate exists in the region, with a strong dry season which lasts 4–6 months, followed by a rainy season with excess humidity. The dominant erosional process is scarp retreat, due to the action of the savanna climate on the lithology of the partly unconsolidated sediments; it has formed deep canyons in the headwaters, and is magnified by tectonic uplift and the lowering of the regional base level. However, some topographic forms have been preserved due to the hardening of the sands by iron oxides. Preliminary thermoluminescence dating of sand from two stratigraphic sections confirms the Pleistocene age of the Mesa Formation (0.5–1 Ma BP), previously suggested by its stratigraphic position and geomorphological interpretation.