WEATHERING OF UNCONSOLIDATED SEDIMENT AND APPARENT AFFECTS ON SURFACE STRATIGRAPHY, CENTRAL GUYANA

The unconsolidated sediments of a buried paleo-valley, in the humid tropical zone on northern fringe of the Amazon rainforest, have been significantly altered by chemical weathering. Historic maps of the surficial materials of central Guyana do not differentiate the Paleogene to Quaternary stratigraphy and place all surface materials into the White Sand Formation of the Corentynes Series. Interpreted to be Plio-Pleistocene in age, these sands are believed to be of marine or fluvial origin. The stratigraphy of the Maple Creek paleo-valley was examined and four lithostratigraphic units were identified including (bottom to top): saprolitic gravel, fluvial gravel, fluvial sand, and ubiquitous surface sand (correlative with the White Sand Formation). This investigation reveals that much of the apparent surface stratigraphy is a product of chemical weathering processes on unconsolidated fluvial sediments.

Work at Maple Creek provides an alternative hypothesis for the origin of the White Sand Formation and suggests a model for the weathering of unconsolidated sediment. Data used to interpret the maturity of these sediments are not necessarily in agreement. For example, poorly sorted and angular sand grains and clasts indicate that both the fluvial and the surface sand units are texturally immature. Most sand grains are irregularly shaped fragments and some clasts are aggregates of quartz crystals. Compositionally however, the sediments are dominantly quartz-rich suggesting a mature sediment. Clay mineralogy indicates that kaolinite is the dominant species below the surface sand whereas quartz dominates the clay fraction in the leached surface sand. Iron oxides commonly precipitate below the contact of the surface sand within the zone of a fluctuating water table. We propose that the homogeneous appearance of the White Sand Formation is the result of the weathering of less stable minerals (i.e. lithic fragments) and their subsequent removal through ground water flow.