Publication Type:Book Chapter
Source:Precambrian sedimentary environments; a modern approach to ancient depositional systems, Blackwell, Oxford, International, Volume 33, p.323-338 (2002)
Keywords:fluvial environment, Metamorphic rocks, metasedimentary rocks, paleoenvironment, Paleoproterozoic, planar bedding structures, Precambrian, proterozoic, ripple drift-cross laminations, Roraima Formation, sedimentary structures, South America, upper Precambrian, Venezuela
Strata of the Uairen Formation, at the base of the Late Palaeoproterozoic Roraima Supergroup in the Gran Sabana, near Santa Elena de Uairen, Bolivar, south-eastern Venezuela, can be interpreted in terms of deposition in a variety of vegetation-free fluvial environments in a foreland basin setting. Sandstones and small to medium pebble conglomerates at the base of the formation reflect erratic deposition from high-energy (proximal) gravelly braided streams in valley-fill and valley-marginal alluvial fan settings. Arid conditions are indicated by the presence of minor diamictites of debris flow origin. Overlying predominantly sandy strata were deposited by sheet floods in a low-relief basin characterized by flashy ephemeral sandy braided fluvial systems in a terminal-fluvial setting. Temporary development of playa lakes may be indicated by thin mud drapes and fine to very fine sandstones with symmetrical ripples. Low angle truncation of laminae may reflect erosion and deposition by sheet floods. Aeolian reworking may be significant in this setting and is indicated locally by ripple cross-stratified silt-free sandstone sets between more poorly sorted sandstones. Medium to very large pebble conglomerate at the base of the upper member of the formation rests on a regionally significant erosion surface, reflecting uplift and/or a dramatic change in climatic regime. Overlying sandstones reflect deposition by sandy and gravelly sandy perennial braided fluvial systems with high width to depth ratios. Strata of the Uairen Formation are succeeded by poorly exposed lacustrine mudstones of the Cuquenan Formation, which are overlain by fluvial and aeolian deposits of the Uaimapue Formation. In Proterozoic and Phanerozoic sequences, thick sets of flat laminated sandstones are commonly interpreted in terms of beach-face deposition. This study shows that this can be incorrect and that such sequences are a common feature of pre-vegetation fluvial settings, especially in arid to semi-arid terminal fluvial systems. Abstract Copyright (2002), International Association of Sedimentologists.
GeoRef, Copyright 2018, American Geological Institute.<br/>2003-074526<br/>Uairen Formation