First-Cycle Quartz Arenites in the Orinoco River Basin, Venezuela and Colombia

Modern first-cycle quartz arenites are forming in the Orinoco drainage basin by at least two distinct mechanisms. Common to both mechanisms is an environment of intense chemical weathering and extended time over which weathering can occur. In the Llanos (Andean foreland basin), extended time is provided by temporary storage of orogenically derived sediments on extensive alluvial plains. In lowland portions of the Guayana Shield, long soil mineral residence times are produced by very low erosion and transport rates, resulting from low relief and tectonic quiescence. Both weathering intensity and the duration of weathering are important in the modification of sand composition. First-cycle quartz arenites are produced in diverse tectonic settings and cannot be used by themselves to infer paleotectonic settings of ancient sequences. Furthermore, although tectonic setting is of great importance in determining sand composition, modifications during transport and deposition can overprint the effects of the tectonic regime. In extreme cases, such as first-cycle quartz arenites, the tectonic signal may be virtually obliterated. Many criteria used in the past to discriminate between first-and multi-cycle quartz arenites are invalid when applied to the quartz arenites of the Orinoco drainage basin. At present, the only unambiguous criterion is that the presence of sedimentary lithic fragments and syntaxial quartz overgrowths is clearly indicative of a multi-cycle origin for at least one component of a sand.