ABSTRACT

Quartz is the most stable natural form of silica, normally resisting weathering. However, Precambrian quartz arenites of the Roraima Group in Venezuela clearly show evidence of abundant quartz dissolution in a highly organic-rich, acidic to neutral meteoric environment. The Roraima sediments occur in flat-topped mesas (200 m high) in a tropical environment of heavy rainfall. The degree of weathering varies from highly indurated, unweathered quartz arenites, away from and deeper than the outcrop surfaces, to highly degraded, crumbly, extremely porous sandstones on the outcrops. Petrographically, the tight arenites show abundant grain welding as a result of pervasive syntaxial quartz cement and sutured grain contacts. In the weathered samples, by contrast, an excellent network of lamellar porosity has formed through the dissolution of quartz cement. The silica dissolution has mainly followed along the compromise boundaries of adjacent overgrowths. The extensive dissolution and mobility of silica may be related to organic acids produced during the biodegradation of terrestrial organic matter. These organic acids or compounds can react with dissolved silica, lowering the activity of silicic acid and accelerating the dissolution of quartz and other aluminosilicates.

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