THE ROLE OF DETRITAL COMPOSITION AND CLIMATE ON THE DIAGENETIC EVOLUTION OF CONTINENTAL MOLASSES - EVIDENCE FROM THE CAMBRO-ORDOVICIAN GUARITAS SEQUENCE, SOUTHERN BRAZIL

The Cambro-Ordovician molassic Guaritas Sequence (Camaqua Basin, south ern Brazil) comprises alluvial-fan and braided fluvial sandstones and conglomerates with intercalated aeolian and lacustrine-deltaic deposit s and andesitic lava flows. The sediments display a complex detrital c omposition derived from plutonic/gneissic, acidic volcanic and low-gra de metamorphic source rocks. This detrital assemblage was strongly mod ified by semi-arid continental near-surface diagenesis. Early cementat ion by hematite, smectite, quartz and calcite, and the relatively limi ted burial prevented strong compaction and preserved some primary macr oporosity in most of the sandstones, whereas the absence of early ceme nts and/or abundance of ductile grains promoted substantial porosity d estruction by compaction and the inhibition of further diagenetic modi fications. The diagenetic dissolution and replacement of volcanic rock fragments and detrital feldspars by clays and albite changed the orig inal framework composition, as well as the tectonic provenance classif ication of the sandstones. Detailed quantitative petrographic study al lowed the reconstruction of the original detrital compositions and the distinction of six different pathways of diagenetic evolution of the sandstones, attesting to the efficiency of this method for diagenetic modelling and provenance analyses of ancient sandstones.