The role of detrital composition and climate on the diagenetic evolution of continental molasses: evidence from the Cambro—Ordovician guaritas sequence, southern Brazil

Abstract

The Cambro-Ordovician molassic Guaritas Sequence (Camaquã Basin, southern Brazil) comprises alluvial-fan and braided fluvial sandstones and conglomerates with intercalated aeolian and lacustrine-deltaic deposits and andesitic lava flows. The sediments display a complex detrital composition derived from plutonic/gneissic, acidic volcanic and low-grade metamorphic source rocks. This detrital assemblage was strongly modified by semi-arid continental near-surface diagenesis. Early cementation by hematite, smectite, quartz and calcite, and the relatively limited burial prevented strong compaction and preserved some primary macroporosity in most of the sandstones, whereas the absence of early cements and/or abundance of ductile grains promoted substantial porosity destruction by compaction and the inhibition of further diagenetic modifications. The diagenetic dissolution and replacement of volcanic rock fragments and detrital feldspars by clays and albite changed the original framework composition, as well as the tectonic provenance classification of the sandstones. Detailed quantitative petrographic study allowed 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.