Fluxos gravitacionais de sedimentos : registro de transformações de fluxo no rifte Santa Bárbara Oeste, Bacia do Camaquã/RS

Abstract

Sediment gravity flows comprise gravity-driven underflows characterized by a large concentration of suspended sediments. Along the downslope transport of sediments some flow transformations (laminar to turbulent flow and vice versa) can take place due to the incorporation or segregation of clay into or out the flow, respectively. These flow changes along a single depositional event indicating hybrid behavior and may produce deposits called hybrid events beds. The Western Santa Bárbara Rift (Camaquã Basin, Brazil) includes both cohesive (debrites) and non-cohesive (turbidites) gravity flow deposits. This research aims to test a possible connection between both types of deposits, to understand their triggering mechanisms and to characterize possible flow transformations taking place along their downslope evolution. A photostratigraphic analysis was performed to identify key stratigraphic surfaces and associated system tracts. Detailed sedimentological and gamma logs (scale 1:20) and outcrops description were performed to identify facies and facies association. During field campaigns, sampling was undertaken to provide material for microscopic (optical and SEM) and X-ray analyses. The stratigraphic record suggests a longitudinal, northeasterward delta system. Facies tract indicates several flow transformations taking place downslope from the distributary mouth bars. The suspended load at the streams mouths produces dense underflows that become bipartite as the flow decelerates and thins out. As soon as the denser interval of the bipartite current freezes, the upper, turbulent layer with high erosion capacity erodes the substrate, incorporates mud and gradually becomes a debris flow. At last a dilute plume develops along the upper layer of debris flows due to flow dilution and deposits a thin turbidite layer above the debrite as a hybrid bed.