Sedimentological and petrographic evolution of a fluvio-lacustrine environment during the onset of volcanism: Volcanically-induced forcing of sedimentation and environmental responses

This work focuses on the transition from the Las Leoneras to Lonco Trapial Formations, in the lower part of the Early Jurassic succession of the incipient rift phase of the Cañadon Asfalto Basin (western part of Chubut Province - Patagonia, Argentina). Twenty lithofacies have been identified and grouped into seven facies associations on the basis of field characteristics (sedimentological and lithological) and optical microscope analysis, from two localities representing proximal and distal locations in the basin. The spatial relationship between all the lithofacies provided a four-dimensional reconstruction of the palaeoenvironmental evolution, showing how the original, clastic sedimentation in alluvial/fluvial and lake environments was modified by short-lived volcanic events during three volcanic cycles, and how the environment reacted after the input of huge amounts of volcaniclastics. Progradation of small deltas and subaqueous lobes, retrogradation caused by rising lake levels, and frequent erosion of valleys were typical processes in this environment. When explosive volcanism began, the original tectono-climatic control on sedimentation was replaced by the volcanic control, and the volcanically-forced sedimentation broke the equilibrium among production, delivery and accumulation of sediments. The nature of the volcanic eruptions and the different propensity of volcanic lithofacies to produce particles of different size and types (lithics, crystals and glass) are also analyzed. The role of volcanism in the production and transport of great volumes of sediments across sedimentary systems needs to be carefully re-examined, and the analysis on the variability in the composition of volcaniclastic deposits must take into account that volcaniclastic particle types may not simply reflect a linear deepening in the dissection of magmatic arcs through time but are often controlled by the style of the eruptions and the lithological variation of the volcanic products.