The sedimentation and tectonics of a marine fan-delta developed on an active continental margin: the Triassic San Félix Formation in the Andes of northern Chile

The Middle Triassic (Anissian) San Félix Formation of northernChile (29° S, 70° 30′W), represents the deposits of an extensional (probably transtensional) basin developed on an active continental margin. Over 4000 m of mainly marine clastic sedimentary rocks are intercalated between volcanic sequences. The thick sedimentary section wedges out over a distance of 12 km, suggesting a steep, fault-controlled basin margin. Distinct lateral and vertical facies changes probably reflect tectonic control on the sedimentation. Most of the clastic material was derived from adjacent active silicic and andesitic volcanoes. Most deposition took place from suspension and subaqueous mass-flow processes in still water on a submarine fan-delta. The deposits of the coarse-grained delta slope include both cohesive and cohesionless debris flows. Further offshore, sedimentation on the muddy prodelta was from suspension and low-density turbidity currents. The common mass-flow events, re-depositing coarse-grained clastic material in a shallow water submarine environment, are indicative of an abundant sediment supply, frequent flood events and steep slopes. Intermittent subaerial exposure is indicated by lake-shore sediments at the base and fluvial sediments at the top of the succession.