During Jurassic times, marine sedimentary and marine and subaerial volcanic deposits accumulated in the western part of the Neuquén Basin (Lonquimay region, Chile, between parallels 38 and 39°S). These rocks conform the Nacientes del Biobio Formation, which comprises three members: Icalma, Lolén-Pacunto and Lonquimay. The Jurassic rock record in this part of the basin started in the late Pliensbachian-early Toarcian, with thick submarine tholeiitic basalts of island arc affinities and intercalated turbidites (Icalma Member). Later, in another place of the basin, basin plain deposits accumulated during Aaienian to middle Callovian times (lower part of the Lolén-Pacunto Member), while in the shelf, during the late Bathonian and early Callovian, pelites with, at least, deposits of a volcaniclastic fan delta or of one or more channels, eventually forming part of fan delta accumulated (Lonquimay Member). Submarine fans (upper part of the Lolén-Pacunto Member) prograded over the basin plain deposits probably during late middle Callovian and Oxfordian times, suggesting a lowering of the base level. At the same time, calcareous deposits accumulated in the shelf (Lonquimay Member). Deep paleochannels were cut and filled (Lolén-Pacunto and Lonquimay Members) into slope deposits and the calcareous shelf, probably during the Kimmeridgian, indicating another base-level lowering event. Shortly after, there was intense volcanic activity that accumulated pyroclastic deposits and lavas, mainly subaerial. During all this time the basin was adjacent to active volcanoes and, at least during the Liassic, it may have corresponded to an intra-arc basin.