THE JURASSIC OF THE NEUQUEN BASIN IN LONQ UIMAY, CHILE - NACIENTES-DEL-BIOBIO FORMATION (38-39-DEGREES-S)

During jurassic times, marine sedimentary and marine and subaerial vol canic deposits accumulated in the western part of the Neuquen Basin (L onquimay region, Chile, between parallels 38 and 39 degrees S). These rocks conform the Nacientes del Biobio Formation, which comprises thre e members: Icalma, Lolen-Pacunto and Lonquimay. The Jurassic rock reco rd in this part of the basin started in the late Pliensbachian-early T oarcian, with thick submarine tholeiitic basalts of island are affinit ies and intercalated turbidites (Icalma Member). Later, in another pla ce of the basin, basin plain deposits accumulated during Aalenian to m iddle Callovian times (lower part of the Lolen-Pacunto Member), while in the shelf, during the late Bathonian and early Callovian, pelites w ith, at least, deposits of a volcaniclastic fan delta or of one or mor e channels, eventually forming part of fan delta accumulated (Lonquima y Member). Submarine fans (upper part of the Lolen-Pacunto Member) pro graded over the basin plain deposits probably during late middle Callo vian 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 (Lolen-Pacunto and Lo nquimay Members) into slope deposits and the calcareous shelf, probabl y during the Kimmeridgian, indicating another base-level lowering even t, Shortly after, there was intense volcanic activity that accumulated pyroclastic deposits and lavas, mainly subaerial. During all this tim e the basin was adjacent to active volcanoes and, at least during the Liassic, it may have corresponded to an intra-aro basin.