Provenance of pyroclastic levels in the back-arc basin in the Northwest of Argentina (Eocene - Pliocene)

COIRA, B.; GALLI, C.I.; ALONSO, R.N.

Mendoza

Congreso; 18th International Sedimentological Congress; 2010

International Associations of Sedimentologist

The interruption of the sedimentation of the Salta Group and the erosion of different levels of the Santa Barbara Subgroup produced by the movements happened during the upper Paleoceno - early Eocene, in the zone of Eastern Cordillera and Subandean Belt. These different erosion levels are filed on the Umbral de los Gallos, where the base of the Oran Group lies on different deposits of the Santa Barbara Subgroup or more older terms. This area behaved as the marginal forebulge of the foreland basin. The dispersion of the Oran Group deposits is wider than Salta Group?s in the oriental and austral portion of the foreland basin and the general structural characteristics do not differ with regard to the design of the cretaceous rift basin. In Luracatao Valley and in the western of the Calchaquí Valley, thrusts would have been produced shaping the western edge of the foreland basin, which would have been the provenance area of the Los Colorados Formation deposits (Payogastilla Group) of middle to upper Eocene age (Payrola Bossio et al., 2009). In the Puna, the tertiary basin of Pastos Grandes has offered paleontological information proceeding from the basal portion of the Geste Formation and it is Eocene age. These intermountainous basins would have evolved together with the foreland basin, in the intra-arc/intra-plateau. During the upper Miocene, with the advance of the Nazca plate subduction to the eastward, thrusts were added in the western edge of the Luracatao-CalchaquíValley. They would have constituted a wedge top, causing the deepening of the Payogastilla Group basin. The paleogene and neogene clastics sequences, in the intra-arco/intra-plateau basin, and in the foreland basin, have many tuffs levels. The ages of these pyroclastic deposits, identified in different profiles of the Pastos Grandes Group, Payogastilla Group, Oran Group and equivalents, show a clear correspondence with explosive events associated with big calderas evolution of the arc and retroarc region of north of Puna and north of Chile. The volcanism reached its climax between 10 my and 2 my, with episodes to 10-9.8 my; 8.4-8.3 my; 6.6-5.6 my; 4-3.5 my and 2-0.4 my. Pyroclastic events of magnitude also were recognized in the south of Puna for similar times, having been  registered, on the other hand, to this latitude in Chile episodes before 26 my and 19-16 my, as to the north of the 22 º S, in the Altiplano - Eastern Cordillera. These differences in time and space of the explosive cenozoic volcanism in Central Andes, have distinctive compositional characteristics of the centers located along the above mentioned andean strip. These pyroclastic records are guide levels to use, characterized by the clastic sequences that lodge them, for the comprehension of the tecto-sedimentary evolution of the back-arc basin. The tecto-sedimentary evolution of the region, which involved the creation of positive reliefs that should have acted as orographical barriers (Proto- Eastern Cordillera, upper Eocene), or those that determined migration of the thrusts (Oligocene to middle Miocene) or those that indicated stages of segmentation of a basin of simple foreland to basins of piggy back (upper Miocene to Pliocene). They are analyzed on the base of the pyroclastic records far away from the cenozoic volcanism in the region. The sum of paleontological evidences and of records pyroclastic allows to define correlations and homologations in the continental sequences of red-beds in the back-arc basin in the norwest Argentine.