Stratigraphy and syndepositional structures of the basal foreland deposits in the northern Valle Calchaquí, NW Argentina

CECILIA EUGENIA DEL PAPA; F, H.; R, M.; J, P.; PETRINOVIC,I,

Barcelona

Congreso; VI International Symposium on Andean Geodynamics; 2005

The geologic processes that concerned the Andean uplift and related basins in the northwest of Argentina from middle Miocene to Recent are quite well known. Before this period, the manner in which the Palaeogene postrift basin evolved to the foreland basin and mainly, in which moment this transformation took place, are still uncertain. In this contribution new stratigraphic and structural data from the Valle Calchaquí are presented. These elements provide information to assess the initial stages of foreland basin and the structures related with the Central Andes uplift. The Valle Calchaquí is a north-south depression and constitutes the limit between the Puna Plateau to the west and the Cordillera Oriental to the east (Fig. 1a). In the northern Valle Calchaquí, a red coarsening up-ward succession of continental strata of 1.500 m of mean thickness, represents the foreland sedimentation. This deposits overlie the postrift sediments of Salta Group (Fig.1b). The sedimentary facies and the stratigraphic position make it possible to correlate this succession with the Quebrada de Los Colorados Formation, identified by Díaz et al (1987) to the south of the valley. This unit represents the initial sedimentation of the foreland basin and its age is still unknown. A Paleogene age is estimated in relation to its stratigraphic position, underlying Angastaco Formation, which is dated middle Miocene by its paleontological record and a Ar/Ar radimentric age of 13.04 Ma (Díaz & Malizzia., 1984; Grier & Dallmeyer, 1990) and by correlation with Gestes Formation (Marrett, 1990) which is  bearer of Palaeogene mammals. The preliminary study of vertebrate fossils (mainly notoungulate mammals) suggests an Eocene age for this unit.