Progressive discordances in the pre-Neogene deposits of Famatina (Del Crestón Formation) of La Rioja, Argentina: and their implication for the evolution of the Andean foreland

Abstract

A set of progressive unconformities in the synorogenic deposits of the Del Crestón Formation that underlie the Angulos Group (Neogene), Central Famatina (La Rioja), are described and analysed. More than 1000 m of relatively proximal alluvial fan strata are strongly fanned forming rotational intraformational unconformities. The strata fan more than 80º, between an overturned domain dipping 60° W and a normal domain dipping 45° E. Six dip domains were determined separated by angular unconformities. The progressive unconformities are developed between the El Crestón Syncline and the Los Colorados anticline for over 15 km. The plane 102º/80º SW that contains all the poles to bedding of rotated strata allows inference of a 13º/10º rotation axis, which approximately parallels the Los Colorados anticline axis, suggesting that its formation was coeval with the development of the anticline. The general pattern of the progressive unconformities is comparable to rotational offlap, providing evidence of formation under accelerated uplifting. Both facies associations and progressive unconformities indicate their position in the proximal foredeep depozone, adjacent to the front of the thrust belt. This is the first documentation of these kinds of stratigraphical relationships in the Famatina Range and allows us to infer an Andean-related deformation previous to 6.9 Ma.