Geology of the Del Creston Formation, Famatina Ranges, Argentina: Paleogene sedimentation in the Andean Foreland?

Abstract

Largely ignored thick (>1700m) dominantly conglomeratic and sandy red-purple strata of the Del Creston Formation in the Andean foreland in the Famatina Ranges (La Rioja Province) are analyzed and redefined. Its paleoenvironment stratigraphical position, age, and significance in the context of synorogenic Andean strata are discussed. The unit can be divided into two coarsening and thickening upward megasequences that record the geological evolution from low-relief to high-relief geomorphological stages, the latter represented by coarse deposits of proximal alluvial fans. Playa lake environments, distal alluvial fans, and low efficiency alluvial fans form the lower megacycle, whereas playa lake deposits, mixed braided systems and high efficiency alluvial fans constitute the upper megacycle. Conglomerate composition links the first megacycle with the erosion of andesitic flows and volcanic edifices, whereas the second cycle would be related to the exhumation of granite bodies that form part of the main framework of the broken foreland. The Del Creston Formation is considered here to be older than the classic Neogene synorogenic deposits (Angulos Group) based on fission track evidence and orogenic activity in other regions of the Central Andes. Hence, the beginning of the Andean broken foreland and its evolution in the Famatina region seems more complex than previously suggested.