Lithofacies, architectural elements and depositional system of the Los Colorados Formation (Eocene - Lower Miocene), Tonco Valley, NW Argentina

Abstract

The development of the Cenozoic foreland basin containing the Payogastilla Group promoted the inversion of the underlying Cretaceous-Paleogene rift basin (Salta Group) in the NW Andes of Argentina. Although these Groups are bounded by an extensive regional unconformity, several deposits of both Groups have been proposed to represent the earliest stage of the foreland basin. Thus, the Los Colorados Formation, basal unit Paypogastilla Group, has been analysed by sedimentology and petrography in the Tonco Valley. These continental deposits were classified into 15 facies and 11 architectural elements which are described and interpreted to represent an ephemeral fluvial-aeolian depositional system developed from the Middle Eocene to the Lower Miocene. The paleopedogical and ichnological observations revelean the shift from semiarid to arid paleoclimate. Petrographic analysis determined feldspatho-quartzose, litho-feldspatho-quartzose and quartzo-feldspathic sandstones with provenance from continental block, recycled orogen and magmatic arc. These new data, in combination with and existing paleocurrent, exhumation data, suggest that the eastern border of the Puna, that is exposed to the west of the valley, was a primary source of sediments for the Los Colorados Formation in the Eocene foreland basin system of the Central Andes.