Linked basin sedimentation and orogenic uplift: The Neogene Barinas basin sediments derived from the Venezuelan Andes

Abstract

Abstract The Venezuelan Andes are an asymmetric, doubly vergent orogen that is flanked on its southeastern side by the Barinas basin. Analyses of sedimentary facies, sandstone petrography, apatite fission-tracks, and magnetostratigraphy were completed on a 1750-m section of the syn-orogenic Neogene Parangula and Rio Yuca formations in the Barinas side foothills of the Venezuelan Andes. Our sedimentary facies analyses record a progression of sedimentary environments from floodplain and floodplain channel deposits through the 560-m thick Parangula Formation transitioning to distal alluvial fan deposits in the lower Rio Yuca Formation and finally to an alternation of distal alluvial fan and two, ∼100-m thick organic-rich lacustrine deposits in the upper third of the section. Major- and minor-mineral petrographic analysis reveals unroofing of the Venezuelan Andes, with quartz arenite composition low in the section succeeded by metamorphic and igneous clasts and potassium feldspar appearing near the base of the Rio Yuca Formation. Apatite fission-track (AFT) analysis of sandstones and pebbles generated ages of 11.2 ± 1.3 – 13.8 ± 2.0 Ma over ∼1100 m of stratigraphic section. Thermal modeling of the detrital AFT and vitrinite data from the lower Rio Yuca Formation indicates exhumation of the source area was occurring by 12–13 Ma, surface exposure at 10–9 Ma, maximum burial by 4–2 Ma and exhumation of the sedimentary package starting 3–2 Ma. Accumulation of the Rio Yuca Formation is contemporaneous with a basinward migration of the deformation front. Regional considerations indicate that the Venezuelan Andes evolved from a primarily singly vergent orogen to its current double vergence over the interval of Neogene-Quaternary sedimentation.