The Oligocene – Early Miocene erg in the western Andean basins: Patterns during the transition from eolian to fluvial sedimentation

This paper analyzes an extended eolian field developed during the Oligocene-Early Miocene along the eastern flank of the Andes. This eolian episode, herein named OMEB (Oligocene-Miocene Eolian Belt), is recorded in several units, including Mariño, Pachaco, Cuculí, Vallecito, El Crestón, Tambería, Angastaco, Quiñoas, Las Chacras, and Vizcachera Formations. Eolian accumulation occurred in five environments: 1. Sand sheet and extradune, 2. Dune field, 3. Dune and draas, 4. Deflationary eolian fields, and 5. Wadi and fore-eolian areas. While sand sheet deposits are composed of horizontally laminated and low-angle cross-bedded sandstones, dune and draas environments comprise giant- and large-scale cross-bedded sets and dune fields by stacked sets of cross-bedded sandstones. In contrast, the deflation field environment exhibits low-relief irregular surfaces carved in dune and extradune environments. Finally, the wadi and fore-eolian deposits correspond to the eolian dune interbedded with fluvial conglomerates and muddy sediments deposited mainly by low-energy fluvial currents on the interdunes. Towards the end of the Early Miocene, the OMEB declined in almost all the foreland area, and fluvial and lacustrine sediments replaced eolian deposits. Although the stratigraphic expression of this change was different along the region, three principal patterns of change are recognized: 1. Different degrees of fluvial incisions, 2. Fluvial replacement of eolian deposits without evidence of significant incision, and 3. interbedding of eolian, fluvial, and occasionally playa-lake deposits. In the first pattern, conglomerates forming lenticular channels abruptly incise into the dune deposits. In pattern 2, fluvial channels are dominantly sandy, and incision surfaces are poorly defined. The third pattern, contrasting with the two previously described, results from the recurrent interstratification of eolian, fluvial, and playa lake deposits. This type of eolian-fluvial transition was studied in detail in the Tambillos creek (Northeastern Sierras de Los Colorados), where a sequential model for the eolian fluvial transition using the position of the water table is proposed. This model allows defining four primary states: 1. Low water table, 2. High water table (below the surface), 3. High water table (at or slightly above the surface) with partial flooding and damming, and 4. Runoff for fluvial channels.