New Mexico Geological Society Annual Spring Meeting — Abstracts

Great thicknesses of eolian dune deposits of Oligocene age crop out in the Chuska Mountains (535 m thick) and in Mogollon– Datil volcanic field (200–300 m thick). 40Ar/39Ar ages indicate eolian deposition in these areas was approximately contemporaneous, with major eolianite accummulation beginning at ~34 Ma and ending at ~27–26 Ma. Probable eolian deposits of Oligocene age ~483 m thick were also penetrated by the Tamara #1–Y well in the northwestern Albuquerque Basin. The beginning of eolian deposition on the Colorado Plateau occurred near the Eocene–Oligocene boundary and corresponds closely to the beginning of significant glaciation in Antarctica and a global decrease in δ¹⁸O composition in benthic marine foraminifera.

Oligocene eolian dune deposits on the Colorado Plateau are thicker than all of the better-known upper Paleozoic–Mesozoic eolianites in the region, except the Jurassic Navajo Sandstone. We interpret that the widely separated Oligocene eolianites in the Colorado Plateau region were probably originally contiguous, and are but erosional remnants of an extensive (~140,000 km² ), regional sand-sea (the Chuska erg). This interpretation is based on: 1) Regional thickness trends of older eolianites in the Colorado Plateau region; 2) Low topographic gradients (< 1.3 m/km) on regional surfaces of major modern ergs, and 3) evidence for a 300–400 m thick zone of saturation that existed during, and shortly after, eolian deposition in the Chuska Mountains.

The Chuska erg represents the final episode of regional Paleogene aggradation on the Colorado Plateau. Our results indicate the reconstructed top of the Chuska erg would lie at a modern elevation of ~3000 m or more. Regional erosion and incision began ~26–25 Ma, soon after Chuska deposition. Major erosion (≥ 1230 m incision) occurred during the late Oligocene and early Miocene, prior to the onset of Bidahochi Formation deposition at ~16 Ma on the south-central part of the plateau. The Bidahochi Formation aggraded ~250 m between ~16–6 Ma, followed by ~500 m of late Miocene and younger incision in the valley of the Little Colorado River. Post-Chuska, pre-Bidahochi (late Oligocene–early Miocene; ~26–16 Ma) erosional denudation of the central and southern Colorado Plateau thus far exceeded later, post-Bidahochi (late Miocene–Holocene; < 6 Ma) erosion in this region.