The Chuska erg: Paleogeomorphic and paleoclimatic implications of an Oligocene sand sea on the Colorado Plateau

Abstract

Great thicknesses of eolian dune deposits of early Oligocene age crop out in the Chuska Mountains of northwestern NewMexico-Arizona (as much as 535 m thick) and in the Mogollon-Datil volcanic fi eld of western New Mexico-Arizona (as much as 300 m thick). 40 Ar/ 39 Ar ages of intercalated volcanic rocks indicate eolian deposition in these areas was approximately synchronous, with eolian accumulation beginning regionally at ca. 33.5 Ma and ending at ca. 27 Ma. Probable eolian sandstone of Oligocene age 483 m thick is also present in the subsurface of the Albuquerque Basin of the Rio Grande rift. The beginning of eolian deposition on the Colorado Plateau corresponds closely to the beginning of eolian (loessic) deposition in the White River Group of the Great Plains and major Oi-1 glaciation in Antarctica, suggesting possible global paleoclimatic control.Successions of Oligocene eolian sandstone 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 suggest that the widely separated Oligocene eolianites in the Colorado Plateau region were probably originally continuous, and thus are erosional remnants of an extensive (~140,000 km 2 ), regional sand sea (the Chuska erg). This interpretation is based on: (1) comparison with thickness trends of older eolianites in the Colorado Plateau region, (2) evaluation of regional topographic gradients of modern ergs, and (3) hydrologic modeling of a 300-to 400-mthick zone of saturation that existed during eolian deposition in the Chuska Mountains.The Chuska erg represents the fi nal episode of Paleogene aggradation on the central and southern Colorado Plateau. Aggradation was driven primarily by trapping of fl uvial sediments on the plateau by development of major volcanic fi elds along the eastern plateau margin. These volcanic fi elds blocked earlier Laramide drainages that had previously transported sediments eastward off the plateau. Following a shift to widespread eolian deposition at ca. 33.5 Ma, constructional volcanic topography induced eolian accumulation upwind of developing volcanic fi elds. Stratal accumulation rates (not decompacted) of eolian deposits were ~28-82 m/m.y.The reconstructed top of the Chuska erg would lie at a present-day elevation of ~3000 m or more, and provides a datum for assessing subsequent erosion on the Colorado Plateau. Major exhumation (≥1230 m) occurred during the late Oligocene and early Miocene, following the end of Chuska deposition and prior to the onset of Bidahochi Formation deposition at ca. 16 Ma on the southcentral part of the plateau. The Bidahochi Formation attained a thickness of ~250 m by ca. 6 Ma, followed by ~520 m of late Miocene and younger erosion in the valley of the Little Colorado River. The depth of late Oligoceneearly Miocene (ca. 26-16 Ma) exhumation of the central and southern Colorado Plateau thus was more than twice that of the late Miocene-Holocene (ca. 6-0 Ma). The timing of initial deep erosion in the Colorado Pla...

Diachronous episodes of Cenozoic erosion in southwestern North America and their relationship to surface uplift, paleoclimate, paleodrainage, and paleoaltimetry

“…In contrast to the Wyoming-Colorado area, the Rocky Mountain erosion surface in the Chuska Mountains of the central Colorado Plateau (Tsaile surface of Cooley, 1958;Schmidt, 1991) was not dissected by fl uvial incision prior to its burial by the Chuska Sandstone during the latest Eocene (ca. 34 Ma; Cather et al, 2008). We therefore interpret the area south and west of the Wyoming-Colorado region to be outside the area of late middle Eocene erosion.…”

“…During the Oligocene there was erg development (Chuska Sandstone and equivalents) on the Colorado Plateau and major volcanism in surrounding areas, both of which resulted in the disruption of former Laramide drainages (Cather et al, 2008). The northern Great Plains were blanketed with loess and volcanic ash of the White River Group.…”

“…Recognition of this phase of erosion on the Colorado Plateau was fi rst enabled by the reconstruction of a major eolian sand sea, the Chuska erg (Cather et al, 2008). During most of the Oligocene (ca.…”

“…5). The reconstructed top of the erg would be at a modern elevation of at least 3.0 km on the central and southern Colorado Plateau (see discussion of the ergreconstruction methodology in Cather et al, 2008) and provides a paleogeographic datum to which the elevation of younger, inset fl uvial deposits can be compared.…”

“…Geologists and geophysi cists, invoking a variety of mechanisms, have interpreted that major episodes of regional uplift occurred during the Late CretaceousEocene Laramide orogeny (McQuarrie and Chase, 2000;Humphreys et al, 2003;Huntington et al, 2010), post-Laramide magmatism (Spencer, 1996;Cather et al, 2008;Roy et al, 2004Roy et al, , 2009Eaton, 2008;Liu and Gurnis, 2010;Cather, 2011a), the middle and late Cenozoic (Sahagian et al, 2002a;Roy et al, 2009), the middle Miocene-Holocene (Eaton, 1986(Eaton, , 1987(Eaton, , 2008Moucha et al, 2009;Pelletier , 2009), on June 5, 2015 geosphere.gsapubs.org Downloaded from and the late Miocene-Holocene (McMillan et al, 2006;Karlstrom et al, 2008;van Wijk et al, 2010;Levander et al, 2011). Diverse uplift mechanisms have been proposed, but most authors have emphasized the role of a single geologic process to explain the majority of uplift.…”