Richard M. Chamberlin

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

Great thicknesses of eolian dune deposits of early Oligocene age crop out in the Chuska Mountains of northwestern New Mexico-Arizona (as much as 535 m thick) and in the Mogollon-Datil volcanic field 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 Oi1 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-m–thick zone of saturation that existed during eolian deposition in the Chuska Mountains. The Chuska erg represents the final episode of Paleogene aggradation on the central and southern Colorado Plateau. Aggradation was driven primarily by trapping of fluvial sediments on the plateau by development of major volcanic fields along the eastern plateau margin. These volcanic fields 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 fields. 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 south-central 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 Oligocene-early 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 Plateau-Southern Rocky Mountains region suggests the beginning of major epeirogenic rock uplift occurred during post-Laramide magmatism.

Alternative perspectives of crustal and upper mantle phenomena along the Rio Grande rift

At various locations along the Rio Grande rift in central and southern New Mexico, USA, heat flow data suggest a rather abrupt change from high values often associated with the rift to lower values characterizing neighboring geologic provinces. With this consideration in mind, we explore possible perspectives derived from seismic cross sections along La Ristra profile crossing New Mexico. In New Mexico, La Ristra transects at ~45° to the generally accepted north-south axis of the Rio Grande rift and more acutely to potential eastern boundaries of the southern rift. Considering foreshortened projections of seismic cross sections, one may be able to better visualize possible advection associated with the Rio Grande rift, although resolution of existing seismic data suggests the narrow, near vertical seismic anomalies in these projections may be hypothetical interpretations. If the foreshortened cross sections are realistic, then upper-mantle upwelling appears to be focused near the Rio Grande rift–Great Plains craton boundary. INTRODUCTION “Ristra is a deep-imaging seismic experiment using naturally-occurring earthquake sources” (RISTRA Research Group, 2007). Analysis of seismic data along the La Ristra profile has provided extremely valuable information regarding the seismic velocity distribution in the crust and upper mantle from southeastern New Mexico and west Texas to northeastern Arizona, USA (Fig. 1) (Gao et al., 2004; West et al., 2004; Wilson et al., 2005b). These analyses have been related to phenomena typically associated with rift tectonics, such as mantle convection and elevated temperature and partial melting conditions in the crust and upper mantle. Several considerations with respect to the resulting seismic cross sections and the relationship to the traversed geological provinces are noted (Fig. 1): (1) the seismic profile is oriented at ~45° to the generally accepted northsouth axis of the Rio Grande rift; (2) south of the Albuquerque Basin, the Rio Grande rift widens considerably into east-stepping basins, including the Oscura Basin and Tularosa Basin in New Mexico and, from geomorphic studies, the Salt Basin extending into Texas; (3) the distance from La Ristra to possible eastern edges of the Rio Grande rift likely increases south of Socorro; and (4) the seismic cross sections represent a swath average of up to ~160 km on either side of La Ristra. These observations suggest that subsurface seismic changes may be unduly smoothed and broadened along La Ristra south of the intersection with the Rio Grande rift in the southern Albuquer-