Sean D. Connell

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.

Mass-balance control on the interaction of axial and transverse channel systems

We present a geometric, sediment mass-balance model for the interaction of axial and transverse alluvial systems in a subsiding basin. By comparing the model result with a flume experiment that employed a simplified half-graben tectonic geometry with axial and transverse sediment sources, we quantify rates of axial-transverse erosional sediment mixing. In the experiment, the lateral migration rate of the axial-transverse boundaries due to the sediment mixing scales with sediment supplies delivered by transverse drainages, but not with water (or sediment) discharge from the axial channel or with tectonic tilting rate. Using an empirical lateral erosion rate, the model shows how sediment supply partitioning among the axial, hanging-wall, and footwall drainages controls the width and the location of the axial-channel belt. Comparing the modeling results with field cases demonstrates that transverse sediment fluxes could slow the axial-channel migration or even reverse the movement against the tectonic forcing.