MIDDLE CENOZOIC QUIESCENCE AND LATE CENOZOIC EXHUMATION-HOW BASE-LEVEL CHANGE THROUGH DRAINGE DEVELOPMENT AND EPEIROGENESIS HAS SCULPTED THE WEST

Several lines of research are making it clear that post-Laramide erosion rates of the interior west have increased in the late Cenozoic. Climate change and epeirogenic uplift have been implicated for this, but we suggest that long-term base level lowering, driven by drainage integration, repositioned ice-marginal streams, and eustatic fall is a third and dominating factor, the effect of which has been a wave of incision propagating from the edges to the core of the region in the late Cenozoic.

The region's long-wavelength topography is dominated by centers of high elevation in the northern Rio Grande Rift and Yellowstone with major drainages radial away from these regions. The thermal/mantle sources of epeirogenic uplift evident in these areas may be less important on the Colorado Plateau, where there is less total rock uplift and erosional isostasy may be a predominant process. In the Colorado River and Rio Grande drainages volcaniclastic deposition concomittant with pedimentation of Laramide ranges prevailed in middle Cenozoic time, followed by unroofing and incision at 100-300 m/m.y. in the Neogene and Quaternary. Support for this comes from our stream terrace studies and erosion estimates based on GIS landscape reconstructions, along with other worker's U-Th/He thermochronology and work using dated volcanic rocks. Importantly, basin-scale erosion follows a pattern of generally headward-decreasing exhumation and terraces that converge upstream in widely-spaced drainages such as the Colorado and Cheyenne rivers. Notable exceptions include places where recent faulting has modified erosion such as along the Colorado River.

Information thus far supports a model where early Cenozoic tectonism provided the initial uplift, but exhumation was hindered by poorly-integrated drainages, distance to base level, and local volcanism, resulting in relative exhumational quiescence in the middle Cenozoic. This latent erosional potential, enhanced at least in regions by epeirogeny, was subsequently tapped by well documented drainage integration (Colorado River and Rio Grande), formation of ice-marginal drainages (Missouri River), and eustatic fall in the Neogene and Quaternary. The effects of late Cenozoic climates are superimposed upon, but may be subordinate to, the effects of base-level fall.