A GEOMORPHOLOGIC PERSPECTIVE ON THE DEBATE OVER THE ANTIQUITY OF GRAND CANYON

Interpreted ages of Grand Canyon range from latest Miocene to latest Cretaceous, with debate centered on interpretation of geologic and thermochronologic evidence. The “young canyon” (<6Ma) hypothesis has long been supported by qualitative geomorphologic evidence: a steep-walled, narrow canyon incised into a low relief plateau evokes a recent incision event. Indeed, elevations of basalt flows relative to stream beds within the canyon and in the surrounding landscape document that the canyon is deepening. Moreover, the narrowness of the Grand Canyon persists not only because canyon sidewalls are steep, but also because profiles of even large tributaries are steep, plunging over pronounced knickpoints as they cross the canyon rim. Such channel profiles are often associated with landscape response to an increase in the rate of baselevel fall. Landscape evolution on the Colorado Plateau, however, is complicated by strong contrasts in rock strength within the largely sub-horizontal stratigraphy. Although a simple structural geometry, the presence of sub-horizontal variations in rock strength can have a surprisingly rich and complex influence on landscape evolution, as we show from 1D and 2D models. First, the rate of local baselevel fall on upstream channel segments is dictated by a combination of the lateral velocity of knickpoint propagation and the dip of resistant rock layers. Second, when a mainstem river incises into a more resistant package of rocks underlying weak rocks, a period of relief production commences even with no change in the rate of mainstem river incision – a lithologically induced period of transient canyon formation. As for an acceleration in baselevel fall, as assumed in the “young canyon” hypothesis, a steep-walled narrow canyon with tributaries that plunge over knickpoints at the canyon rim and a surrounding topographic bench characterized by very low erosion rates is predicted. In addition, preliminary rock strength data collected by us and others suggests the possibility that lithology could play an important role in forming and maintaining the morphology of the Grand Canyon. Finally we summarize available data on the relationships among topography, rock strength, and erosion rate that will be required to test between the “young” and “old” canyon hypotheses on geomorphologic grounds.