A PALEOZOIC LANDSCAPE IN THE ROCKY MOUNTAINS

Unaweep Canyon of western Colorado is a particularly enigmatic feature. The Precambrian gorge carves a wide, deep path through the Uncompahgre Plateau (Colorado) and houses two underfit creeks emanating from a divide within it. Hypotheses for canyon genesis typically involve late Cenozoic fluvial or glacial processes and neotectonic warping to account for the canyon's curious elevation and longitudinal profile. However, new geomorphic, sedimentologic, palynologic, and paleomagnetic data indicate that the inner gorge of the canyon dates from the late Paleozoic. Within the Precambrian-hosted gorge, tributaries cross-cut by Mesozoic strata suggest a pre-Mesozoic age, as do unmapped strata within the western canyon that bear late Paleozoic palynomorphs (in the absence of an obvious source) and an exclusively Precambrian provenance. Core recovered from wells drilled in the western canyon indicate a thick (~330 m) fill with three distinct units: The upper two units contains a small but distinctive contribution from Tertiary volcanic clastics, recent pollen, and exhibit steep paleomagnetic inclinations, reflecting a recent (late Cenozoic) age. The basal unit bears late Paleozoic palynomorphs, a Precambrian provenance, and shallow paleomagnetic inclinations, consistent with the hypothesis that Unaweep Canyon dates from the late Paleozoic.

Landscapes are typically thought to be young, particularly in tectonically active regions. Hence, the existence of Unaweep Canyon within a region of substantial Paleozoic and Cenozoic tectonism is odd in the context of commonly held models of landscape evolution. It negates the need for neotectonic uplift of the Uncompahgre Plateau, but reflects late Paleozoic carving of the Precambrian gorge, followed by rapid burial and eventual exhumation of the Paleozoic canyon, probably by evolution of the Gunnison River. Moreover, its preservation and form bear on aspects of intraplate tectonism and equatorial climate of the late Paleozoic.