THE MYSTERY OF UNSTABLE AND TRANSIENT PLEISTOCENE INCISION IN THE COLORADO PLATEAU

Active and complex erosion persists even in tectonically quiescent landscapes in response to controls such as lithologic contrasts, mantle dynamics, and climate-change. The Colorado River, which traverses the tectonically stable central Colorado Plateau, presents a world-class example of these controls operating during transient incision. We present new fluvial incision histories, terrain analyses, and response-time modeling of the Colorado River through the central Colorado Plateau to better resolve these issues.

Luminescence and cosmogenic nuclide burial dating of fluvial terraces indicate a common, Early-Mid Pleistocene (~1.5 to 0.3 Ma) period of mysteriously slow incision (baselevel high) and a subsequent pulse of rapid incision of ~250 m over the last ~350 kyr that is propagating upstream. Terrain analyses agree with the observed magnitude of incision and indicate that the subsequent, rapid incision is transient and related to 1.5 km baselevel fall from integration through western Grand Canyon. The response-time model reveals that this transient baselevel signal would have reached the study area ~1 Ma, matching the onset of rapid incision recorded by the fluvial terraces. The erosional hiatus may record a planated landscape of unknown spatial extent graded to the Colorado River before the propagation of this baselevel fall. In detail, this transient signal appears to have been partitioned into multiple waves across the landscape due to local geologic controls such as lava damming, salt tectonics, and heterogenous bedrock.

Moreover, baselevel fall from river integration diffuses upstream and only accounts for about a quarter of the total 2-3 km of late Cenozoic incision in the central Colorado Plateau evident from thermochronology and landscape reconstructions. This demands an unknown baselevel driver for significant exhumation in the Pliocene. Future work in the region should focus on solving the mystery of the Pliocene pulse of exhumation and the Early-Mid Pleistocene pause in incision.