TRACING ANCIENT LANDSCAPES USING THERMOCHRONOLOGY AND DETRITAL ZIRCON AND SANIDINE STUDIES OF PALEORIVERS ON THE SOUTHERN COLORADO PLATEAU

Dutton’s (1882) “Great Denudation” involved the lateral erosion of Mesozoic strata northwards from the rim of Grand Canyon via cliff retreat. He described a “great stairway” of alternating treads of erodible strata and steps of resistant strata now known as the Grand Staircase and hypothesized that incision by ancient rivers exposed the weak layers, undercutting the resistant layers to drive cliff retreat. Here, we use linear inverse modeling in HeFTy of 52 samples with apatite thermochronometric data to determine continuous cooling histories and combine them with dated paleosurfaces and paleoriver deposits to reconstruct lateral cliff retreat and volumetric loss across the southern Colorado Plateau. We find three main times of denudational cooling, each associated with remnant paleoriver deposits. Significant fluvial deposits on the southwestern Colorado Plateau include the 65-55 Ma Music Mountain Formation and the 25-19 Ma Buck and Doe Conglomerates. The combination of new and published detrital zircon and detrital sanidine data define provenance and potential sinks of these two formations, each of which document episodes of aggradation and incision between drainage reorganizations. The Laramide orogeny (70-50 Ma) and incision of Paleocene fluvial valleys and subsequent deposition of the Music Mountain Formation induced 10s-km-scale cliff retreat. The 35-15 Ma ignimbrite flare-up, initiation of Basin and Range extension, and incision of the East Kaibab paleocanyon induced 100-km-scale cliff retreat; the Buck and Doe Conglomerate is a remnant of temporally associated rivers. Post-6-Ma river integration of the Colorado River system through Grand Canyon, and young uplift, are driving ongoing deep incision and 50-km-scale cliff retreat. We conclude that three periods of drainage reorganization, each most likely driven by epeirogenic uplift and/or base level fall, drove episodic lateral denudation via cliff retreat. The Colorado Plateau region is thus type example of epeirogenic controls on long-term geomorphic re-shaping of orogenic plateaus and the interaction of cliff retreat with fluvial incision.