HISTORY OF HEADWARD EROSION OF THE GRAND CANYON FROM A STUDY OF SPELEOTHEMS

Establishing the timing of incision across the Grand Canyon can delineate the process by which the Grand Canyon formed, e.g. simple uniform incision vs. headward erosion, and has implication for the different models suggested for the evolution of the canyon and tectonic history of the Colorado Plateau. Cave deposits (speleothems) may provide direct evidence for the timing of incision of the Grand Canyon. A detailed study of the deposits in Grand Canyon caves, now in its sixth year, has identified a sequence of deposition before, during, and after downcutting of the canyon. Speleothems found within artesian Redwall caves, from oldest to youngest, can include: (1) druses of large scalenohedral and rhombohedral calcite and manganese and iron oxides that predate the canyon, (2) mammillary crusts, folia, and cave rafts that formed at or near the water table, and (3) dripstone (stalactites, stalagmites, etc.) and flowstone that formed after the water table had descended below the level of the Redwall Limestone cave horizon. By determining absolute dates for these different types of speleothems, especially mammillary crusts, the past positions of the water table across the canyon can be determined.

The caves of the Grand Canyon almost exclusively formed in the Redwall-Muav aquifer. Both the Redwall and Muav limestones are exposed along the walls of the Grand Canyon for most of its entire >300 km length. Preliminary combined U-Pb (two isochron dates) and U-series data (mostly looking at ²³⁴U excesses) indicate older incision ages ~15-20 Ma in the western end of the canyon and younger ages in the eastern end (~1-5 Ma). The age pattern suggests headward erosion as the mechanism for erosion of the Grand Canyon. Thus, the Colorado River must have cut through the Redwall Limestone on the west end of the canyon early in the canyon's history, with sequentially younger truncation of the Redwall eastward and then northward up Marble Canyon to where the Colorado River intersects the Redwall Limestone today. The age of mammillary crusts in Marble Canyon should be younger still. Additional U-Pb and U-series data will provide a high-resolution history of incision of the canyon and will provide critical constraint for the overall evolution of the canyon and the Colorado Plateau.