Bridging the Gap: Reconstructing the Evolution of the Grand Canyon from the Dating of Speleothems

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The Grand Canyon is one of the most impressive geologic features on earth and one of the most, if not the most, recognized of our National natural landmarks. While a general consensus exists that the canyon formed since sometime after the middleMiocene (20 Ma), remarkably little absolute chronology on the evolution of the Canyon, and in particular the incision history, exists currently. Study of travertines and basalt flows at or near the bottom of the canyon has successfully produced incision rates, but only for the last half a million years. Surface materials deposited coeval with incision are rare or non-existent between 100 and 1000 meters above the Colorado River throughout the Grand Canyon. Caves in the Redwall Limestone exposed by the canyon, however, contain well-preserved deposits (speleothems) that are coeval with the incision of theGrand Canyon. Of these cave deposits, four particular speleothem types, mammillary coatings, folia, cave rafts, and gypsum rinds, are direct indicators of pre-existing water table levels. Many of these speleothems have unusually high uranium content and are thus ideally suited for combined uranium-series and uranium-lead chronology. The position of the water table can therefore be determined by dating these speleothems, yielding incision rates in different parts of the Grand Canyon. The overwhelming benefit of this method is that average incision rates can be obtained for periods of several million years, and for several hundred meters of vertical relief.Results from our pilot study show that these water table-indicator speleothems (especially mammillary coatings) are common and well preserved in the caves of theGrand Canyon. Preliminary U-series and U-Pb results demonstrate that mammillary coatings collected from Grand Canyon Caverns (at the southwestern end of the GrandCanyon at a higher elevation) formed ~19 Ma. This marks the timing of a drop in the water table through Grand Canyon Caverns suggesting early incision of the southwestern margin of the Colorado Plateau, consistent with the incision record from dating of basalt flows by Young (2004). Near Grand Canyon Village (eastern end of canyon), a mammillary coatings has been dated at ~3 Ma. The mammillary coatings are forming just prior to incision of the Redwall Limestone by the ground water associated with the Colorado River system. Recently exposed caves in Marble Canyon (e.g., FallsCave) should therefore have mammillary coatings with very young ages. In addition to the chronology, 87Sr/86Sr isotopic values of two of the dated mammillary samples show significant shift and suggest evolution of groundwater over the last 20 Ma, perhaps related to removal of overlying clay-rich deposits or caused by groundwater mixing. The fundamental process to be investigated by this proposed study is the rate of incision of the Grand Canyon, at several localities and from east to west, using U-series and U-Pb dating of calcite mammillary speleothem coatings. The proposed study includes two years of intensive field trips and concurrent U-Pb and uranium-series isotopic work. The investigators and their collaborators have deep expertise both in the field and laboratory based work. The Radiogenic Isotope Laboratory at the University of New Mexico has done extensive U-series and U-Pb isotopic work on carbonates.The value of this proposed study goes beyond geologic considerations. Almost half of the river miles in the canyon are within the boundaries of Grand Canyon NationalPark, perhaps the most popular and most visited national park in North America. The potential interpretative value of this proposed research is far reaching both scientifically and publicly. Another very important societal benefit is the link with the local Native Nations (please see letters of support). Although they lack local expertise for the kind of work proposed here, they recognize its value for their stewardship effort, thus are eager to work with us. We hope that part of the involvement will be in the form of student training around the project.