REASSESSING A CONSTRAINT ON TIMING GRAND CANYON INCISION USING MODERN GEOCHRONOLOGY

The timing and mode of integration of the Colorado River system is central to understanding the evolution of the western United States. However, the time at which the Colorado River started incising the western Grand Canyon is still a debated topic, with some disagreement between geologic interpretations and thermochronologic methodologies. Some cooling histories measured using the (U-Th)/He and ⁴He/³He thermochronometers in apatite grains from the modern canyon base which suggest that the western Grand Canyon was near modern depths by 70 Ma. This thermochronology is at odds with late Miocene deposits downstream of the mouth of the canyon which show that the Colorado River did not flow through this area prior to 6 Ma. Additionally, fluvial gravels on the Shivwits Plateau, north of the canyon, have been reported to contain possibly southerly-derived clasts, precluding the existence of the western Grand Canyon at the time of gravel deposition (~6 Ma). In this study, we reassess these deposits using modern geochronologic measurements to determine the age of the deposit and the presence (or absence) of clasts from south of the Grand Canyon. ⁴⁰Ar/³⁹Ar analysis of a basalt clast entrained in the upper deposit support a maximum age of gravel deposition at ~5.36 Ma. We could not identify southerly-derived clasts, which permits, but does not require, the existence of Grand Canyon prior to the age of deposition of the gravels at 5.36 Ma. Any exotic clasts may have been derived from the San Juan Mountains by the ancestral upper Colorado River drainage system. The ambiguity would be removed by finding extra-regional clasts derived unequivocally either from south of the Grand Canyon or from the San Juan Mountains, however careful excavation and additional characterization of clast composition within the Grassy Mountain gravels would be necessary.