Syntectonic deposition and paleohydrology of the spring-fed Hualapai Limestone and implications for 5-6 MA integration of the Colorado River system through Grand Canyon : evidence from sedimentology, geochemistry and detrital zircon analysis
The Hualapai Limestone, at the western edge of the Colorado Plateau, provides the best sedimentary record available for 12 to 6 Ma at the mouth of the Grand Canyon. Because it directly underlies the first Colorado River gravels, this unit is a key element for understanding the integration of the Colorado River from the Colorado Plateau to the Basin and Range province, and the early paleogeography of the Grand Canyon region. This study combines a tectonic investigation with a stratigraphic and sedimentologic analysis that includes new geochemistry, tephrochronology, and detrital zircon analysis to examine variations of the sedimentary and tectonic records from the Hualapai Limestone basins. Thickness variations, with progressively thicker deposits towards the east in two of the four basins, indicate syntectonic deposition of the unit in half grabens formed above listric faults with 5-11 km depth to detachments. A sedimentary facies analysis highlights that the Hualapai Limestone was deposited in spring-fed lake and marsh systems fed by groundwater similar in composition to modern Havasu Creek and western Colorado Plateau groundwater. Stable isotope analysis of carbon and oxygen suggests gradual increase in meteoric water input through time. Sr isotopes, though variable, show an up-section decrease in 87Sr/86Sr and confirm a freshwater origin for the Hualapai Limestone. These data, plus facies analysis, suggest that waters that fed the Hualapai Limestone contained a significant component of endogenic spring inputs. Detrital zircon data for the Grand Wash trough indicate that red siltstones that underlie and interfinger with the Hualapai Limestone, from 13 to 6 Ma, were not derived from the Colorado Plateau, but likely from the Kingman Arch to the south. Western basins contain a more diverse suite of detrital zircons, suggesting possible connections to a northern Paleo Virgin River source. Tephrochronologic analyses show a 12 Ma ash near the base of the unit, extending the basal Hualapai date. We propose that the Hualapai Limestone was a long-lived (12-6 Ma) groundwater-fed series of lake and marsh systems that were deposited in syntectonic half grabens via spring vents along faults and discharge along the dissected aquifer of the Grand Wash Cliffs.
Hualapai Limestone, Colorado River, Grand Canyon, geochemistry, detrital zircon
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