Dissected Hydrologic System at Grand Canyon: Interaction Between Upper and Lower World Waters in Modern Springs and Ancient Travertines

点击翻译按钮获取中文摘要

Crossy0310323 The spectacular landscape at Grand Canyon provides a unique view of a deeply dissected (1-2 km deep) aquifer system. Active springs and Quaternary travertines provide key data for understanding the hydrochemistry of the Colorado Plateau, as well as a record for understanding how this system has evolved and interacted with Quaternary canyon incision. Our preliminary data suggest the modern and ancient travertine-depositing springs, and the Colorado River itself, record a "tale of two waters": a mixing of deeply-circulated waters rising along faults with surface- and groundwaters of the plateau. We propose to combine mapping, water and gas analysis, and travertine geochemistry to test this model. Travertine occurs where major spring-producing horizons (such as the Muav Limestone-Bright Angel Shale contact) intersect Laramide and Precambrian basement-penetrating faults. We suggest on the basis of a pilot study of spring water chemistry and gas analysis that large volumes of travertine are produced from deep saline waters that are rich in CO2 and higher in 87Sr/86Sr than waters derived from the plateau aquifers. Spring waters in two travertine-forming localities record high He/Ar and He/N2, and 3He/4He ratios of 0.15 Ra suggesting contributions of mantle-derived He and its carrier gas CO2 potentially from nearby coeval volcanic fields. 87Sr/86Sr in central Grand Canyon springs ranges from 0.710 to 0.735 and we will test whether radiogenic Sr contributed by deeply-circulated waters can account for previously observed increases in Colorado River 87Sr/86Sr between Lee's Ferry and Lake Mead. Broader scientific impacts of the work extend to the longer-term (Plio-Pleistocene) record of the Colorado Plateau: we propose that the radiogenic Hualapai Limestone (~5 Ma) in the Grand Wash trough was formed by spring waters similar to those seen in today's travertine localities (87Sr/86Sr of Grand Canyon travertine is comparable to values reported for the Hualapai Ls.). Rapid travertine deposition in selected localities has also caused pervasive cementation and preservation of Quaternary terrace sequences and this allows direct U-series dating of aggradation events and calculation of incision rates. Dating so far suggests that major pulses of travertine accumulation occurred over the last 350 ka. Previous workers suggested that travertine growth episodes correspond with glacial events/ regional wetness. However, modern spring discharge variations correspond to microseismicity, suggesting that seismicity/volcanism influence the movements of deep water up faults. The travertine record, combined with spring water chemistry and geochemical analysis will provide a quantitative understanding of both the Quaternary history and evolving hydrologic system of the region.