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Late Miocene aquifer beneath southwestern Colorado Plateau, a precursor to the Grand Canyon reach of the Colorado River. Part II, Sr isotopes: Abstract
Voluminous limestone in the Hualapai member of the Muddy Creek Formation, 11-5 Ma (M.A. Wallace, 1999), was deposited entirely by spring-discharge along the Grand Wash Cliffs at the Basin Range-Colorado Plateau border, northwestern Arizona (see abstract, Part I, Schmidt, this volume). The spring water discharged from a large, Late Miocene carbonate-rock aquifer beneath the southwestern Colorado Plateau between the Kaibab Upwarp and the Grand Wash Cliffs.
The Hualapai limestone has a high radiogenic Sr 87/86 of 0.7145 %o, the same as the Sr ratio of the paleoaquifer water from which it was deposited. Along a 30-km reach of Grand Canyon below South Rim, strontium ratios in present-day spring discharges are also radiogenic, 0.711 to 0.715 (Margot Truini, written commun., 2000). These ratios suggest that the Paleozoic carbonate rock of much of the Grand Canyon region may contain abnormally high radiogenic Sr, and that ground water flowing through this altered rock acquires high radiogenic Sr. By comparison, normal Paleozoic marine limestone has Sr ratios of 0.708–0.709. Probably, the Paleozoic rocks beneath the Grand Canyon region were inconspicuously altered by low-temperature, hydrothermal solutions enriched in highly radiogenic Sr derived from the underlying Precambrian igneous and metamorphic rocks. This unrecognized alteration might have coincided with uranium and other mineralization of some of the hundreds of large breccia pipes located in and near Grand Canyon. Mineralization age is uncertain and ranges from Laramide-Sevier to Mississippian age.
The Sr alteration of the Paleozoic limestone of the Grand Canyon region is also affirmed by a large increase of the Sr-isotope ratio of the modern surface water of the Colorado River; Upstream of Grand Canyon the Sr ratio of river water is about 0.7092, but downstream it is about 0.7106 (P. J. Patchett, written commun., 2000). For a large river, this radiogenic Sr increase is pronounced, but seems generally proportional (1) to the ground-water volume and (2) to the above-suggested widespread Sr-isotope content of the present-day spring discharge into the river in Grand Canyon. Modern upper Colorado River surface water within the Grand Canyon presumably mixes with discharging ground water having a high radiogenic Sr content derived from altered Paleozoic carbonate rock adjacent to the Grand Canyon. In a similar situation, some of the Late Miocene, ancestral upper-Colorado-River water was recharged and flowed through the same altered Paleozoic carbonate rock and became the high radiogenic-Sr ground water of the paleoaquifer that deposited the Hualapai limestone.
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