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Provenance Investigation of the Tertiary “Rim Gravels”: Implications for Cenozoic evolution of the Colorado Plateau

Brian Tillquist
San Diego State University San Diego California
[email protected]

Late Cretaceous-Paleocene (Laramide) compression subjected central and northern Arizona to at least 1200 m of uplift followed by deposition of Tertiary Rim gravels across the topographically high southern margin of the Colorado Plateau. The record of Cenozoic sedimentation on the Colorado Plateau is fragmentary, yet provides insight into reorganization of drainage patterns since the late Cretaceous. Rim gravels were deposited by northeast-flowing drainages that originated in the present topographically lower regions along the southern edge of the plateau. To better understand the topographic evolution, a provenance study was conducted on the Rim gravels volcanic clasts. Fifty-one clasts from localities in northwest Arizona (Long Point) to west-central New Mexico spanning a distance of almost 400 km are uniformly dacite-rhyolite in composition. Three clasts collected near Long Point yielded Middle Jurassic zircon U-Pb ages (160.4±4 Ma, 161.3±3 Ma, and 163.0±3 Ma). These ages are older than previously reported K-Ar volcanic clast dates from Rim gravels that ranged from 117-65 Ma. Rim gravel clasts are likely derived from the Jurassic arc that extends throughout SE California, Arizona, and mainland Mexico. Long Point clasts are dominated by ultra high-K rhyolites (K2O ranging from ~9 to 12 wt %) with Na2O contents <1wt% suggesting potassium metasomatism. Although normal K2O values typical of the arc range from 3-7 wt%, there are arc localities in southern Arizona and central Mexico with similarly high-K values. This may offer key indicators of Cenozoic paleoflows and provide new insight towards the paleogeographic model and Cenozoic deposition of the Colorado plateau.


AAPG Search and Discovery Article #90157©2012 AAPG Foundation 2012 Grants-in-Aid Projects