Abstract

Radioisotopic dating of detrital minerals in sedimentary rocks can constrain sediment sources (provenance), elucidate episodes and rates of ancient orogenesis, and give information on paleogeography and sediment-dispersal patterns. Previous approaches have been restricted to the application of a single technique, such as U/Pb or fission-track dating, to detrital grains. These methods provide crystallization and cooling ages, respectively, of sediment sources (terranes). However, evidence for source regions from a single technique can be ambiguous because candidate source terranes often have similar ages for a given radioisotopic system. This ambiguity can be avoided by applying multiple radioisotopic systems to individual detrital grains. Here we present a method for measuring both (U-Th)/He and U/Pb ages of single crystals of detrital zircon, providing both formation and cooling ages (through ∼180 °C). We applied this technique to zircons from the Lower Jurassic Navajo Sandstone, which represents one of the largest erg deposits in the geologic record. A large fraction of these zircons was derived from crust that formed between 1200 and 950 Ma, but cooled below ∼180 °C ca. 500–250 Ma. This history is characteristic of Grenvillian-age crust involved in Appalachian orogenesis (and subsequent rifting) in eastern North America. Our finding requires the existence of a transcontinental sediment-dispersal system capable of moving a large volume of detritus westward (modern coordinates) throughout the late Paleozoic and early Mesozoic.

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