Upper Triassic sandstones in diverse locations in eastern California, southern Arizona, and on the Colorado Plateau (USA) yield detrital zircons that are remarkably similar in age and geochemistry, leading to the hypothesis that they are temporally related and were derived from similar sources. Volcaniclastic sandstone from the lowest Vampire Formation in eastern California, the Sonsela Member of the Chinle Formation at Petrified Forest National Park, northeastern Arizona, and the herein-named Waterman formation in southern Arizona yield zircons that range in age from ca. 205 to ca. 235 Ma. Together with the similar range of ages, these zircons uniformly have Th/U ratios between ∼0.2 and 2. In addition, the Waterman formation contains zircon grains with an age range from ca. 225 to 250 Ma, but with markedly lower Th/U ratios of 0.1–0.2, and a distinctively older group with ages to ca. 280 Ma. In a general sense, variations in Hf concentrations and Yb/Gd ratios support the discrimination of grains based on age and Th/U.
We use age and geochemical data from the zircons to infer that these units capture a slice of time during development of the early Mesozoic Cordilleran magmatic arc along western North America. Plutonic rocks that record magmatism in the arc are Permian–Triassic in age, and match zircon ages in the detrital grains, thus providing a view of which parts of the arc were actively eroding into the stream systems that deposited the three units. Streams diverged from a common source that maintained a relatively uniform magma composition over time, as indicated by a narrow range of Th/U values, as well as tapping a somewhat different source evidenced by a grouping in which Th/U ratios are lower. Once the streams left the highlands of the arc and the depocenter of the lowest Vampire Formation, they diverged, such that one flowed to the area of the Colorado Plateau while the second flowed toward southern Arizona. At the same time, a stream system originating in the older, Sonoran part of the arc flowed from the south into southern Arizona.
- Received 11 August 2012.
- Revision received 1 February 2013.
- Accepted 13 March 2013.
- © 2013 Geological Society of America