(U-TH)/HE THERMOCHRONOMETRY OF DETRITAL ZIRCON IN THE EOLIAN NAVAJO SANDSTONE, SOUTHWESTERN UTAH

The Early Jurassic Navajo-Nugget-Aztec sandstones represent one of the largest eolian sedimentary deposits in North America, with an estimated original volume exceeding 10⁵ km³. Surprisingly little is known about the provenance of these units and the paleogeographic context of their deposition. To address these issues, we measured (U-Th)/He ages of detrital zircons from the Navajo Sandstone in southwestern Utah, where the unit has a thickness of nearly 700 m. Zircon He ages were measured from two samples, one near the base of the unit and one near the top. Preliminary age determinations show a large range in ages, from Middle Triassic (237 Ma) to Late Proterozoic (560 Ma). There are no significant differences in the age distributions from the samples, suggesting that the source terrane was effectively constant during the approximately 6 million years of deposition.

Zircon (U-Th)/He ages typically record cooling of samples below a closure temperature of 160 ± 20 °C, corresponding to crustal depths of approximately 5-7 km. These ages likely relate to cooling of zircons due to erosional unroofing of their source area(s). Our preliminary data do not cluster around a particular time, suggesting that the source material was derived from continual erosion throughout the Paleozoic rather than during one or more discrete periods of rapid tectonic or magmatic activity. However, more data are required to confirm this interpretation.

The lack of Jurassic (U-Th)/He ages supports previous interpretations that the Jurassic arc to the south and west did not contribute a significant amount of sediment to the erg. Paleocurrent indicators suggest that the prevailing wind direction was generally southward (present coordinates) during the early Jurassic, leading previous workers to hypothesize a source terrane on the craton to the present day north and east. The old ages for many of the zircons require that they resided in the upper levels of the crust for hundreds of millions of years prior to deposition in the Navajo. These facts, plus the extreme maturity of the sediment, suggest that the immediate source may have been siliciclastic sedimentary rocks to the modern north and/or east that were able to store the Navajo material in the shallow crust throughout the Paleozoic.