DETRITAL ZIRCON ANALYSIS IDENTIFIES RECYCLED EUREKA CLASTS IN THE POWDER RIM GRAVEL (LATE MIOCENE) OF SOUTHERN WYOMING

A pilot study of detrital zircon spectra from well-rounded quartz arenite clasts in the Powder Rim Gravel of southern Wyoming tests the hypothesis that the clasts were derived from areas other than the Uinta uplift. Previously, the gravel, which overlies thick-bedded ~24-7 Ma eolianite of the Browns Park Formation, had been interpreted as a north-sloping piedmont gravel derived soley from the Uintas. The spectra convincingly show that many of the clasts are Eureka Quartzite (Ordovician), a unit with a very distinctive detrital zircon spectrum that is conspicuously absent from the Uintas and southwestern Wyoming. The most likely source is a conglomerate containing abundant Eureka clasts in the middle Albian Cedar Mountain Formation of east-central Utah (Lawton et al., 2010: Geology, v. 38, p. 463–466). The nearest alternate potential sources are thin, sparsely exposed Ordovician outcrop belts in north-central Wyoming or eastern Idaho. These are highly unlikely as sources since the Powder Rim Gravel was transported to the north and contains a large proportion of distinctive Uintan (ie. southerly derived) clasts.

Our new data supports the interpretation that Powder Rim Gravel is the remnant of a major, north-flowing trunk stream with headwaters extending at least as far south as east-central Utah. This is significant because all other post-Browns Park Formation gravels in the area were integrated with the modern Colorado – Green River system. The Powder Rim Gravel probably represents the youngest record of north-flowing drainage of the Colorado Plateau prior to the latest Miocene – Pliocene uplift of southern Wyoming which ultimately lead to formation of the modern Colorado River. The Gravel also provides evidence of the north-flowing streams that are needed to explain the Pacific Northwest fish fauna that inhabited the 16-6 Ma Bidahochi Formation of northwestern Arizona.