U-PB DETRITAL ZIRCON PROVENANCE OF THE RINGOLD FORMATION WITH IMPLICATIONS FOR THE ANCESTRAL COLUMBIA AND SNAKE RIVER DRAINAGE AND REGIONAL TECTONIC UPLIFT

The Miocene – Pliocene Ringold Formation, in central Washington, has been an important marker for understanding river drainage reorganization of the ancestral Columbia, Snake, and Salmon/Clearwater Rivers. Previous work on the evolution of the Snake River Plain and Pasco Basin fluvial networks has lead to a debate on whether the Snake River flowed through or bypassed the Pasco Basin prior to Pliocene incision through Hells Canyon. In this study, we provide new detrital zircon datasets from the Ringold Formation and modern river systems in central Washington and western Idaho to clarify the history of drainage reorganization with implications for regional tectonics.

We analyzed fluvial sandstone samples from the Ringold Formation on the north side of the Saddle Mountains anticline for detrital U-Pb zircon provenance. U-Pb zircon dating of interbedded tephra indicates that sandstone samples were deposited between 7.0 – 3.4 Ma. Correlation with the nearby Taunton section shows that the Taunton local fauna are older than previously interpreted and are coeval with isolated local fauna on the south side of the Saddle Mountains. For comparative analysis, we performed U-Pb detrital zircon dating on modern sediments from the Columbia, Okanogan, Spokane, Methow, Yakima, and Salmon Rivers, and supplemented this with existing U-Pb zircon age spectra from the Snake River Plain.

Detrital zircon provenance studies, along with extensive paleontological work on Miocene to Pliocene lacustrine and fluvial strata in Washington and Idaho, show that the Ringold strata on the north side of the Saddle Mountains were derived from the Snake River and salmonoids were able to migrate to the Taunton locality, whereas the lacustrine strata on the south side of the Saddle Mountains were isolated by a localized hydrographic and/or temperature barrier. Therefore, the Columbia River captured the Snake River prior to Pliocene time and no major obstructions barred salmonoid migration from the Pacific Ocean to the Pasco Basin. We suggest that deposition of the low gradient fluvial and lacustrine Ringold strata was facilitated by tectonic uplift along the Wallula fault system during late Miocene – Pliocene time. Rapid incision of the Columbia Plateau marks the end of Ringold deposition and may be due to a shift in regional tectonic strain accommodation.