RINGOLD FORMATION SEDIMENTOLOGY, PALEONTOLOGY, AND PROVENANCE: IMPLICATIONS FOR THE ANCESTRAL RIVERS OF THE PACIFIC NORTHWEST
We analyzed fluvial sandstone samples from the Ringold formation on the north side of the Saddle Mountains for detrital U-Pb zircon provenance. Above and below the sampled sandstone, we dated interbedded tephra layers that bracket the time of sandstone deposition between 7.0 and 3.4 Ma. Importantly, these new ages show that the Taunton fish fossils, which are an important assemblage for interpreting the Columbia and Snake River drainage history, are older than previously thought. For comparative analysis, we dated detrital zircons from modern Columbia, Okanogan, Spokane, Methow, Yakima, and Salmon River sands, detrital zircons from various Miocene – Pliocene aged fluvial units throughout Oregon and Washington, and supplemented this with existing detrital zircon ages from the Snake River Plain.
Our statistical analysis of detrital zircon datasets, along with extensive paleontological data from Washington, Oregon, and Idaho, clarify and resolve outstanding questions about drainage reorganization. We show that there was a fluvial connection between the Snake River Plain (Idaho) and Pasco Basin (central Washington) before 3 Ma. This might indicate that either the Miocene Snake River flowed along a similar path as it does today, or that the Snake River Plain was connected to Pasco Basin via an alternative route, such as through the Salmon River. Whether the current Snake River path through Hells Canyon was carved before 3 Ma is still an open question.