Lydia M. Staisch

Ringold formation sedimentology, paleontology, and provenance: implications for the ancestral rivers of the Pacific Northwest

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The Miocene–Pliocene Ringold formation has been an important marker for understanding where and when the ancestral Columbia, Snake, and Salmon/Clearwater Rivers flowed. Over the past century, many researchers have provided important insights into the river history, and most studies have focused on fish fossils and river cobble provenance as evidence. However, the details and mechanisms for river reorganization are still debated and have key implications for regional tectonic and volcanic activity. To test various proposed and possible ancestral drainage patterns, we provide a new dataset of detrital zircons, which provide a unique fingerprint to identify possible source terranes.

 

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 Pend Oreille River. Whether the current Snake River path through Hells Canyon was carved before 3 Ma is still an open question.