PINPOINTING PALEO-DRAINAGE DIVIDES AND RIVER CONFLUENCES USING DETRITAL ZIRCON PROVENANCE ANALYSIS

The Snake and Columbia River systems have undergone substantial drainage network reorganization over Miocene and Pliocene time. Detrital zircon provenance and sedimentological characteristics in depositional basins in the western Snake River Plain (SRP) and Columbia Basin indicate that the Yellowstone hotspot and crustal densification/subsidence of the eastern SRP are important geodynamic mechanisms affecting evolution of the modern drainage network. Important questions remain about the downstream river network, the spatio-temporal evolution of major drainage divides, and the timing and mechanism of Hells Canyon incision, North America’s deepest canyon.

In this study, we expand on previous detrital zircon provenance analyses from these regions and present new work from the Glenns Ferry formation at the Hagerman fossil beds in the SRP, Clarkston Heights gravels downstream of the Snake and Clearwater confluence, and the Ringold formation located in the central Columbia Basin. Detrital zircon ages from Miocene-Pliocene strata are compared to tributaries of the Snake and Columbia river systems to ascertain which portions of the landscape contributed to prior drainage networks and how these have changed over time and space.

Preliminary results show that the ~4.3 – 2.5 Ma Glenns Ferry formation at Hagerman fossil beds were mainly sourced from the nearby Wood and Big Lost River systems; however, Glenns Ferry strata < 3.1 - 3.7 Ma show an increasing component of far-traveled zircons from the southern and eastern SRP. In the Columbia Basin, northern Ringold formation samples of similar age show a strong affinity towards eastern SRP tributaries, specifically the Little Lost River, Henry’s Fork, and upper Snake River, suggesting a drainage divide between the Big and Little Lost rivers. In contrast, Ringold formation samples collected in the southern Columbia Basin show a clear increase in Clearwater River contribution. The Clarkston Heights gravels are best fit with a source model that includes portions of the western SRP, but not the eastern SRP. This suggests that the Clarkston Heights gravel was deposited prior to the reconnection of the eastern and western SRP at 3.7–3.1 Ma but that Hells Canyon was a throughgoing fluvial route between the western SRP and Columbia Basin before defeat of central SRP drainage divide.