GRAND TETON TO GRAND CANYON, PLIOCENE COURSE OF THE GREEN RIVER IN THE CENTRAL ROCKY MOUNTAIN, USA

The Green River basin of southwestern Wyoming was captured from a well-established late Miocene connection with the composite Snake-Klamath-Sacramento Pacific northwest drainage sometime between the latest Miocene and 620 ka. That this diversion included an interval of time when the Green paid tribute to the Gulf of Mexico via the Platte, already strongly challenged by ichthyologic evidence, is rejected based on a reevaluation of the river gravel terraces in the Great Divide basin of south-central Wyoming that had previously been interpreted as evidence that the Green once transported clasts derived from distinctive western Wyoming sources into southeastern Wyoming and then integrated with the Platte. The gravels are reinterpreted as deposits of a river system that flowed south into the Yampa and then into the lower Green.

Diversion of the upper Green from its connection with the Snake and integration with the Colorado probably occurred during the earliest Pliocene. At this time, extensive silicic calderas inflated and blocked the upper Snake River plain, and renewed southwest-directed thrusting along the combined Gros Ventre – Wind River thrust system combined to achieve the capture. Thrust loading of the northern Green River basin encouraged drainages of the proto Jackson Hole area to flow south where they quickly integrated with drainages of the upper Colorado. The sudden surge of hydrologic power to the south across the Colorado Plateau caused Lake Bidahochi, which had previously been a high, inland sea with only minor inlets, to rapidly fill and spill over the Kaibab uplift, forming the Grand Canyon between 5.5 and 5.2Ma.

Later Pliocene uplift of the Grand Tetons and formation of Jackson Hole probably diverted some headwater drainages back into the Snake resulting in slightly diminished discharge of the combined Green – Colorado system. This fits with information derived from models that suggest flow in the lower Colorado was greater during canyon incision and integration with the Sea of Cortez than it is today.