James W. Sears

Dept. of Geosciences, University of Montana, Missoula, Montana 59812, USA

ABSTRACT

Remnants of fluvial sediments and their paleovalleys may map out a late Oligocene–early Miocene “super-river” from headwaters in the southern Colorado Plateau, through a proto–Grand Canyon to the Labrador Sea, where delta deposits contain microfossils that may have been derived from the southwestern United States. The delta may explain the fate of sediment that was denuded from the southern Colorado Plateau during late Oligocene–early Miocene time.

I propose the following model:

1. Uplift of the Rio Grande Rift cut the southern Colorado Plateau out of the Great Plains at 26 Ma and tilted it to the southwest.
2. The upper Colorado River and its tributaries began as consequent streams that flowed down the structural plunge of the basin toward the southwest corner of the Colorado Plateau, where the river passed through a Paleogene canyon.
3. The river turned north in the Lake Mead region to enter Paleogene rifts of the eastern Great Basin.
4. NE-trending grabens across the Idaho and Montana Rockies provided the final link to the Great Plains, where the Miocene drainage joined the “Bell River” of Canada, which drained to the Labrador Sea.
5. Faulting and volcanism began to segment the paleo-river by ca. 16 Ma.
6. Faulting dammed Miocene Grand Canyon, creating a large ephemeral lake that persisted until after 6 Ma, when the Colorado River was captured by the Gulf of California.
7. The resulting shortcut to sea level greatly increased the gradient of the Colorado River, leading to headward incision of the Inner Gorge of Grand Canyon along the trace of the Miocene bedrock valley floor and renewed late Miocene-Holocene erosion of the Colorado Plateau.
8. The Yellowstone hotspot cut the river off in Idaho after 6 Ma.
9. Pleistocene continental glaciation destroyed the Canadian Bell River and diverted Montana’s drainage into the modern Missouri River.

Manuscript received 3 April 2013; accepted 30 June 2013

DOI: 10.1130/GSATG178A.1