Monday, November 04, 2013

More Details About the Grand Canyon/Bell River Connection

Did the Grand Canyon form 23 million years ago instead of 6 million, as proposed by some geologists? See this updated posting for an interesting scenario in the Colorado River's evolutionary history.  **JUST ADDED: a complete scientific article on this proposal has just been published in GSA Today. You can access the article here where a pdf link is available.

Readers of this blog will recall my last posting about a proposal made by Dr. Jim Sears (University of Montana) at the Annual Meeting of the Geological Society of America in Denver last week. In it he proposed that the ancestral Colorado River might have flowed west through an early incarnation of the Grand Canyon to an area near Nevada, then turned northward toward Idaho and Montana, eventually spilling into the Saskatchewan arm of the pre-Pleistocene (before 2.5 million years ago) Bell River system. The Bell River was an Amazon-scale river system that drained about one-third of the North American continent before glaciers in the last 2.5 Ma reorganized the drainage system.

Here is a map of the Bell River system in pre-Pleistocene Canada, (with an arm of it added by Dr. Jim Sears and colored in red and light blue). It shows how this arm may have run through the Grand Canyon toward Nevada, then along a rift system to Idaho and Montana. The evidence for this? In southwest Montana are Miocene age sediments (about 23 to 17 million years old) in the modern upper Missouri River system. They contain fluvial gravel that is called the Sixmile Creek Formation and they indicate northeast flow directions. They also contain clasts of rocks that are exposed in Nevada. Recent mapping shows that the Sixmile Creek rocks are distributed across 500 miles from the Snake River Plain to the Saskatchewan River area.

This arm of the Bell River thus crossed the present-day Snake River Plain and the Idaho/Montana Continental Divide to connect with the Saskatchewan arm of the Bell River. Note a greatly diminished Mississippi River in the pre-Pleistocene United States.

The Bell River flowed eventually to the area of modern day Hudson Bay and the Hudson Strait to the Labrador Sea. It ended in the Saglek delta which is the largest depositional basin between Baffin Island and Florida. It contains a 5-mile thick section of sediment. Recently released geoinformation from oil companies shows that the Saglek deposits contain Pennsylvanian age (about 300 million years ago) pollen fossils that were transported from their source rocks by this ancient river. Sears surveyed the drainage area of the Bell River and suggested that the likely source area for these transported fossils was the Supai Group rocks in Grand Canyon. One might think that other Pennsylvanian age rocks could have also supplied the material to the Saglek basin but they reside south of the modern Great Lakes and thus on the other side of the drainage divide.

Of course, this idea is not a slam dunk immediately. Further testing of the idea is necessary. But it does dovetail with other proposals by Karl Karlstrom and Brian Wernicke that a paleocanyon in eastern Grand Canyon might have been present during the Miocene.

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