Great Divide Basin: A big tiltmeter?
One of the treat of driving across the western U.S. from time to time is being reminded of puzzles. One is the Great Divide Basin, conveniently located on interstate 80 in southwestern Wyoming. The basin is like the Great Basin: once water goes in, it doesn’t come out (at least not on the surface). You sort of cross the Continental Divide twice: once to leave the Atlantic, and a second time to enter the Pacific drainage. Now the Great Basin has no outlet to the ocean for two reasons: one is the extreme aridity of the region thanks to the Sierra Nevada sucking off all the moisture from the Pacific. The other is that it is tectonically active, so basins are being created by normal faulting that would have to be filled with water before the water could get out of the basin. But how does this work for the Great Divide Basin?
The area is supposed to be inactive tectonically, and the basin is too large for something like wind erosion to have carved it out. There is so little relief that it is hard to imagine something like a landslide filling an outlet.
From a casual look in driving across, it seems that the basin is more deeply eroded in the east than the west; the east edge is decidedly structural, with near-vertical strata just west of Rawlins. This seems like a basin that used to drain to the east; it might have had a strong knickpoint on a more resistant stratum in the tilted sediments. But to make this basin closed, it had to tilt back to the west (east up, west down). Turns out this quick and dirty guess is probably not too far off.
A quick literature search turned up a paper by Paul Heller and colleagues that makes an interesting suggestion: erosion of the North Platte to the northeast caused everything to flex upward, in this case just enough to keep water from the Great Divide Basin to make it out through the two gaps in the northeast side of the basin that seem to be at the ends of where the old river ran. Erosion would have to be exceedingly slow for this to work, so a key part is that the basin is very arid. This model makes other predictions (the Hanna Basin should tilt to the southeast, for instance) and erosion to the west by the Green River isn’t included, and the flexural rigidity needed seems low compared to other estimates, so maybe there is more work to be done. But it is a cute idea and certainly an interesting tilt meter in a place where it is hard to measure geologic tilts.