The La Plata Mountains as seen from above the author’s

Durango Bill's

Paleogeography (Historical Geology) Research

Appendix to the Evolution of the Colorado River and its Tributaries (Part 2)

Southwest Colorado Regional Summary

Bill Butler

   The area bounded by the Animas River on the east, the Wilson Peaks to the north (and including the La Plata Mountains and Dolores Anticline), stretching westward into Utah and southward into New Mexico appears to have undergone a region wide uplift beginning during the Oligocene and continuing into the Miocene. (Please see the paper detailing evidence of an intracontinental magmatic zone extending from the San Juan Mountains westward into Nevada at: ) The most pronounced increases in elevation were the (renewed) uplift in the La Platas (and further northward in the Wilson Peaks) westward to Utah’s Abajo Mountains. The original La Plata uplift was local in nature and resulted from igneous intrusions in the Late Cretaceous, but considerable additional increase in elevation seems to have occurred with the more recent event. The degree of this regional uplift tends to diminish gradually as you move outward from the uplift centers, but a sharper downwarp toward the Animas River defines the eastern boundary.

   Before this regional uplift, the area west of the Animas River was significantly lower than it is now. In the early Oligocene (and earlier), river drainage near the Utah border was from south to north toward the old Lake Uinta lowlands. The only remnant of this old drainage is the Dolores River. As a consequence of the regional uplift being greater from the Wilson Peaks and La Plata’s westward to the Abajo Mountains (with lesser uplift to the south), the old drainage that had been south to north was reversed to become north to south about late Oligocene to early Miocene time. The ancestral San Juan River, which had flowed west northwestward across the region (and then north using the present path of the Dolores River), was forced to find a new route further south in New Mexico. The upper Dolores River was left as the sole owner of the old riverbed. The best estimate of the breakpoint in river patterns is about 25 to 30 million years ago on (or near) the Oligocene/Miocene boundary.

   (As a note to the reader, there are alternate models for the history of river patterns and mountain building in southwest Colorado. What is presented here is the author’s interpretation. It should be evaluated in this light as opposed to “generally accepted knowledge”. It may take many years and research techniques not available today to establish the whole story.)

Also please see Dolores

Return to the Physics of Mountain Building Page (Part 1)

Continue to the La Plata Mountains, Upper Dolores River, and ancestral San Juan River Page (Part 3)

Return to the Main Appendix Page for the Evolution of the Colorado River

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