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:
http://geology.cr.usgs.gov/pub/bulletins/b2158/B2158-4.pdf
) 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