PERMFILE67019

• Permit No: C1981038
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: Volume 4 Alluvial Valley Floor Appendix & Alluvial Valley Phase I Report
• Media Type: D
• Archive: No

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Regional Geomorphology
RESULTS AND DISCUSSION
The Mancos shale and overlying Mesaverde formation of Cretaceous age are
exposed along the slopes of the valley of the North Fbrk of the Gunnison River.
Mesaverde outcrops extend northward from the river along all the major drainages
in the lease area, ranging from isolated, vertical sandstone ledges to extensive
exposures of interbedded sands and shales. 4hese beds dip 2° to 5° to the NNE.
South of the river, beyond the bordering Cretaceous strata, are eroded peaks of
an intrusive monzonite stock which reach elevations up to 11,000 feet, compared
to ridge elevations of 8,000 feet to 8,600 feet over most of the lease area.
North of the lease area, the topography rises to elevations exceeding 9,000
feet, with remnants of the Wasatch formation of Eocene age overlying the
Mesaverde formation.
Basaltic rubble consisting of subarcgular to subrounded boulders up to four
feet in dimension in a matrix of clayey soil covered the original post Eocene
topography of the Wasatch remnants and all of the associated erosional surface
on the Mesaverde formation at the level of the top of the ridges below the
rubble in the lease area. Volcanism of late Eocene to Pliocene time which
produced the basalt floor of the Grand Mesa was the source of this rubble.
There are volcanic necks in the t4asatch area to the north and in the monzonite
peaks to the south of the lease area.
Homogeneity of the basaltic rubble, and the size and shapes of the large
boulders indicates gravitational force, aided by water saturation of the
sediments, moved the blanket deposits from nearby highlands. 'This phase of
development may have been initiated in Pliocene time and accelerated during
Pleistocene time, with subsequent incision of steep walled canyons through the
rubble blanket into the Mesaverde formation. Much of the original blanket covet
migrated down the steep slopes of the canyons as hummocky landslide deposits and
partially filled the deep channels.
As the bounteous water of the Pleistocene decreased, th~~ landslide
topography became stable, and finer sediments were deposited in the lesser