PROJ00505

• Contract/PO #: C153419
• Contractor Name: San Miguel Water Conservancy District
• Water District: 0
• County: San Miguel
• Bill Number: XB 99-999
• Loan Projects - Doc Type: Feasibility Study

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Bedrock is essentially horizontal to gently rolling, with notable exceptions in

the east and west portions of the region. A generalized stratigraphic column

of the San Miguel region is shown on Drawing 1124-2. The eastern margin of the

region has been partially influenced by the dome building events of the San

Miguel and San Juan Mountains, as evidenced by the presence of igneous

intrusive bodies, merging of the nearly flat-lying beds with structures

originating to the south and east, and local steepening of some beds (Bush and

others, 1960). The western end of the region is distinguished by the linear

graben-like, salt and gypsum-cored collapsed anticlines of Paradox and Gypsum

valleys, as well as the broad northwesterly-trending, simple folds that flank

the collapsed salt structures, such as the Dry Creek Basin anticline and

syncline, the Nucla syncline and an unnamed anticline to the northwest of

southeast Paradox Valley.

Two groups of potentially active faults are identified as No. 90 and 92 by

Kirkham and Rogers (1981) of the C.G.S. in the western portion of the region,

and flank Paradox Valley and Big Gypsum Valley, respectively. These faults are

located 21 miles or more west and northwest of Cone Reservoir. As these faults

result from salt flowage and solution, it is unlikely that earthquakes

generated by these faults would exceed a magnitude of 4 or 5. Movement on many

of these faults is predicted to occur as creep. It is not known if the

subsurface faults that initiated the formation and later deformation of the

salt anticlines are still active. These deep, subsurface faults should be

thoroughly investigated before any critical or sensitive structures are built

in this region.

Cone Reservoir is located about 14-17 miles south of some unnamed, late

Cenozoic-age faults situated on the southwest flank of the Uncompahgre uplift

which represent the nearest potentially active faults. The most recent

movement of No. 88 is Quaternary. The peak horizontal acceleration produced at

the Cone site should not exceed .27g in rock and .26g in stiff soils based on

estimates of the maximum credible earthquake F;enerated by the potentially

active fault No. 88 (Kirkham and Rogers, 1981), which is predicted to have a

magnitude of 6.8. The above estimates basedon published empirical procedures

by Seed and Idriss (1982), and Slemmons (1977).

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