C153379 Feasibility Study

• Contract/PO #: C153379
• Contractor Name: Brook Forest Water District
• Contract Type: Loan
• Water District: 9
• County: Jefferson
• Bill Number: SB 80-67
• Loan Projects - Doc Type: Feasibility Study

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deposits outcrop wi thin 25 miles of the Hardin Dam site, none of them are

located within the proposed dam or reservoir area.

The Laramide Orogeny caused extensive folding and faulting of the

Cretaceous and earlier rocks along the Front Range. These faults and folds,

however, had little effect on the pre-Tertiary rocks in the Hardin area.

Attitudes measured in the Fox Hills Sandstone at the proposed dam site indi-

cated strikes ranging from N30W to N45W and dips ranging from 2 to 12 degrees

to the southwest. No major joint sets were recognized in the Fox Hills

outcrops and no major faults have been mapped in the area. This upstream dip

and the lack of extensive joints or faulting should prevent significant

seepage through the Fox Hills abutments at the Hardin Dam site. The rela-

tively impermeable claystone and siltstone of the upper transition zone of the

Pierre Shale should also provide an adequate dam foundation.

4. Surficial Geology

The sand and gravel carried onto the eastern plains of Colorado during

the Pleistocene (Chapter III, B.2) formed a series of alluvial pediments,

terraces and valley fills which still exist along the South Platte River

(Table III-I). The major high-level, pediment gravels -- the Nussbaum,

Rocky Flats, and Verdos -- occur only as isolated hilltop remnants of well-

weathered, carbonate cemented gravel or sand. These hilltop exposures range

from 120 to 440 ft above the modern level of the streams (Gardner, 1967).

Although unimportant as a source of aggregate due to the amount of weathering

and small extent of the deposit, Rocky Flats Alluvium has been mapped 5

miles southeast of the proposed Hardin Dam site. This exposure, located in

Section 3, T3N, R61W (Gardner, 1967), occurs in a gravel pit 200 ft above

the South Platte. Several small exposures of Verdos Alluvium cap two hills

3 miles southeast of Kersey (Schwochow et al., 1974) at an elevation of

120 ft above the present South Platte channel (Figure 111-1). No exposures

of Nussbaum Alluvium are known in the Hardin Dam Project vicinity.

Lying topographically lower than the three gravel-covered pediments

are three Pleistocene sand and gravel terraces -- the Slocum, Louviers

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