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volcanics are now known as the Conejos Andesites and Breccias and the Treasure Mountain Quartz Latite. Subsequent erosion again leveled this area creating the San Juan Peneplain. Still later in the Tertiary Era there were regular basaltic extrusions and the area was again elevated and tipped slightly to the east, bringing on a new cycle of erosion in which the present canyons and valleys were created.

4. Geology of Dam Site and Reservoir. - There was very little overburden material in the dam foundation area at the time of construction and the relatively thin accumulations of talus and soil were easily excavated from the dam and dike foundation area. A relatively light stripping of weathered and fractured rock was required. Figure 2 is a geologic map of the dam site area.

Much of the exposed rock throughout the dam site area appears severely fractured, and there are apparent localized shear zones and faults which contain highly stained or mineralized vein material. The quartz vein filling contains vugs or openings along old fracture planes but these openings are believed to be discontinuous or unconnected. However, except for these localized zones, explorations showed that the rock is generally fresh and unweathered, and the joints and fractures are remarkably well healed within 2 or 3 feet from the surface exposures. It is believed that seepage will be effectively controlled by the grouting performed, as the few unhealed or open fractures were found to be generally clean and free from mud or other disintegrated material.

Whether or not the visible faulted or mineralized zones will in the future transmit appreciable quantities of water remains to be seen. The angle drill holes which were intended to explore these features, in most cases either missed the disintegrated rock or crossed the fault too near the surface to obtain good core recovery or good percolation tests. These holes did, however, prove the zones affected by the faults to be relatively narrow. Rock alteration or intense fracturing is limited to within a few feet of the vein material,

For the convenience of description, the principal shear zones have been designated as faults 1, 2, and 3 (fig. 2). Fault 1 crosses the river beneath the upstream toe of the dam and enters the draw into which water from the spillway will be discharged. The affected portion of the left abutment is a zone about 4 feet or less in width. Iron-stained quartz and clay gouge were excavated from this zone. The fault is believed to be insignificant as far as leakage or stability are concerned and too distant from the proposed spillway structure on the ridge to induce any serious problems. Holes drilled in the vicinity of the spillway draw were in good rock and it is believed that water can be discharged into the unlined channel below the toe of the dam. The exact location of the fault on the taluscovered slope on the right side of the river is uncertain, but it is believed that it lies in the vicinity of the inlet of the outlet tunnel. Fault 2 is downstream and nearly parallel with fault 1. It is exposed by a series of prospect holes in the quartz vein material and fault gouge and shattered rock. Fault 3 is not clearly defined but has been designated as a fault to account for the sheared and jointed rock more or less parallel to the river near the outlet tunnel line. The upstream extension of this sheared zone is uncertain. Other minor faulted zones probably exist but are not apparent.

Two holes were drilled to locate the top of sound rock for the inlet of the diversion tunnel. The tunnel outlet was located in a steep bluff of exposed rock, the character of which could be discerned without drilling. A slight shifting of exploration holes would have shown wide or varied results as the rock in the area is severely jointed and sheared.

. In the vicinity of the spillway, located on the ridge between the dam and dike, sound rock is exposed over virtually the entire ridge and through most of the draw leading to the river below the dam. Except as may be affected by the localized faulted zones, it is anticipated that the rock will be resistive to any appreciable scour by spills of water discharged a safe distance from the dam. For this reason a lined stilling pool for the spillway was not warranted.

5. Embankment Materials. - (a) Investigations. --The preliminary search for embankment construction materials began in 1940. Investigations continued intermittently until 1950. Preliminary investigations of a reconnaissance nature located 11 possible sources

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volcanics are now known as the Conejos Andesites and Breccias and the Treasure Mountain Quartz Latite. Subsequent erosion again leveled this area creating the San Juan Peneplain. Still later in the Tertiary Era there were regular basaltic extrusions and the area was again elevated and tipped slightly to the east, bringing on a new cycle of erosion in which the present canyons and valleys were created.

4. Geology of Dam Site and Reservoir. - There was very little overburden material in the dam foundation area at the time of construction and the relatively thin accumulations of talus and soil were easily excavated from the dam and dike foundation area. A relatively light stripping of weathered and fractured rock was required. Figure 2 is a geologic map of the dam site area.

Much of the exposed rock throughout the dam site area appears severely fractured, and there are apparent localized shear zones and faults which contain highly stained or mineralized vein material. The quartz vein filling contains vugs or openings along old fracture planes but these openings are believed to be discontinuous or unconnected. However, except for these localized zones, explorations showed that the rock is generally fresh and unweathered, and the joints and fractures are remarkably well healed within 2 or 3 feet from the surface exposures. It is believed that seepage will be effectively controlled by the grouting performed, as the few unhealed or open fractures were found to be generally clean and free from mud or other disintegrated material.

Whether or not the visible faulted or mineralized zones will in the future transmit appreciable quantities of water remains to be seen. The angle drill holes which were intended to explore these features, in most cases either missed the disintegrated rock or crossed the fault too near the surface to obtain good core recovery or good percolation tests. These holes did, however, prove the zones affected by the faults to be relatively narrow. Rock alteration or intense fracturing is limited to within a few feet of the vein material.

For the convenience of description, the principal shear zones have been designated as faults 1, 2, and 3 (fig. 2). Fault 1 crosses the river beneath the upstream toe of the dam and enters the draw into which water from the spillway will be discharged. The affected portion of the left abutment is a zone about 4 feet or less in width. Iron-stained quartz and clay gouge were excavated from this zone. The fault is believed to be insignificant as far as leakage or stability are concerned and too distant from the proposed spillway structure on the ridge to induce any serious problems. Holes drilled in the vicinity of the spillway draw were in good rock and it is believed that water can be discharged into the unlined channel below the toe of the dam. The exact location of the fault on the taluscovered slope on the right side of the river is uncertain, but it is believed that it lies in the vicinity of the inlet of the outlet tunnel. Fault 2 is downstream and nearly parallel with fault 1. It is exposed by a series of prospect holes in the quartz vein material and fault gouge and shattered rock. Fault 3 is not clearly defined but has been designated as a fault to account for the sheared and jointed rock more or less parallel to the river near the outlet tunnel line. The upstream extension of this sheared zone is uncertain. Other minor faulted zones probably exist but are not apparent.

Two holes were drilled to locate the top of sound rock for the inlet of the diversion tunnel. The tunnel outlet was located in a steep bluff of exposed rock, the character of which could be discerned without drilling. A slight shifting of exploration holes would have shown wide or varied results as the rock in the area is severely jointed and sheared.

In the vicinity of the spillway, located on the ridge between the dam and dike, sound rock is exposed over virtually the entire ridge and through most of the draw leading to the river below the dam. Except as may be affected by the localized faulted zones, it is anticipated that the rock will be resistive to any appreciable scour by spills of water discharged a safe distance from the dam. For this reason a lined stilling pool for the spillway was not warranted.

5. Embankment Materials. - (a) Investigations. --The preliminary search for embankment construction materials began in 1940. Investigations continued intermittently until 1950. Preliminary investigations of a reconnaissance nature located 11 possible sources

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