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Heron Dam

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The damsite is situated on the northeast flank of the North El Vado Dome, which is a simple elliptical dome about 1.5 miles in diameter.  The dome has relatively little deformation, with a few small faults and radial concentric joints.  Many of the joints are slackensided, but seldom show an offset of bedding planes.  At the damsite, the joints strike N-S at a few inches to one foot apart (radial joints) and strike E-W at about 50 to 100 feet apart (concentric joints).  Some of the joints are open 1/8 to ¼ inch in sandstone beds, but are tight in shale beds.  Two normal faults are present 500 to 1000 feet upstream of the dam; one strikes E-W, about 2 miles long, with displacement varying from 2 to 50 feet and the other strikes N-S, about 1.1 miles long and has displacement of up to 75 feet.  At the damsite, Willow creek has eroded its canyon through the Dakota formation (siliceous sandstone and carbonaceous shale) and into the upper part of the Morrison formation (Jurassic fine to medium Clayey sandstone and sandy shale).  The contact between the Dakota and Morrison formations was found at elevation 6958 in the right abutment and elevation 6948 in the left abutment.  The Morrison sandstone and shale comprising the dam foundation were described as fine to medium grained, clayey, firm to moderately hard, friable, jointed, brown, green and gray (for the shale).  An apparent dip of 5o upstream (N 11o 30' E) was measured in the Morrison formation, with most of the beds tight and moderately bonded.  The beds of shale and sandstone are interleaved and interfingered.

Last updated: Oct 30, 2008