JOINTS, FAULTS, ANGULAR UNCONFORMITIES-HARLAN COUNTY LAKE, NEBRASKA

A major flood along the Republican River in Nebraska in 1935 led to construction of Harlan County Dam by the U.S. Army Corps of Engineers. The dam was completed in 1951. Filling of the reservoir resulted in wave erosion of Cretaceous and Miocene bedrock and Quaternary sediments along the shore of Harlan County Lake. Cross sections and maps published by the Corps show sets of NW and NE striking joints and faults in the Cretaceous bedrock beneath the floor of the valley at the dam site. There are only a few later brief written and oral reports of faults in bedrock observed at the surface in the area.

In the summer and fall of 2001 pool levels dropped to the point where it was possible to walk along most of the shoreline of the lake. Joints in bedrock and in some Quaternary sediments are mostly in three sets striking NW, NE, and nearly parallel to the shoreline. On the south side of the lake extensive exposures of Cretaceous bedrock are cut by faults. These faults also strike NW and NE just as do those reported earlier at the dam site. Most faults are normal faults, but some reverse faults are also present. Most bedrock appears to be block faulted. Only one of the faults appears to possibly continue into overlying Quaternary deposits. An angular unconformity between the Late Cretaceous Niobrara Formation and overlying Pierre Shale occurs on the SW side of the lake. Another of these unconformities was reported previously just east of the dam on the south side of the Republican Valley.

Using the 2001 low pool level as a datum, I mapped the general distribution of formations cropping out along the shore at that datum. The Ash Hollow (Miocene), Pierre Shale, and Niobrara formations crop out along parts of the south shore. The oldest unit (Niobrara) appears to coincide with highs on the structural contour map of the area drawn on the top of the Pennsylvanian Lansing Group, while younger rock units appear to coincide with lows. The angular unconformities are evidence of post-Niobrara/pre-Pierre deformation. Thus, deformation may have been episodic.