JOINTS, FAULTS, ANGULAR UNCONFORMITIES RECONSIDERED-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 1952. Filling of the reservoir resulted in wave erosion of Cretaceous and Miocene bedrock and Quaternary sediments along the shore of Harlan County Lake. During most years pool level remained high enough that continuous access to the shore was difficult.

An abnormally dry period began in 1999 and continues today. Current pool level in December 2002 is more than 20 ft below normal high pool level so it is possible to walk safely along the entire shoreline of the lake and to view numerous structures in the bedrock. 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. So far 14 normal faults and 4 reverse faults have been identified. Some of the normal faults form graben strutures. The faults strike NW and NE. Only one of the faults may extend into younger deposits. An angular unconformity between the Late Cretaceous Niobrara Formation and the overlying Pierre Shale occurs on the SW side of the lake. Another of these unconformities was reported previously by V. Souders just east of the dam on the south side of the Republican River.

Using high 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 by R.R. Burchett 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 in the area may have been episodic.