FAULTS, ANGULAR UNCONFORMITIES, ANCIENT LANDSLIDES, AND PALEOTOPOGRAPHIES-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 NE and NW striking faults in the Cretaceous bedrock beneath the dam site. There are only a few later brief written and oral reports of exposed faults observed in other parts of 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. On the south shore exposures of Cretaceous bedrock are cut by faults. These faults also strike NE and NW and are mostly normal faults (horst and graben structures) although a few reverse faults also have been discovered. Offsets of 10 or more meters appear to have occurred along some of the faults. None seems to cut younger beds. Broad and gentle local folding of the Cretaceous beds has also been observed in a few places. Angular unconformities between the Late Cretaceous Niobrara Formation and overlying Pierre Shale occur on the SW shore of the lake and at a site about 0.6 mi east of the dam on the south side of the river. In places the Pierre Shale was subject to post-Cretaceous landsliding prior to deposition of younger stata. Miocene Ogallala Group strata and younger Quaternary sediments were deposited in low spots on land surfaces developed on the faulted and folded Cretaceous strata during multiple cutting and filling episodes. The Ogallala Group strata fill valleys cut into the Cretaceous bedrock to pool level in at least three places on the south side of the reservoir. Cretaceous Pierre Shale bedrock has only been found at one site on the west side of North Cove on the north side of the lake. The shale beds at this site dip to the southwest at about 10 degrees.