COMPARISON OF TENSILE FEATURES IN LOESS AND BEDROCK OF HARLAN COUNTY RESERVOIR

Cretaceous strata and loess deposits in the Harlan County Reservoir area of south-central Nebraska display a variety of tensile features. Previous work has documented preferred direction of fractures in younger loess units in the Omaha and Broken Bow areas of Nebraska. This study considers and compares preferred strike orientations of veins and joints in Cretaceous Pierre shale & Niobrara Chalk and fractures in overlying Quaternary loess.  There is also a strong preferred direction of drainage in the Harlan area that trends roughly north-south. Using a multiple working hypothesis we are exploring if the loess fractures are a result of neotectonics, topography, or inherited from older, underlying fracture sets.   Fractures in loess from two sites separated by approximately 10 miles show strong preferred orientations of 10-20 degrees NNE, and a less dominant orientation of the loess fractures exists in the 110-130 degrees ESE direction. Veins and joints within the Pierre Shale and Niobrara chalk, along an approximately 2.5 mile shoreline outcrop, display strong preferred orientations in the 170-180 degrees SSE direction and a less dominant orientation in the 50-60 degrees NE direction. Preliminary analysis indicates the north-south fractures within the loess may be sub-parallel to the underlying joints and veins in the Pierre shale and thus may be inherited. The 110-130 degrees ESE trending set of loess fractures are not parallel to the underlying vein/joint set and therefore another explanation is needed for this fracture direction. Previous work shows dominant vein and joint directions in the bedrock varies locally. If inherited, one set of loess fractures should co-vary with the dominant bedrock joint/vein direction. Continued questions are: a) How does the orientation of the loess fractures relate to a present day stress field, and b) What is the relationship between the loess fractures and the strong preferred direction of drainage?