THE GEOLOGY OF FRACTURES IN FINE-GRAINED MATERIALS IN OHIO

Studies of fractures in cores, test pits, and natural outcrops have lead to several generalizations about the frequency, depth, geometry, locations, and genesis of fractures in various fine-grained geologic materials. Shallow fractures in non-lodgement facies are most common in the weathered zone (above 4 m depth). Shallow fractures that are vertical join to form polygons when viewed on a horizontal plane; the area of the polygons increases with depth. Shallow horizontal and subhorizontal fractures are irregular planes and their spacing increases with depth from millimeters to centimeters apart. Shallow fractures form primarily by desiccation and freeze-thaw. Fractures in unweathered lodgement till are parallel, low- to high-angle; viewed on a horizontal plane they do not form polygons. They form by shearing. Shallow fractures are ubiquitous, found in every core, test pit and outcrop observed. Below the surface-weathering zone fractures are less common and are found in some lodgement tills and in buried soil/weathering zones. Predictions about the occurrence of fractures across the landscape are based on the idea that similar facies, geomorphic settings, geologic history, and physiography produce similar fractures. For example, sheared lodgement till would be more common in the Glaciated Allegheny Plateau, where bedrock topography has greatly affected basal glacial movement, than in the Ohio till plain. In contrast, there would be a low probability of deep shear fractures in a thick surficial lacustrine unit, except where it was overridden by ice.