Structure plan of clastic dikes

Abstract

Conclusions drawn from field observations are that vertical sandstone dikes of the White River Badlands, South Dakota, originated as open fractures that were filled with debris supplied by erosion of overlying formations. One existing explanation is that the dikes formed in deep desiccation cracks which developed at the surface as the silts and clays of the Oligocene White River group became consolidated. Evidence from a recent study of a small area of clastic dikes is incompatible with the desiccation‐crack theory of origin. The pattern of intersecting dikes, when mapped, resembles a fracture system rather than having a hexagonal arrangement normally associated with desiccation cracks. Shear fractures appear on the map intersecting at angles of 54° to 71°, the acute bisectrix varying in strike from N 45°W to N 54°W. One dike occupies a major tension fracture, strikes N 50°W, and extends vertically through two beds of strikingly different lithology, a condition unlikely to occur with desiccation cracks. South of the area, normal faults of small vertical displacement, trending northwest, suggest minor regional tension, which may well have caused the development of a fracture pattern as displayed by the dikes. If these fractures were open at the surface, subsequent filling from above would follow, resulting in numerous intersecting clastic dikes.