FAULTS AND VEINS IN WHITE RIVER GROUP STRATA OF TOADSTOOL GEOLOGIC PARK, NORTHWEST NEBRASKA

An array of normal faults and veins are well exposed in the badland topography of Toadstool Geologic Park in northwest Nebraska within the Chadron and overlying Brule formations of the White River Group (late Eocene to early Oligocene). The Toadstool Fault – a large northeast striking fault – has been linked to the Colorado Lineament, but otherwise little is known about these structural features.

A GIS database of these structures is being constructed. Along with GPS position, fault parameters describing orientation, slip direction, throw, fault zone thickness, and fault rock type are recorded at points along each fault. For veins, orientation, length, width, vein textures, and composition are recorded. At this point the database includes >400 points on 138 faults and about 200 entries for veins.

Several different fault trends are evident and faults show non-planar geometries. Most faults strike northeast and north-northeast but subordinate sets strike north, northwest, and east. Some fault sets exhibit regular spacing. En echelon patterns of small faults and or veins appear to link the tips of adjacent faults. Fault fill consists of chalcedony, calcite spar, and micrite but is not homogeneous across the area and exhibits patterns that may reflect variable fluid geochemistry. Veins occur in distinct ‘patches’ and are not uniformly distributed. Some vein arrays occur at the tips of faults (representing ‘frozen’ propagation geometries), while others truncate at a high angle against faults.

This preliminary work has revealed a more distributed and complex pattern of brittle strain in the Toadstool area. The northeast striking faults parallel the Colorado Lineament and underlying Precambrian trends. Other fault orientations parallel a Black Hills trend. Subsequent research and analysis should help reconstruct the deformation events and place them into the geological history of the region and help document the nature of intraplate strain.