CLASTIC DIKES IN THE CEDAR PASS AREA, BADLANDS NATIONAL PARK, SD

Clastic dikes in the South Dakota Badlands are still somewhat enigmatic. The dikes are up to 10s of cms thick , can exceed 300m in strike length, locally cut through the Sharps formation, and vary significantly in thickness, grain size, and orientation.. This report discusses observations in the Cedar Pass area. Individual dike are typically composite features, with multiple vertical bands of internal fill (1-4 cm wide) that vary from mud to medium sand. Brittle internal cross cutting relationships indicate multiple opening events for individual dikes. Geometries suggest that earlier fill must have lithified before subsequent intrusions.

Two preferred sub-orthogonal strike directions exist (30-40 and 120-130) in the Cedar Pass area, but a significant percentage do not belong to any trend. Cross cutting relationships indicate the two directions are coeval, and some dike geometries indicate simultaneous opening of orthogonal directions. Some of the 120-130 dikes have green wall rock alteration absent in other dikes, and these tend to crosscut other dikes. The orthogonal pattern is aligned with local normal faults and a subtle monocline. One dike was observed to cut a fault.

A variety of internal features occur. Marginal conical forms that V inwards and upwards, sometimes grading into massive, coarser interior, consistently suggest upward flow. Abundant cross stratification occurs in dikes. Calcite cement locally occurs, and in places gypsum veins occur.

The dikes are sub-vertical and taper downwards. They are dilational, but sometimes wall geometries do not match, suggesting post-parting modification. The sediment sources are not evident. Subtle folding (sub-horizontal axis) of the dikes is consistent with post intrusion compaction. Within a thick, persistent white layer (Rocky Ford Ash?) dikes tend to splay and split more, and often a 5-10cm of green wall rock alteration exists around the dikes for a few meters below this layer. Cementation nodules also deflect the dikes locally, and therefore predate dikes. The composite character, cross cutting geometries, and multiple opening directions suggest protracted dike formation—a history consistent with a structural diagenetic origin. The green alteration associated with some dikes suggests reducing fluids were mobilized later in the evolution of the system.