ARE CLASTIC DIKES IN BADLANDS NATIONAL PARK A RESULT OF STRUCTURAL DIAGENESIS?

Abundant clastic dikes are a local common occurrence in the Tertiary strata of Badlands National Park (BNP). Their origin is debated. A model for their formation should include the following traits: 1) At some localities the dikes show well developed preferred strike orientations, however, at others they do not; 2) They consistently taper downwards, and are most abundant in the Sharps Formation, but some traverse the Brule Formation to reach the top of the underlying Chadron Fm.; 3) Dike widths vary from tens of centimeters to discernable tips millimeters wide; 4) The sand fill displays complex internal structures that consistently indicate upward fluid flow; 5) Many dikes have clear evidence for multiple episodes of sediment fill, which varies from mud to coarse sand dominated. This indicates that different dikes and through time individual dikes had different sediment fill sources which developed through multiple episodes of opening and fluid flow; 6) Some dikes display green alteration zones in adjacent sediments that are many times the width of the dike, suggestive of larger fluid fluxes along the dikes and associated reduction; 7) Clastic dikes either precede or are coeval with strati-bound horizons of chalcedony veins. Chalcedony veins parallel or are perpendicular to nearby clastic dikes. The chalcedony veins show structures clearly indicating vertical shortening consistent with deformation during compaction; and 8) Smaller dikes show small folds or wrinkles with a horizontal axis that may indicate they have also been shortened. Thus, dike formation occurred pre- or syncompaction.

Recent literature (Davies et al., 2006) suggests that clastic dike complexes (injectites) found in marine environments can form as a result of shallow silica phase diagenesis and associated release of bound fluids. We suggest that the clastic dikes in BNP may have formed in a similar manner. Such an origin could explain the local lack of preferred orientations, the timing of formation relative to compaction and chalcedony vein formation, multiple episodes of development, and their concentration at higher stratigraphic levels.