Harmon D. Maher , University of Nebraska (Omaha)
Robert Shuster , University of Nebraska (Omaha)
The following points update some of the scientific findings relayed in the last report.
1) Both stratiform vein horizons in the Brule Formation at Scotts Bluff show a well developed preferred orientation trend at 175 degrees, oblique to unornamented joints in the enclosing strata which trend at 87 and 137 degrees, indicating a polyphase fracture history. The roughly north-south fracture trend is a direction seen at a number of other sites and is of regional significance. Statistical modeling of the vein strike distribution, however, suggests a significant (circa 50%) uniform component is needed to explain the overall strike distribution. The simplest explanation for a combined preferred orientation coupled with a uniform component and the stratabound character of the veins is formation due to diagenetic processes, but with partial organization and contribution to the veining from an anisotropic stress field (primarily, a 95 degree trending minimum principal stress). A late stage diagenetic replacement of earlier gypsum by calcite is pervasive.
2) A new site of clastic dikes studied in northwest Nebraska strongly reinforces a linkage between clastic dike formation, silica mobilization and chalcedony veins suggested by observations elsewhere. Irregular chalcedony veins are partly brecciated within a green mudstone matrix along significant intervals of the dikes. Otherwise, chalcedony veins are absent at this site. Local normal faulting clearly postdates the clastic dikes, and is associated with late stage calcite mineralization and diagenesis. The clastic dikes occur near the base of the Brule Formation, a deeper stratigraphic position than is typical for the well developed clastic dikes of Big Badlands National Park. Three preferred orientations exist, including a roughly N-S direction. Cross-cutting relationships suggest the different directions are roughly coeval and the frequency similar, permissive of the idea that horizontal strain was homogenous due to a diagenetic shrinkage origin.
3) A major challenge with understanding the clastic dikes in the Tertiary strata is identifying where the source material came from. An XRD comparison of the clay mineralogy within a dike and of the enclosing sediment is in progress, with initial results indicating a similarity that permits the enclosing sediments to be a source. Abundant lithic clasts suggest that lithified wall rock material is incorporated into the clastic dikes. At the northwest Nebraska site clastic dikes were observed to pinch out upwards, incompatible with infilling from surface sediments. Detailed petrographic analysis of the clastic dike fill is underway.
4) XRD analysis of chalcedony bearing horizons reveals a complex signature with abundant smectite clays, but that horizons with chalcedony vein development are richer in mixed smectite-illite or illite. This suggests an unconventional smectite to illite transformation process since these sediments were not buried deeply enough to reach typical transformation temperatures described in the literature.
We would like to acknowledge Dr. Rachel Benton for help in understanding the Badlands National Park geology and for oversight of the park research permit, and Dr. Mary Ann Holmes for sharing her expertise in clay mineralogy. Of course, all errors are ours. Finally we would once again like to thank the very enthusiastic and motivating students who chose to engage in this project.