Reports: UR849405-UR8: Shallow Fracture Formation and Fluid Mobilization During Diagenetically Driven Deformation: the Tertiary Badlands Chalcedony Vein Systems of South Dakota and Nebraska

Harmon D. Maher , University of Nebraska (Omaha)

Robert Shuster , University of Nebraska (Omaha)

The focus of this research project largely accomplished through undergraduate research participation, is the origin of chalcedony vein and clastic dike arrays in the Tertiary White River Group of South Dakota and Nebraska, with a focus on the role that diagenetically driven deformation has played. Two new undergraduate students engaged in and were supported by this research project this summer (2011). Three other students continued or are continuing their efforts from the previous year, with two finishing their senior theses and one very nearly finished (4 theses total to date related to this project). All three students have been accepted into geology graduate programs, and will start their associated studies this fall. A total of 14 students have been involved and supported to date in a combination of field work, thin section petrography, XRD analysis, structural analysis, and GIS project construction related to this project.  Field work this summer focused on: a) completing collection of data from the Cedar Pass clastic dike assemblage in the Badlands National Park and updating of the existing GIS project; b) mapping and sampling a suite of clastic dikes with abundant chalcedony in them at a site in northwest Nebraska, and determining their relationship to local faulting; and c) on investigating a new locality of clastic dikes and conducting reconnaissance work, looking for chalcedony veins and other clastic dike localities, in the Pawnee Butte area of northeast Colorado. One student presented his senior thesis results at the Rocky Mountain GSA sectional meeting in the spring of 2011, and a student-authored abstract summarizing some of this summer’s results has been submitted for the upcoming national GSA meeting (Minneapolis). Our experience with this project strongly reinforces our belief that undergraduate research experiences help attract and retain a higher quality of student, and helps provide them with experience and credentials that aid their continued career development at a particularly opportune time as interest in and prospects for geoscience careers grows. One manuscript is in review and two others are in preparation.

     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.

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