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)

This is the final report for this grant. Over the 4 year duration undergraduate research has been supported by this award in the following ways: 20 students have been funded to engage in undergraduate research projects in 7 different sites in 3 states, 5 senior theses have been finished and 2 are in progress, and 6 Geological Society of America conference presentations have been made by students (and 3 by faculty).  Of the 20 students 5 are presently in graduate programs in Geology. A framework for structuring an undergraduate research program has been refined and includes the following elements: A) an evolution from initial collaborative data collection and assessment in the field to delineation of individual subprojects towards the end of field work that students then chose to write a final field report on (thus delineating individual responsibility and ownership within the team effort),  B) delineation of a sequence of next-step projects (e.g. XRD analysis of samples collected) so that those who completed a field project and report could continue to be involved, culminating for some in a senior thesis,  C) purposeful mixing of students at different stages of their geoscience education on research teams,  D) recruitment of  students with previous experience in the project in subsequent years to help set expectations and mentor,  E) the construction of a participant-accessible central repository for training materials, completed project reports, data and resources so that students could see examples of and build on other student’s products, and F) close mentoring of the students at all stages.  Boiled down, the framework involves a blending of students at different points in their undergraduate careers, a mix of collaborative and individual efforts, and a progression of possible levels of involvement. While each undergraduate research project is unique, we believe that many of the elements mentioned above can be translated to other projects.

A fundamental scientific conclusion of our work is that diagenetically driven deformation is a significant driver of fracturing in the Tertiary strata of the Great Plains in a complex way.  Specifically, the spatial and orientation distributions of chalcedony vein arrays are of a character best explained by syneresis during diagenesis and attendant silica mobilization. The same case can be made for clastic dikes that are locally abundant in these strata.  The orientation distributions from locality to locality varies from uniform/random to well organized into multiple preferred directions, reflecting different stress field environments at the time of syneresis.  This highlights the possibility that such a fracture generation mechanism is more common than presently recognized as the conventional interpretation for well-organized fracture patterns is that the loading is related to tectonic strains in some way.  Petrographic work has identified a complex and multi-stage diagenetic history that includes: a) initial pedogenic gypsum, smectite and carbonate production, b) a subsequent stage of extensive silica mobilization, clay growth (likely associated with U mimneralization), and c) a later stage of carbonate cementation and replacement.  Zeolite growth may be associated with both stages b and c.   Clastic dikes are coeval with stage b.   The previous pedogenic history likely strongly influences the subsequent diagenetic history. XRD analysis has identified a distinctive stratigraphic/depth distribution of smectite versus mixed smectite-illite phases, the development of the latter of which may have contributed to chalcedony vein formation.  1 peer-reviewed paper has been published and two are in progress. 

This research has evolved into two projects that we intend to continue pursuing.  First, is investigation of the role that diagenesis may have played in the development of an extensive suite of normal faults in the White River Group that are unconformably truncated by overlying Arikaree Group strata in the Slim Buttes area of NW South Dakota.  Work by a group of four undergraduates this summer documented the presence of a detachment immediately underneath a particularly thick and smectite-rich mudstone layer in the basal White River Group. The footprint of this normal faulting is over 15 km in a cross-strike direction, and the deformation appears to be constrained to a stratigraphic interval some 100-150 m thick. Overall the deformation bears some resemblance to gravity glide/spreading such as is exemplified by the Heart Mountain detachment. Continued work this coming summer will focus on the role that diagenesis and overpressures may have played in the formation of these structures.  The second project was also initiated this summer and focuses on a suite of normal faults and related vein structures in the underlying Pierre Shale and Niobrara Chalks that we also believe is related to diagenetically driven deformation.  Field work was focused in the Francis Case Lake area of South Dakota.  We believe that the faulting is part of a regional polygonal fault system that extends down into Kansas, and which other workers have identified in the subsurface of the Julesberg-Denver basin.  A poster will be presented at the Denver Geological Society of America meeting on this proposal this fall.