Harmon D. Maher, University of Nebraska (Omaha)
Robert Shuster, University of Nebraska (Omaha)
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.