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. One new undergraduate student and an undergraduate who had previously participated were supported by this research project this summer (2012). (4 senior theses completed total to date related to this project, with one in progress). A total of 16 students (8 women, 8 men) 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. One manuscript has been published in the peer reviewed geologic journal Lithosphere and two others are in preparation.
Field work this summer focused on: a) collecting a larger suite of samples for XRD analysis from the base of the Chadron Formation up through the Brule Formation strata in the Toadstool, NE area; b) collection of orientation data from joints in sandstones of the Brule Formation to more fully understand the fracture history and context for chalcedony vein formation; c) extending mapping and sampling from the Chadron Formation strata in the Monroe Creek, NE area which had been worked in during the previous summer; d) documenting preferred directions of fractures across the White River and Airkaree Group contact at Jailhouse and Courthouse rock; and e) conducting reconnaissance fieldwork to the west of Monroe Creek that identified additional sites of normal faulting, and stratabound vein formation that may be a result of diagenetically driven deformation. Twenty five samples of those collected at Toadstool Geologic Park have been sent of for clay mineral identification and composition by XRD analysis and are the basis of a senior thesis proposal focused on understanding the distribution of mixed smectite-illite in the White River Group. Mapping in the Monroe Creek area identified a suite of distinctive vein/dikes filled with an unknown whitish mineral (hand specimen properties suggest it is a zeolite) with strike lengths in excess of 40 meters and widths of several cms. These unusual features will be the focus of another undergraduate research project.
We continue to collect evidence 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.
The following points update some of the scientific findings relayed in the last report.
- Work has been put into developing a diagenetic model/history for these rocks. At the moment this includes: 1) early pedogenic alterations (e.g. smectite production from volcanic ash, pedogenic gypsum and calcite growth); 2) a subsequent diagenetic ash alteration and a clay-silica cement and substantial mobilization of silica to produce chalcedony veins and some U mobilization; and 3) subsequent development of coarse calcite cement associated with freshwater aquifer penetration. Of special interest is the stratigraphic distribution and timing of mixed smectite-illite production which is associated with phase 2 above, and is a focus of the ongoing senior thesis. We also hope to gain clearer understanding if phase two is associated with burial during deposition of (?) the White River Group strata or during Arikaree Group strata deposition.
- Reconnaissance work at Slim Buttes, SD was conducted over the summer and strongly confirms the literature that describes a significant pre-Arikaree Group deformation event that affects White River Group strata, which were at least partly lithified during deformation. This clearly demonstrates the possibility of significant diagenesis occurring syn-White River Group deposition and prior to Arikaree Group deposition, although it seems unlikely that the deformation at Slim Buttes is diagenetically driven.
- Additional mapping at Monroe Creek clearly demonstrates that here the clastic dikes show large scale curvature of up to 90 degrees strike change over about two hundred meters.
- Expanded multiple working hypothesis framework. The curvature is constant along the dike length, and thus this pattern is not a tip curl, but suggestive of fracture growth in a stress field without a strong consistent tectonic component.
- About 300 orientations on joints in Brule Formation sands in Toadstool Geologic Park were collected and analyzed, to compare with the fault and vein orientations measured during previous field seasons. A complex orthogonal pattern resulted, that seems unrelated to (and likely post-dates) chalcedony vein formation, and may be related to a ENE oriented fault phase.
- It is becoming clearer that the fracture history of Tertiary strata in the Great Plains is both polyphase and polygenetic, and therefore a multiple working hypothesis framework has been developed that looks at the intersection of loading path (e.g. increasing pore pressure) and geologic trigger (geologic burial).
- Finally, initial work has uncovered an array of distributed small-scale normal faults and stratabound veins in the Cretaceous Pierre shales and Niobrara chalks of northern Nebraska and southern South Dakota at three widely separated sites in the central Great Plains. There is some indication these may be of Tertiary age, and based on our work with the Tertiary strata, and the fact these Cretaceous strata also have the appropriate composition, we believe these deformation features may also be diagenetically driven, and are initiating a project that focuses on these rocks and this possibility. This new project is a direct outgrowth of this PRF funded project, and funding is being sought from NSF.
Finally we would once again like to thank the very enthusiastic and motivating students who chose to engage in this project.