Great Plains Fracture Study

H. D. Maher Jr. & Robert Shuster, Department of Geography and Geology, University of Nebraska at Omaha

Component projects

The larger Great Plains Fracture Study at present can be divided up into the five component projects described briefly below. Please contact me for more details if you have questions or are interested. This is an evolving document.

Fractures in loess deposits: This project started out as a local structural geology course field exercise when a blizzard prevented us from traveling to Baraboo, Wisconsin. Surprisingly, instead of being random or uniform, the columnar fractures in loess exposures had well developed preferred orientations, some of which were aligned with the present day stress field, suggesting they are neotectonic in some fashion.

These are ray diagram plots of loess fracture strikes from the Hummel Park area in northern Omaha (left, n = 292) and the Crescent, Iowa area (right, n = 128). The red curve is the field data, with each ray value representing the number of readings within in a 30 degree sector centered on the that particular ray, for ray intervals of 1 degree. These types of plots avoid some of the pitfalls associated with rose diagrams. The blue lines represent statistical models developed for each distribution. For the Hummel site the primary model distribution component is a preferred orientation at 59 degrees and a uniform component of 59%. For the Crescent site the primary distribution components are preferred orientations at 72 and 160 degrees azimuth, and a uniform component of 29%. The 160 direction is likely under represented because the outcrop surfaces sampled (road cuts) ran north-south. For more details on the modeling please contact me. Data exists for 11 additional sites.

Chalcedony veins and clastic dikes in White River Group strata: We initially studied these with the hope of detecting preferred orientations that could be related to the regional stress-strain history, and soon discovered a somewhat more complicated, but interesting story. The chalcedony veins are distinctly stratabound, and show quite variable strike distributions. In some places the distribution is close to uniform, whereas in others it is best explained by two to three coeval sets. The simplest explanation for being stratabound and uniform is that they are the products of diagenetically driven deformation (syneresis) associated with the swelling clays common in these strata, and that in some cases locally regional stresses organized them into 2-3 preferred directions. A manuscript describing this is in review. Some further investigation and other examples in the literature led us to the idea that the clastic dikes may be of similar origin. Funding to investigate diagenetically driven deformation in the White River Group strata was obtained from the Petroleum Research fund, which we thankful for.

Road cut near Sheep Mountain Table, Badlands National Park, South Dakota, showing the rubble piles of chalcedony veins outlining their distribution within the White River Group strata. This chalcedony vein horizon is located just below the Brule-Chadron formational contact.

Ray plots and associated model as described above, for two chalcedony vein sites, the left from between the north and south units of Badlands National Park, and the right from a sight in NW Nebraska. In some statistical tests the null hypothesis that the Imlay site has a uniform distribution can not be rejected. At the Rock Bass site the distribution is clearly non-uniform and has a well developed model component at 115 degrees, which is closely parallel to a major fault set in the area. Interestingly, at the Rock Bass site the faults clearly postdate the chalcedony vein formation.

Normal faults in Tertiary strata: The Toadstool fault in Toadstool Geologic Park in NW Nebraska is perhaps the best known fault in the Tertiary strata. Subsequent work (fueled by a variety of undergraduate researchers) has identified a rich array of distributed faults in the area with a clear two-stage history. A manuscript is in preparation.

Photo of fault surface at Toadstool Geologic Park, showing well developed dip-slip slickensides developed on a chalcedony vein surface. Note the mechanical pencil for scale. The field relations clearly indicate that chalcedony vein formation was roughly synchronous with one phase of the faulting. Because of the mineralization the faults are well exposed as fins such as this one, providing an excellent natural laboratory in fault geometries and mechanics. Many very interesting fault tip geometries are evident at Toadstool.

Stereonet plot of poles to faults (normal) from the Toadstool Geologic Park area. The distribution is complex, but can be divided into two basic components (with the aid of field observations). An earlier symmetric, conjugate set that accommodated extension in a 105-285 azimuthal direction, and a later asymmetric set, almost at right angles to the first, with south-side down offset predominating. At other sites in NW Nebraska the more east-west set dominates.

Example GIS map plots of fault strikes (in black) and associated chalcedony vein strikes (in red) from two subareas of the Toadstool study area. The chalcedony veins form parallel, perpendicular and oblique to the fault trends.

Normal faults and vein arrays in the Niobrara chalk and Pierre shale: Bob Diffendal described a suite of faults (mostly normal) in the Niobrara chalks and Pierre shales of Harlan Reservoir in south-central Nebraska. We also mapped gypsum vein and other fracture sets (see documents below) along the shore. Recently we have found similar faults in the Niobrara chalks of Lewis and Clark Reservoir in northeast Nebraska, with some similar orientations. Research at this site is ongoing. Some evidence indicates that these are Tertiary in age (put pre-Ogallala Group). The abundance of swelling clays in these invites the proposition that these normal faults and gypsum vein arrays are also related to diagenetically driven deformation. Gypsum veins are common in these strata elsewhere, but their orientations, timing and significance is unexplored as far as we know, and so we plan to investigate them at other localities.

View of normal fault offsetting ash bed in the Niobrara chalks along the shores of Lewis and Clark reservoir. Thin white vein material in the fault zone is a combination of coarse calcite spar and gypsum, and slickensides are well developed. A close look will find a vein array in the footwall block immediately below the ash. The ash layer has been squeezed out in the immediate hanging wall area due to lateral flowage.

Fracture systems associated with Mississippi Valley mineralization: Some joints in Paleozoic rocks exposed along the Missouri and Mississippi River corridor have sulfide mineralization, part of the well known Mississippi Valley mineralization. The mineralization is associated with the expulsion of deep crustal fluids from the Appalachian orogenic welt that developed during the Alleghanian orogeny. The advantage of working with this fracture system is that the mineralization helps to constrain the age of the associated joint. This project is the core of Chris Sautter's undergraduate research project that is being funded by a UNO FUSE (Funds for Undergraduate Scholarly Experience) award. There are two basic components of Chris's research: a) a literature search on the associated fracture directions, b) field work in the Missouri River corridor area to characterize joint patterns in the Paleozoic rocks, with a special focus on those fractures with sulfide mineralization along them.

 

Documents related to Great Plains Fracture Study

Below are some links to documents that summarize some of the work done to date on fracture systems in the Great Plains.

July 2011