THE TOADSTOOL FAULT COMPLEX

William Moak

 

INTRODUCTION

A fault is a fracture in the Earth’s crust along which movement occurs. The rock on one side of the fault may move up or down or sideways relative to the rock on the other side. Faults are planes but are often visible as linear features when exposed at a surface such as on the ground or in an outcrop. Faults are often characterized by the relative motion of the earth on one side of the fault to that on the other side. Normal, reverse, thrust, and strike-slip faults are the response to different stresses on the crust. Faults are described in terms of this motion and by their size, orientation, and the nature of the material that fills the fault.

In this study, we examined a fault complex in Toadstool Geologic Park in northwest Nebraska. These faults occur in sedimentary rocks that are late Eocene to early Oligocene in age (perhaps 30 to 40 million years old). These faults are exposed in the badland topography of the area. Many can be traced for tens or even hundreds of meters. We collected data in the field by measuring and describing various parameters at selected points along each fault. We then used a geographical information system to map and display the data. These maps were examined for patterns within and between the different fault parameters. These patterns may help identify the strains on the Earth’s crust that produced the fault complex. They may also help construct a model of the faulting events and help tie these events into the geological history of the region.

This study is a work in progress. We have just started to produce and analyze maps of the fault parameters. The ideas presented in this report are only preliminary and require further analysis.

PURPOSE

The purpose of this study is to understand and model the fault complex at Toadstool. We want to determine why the faults occurred and how they behaved. We would like to reconstruct the geochemistry and movement of fluids through these faults. Finally, we would like to determine when the faulting occurred and place the events at Toadstool in the context of the larger geological history of the region. The tasks required to achieve the objectives include: (1) application of geographic information system (GIS) technology to map and spatially analyze the fault data; (2) search for patterns in the fault data; (3) look at the relationships between different fault parameters; (4) conduct a literature search of major fault events in the area; and (5) construct a model that accounts for the patterns and relationships that are observed. This report will address the first two tasks.

BACKGROUND

Toadstool Geologic Park is located in northwest Nebraska and is accessible from Nebraska Route 2 about 17 miles north of Crawford. The park is set in badlands carved out of the Oglala National Grasslands (see Figure 1). Badlands are an erosional feature characterized by a hilly topography with closely spaced, V-shaped valleys and narrow ridges. Badlands are typically void of vegetation above the thin sage and grasses that dot the valley floors. Consequently, any faults that are present are often exposed and easily traced along the surface.

The badlands are composed of sedimentary rocks that formed from layers of sediment and volcanic ash laid down during the early Tertiary Period. Distinct layers or rock are classified into basic units called formations. A formation may be further divided into members and two or more formations are sometimes combined into larger units called groups. The stratigraphy of an area refers to the bottom-to-top assemblage of the different rock layers. A portion of the Tertiary stratigraphy in northwest Nebraska is listed below.

PARTIAL TERTIARY STRATIGRAPHY IN NORTHWEST NEBRASKA

EPOCH GROUP FORMATION MEMBER
Miocene Arikaree Harrison

Monroe Creek

Gering

 
Oligocene

 

 

Eocene

White River Brule

 

 

Chadron

Chamberlain Pass

Horn

Whitney

Orella

 

 

In the absence of any structural upheaval, the oldest rocks are at the bottom of the column and the youngest, most recently deposited rocks are at the top. The different units of rock are attributed to different depositional environments. In addition to the sediments from which they formed, these rock layers are also characterized by the climate and plant and animal life of the period. In fact, fossils are often used to identify particular layers and sometimes to establish an age for that layer.

Sometime after the strata were in place, small streams began to carve valleys into the landscape which expanded by headward erosion. This process, aided by an arid climate punctuated by heavy but short-lived rainfall, continued over time to produce the badland topography that we see today. The rocks exposed at Toadstool are from the Chadron and Brule formations of the White River group. The strata consist of interbedded sandstones, siltstones, and mudstones as well as layers of volcanic ash. As it weathers, the ash is transformed to clay which gives exposed surfaces a characteristic popcorn-like texture as the clay expands and contracts. Some layers include continuous, horizontal, reddish bands which probably represent soil horizons. These occurred during stable periods when the surface was not subject to burial or erosion and soils had an opportunity to form. The climate was arid by this time and the red coloration resulted from the oxidation of iron in the soil. The Chadron is a thick-bedded and light tan, green, or white-colored, siltstone. A darker band marks the top of the Chadron at the contact with the overlying Orella member of the Brule formation. The Orella is whitish at its base but multicolored above and includes many small ledges of more resistant sandstone. Sometimes the sandstone fractures into blocks which are rounded by the wind and rain over time. The elements also excavate the softer, underlying rock and remove all but the thin column protected by the sandstone block above it. The result is a large toadstool like features from which the park derives its name. In some of the higher hills at Toadstool, the Orella is capped by the Whitney member of the Brule. Several faults are exposed in the Chadron formation and Orella member of the Brule formation.

A fault is a fracture in the Earth’s crust along which movement occurs. The rock face below the fault is called the footwall and the rock face above is called the hanging wall. Consider the end of a brick through which a diagonal cut has been made from the upper left-hand corner to the lower right-hand corner. In this case, the upper half of the brick to the right of the "fault" would be the hanging-wall block and the lower half to the left of the cut would be the footwall block. Faults are classified according to the relative motion of the two rock faces. For instance, normal faults occur when the hanging-wall block moves down relative to the footwall block. If we simulate this motion with our hypothetical faulted brick, we will note that the total width of the brick increases while the height of the brick decreases. Thus normal faults lead to horizontal extension and vertical shortening. Normal faults are the response to tensile or extensional strain within the Earth’s crust. All the faults in Toadstool are normal faults.

Faults are treated as planar features although they have thickness and may be curved, rippled, or torn and thus exhibit a three-dimensional character. The fault zone or gap between the hanging wall and the footwall are filled with breccia, sediment, or hydrothermal minerals which precipitate from fluids flowing through the fault. Over time, these materials will lithify into rock. At Toadstool, the faults are most commonly composed of chalcedony (an amorphous variant of quartz), calcite, and micrite (a fine-grained lime mud). Fault planes are often visible as linear features when exposed at a surface such as the ground or an outcrop. At Toadstool, the fault rock is more resistant to weathering than the siltstones in the footwall and hanging-wall blocks. Consequently, the fault often appears as a thin, continuous fin protruding from the surface.

Faults are described in terms of their orientation, size, motion, and the texture and composition of the fault rock or fill. The orientation of the fault plane is determined by strike and dip measurements. Strike is the compass direction of the line created by the intersection of the fault plane and a horizontal surface, real or imagined. Dip is the angle at which the fault surface dips away from a horizontal surface and the direction toward which the fault is dipping. For example, a fault might strike 60°east of north and dip at an angle of 65° to the southeast. Fault thickness refers to the thickness of the fault-zone material. Thickness is often variable and expressed as a range (e.g., 2 - 4 cm). Fault movement may be reconstructed from several indicators if they are available. Offset of a feature such as a distinctly colored soil horizon on each side of the fault records the amount of movement along the fault. Striae—grooves carved into the fault rock by the footwall or hanging wall—indicate the direction of movement. Slip trend is the horizontal compass direction toward which the striae dip. Slip plunge is the angle of the striae from a horizontal surface. Finally, the texture and composition of the fault rock are noted. These parameters are important to reconstructing the geochemistry and movement of fluids through the faults.

DATA

Fault Data. Fault data were collected on three separate visits to Toadstool. Geologists working individually or in pairs investigated assigned areas to locate faults and measure or classify specific parameters at several points along a fault. At present, we have 400 data points (see Figure 2). The data were compiled in a Microsoft Excel spreadsheet. Some parameters were originally recorded in different units and subsequently standardized in the spreadsheet. The table below summarizes the data in the spreadsheet.

FAULT DATA DESCRIPTIONS

FAULT PARAMETER EXAMPLE
Data point identifier - unique identifier that includes reference to specific fault A-1
Collection date - recorded as month and year Apr 02
Latitude - determined by GPS and recorded in decimal degrees 42.8611
Longitude - determined by GPS and recorded in decimal degrees -103.5841
Elevation - determined by GPS and recorded in meters above mean sea level 1165
Fault strike - orientation of the exposed fault plane recorded in degrees clockwise from north 192
Fault dip - angle between a horizontal surface and the fault plane recorded in degrees. The "right-hand rule" applies here: the fault dip direction is 90 degrees to the right of the strike direction 53
Slip trend - direction of movement of the rock on one side of the fault as projected on a horizontal surface; recorded in degrees clockwise from north 320
Slip plunge - angle between a horizontal surface and the movement direction; recorded in degrees 62
Relative motion of hanging wall - sense of vertical displacement relative to the footwall; recorded as "U" for up or "D" for down U
Throw - Amount of displacement between the footwall and hanging wall recorded in meters 2.3368
Fault zone thickness - the thickness of the fault "plane" recorded in centimeters. The thickness is expressed as a range when the thickness is variable 5.715
Footwall strata - the rock unit composing the footwall Chadron
Hanging wall strata - the rock unit composing the hanging wall Chadron
Fault rock type - the nature, texture, or characteristic of the fault rock. Descriptors include "b" (breccia), "st" (striae), "sl" (slickensides), "c" (comb structure), "se" (septa), "v" (vuggy), "z" (zoned), "mg" (multiple generations), and "o" (other) b
Fill composition - type and nature of mineral(s) present in the fault. Descriptors include "gch" (gray chalcedony), "wch" (white chalcedony), "bch" (brownish chalcedony), "cs" (calcite spar), "gm" (green micrite), "bm" (brown micrite), "cy" (clay), or "o" (other) chal,cs,gm
Comments - additional observations.  

The data described above were often recorded in the field using different units (e.g., latitude in degrees-minutes-seconds, elevation in feet, fault thickness in inches). However, all data were converted to the units specified above.

Vein Data. Veins are fractures in the rock that have been filled with hydrothermal minerals. Like faults, veins are planar surfaces and are described in terms of strike and dip, thickness, and fill texture and composition. However, there is no movement of the rock along the vein. Veins also provide insight into the strain on the rock as well as the geochemistry and fluid dynamics of the environment. Vein patterns and composition may reveal interesting relationships between the faulting and veins. Vein data were collected for a related project and are displayed in some of the figures in this report

Aerial Photos. Digitized and ortho-rectified aerial photographs and the corresponding world files were downloaded from the Microsoft TerraServer. These images were jpeg files prepared by the U. S. Geological Survey. They were photographed on May 3, 1993 and digitized on Sep 2, 1998. They are projected in the Universal Transverse Mercator (UTM) Zone 13 on the North American datum (NAD) of 1983. Images used in this report have a resolution of 8 meters. Finer resolution data are available and are used for some detailed analyses. The world file is a small text file that contains the upper left-hand coordinates (easting and northing) of the image and the image resolution. The world file is necessary to properly position the image on a map. Each image must have an accompanying world file with the same file name but a ".jpw" file extension. The aerial images were used as a backdrop for some of the maps (see Figure 2).

Digital Elevation Model (DEM) Data. 10-meter DEM data were used. These are tif files and must also be accompanied by a world file.  The DEM images were also used as a backdrop for some of the maps (see Figure 3).

METHODOLOGY

Data were collected in the field and then keyed into a MicroSoft Excel spreadsheet, as indicated earlier. Some of the data were then modified as described below.

1. Longitude coordinates (all in the Western hemisphere) were made negative to conform to mapping convention.

2. Some strike and slip trend directions were corrected for declination angle. Usually, we made the proper adjustments to our compasses before we collected data. However, on one occasion the data were modified to account for the proper declination angle after we left the field.

3. Units of measurement were standardized to those specified in the data table.

4. The average fault zone thickness was calculated. Fault zone thickness is often variable and recorded as a range. The average thickness was calculated in these cases in order to provide one value for mapping. However, we subsequently decided that the maximum thickness at each point was more revealing.

5. Strike directions were modified to the "right-hand rule" convention. Strike refers to the orientation of a line and can be reported as one of two complimentary compass directions (e.g., 60° or 240°). However, dip is always reported as the down-slope compass direction. The right-hand rule specifies the unique strike direction for which the dip—always perpendicular to strike—slopes to the right. This was necessary because the symbol used to plot strike and dip invokes the right-hand rule.

6. I experimented with different ways to express the fill composition to facilitate mapping. Initially, I tried applying a classification scheme based on composition: 1=chalcedony; 2=calcite spar; 3=chalcedony and calcite spar, etc. However, I subsequently broke the hydrothermal fill down into four separate categories: chalcedony, calcite spar, micrite, and other. This allowed me to map each type of fill separately and produce a more meaningful visualization of how the hydrothermal fill changes from point to point.

The spreadsheet was saved as a dBASE IV file for use by the geographical information system (GIS). It was necessary to format all numerical data the same (e.g., 5 decimal places) before saving. The spreadsheet contained data for 404 points.

The data were then loaded into ESRI ArcMap 8.3. Starting with an empty map, the fault data were added to create a new layer. The data point locations were mapped by selecting the "Add XY data" tool and designating the longitude field as X and the latitude field as Y. A predefined coordinate system (UTM Zone 13N on the NAD 1983) was selected for the layer to match the coordinate system of the aerial photos and DEM images. The new layer was saved as a shape file in ArcMap. The shape file allows the user to zoom in and out and use the "identify" tool—both useful capabilities in analyzing the data. Several layers were created from the shape file to display various fault parameters. These are described below.

1. Strike and dip. A strike and dip symbol was selected from the list of geology symbols. This symbol () consists of a line segment representing strike with a short, perpendicular tic mark extending from the middle to represent dip. The symbols were adjusted to the correct strike by selecting the rotation option and selecting the strike field to determine the rotation angle. The geographic reference frame (in which 0° represents north) was used. Vein data were also mapped using the strike and dip symbol rotated to the strike orientation. (See Figure 4)

2. Fault zone thickness. The maximum fault zone thickness was displayed as graduated symbols (circles). The values were classified into five categories using the natural breaks scheme. The symbol sizes were adjusted to accent the differences. (See Figure 5)

3. Fault fill. We were interested in looking for patterns in the fault composition and how it changes across the area. The three main fault minerals—chalcedony, calcite spar, and micrite—were mapped as separate layers using different symbols. In each case, the selection tool was used to plot the appropriate symbol only for those points where that mineral was found. (See Figure 6)

4. Slip trend. Finally, the slip trends were mapped to provide a sense of the motion direction that occurs along these faults. Slip trends were depicted by arrows oriented to the slip direction using the rotation function. (See Figure 7)

New layers were created for the 8-meter aerial images. Two images were used to provide complete coverage of our area of interest. Each image file was added as new data and ArcMap automatically accessed the appropriate world file to georeference the data. (Although this procedure is simple and straightforward, it was the most difficult hurdle I experienced using ArcMap.) Initially, a mosaic of two dozen 4-meter images was used. However, it was cumbersome to work with this many images and the courser resolution was selected. The finer resolution images are still used for smaller scale analyses.

A new layer was also created for the 10-m DEM image. This image was added as new data and ArcMap used the appropriate world file to georeference the image.

The aerial and DEM imagery provide appropriate backgrounds for the fault data discussed above. However, they serve other functions as well. First of all, they verify the proper geolocation of the data. Field notes and recollection of the terrain help confirm that the data are positioned correctly with respect to the imagery. Secondly, the images help put the faults into context by allowing the observer to view the faults in situ, albeit from altitude. Finally, the images are tremendously useful in planning subsequent data collections by suggesting where to look, what to look for, and how to get there.

ANALYSIS

Studying faults in the field fosters an understanding at the small scale; say tens or hundreds of meters. Walking along the fault, you see changes in its orientation, dip, thickness, or composition. But it’s challenging to develop a mental "big picture" of all the faults you observe and note how they behave as a group and possibly interact with one another. ArcMap has provided that big picture. Now, at a glance, you can see patterns emerge from dozens of faults. The maps produced with ArcMap have confirmed some hypotheses and sparked the formation of others. Some of the initial observations and ideas obtained from the ArcMap visualizations are discussed below. Most of these ideas require more investigation, research, and thought.

Several patterns are evident from the map of fault strike and dip measurements (Figure 4). First of all, individual faults can be extrapolated from the point data. These display several different orientations. The dominant orientation is to the north-northeast, particularly with the faults to the north. The main Toadstool fault near the center of the figure and to the right is oriented more toward the northeast. Two more northeast trending faults to the southeast of the Toadstool fault are likely related to this event. To the west of these are a series of parallel northwest trending faults. The perpendicular orientation of these faults to the Toadstool fault suggests that these resulted from a significantly different strain environment. It would be revealing to find some interaction between these two fault sets, perhaps one fault truncating another. The interesting Z-shape pattern on the west side of the figure also reveals the intersection of two different fault sets. The orientations of these faults are important because they reveal the strain pattern on the crust and may help tie the activity here to larger scale geologic trends such as the Black Hills trend and the Wyoming lineament.

The strike and dip map also reveals other patterns that merit more scrutiny as this study proceeds. We notice a regular spacing between many of the parallel faults. For instance, four nearly parallel north-northeast trending faults slightly above the center of the figure appear to be equally spaced. This may help formulate a model for how the faulting occurred. Also of interest is the interface between faults. In the field, we have observed an en echelon pattern between two parallel but offset faults. This consists of a series of short, sigmoidal fault segments that traverse the gap between the tips of the two faults. This pattern may show up on a smaller scale. Finally, there is a large concentration of veins at the bend of a large fault in the upper left-hand corner of the figure. The veins may result from a concentration of stress between the two legs of the fault.

Fault-zone thickness reveals that some faults are bigger (wider) than others (Figure 5). The widest faults are parallel to the main Toadstool fault. Studies of others faults have revealed a linear correlation between fault zone thickness and displacement (throw) along the fault. Linear regression of thickness versus throw for all the Toadstool data points showed weak correlation between the two parameters. However, an analysis of maximum thickness versus maximum throw for each fault may demonstrate a stronger correlation. Some faults are uniformly thick or thin but many more exhibit variations of thickness along strike. Several of these are thick in the middle and taper off toward the tips. These thickness patterns are of interest because they may suggest a mechanism for the faulting.

The map of fault fill composition (Figure 6) reveals that fault fill is not homogeneous across the area. You get a sense of these changes in the field but the mapped data clearly displays several patterns. Chalcedony is uniformly present across the northern half of the study area but nearly absent in the south. The calcite and micrite patterns are not as consistent. Both minerals are found across the study area but are conspicuously absent from some faults. For instance, the large v-shaped fault (center east) that strikes to the northwest and then to the northeast has little calcite whereas a series of faults to the west of that have little micrite. These patterns may help reconstruct the geochemistry and movement of fluids in the fault environment. One limitation of this depiction is that it only reveals whether or not a mineral was present. It would be interesting to see how the ratios of the different minerals change along a fault.

The final figure of slip trends (Figure 7) shows the movement of the hanging walls. This illustrates how the rock is moving in response to the strain pattern. At Toadstool, nearly all the slip is down dip meaning there is little lateral movement along the faults. The range of slip orientations confirms that the strain pattern here is complex—movement in several different directions was necessary to relieve the stress.

CONCLUSION

This study examined the fault complex exposed at Toadstool National Park. Data collected in the field were mapped and displayed using GIS technology. The resulting visualizations provide a big picture of the patterns in the data that will help reconstruct the faulting events and place them into the context of the geological history of the region. The mapped data are very revealing. They have verified some hypotheses and prompted the development of others.

So far, only initial analyses of the data have been performed. More detailed analyses and examinations of smaller scale patterns will follow. I will look into the spatial analyst toolbox in ArcMap to see if there are tools that will help quantify some of the patterns that we have seen (e.g., dominant strike orientations or the regular spacing of parallel faults). I will also continue to modify the maps in order to accentuate the patterns that we have recognized or to reveal new ones. Finally, we will continue to add data to our database with each trip to Toadstool.

The pattern of faults at Toadstool is complicated and reveals a complex history of strain on the Earth’s crust. But the power of GIS is helping us to decipher the pattern and reveal some of the dynamic geological history of the area.

ACKNOWLEDGEMENTS

I would like to thank Dr. Harmon Maher of the UNO Geology Department for his guidance, patience, and enthusiasm for this project. I would also like to thank Dr. Maher’s structural geology students who have walked the faults at Toadstool and helped expand the ever-growing database.