For larger outcrops, differences are generally less, but still significant. The Laramide orogeny was characterized by a NE–SW compression (Engebretson et al., 1985; Bird, 2002). High‐resolution lidar data hill shade image and geologic outcrop mapping. [45] Before continuing the comparison between existing and new surface models, it is important to consider more thoroughly the implications of assuming a parallel fold in order to project data points to the position of a single folded surface. Southeast of this point, only scattered outcrops, insufficiently distributed to accurately constrain fold geometry, are available. [15] It is important to note that the mapping of outcrops and measurement of bed orientation were performed at different scales. The black Thermopolis shale is exposed in the adjacent, partially faulted northwest-plunging syncline. Cross bedding makes strike and dip measurements difficult to obtain on many outcrops; however, this unit forms the most extensive outcrops in the study area (e.g., Figure 2b). Thus, each interpolated surface is most precise nearest where physical outcrops of the surface are found. [2007b], by printing hill shade images generated from the 1 m DEM on a 42 inch plotter at a scale of roughly 4000:1, on which individual outcrops more than 2–3 m across may be identified. An anticline is a geologic formation where the strata slope downward on either side of a crest, and this particular mountain is one of the more famous examples of this kind of formation. The sandstone at the base of the Frontier Formation is the youngest ridge‐forming unit exposed in the extent of the ALSM survey. [66] Extraneous low and high points (e.g., multipath and fog, respectively) were eliminated from the data set, and a filtered bare earth model (vegetation and buildings removed) was generated using Terrasolid's software TerraScan. [13] Mapping was performed in the manner of Mynatt et al. [1996] (Figure 11c) and therefore enables a direct comparison to the model generated by sequential cross sections. From the raw point cloud of processed laser returns, two digital elevation models with 1 m resolution were assembled. [44] The top Tensleep Sandstone model should be very precise near the fold hinge, where the Tensleep Sandstone is exposed, but is subject to greater error around the periphery where outcrops high in the stratigraphic column (i.e., lime green) are projected nearly 1 km down section. A high‐resolution version of this map is available in the, Schematic illustration of the stratigraphic thickness calculation (. Sheep Mountain Anticline is a basement cored, doubly-plunging, asymmetric fold on the east limb of the Bighorn structural basin. Geophysics, Geomagnetism Despite minor subjective data manipulations necessary to overcome limitations of the parallel fold model and numerical instabilities, which resulted in significant artifacts in the new surface model, the new method is based upon a specific mathematical formulation constrained by sound data and therefore better satisfies this fundamental requirement of scientific methodology. We acknowledge that multilayer folding to nearly vertical limb dips is a more complex physical process than the small deflection of a single elastic layer. In the base Sundance model, it is the points representing Paleozoic surface outcrops, projected upward, through the Chugwater, that contribute error. [2007] use differential GPS to constrain the geometry of small folds and small patches of larger folds. The surface sedimentary rock formations that are exposed in the fold, range in age from 66-360 million years. Thus, we would expect to observe forelimb thinning of at least 30%, and locally greater than 60%, relative to thicknesses observed in the backlimb. Fold characteristics along cross sections may be omitted or misinterpreted if data points are not sufficiently dense or if the underlying assumptions of fold kinematics on which the cross section is based are incorrect [Woodward et al., 1989]. Geologic map of the Sheep Mountain--Little Sheep Mountain area, Big Horn County, Wyoming / U.S. Department of the Interior, U.S. Geological Survey ; by Robert L. Rioux. At the northwest end of the section where ALSM constraint from the Gypsum Springs Formation is available, the fold model returns to a more parallel form. Photographs of a selection of mapped bedding surface outcrops are presented in Figure 2. Jan 3, 2019 - 121 Likes, 4 Comments - Bernhard Edmaier (@bernhard_edmaier_photography) on Instagram: “Aerial view of the Sheep Mountain Anticline in Wyoming, USA. Subtle features of the fold between cross sections are not captured by these methods. Geometry of Sheep Mountain Anticline [14] Sheep Mountain Anticline is a basement‐cored, doubly plunging, asymmetric fold (Figures 1 and 3). Gypsum Springs data points are discarded despite this, because doing so addresses the primary issue of unrealistic waviness and because of their paucity relative to data points from older Paleozoic surfaces. However, mapping was difficult because most outcrops are not accessible, making it difficult to identify whether exposed surfaces are indeed the top of the Madison Limestone, or if upper layers of the continuous and uniform limestone have been eroded. Where ALSM constraint points are not available to enforce the parallel fold model, MCS interpolation need not generate parallel surfaces. NCALM used an Optech Gemini airborne laser terrain mapper (ALTM) system, mounted in a twin‐engine Cessna Skymaster. Geology -- Wyoming -- Albany County -- Maps. [31] One apparent limitation of the MCS approach is for chevron folds, in which deformation tends to concentrate within narrow zones of high curvature, which separate relatively planar dip domains [e.g., Carrera et al., 2009; Suppe, 1983]. In the Forster et al. Repeat session calculations performed using OPUS (http://www.ngs.noaa.gov/OPUS/) resulted in station coordinate differences of less than 2.2 cm in both horizontal and vertical position. For this reason, the observation grid area on which the interpolated surface is calculated (Figure 1b) is restricted in this area. The balanced cross section approach is limited in this region because the two folds with nonparallel axial surfaces exhibit deformation that is not plane strain. Thus, cross validation is unable to account for model uncertainty in more extensive regions lacking data constraint. Also, because outcrops are inaccessible, strike and dip measurements were not obtained on most of these surfaces. About six other discarded points were scattered around the periphery of the study area. Down‐section projection paths of points in the forelimb and backlimb cross, which is geometrically inadmissible. . Sources include the clay and gypsum dunes in the Bighorn Basin, as well as exposed gypsum in the Sheep Mountain anticline to the southwest, on the slopes of the Pryor Mountains to the northwest, and in the Cody area on the west side of the Bighorn Basin (Reheis 1987). [42] Two new surface models have been generated by MCS interpolation honoring projected ALSM data points that represent bedding surface outcrops and strike and dip measurements scattered across SMA. The inset shows the location, within Wyoming, of the area shown in the figure. Mynatt et al. Average differences between the two calculated trajectories were 2.5 cm vertical and <1 cm horizontal. Planets, Magnetospheric Laramie, WY. situated immediately west of the area of study, and Polecat Bench. Please check your email for instructions on resetting your password. Abundant thickness measurements from both the backlimb and forelimb, spanning a relatively wide range of dips in the thin, competent beds of the Paleozoic Amsden, Tensleep, and Phosphoria formations (Figure 6b) demonstrate that the parallel fold model is quite appropriate there. These outcrops provide scattered data on the location and orientation of the folded surfaces. Learn more. Color shaded polygons represent the extent of mapped bedding surface outcrops, and red dots represent locations of strike and dip measurements. Origin of magnetization in the Phosphoria Formation at Sheep Mountain, Wyoming: A possible relationship with hydrocarbons R. H. Benthien School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma Geologic map of the Sheep Mountain--Little Sheep Mountain area, Big Horn County, Wyoming. Shown above is a railroad cut that runs through this northwest-southeast. Absolute value of the difference between (a) dip and (b) strike measured in the field with a compass and calculated from ALSM data points extracted from outcrops versus outcrop area. The Gypsum Springs Formation is the youngest resistant, outcrop‐forming unit exposed in the forelimb study area. Histograms show the distribution of model misfit, represented by RMS error, for (a) leave‐one‐out (outcrop‐by‐outcrop) cross validation of the interpolated surface and (b) the smoothed (truncated SVD) interpolation using the complete decimated data set, also on an outcrop‐by‐outcrop basis. Open markers represent measurements taken in the backlimb, and solid markers represent forelimb measurements. Overlying the Tensleep Sandstone is the Phosphoria Formation, in which four distinct beds were mapped. Models of folded surfaces are necessary input for curvature analysis, which has been used to describe folded surfaces geometrically [Bengtson, 1981; Bergbauer and Pollard, 2003; Bergbauer et al., 2003; Lisle, 1992; Lisle and Robinson, 1995; Lisle and Martinez, 2005; Mynatt et al., 2007a; Pearce et al., 2006; Roberts, 2001], and as a proxy for strain [Bevis, 1986; Ekman, 1988; Lisle, 1994, 1999; Nothard et al., 1996; Samson and Mallet, 1997; Wynn and Stewart, 2005] and for fracture density and orientation [Allwardt et al., 2007; Bergbauer, 2002; Fischer and Wilkerson, 2000; Hennings et al., 2000; Lisle, 1994; Murray, 1968; Thomas et al., 1974]. Thickness there should be equal to the stratigraphic thickness (366 m, Figure 3), but comparison of contours in Figure 11 suggests thickness in excess of 400 m. Isopach contours representing the thickness between modeled surfaces are difficult to comprehend due to steeply dipping strata across much of the fold, so we use cross sections of the fold (Figure 13) to evaluate whether we successfully interpolate two parallel surfaces. Number of times cited according to CrossRef: Vertical stress history and paleoburial in foreland basins unravelled by stylolite roughness paleopiezometry: Insights from bedding-parallel stylolites in the Bighorn Basin, Wyoming, USA.. In parallel folds, dip isogons are perpendicular to bedding and bed‐perpendicular thickness of folded units is uniform. Constant projection distances to the base Sundance Formation are presented on the right side of Figure 3. Beyond the forelimb syncline, imprecise constraint could be gathered from observations such as the extensive surface exposure of the Chugwater Shale in the forelimb syncline, which provides some bounds on the location of adjacent surface contacts. Therefore, we treat bed thickness as everywhere uniform, with the exception of accounting for subtle thinning of the Chugwater Shale in the forelimb. The lack of correlation between bed thickness variation and local bed dip (Figures 6b and 7b) is consistent with this conclusion. Several surfaces are less continuous, but were mapped because they are in a similar stratigraphic position to other mapped surfaces, and are exposed in areas where the adjacent surface does not provide good outcrops. It is found immediately below a thick, distinct bentonite bed, which is mined locally. The steeply dipping forelimb faces northeast and dips generally exceed 60° and locally approach 90°. [67] The complete technical report on acquisition and processing of ALSM data is available by contacting the authors or directly from NCALM (http://www.ncalm.org). While explicit data points were unavailable to constrain the surfaces in these areas, qualitative field observations make it clear that the syncline continues to the northwest and southeast, as in the Forster et al. Rocks exposed in the study area range from Mississippian through Cretaceous in age, and span more than 1 km of the stratigraphic section (Figure 3). [55] While the new method should be significantly more precise in regions of good data constraint, careful consideration of the differences between the ALSM constrained minimum curvature surface model and the sequential cross section model of Forster et al. @article{osti_6084236, title = {Surface and subsurface analysis of Sheep Mountain anticline, Wyoming}, author = {Abercrombie, S.}, abstractNote = {The Sheep Mountain area, in the southwest Wind River Basin, is the up plunge closure of the Derby Dome-Winkleman Dome producing trend of an echelon folds which comprise the first line of folding down the northeast flank of the Wind River Mountains. Sheep Mountain. Rights Statement The University of Wyoming provides access to these public domain materials for educational and research/scholarly purposes. Following this, ALSM constraints within the threshold distance of the remaining slope constraints are discarded and finally the decimation algorithm is applied to all remaining ALSM points. Auxiliary material for this article contains a high‐resolution version of Figure 4a. Its NW–SE trend is similar to that of many folds within the Rocky Mountains, although some others formed at]. [68] The authors thank Fil Nenna and Solomon Seyum for assistance with field mapping and Michael Sartori and Ionut Iordache of NCALM for assistance with ALSM planning, acquisition, and processing. Light gray triangles have high aspect ratio and are omitted from calculation. [12] Bedding surface outcrops corresponding to 14 different weathering resistant stratigraphic surfaces were mapped on the ALSM data in order to obtain their precise 3‐D geospatial locations (Figures 3 and 4). Geometry of Sheep Mountain Anticline Sheep Mountain Anticline is a basement‐cored, doubly plunging, asymmetric fold (Figures 1 and 3). Sheep Mountain (11,244 feet (3,427 m)) is located in the U.S. state of Wyoming. Regularization is critical to generate a smooth interpolated surface from noisy data. Sheep Mountain forms a 5-mile (8 km) long ridge in the Gros Ventre Range and is easily seen from Jackson Hole. We calculate the orientation of the surface from ALSM data as the mean gradient (magnitude and orientation) of all triangles for a given outcrop. Although the study area is limited to the northwest and southeast, data points beyond the limited area of the observation grid are used for interpolation constraint. In one sense, this approach gives preference to the slope constraints; however, because slope constraints are much fewer than spatial constraints (282 versus 514,342), it provides a more balanced distribution of each type of constraint. [34] Although finite difference implementations of MCS interpolation provide improved stability and increased computational efficiency relative to this method (G is a dense matrix and often close to singular), the Green's function implementation offers the ability to constrain surface gradients, in addition to standard spatial constraint from ALSM data points [Sandwell, 1987; Wessel and Bercovici, 1998]. Plots of thickness for several stratigraphic intervals taken along the fold trend suggest that this variability is not particularly systematic (Figures 6 and 7). However, finer contours, which are omitted for the sake of figure clarity, reveal that even where ALSM data coverage is good, differences between the two models commonly range from 20 to 30 m. 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