Alkali Anticline, WY

Purpose and Introduction

The purpose of this project is to give you experience mapping folded sedimentary rocks. You will also be introduced to air photo interpretation, and be given air photos of the area to assist in your mapping. Folds in this area are classic, Laramide-style, basement-involved folds that trend northwest-southeast. The general subsurface geometry of these kinds of structures is well known because oil is produced from several Paleozoic and Mesozoic formations in the Bighorn Basin. The petroleum traps occur in the crestal areas of folds, or in association with facies changes or in stratigraphic pinchouts in fold limbs. This area is also a major source of bentonite. Bentonite was surface mined (and reclaimed) from the Thermopolis Formation in the southern portion of the project area.

The field area is located on Bureau of Land Management property along the eastern edge of the Bighorn Basin, and comprises 2-3 large-scale anticlines and synclines that at the surface, mostly involve Mesozoic strata; Paleozoic strata are primarily in the subsurface. The principal anticline in the area is named Alkali anticline. The stratigraphic column and associated unit thicknesses you should use for your cross section are as follows:

Formation/Unit	Thickness (ft)
Cody	calculate from your map
Frontier	calculate from your map
Mowry	calculate from your map
Muddy	calculate from your map
Thermopolis	500’
Cloverly	180’
Morrison	350’
Sundance	350’
Gypsum Springs	200’
Chugwater	550’
Dinwoody - Phosphoria	300’
Tensleep – Amsden (combine)	370’
Madison	450’
Bighorn	250’
Gallatin – Gros Ventre	400’
Flathead	100’
Precambrian undivided

Helpful Hints

• In the Bighorn Basin, two main kinds of faults often form in association with map-scale folds: basement-involved, contractional faults that underlie and uplift the entire structure, and steeply dipping, cross-strike, extensional faults that often have displacements of only a few meters or tens of meters. The latter type will be exposed at the surface in this area, whereas the former will only exist in the subsurface. Review the relations between apparent fault separation and true fault displacement.
• consider the various models (e.g., folded basement, faulted basement, trishear, etc.) for how to interpret the subsurface structure beneath the major folds; use the one that best matches your field data.
• Be consistent with your subsurface interpretations: faults should not rapidly change dip or displacement along strike.
• This area is almost 100% exposed, so you should have no trouble finding places to collect structural data. Plan to have at least 25 data that are evenly distributed over the map.
• Use your air photos to plan your mapping strategy, and to logically project contacts through areas you did not visit.
• A more detailed stratigraphic column of the surface-exposed rocks is given in the adjacent figure. Use this column to help you locate yourself in the stratigraphic section.
• Draw your cross sections down to sea level (0’ elevation).

Project Components and Layout

Follow the Guidelines for Map and Cross Section construction when preparing your project. The map, explanation and cross section should be constructed on the paper you are provided. You are free to design the layout of these items any way you like, but they all must fit on the same sheet of paper.

At the end of this project you will turn in:

1. a final colored and inked map,
2. the map explanation with brief descriptions and formal names for each map unit,
3. two geologic cross sections assigned by the instructor(s),
4. your field notebook,
5. your field map, and
6. a synoptic stereoplot and data table showing the poles to bedding in the Alkali anticline, as well as the calculated trend and plunge of the fold axis (a p-diagram).