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Steerhead Ranch, WY

Purpose and Introduction

The purpose of this project is to expose you to the mapping of faults, Paleozoic bedrock and syntectonic growth strata associated with a classic Laramide uplift. You are given two field days and one drafting day to complete a geologic map and two cross sections of a portion of the Steerhead Ranch, roughly ten miles west of Buffalo, Wyoming. The area consists of Archean igneous and metamorphic basement rocks nonconformably overlain by Paleozoic (Cambrian-Pennsylvanian) cratonic sedimentary rocks. Both Archean and Paleozoic rocks were deformed during the Laramide Orogeny and numerous faults of various orientations and slip sense occur within the area. The principal structure of the area is the Clear Creek Thrust, one of the major range-bounding faults that carries the Bighorn Mountains above it. A thick succession of syntectonic Tertiary gravel occurs within both the hanging wall and footwall of the fault.

When constructing your map and cross sections, be mindful of what we know about the regional structural geology. The map area is largely a hanging-wall anticline developed above a basement fault and any subsidiary faults and folds should have geometries similar to those documented in similar structures. Tear faults and extensional faults both exist in this mapping area.

Helpful Hints

  • review the criteria for recognizing faults in the field (e.g. stratigraphic omission or duplication, abrupt changes in strike, juxtaposition of units not normally adjacent).
  • review and understand the 3-D geometries of tear faults and extensional faults associated with Laramide uplifts - don't interpret structures that differ vastly from these well-documented types without very compelling evidence.
  • draw schematic maps and cross sections in your field notes to help you figure out complex geometries or structural relationships.
  • the resolution of your map constrains what you can put on it; in other words, if a feature is too small to appear on your map (e.g. a fault with 5' of offset), then you can't map it!

Project Components, Guidelines and Layout

Follow the Guidelines for Map and Cross Section construction when preparing your project. The map, explanation and both cross sections should be constructed on the paper 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.

When mapping in the field or drawing your cross sections, observe the following guidelines:

  • map all gravel in the area as Tertiary Wasatch (Tw) Formation.
  • map the Gallatin and Gros Ventre formations as one unit, the Gallatin – Gros Ventre (Cgg)
  • fill in the footwall of your cross section with a "normal" stratigraphic sequence.
  • draw cross sections down to a depth of 4000' above sea level.

At the end of this project you will turn in:

  • a final colored and inked map,
  • the map explanation with brief descriptions and formal names for each map unit,
  • two geologic cross sections assigned by the instructor(s),
  • your field notebook, and
  • your field map