1. Steerhead Ranch

2. Purpose Mapping lower Paleozoic rocks
Mapping syntectonic growth strata
First exposure to mapping faults

3. General geology
Straddles the boundary between basement uplift and the adjacent Powder River Basin
Hanging-wall anticline thrust over Tertiary basin fill strata.
Major reverse fault with subsidiary normal and tear faults in the hanging wall.
Clear Creek Thrust ma well have override a land surface and its own syntectonic growth strata.

6. Local stratigraphy
Nelson (1968)
Use regional values elsewhere

7. How do we recognize faults?
Stratigraphic duplication
Stratigraphic omission
Truncation of contacts or other structural grains
Abrupt changes in strike and dip
Fault rocks
Breccia
Gouge
Streams may follow faults
Misaligned ridges and valleys

8. Extensional faults

9. Tear faults

10. Splay faults

11. Composite structure

12. How do faults terminate
Edge of the map or cross section!
Another fault
Splaying into several smaller faults
Zone of ductile deformation….here the Gallatin-Gros Ventre

13. Helpful hints Look for evidence of faults. Don’t imagine them
Use schematic maps and cross sections to help figure out complicated field relations
This structure can’t be solved with fewer than seven faults.
Be mindful of the relationship of structure to topography.
Remember your map resolution
You can’t map a fault with only 5’ of offset

14. Mapping/Section guidelines
Map all gravel as Tertiary Wasatch (Tw)
Remember it is in the hanging-wall and footwall
THINK about the Tw thickness and what may be underneath
Map the Gros Ventre and Gallatin as a single unit (Cgg). Its attitude may not reflect the big picture structure.
Different units will be in the upper plate along the trace of the Clear Creek Thrust
Both your sections must agree on lower plate stratigraphy and fault geometry

15. What you need for tomorrow
Positive attitude
All field gear
All mapping implements
Liability waiver
Patience….this area is easy to cover. Don’t do it too quickly or you will miss things. Sit a lot….draw cross sections!
The Hole, Confusion Hill, Coyote Peak
Be prepared for the walk of shame….

16. The plan for tomorrow