1. Lecture 1: Introduction
GEOL 335 Spring 2008
Alkali Anticline an introduction to mapping folded strata, distal foreland basin stratigraphy and hydrocarbon occurrence

2. Logistics Leave at 7:30 am tomorrow.
1.5 hour drive to field area….time to sleep.
Be ready to map when you arrive.
Leave field by 4:30 pm.
It is going to be hot. 89 is the forecast high for Saturday and 95 for Sunday. We will reevaluate our logistics each day.
There is no shade here. Bring plenty of water and sunscreen.
Quick grocery and gas stop on the way home.

4. Lecture 1: Introduction
GEOL 335 Spring 2008
Regional geology
Eastern margin of the Bighorn Basin
Exposed Cretaceous sedimentary strata
Cloverly through Cody at surface
Basement-involved, contractional, fault-related fold
Moderate-sized (1-2 km wavelength)
Minor associated faults

5. Outcrop patterns for folds
Lecture 1: Introduction
GEOL 335 Spring 2008
Outcrop patterns for folds
Two general types of folds are anticlines and synclines
Plunge direction can be determined by map pattern “closure”

6. Examples of plunging folds
Lecture 1: Introduction
GEOL 335 Spring 2008
Examples of plunging folds

7. Lecture 1: Introduction
GEOL 335 Spring 2008

8. Lecture 1: Introduction
GEOL 335 Spring 2008

9. How do we recognize faults?
Lecture 1: Introduction
GEOL 335 Spring 2008
How do we recognize faults?
Stratigraphic duplication
Stratigraphic omission
Truncation of contacts
Abrupt changes in strike and dip
Fault rocks
Cataclasite
Mylonite
Drainageways may follow faults
Springs may emerge from faults

10. Outcrop patterns for faults
Lecture 1: Introduction
GEOL 335 Spring 2008
Outcrop patterns for faults
Faults may be normal, reverse or strike-slip
Apparent offset depends on:
Fault orientation
Sense of fault slip
Orientation of offset markers
Apparent offset can be very confusing
Most common is to have an apparent strike-slip offset
Can get this even if the fault has no strike-slip component of motion!

11. Interpreting fault slip sense
Lecture 1: Introduction
GEOL 335 Spring 2008
Interpreting fault slip sense
When dipping layers intersect and are offset by a planar, subvertical, dip-slip fault…
Units on the upthrown side of the fault will be offset in the direction they dip

13. Regional structural style
Lecture 1: Introduction
GEOL 335 Spring 2008
Regional structural style
Crustal duplication forms basement arches
Bighorn Range, Wind River Range, Beartooth Range
Structural complexities form smaller basement uplifts
Alkali anticline, Sheep Mountain anticline, Grass Creek
Erslev, Hennings and Zahm (2001)

14. Regional structural geometry
Lecture 1: Introduction
GEOL 335 Spring 2008
Regional structural geometry

15. Backlimb tightening folds
Lecture 1: Introduction
GEOL 335 Spring 2008
Backlimb tightening folds
Sheep Mountain anticline (Stanton, 2002)
Complex sequence of faulting
Relatively small displacements
Tip-line folds

16. Structures in our map area
Lecture 1: Introduction
GEOL 335 Spring 2008
Structures in our map area
Contractional, basement-involved, fault-related folds
Generate the larger-scale, backlimb-tightening folds
Offsets of 10s to 100s of meters
Extensional and strike-slip faults
Maybe basement involved?
How can you tell?
Accommodate local strains during folding
Along-strike stretching; outer arc stretching
Differential shortening along strike (tear faults)
Offsets of meters to 10s of meters

17. Stratigraphy in our map area
Thermopolis Formation – Forms the core of the structure. Mainly black shale with bentonite near the top. Weakly resistant.
Mowry Formation. Laminated silicified mudstone, and bentonite. Some thin sandstones and siltstone. Steep, resistant slopes.
Frontier Formation. Mudstone, sandstone, conglomerate and bentonite. Open marine offshore, shoreface, and prodelta depositional environments. Peay Member, Alkali Member, and Torchlight Members.
Cody Formation. Deep marine shale (Pierre in SD)

22. Marching orders This will be your opportunity to shine!
You will work ONLY with your mapping partner. Any discussions with anyone other than your partner will result in a grade penalty.
At the end of the first day, you will turn in your field map.
Your field map will be returned to you when we begin mapping on Day 2.
At the end of Day 2, you will turn in your geologic map and cross section, which will be graded as before; there will be no drafting day.

23. Lecture 1: Introduction
GEOL 335 Spring 2008
Helpful hints
Carefully assess fault displacements from apparent separation
Interpret the subsurface based on your data
Occam’s razor!
Be consistent and logical with fault geometries and kinematic significance - admissibility criterion
Lots of structure data and field observations! Plenty of time for this.
Thicknesses will be determined from the mapped contacts.