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

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.

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

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”

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

Lecture 1: Introduction GEOL 335 Spring 2008

Lecture 1: Introduction GEOL 335 Spring 2008

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

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!

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

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)

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

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

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

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)

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.

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.