Alkali Anticline
an introduction to mapping
folded strata, distal
foreland basin stratigraphy
and hydrocarbon
occurrence
Logistics
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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.
Regional geology
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Eastern margin of the Bighorn Basin
Exposed Cretaceous sedimentary strata
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Cloverly through Cody at surface
Basement-involved, contractional, fault-related
fold
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Moderate-sized (1-2 km wavelength)
Minor associated faults
Outcrop patterns for folds
Two general types of folds are anticlines and
synclines
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Plunge direction can be determined by map pattern
“closure”
Examples of plunging folds
How do we recognize faults?
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Stratigraphic duplication
Stratigraphic omission
Truncation of contacts
Abrupt changes in strike and dip
Fault rocks
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Cataclasite
Mylonite
Drainageways may follow faults
Springs may emerge from faults
Outcrop patterns for faults
Faults may be normal, reverse or strike-slip
 Apparent offset depends on:
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Fault orientation
Sense of fault slip
Orientation of offset markers
Apparent offset can be very confusing
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Most common is to have an apparent strike-slip offset
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Can get this even if the fault has no strike-slip component of
motion!
Interpreting fault slip sense
When dipping layers intersect and are offset by a
planar, subvertical, dip-slip fault…
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Units on the upthrown side of the fault will be offset in the
direction they dip
Regional structural style
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Crustal duplication forms basement arches
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Bighorn Range, Wind River Range, Beartooth Range
Structural complexities form smaller basement
uplifts
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Alkali anticline, Sheep Mountain anticline, Grass Creek
Erslev, Hennings and Zahm (2001)
Regional structural geometry
Backlimb tightening folds
Sheep Mountain anticline (Stanton, 2002)
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Complex sequence of faulting
Relatively small displacements
Tip-line folds
Structures in our map area
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Contractional, basement-involved, fault-related
folds
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Generate the larger-scale, backlimb-tightening folds
Offsets of 10s to 100s of meters
Extensional and strike-slip faults
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Maybe basement involved?
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Accommodate local strains during folding
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How can you tell?
Along-strike stretching; outer arc stretching
Differential shortening along strike (tear faults)
Offsets of meters to 10s of meters
Stratigraphy in our map area
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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
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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.
Helpful hints
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Carefully assess fault displacements from
apparent separation
Interpret the subsurface based on your data
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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.