CAVE THRUST FAULT OF SOUTHWEST MONTANA

 

FIELD TRIP STOPS – THIS FIELD TRIP STOP ILLUSTRATES TECTONIC TILTING, FOLDING, AND THRUST FAULTING.

 

LOCATION: Found east of Cardwell, MT and about two miles south of LaHood Park, MT in the Canyon of Jefferson River, exposed along HWY 2.

 

GEOLOGIC FEATURES: Paleozoic and Mesozoic strata; Tilted strata; Folded strata; Overturned Folds, Thrust Faulting

 

DESCRIPTION: In the Jefferson River Canyon, Cambrian rocks (Meagher LS and Wolsey Shale) as well as Conglomerates of the PreCambrian LaHood Fm. are thrust over younger strata that are tilted towards the northwest and, closer to the fault, are folded into an overturned syncline whose axial plane also dips to the northwest. The fault plane of the Cave Thrust dips to the northwest. South of the fault trace, the tilted strata include the Mississippian Mission Canyon Limestone, the Mississippian to Pennylvanian orange sands and silts of the Amsden Fm., and the quartz sandstones of the Pennsylvanian Quadrant Fm.  The overturned beds include the Permian Phosphoria Fm, and the Jurassic Ellis Group and Morrison Fm. An Andesite Sill of Cretaceous age is also injected within the upper Mission Canyon beds. Faulting and folding are associated with the Laramide Orogeny

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STUDENT QUESTIONS:

(1) What is a Sill? Where does it occur in Jefferson River Canyon?

(2) How does a Syncline differ from an Overturned Syncline.

(3) What is an Axial Plane of a Fold?

(4) What is a Gravel?  What sedimentary environment is suggested by a conglomerate with rounded gravel-sized particles? 

(5) CHALLENGE: Using both words and cross-sections, illustrate the progressive geological evolution of the rocks in the Jefferson River Canyon beginning with sedimentation in the PreCambrian and ending in the present-day outcrop.

 

SELECTED  REFERENCES:

-Bentley, Callan. (6/12/2010).(5) “The LaHood Comglomerate.” AGU Blogosphere. Accessed on 4/27/20: https://blogs.agu.org/mountainbeltway/2010/06/12/lahood-conglomerate/.

-Bentley, Callan. (2/18/2011). “Friday fold: Jeffereson River Canyon.” AGU Blogosphere. Accessed on 4/27/20: https://blogs.agu.org/mountainbeltway/2011/02/18/friday-fold-jefferson-river-canyon/

-"Montana Geology '05, Jefferson County". Montana Bureau of Mines and Geology, Montana Tech of the University of Montana. Accessed on 4/25/20: https://www.mbmg.mtech.edu/pdf-calendars/2005calendar.pdf

PHOTOS:

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SE

NW

Figure 1 - A geological interpretation of the outcrop in the Jefferson River Canyon. Note the Tilted Strata, Overturned Syncline, and Cave Thrust Fault

M

K

M-Pn

Pn

M

Figure 2 - Photo of the left side of Figure 1. M = the Mississippian Mission Canyon Limestone K = Cretaceous Andesite Sill;  M-Pn = Mississippian-Pennsylvanian Amsden Fm.; Pn = Pennsylvanian Quadrant Fm.

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Pn

Pr

Pr

J

Cave Fault

Figure 3 - Photo of the middle of Figure 1.  Pn = Pennsylvanian Quadrant Fm.; Pr = Permian Phosphoria Fm.; J = Jurassic Ellis Group and Morrson Fm.  Arrows indicate the relative motion along the Cave Thrust Fault

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C

pC

Cave Fault

Figure 4 - Photo of the right side of Figure 1. C = Cambrian Meagher and Wolsey Fms.; pC = PreCambrian LaHood Conglomerate. Arrows indicate the relative motion along faults,

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Figure 5 - The LaHood Conglomerate composed of rounded gravels. Sediment has been interpreted to be deposited in nearshore environments of the PreCambrian Belt Series.