FOLD TESTS AND FIELD CAMP

An important aspect of John Geissman’s career has been his role as an educator of numerous undergraduate students in field mapping courses. These field camp courses have long been examples of an active-learning pedagogical approach, and continue to be valuable capstone experiences for geoscience undergraduates. This presentation will include field course assignments aimed at developing skills in the collection of oriented samples, and examples of the purposes and uses for oriented samples in geoscience field work and studies. Additional examples of post-field camp courses and student research projects include paleomagnetic and magnetic anisotropy projects, which are used to develop and test learning outcomes for a geoscience program. Two field mapping locations will be used in these examples, one in strata deformed within the southern portion of the Helena Salient near Dillon, MT, and the other in folded volcanic and volcanoclastic rocks of the Blue Mountains near Mitchell, OR. In both locations the role of vertical axis rotation to produce fold and thrust belts with arcuate map-patterns can be investigated.

The Laramide fold and thrust belts in SW and central Montana are classic areas to study these deformation features. This study focuses on two areas near Dillon, MT (Argenta and Block Mountain) to examine patterns and spatial extent of vertical axis rotation. Paleomagnetic results from the Triassic Dinwoody Fm and the Cretaceous Kootenai Fm, as well as Eocene volcanics of Block Mountain will be presented. Initial results from the Kootenai Fm have paleomagnetic directions that pass the paleomagnetic fold test: the tilt-corrected mean is D = 23, I = 68, α₉₅ = 10, k = 31. Using a North American reference pole for early Cretaceous time, Dx = 332, Ix = 69- resulting in a relatively large CW rotation of 51 +/- 20 degrees.

The Eocene Clarno volcanics near Mitchell, OR provide an opportunity to explore the timing of magnetization and vertical-axis rotation of these rocks, as well as the use of AMS data to decipher emplacement directions recorded by volcanoclastic flows and ignimbrite deposits. For both studies paleomagnetic fold tests, and comparison of emplacement/flow directions in in-situ and tilt (and rotation) corrected coordinates serve as valuable examples of collection and application of structural data in field courses.