Eocene Tectonic Evolution of the Teton Range, Wyoming

What: The question we are addressing is how are the Eocene Heart Mtn. detachment, with inclusive Eocene quartzite conglomerates, and the plethora of other Eocene quartzite conglomerates (numerous locations and paleocurrents) related to the contemporaneous Laramide uplifts. Hominy Peak is key. How can there be Heart Mtn. lahars with Madison limestone clasts on the WEST side of the Tetons?? The project will combine structural sedimentology and structural geology in a tight geographic area.

When: early July – early August, 2010 (exact dates TBD).

Who:  The project will involve 3 students and one faculty member – Prof. John Craddock, Macalester College.

Where: The grand plan will be to have a reconnaissance trip through the Heart Mtn. structures, sampling the appropriate Eocene conglomerates (Sites 1-3, 5, 8-11 on map), and end up at our base in Jackson Hole. Then we’ll spend 7-10 days around Hominy and Fourellen Peaks (sites 4, 6 on map), regroup in Jackson Hole, then work near Buck Mtn. (site 7, on map) for a week, and regroup. Initial zircon separations (chipmunk, disc mill, Wilfley table) will be done in Laramie, then the group will disburse.

Project Description and Goals

Orogenic shortening is accommodated by hangingwall folding above thrust faults, and the Sevier belt (late Jurassic-Eocene) records this shortening as an orderly “younging toward the craton” sequence of thrust motions with dated synorogenic sediments (Armstrong and Oriel, 1965; Dorr et al., 1977, Wiltschko and Dorr, 1983, Craddock, 1992). The transition from Sevier, east-vergent thin-skinned shortening, with 45° slab dip to the west, to west-vergent, thick-skinned shortening, occurred with decreasing slap (5°) dip in the Eocene (Bird, 1988). These crustal-scale offsets of Archean crust are dated by synorogenic deposits (Gries, 1983; DeCelles et al., 1991; Fuentes et al., 2009) and fission track studies (Roberts and Burbank, 1993; Crowley et al., 2002), and many Laramide uplifts preserve a curious peneplain surface at high elevation (Smith and Seigel, 2000). The zone of overlap between thin and thick-skinned structures is in the vicinity of Jackson Hole and the Teton Range. The Heart Mountain detachment system, arguably the largest volcanic landslide deposit (Malone, 1995 and 1996; Craddock et al., 2009; Malone and Craddock, 2008) in the world, is also an Eocene event in the vicinity of Jackson Hole and the greater Yellowstone-Absaroka volcanic province. Figure 1 is a location map with specific field sites identified; this is correlated to the student projects.

Our goals are to study the deformation (strain) of the Eocene uplift of the Teton range, and the detrital zircon record (provenance) of the numerous Eocene quartzite conglomerates in the overlap zone of the Sevier and Laramide orogenies.

Student Projects

One student will handle the detrital zircons of Eocene conglomerates, another student will be the fault-fold strain person (Fourellen area; Site 6), and the third student will cover the fault-fold strain near Buck Mountain (Site 7).

Sample Preparation

Final zircon separations will be done at Macalester, and sent to Tucson. Oriented thin sections will also be prepared at Mac, and sent out.

Post-Field Analysis

We’ll convene in Tucson to analyze the detrital zircons in the fall. The student doing twinning work (Fourellen fold) will have to do this at Mac, whereas the student working on Buck Mtn. (quartzites) can work independently as a U-stage is not needed, only a microscope and software.

Field and Physical Guide

We will be camping and/or backpacking much of the month. There will be strenuous daytrips to see the geology of Fourellen and Hominy Peaks, and Buck Mountain.

Recommended course background

Structural geology, sedimentology, mineralogy

Initial Reading

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