6/24-6/25

After leaving base camp at the Iowa State Field Camp in Shell, we drove upstream through the canyon to take a look at Shell Falls. The folding in Shell Canyon triggered a discussion on how the structure of the Big Horns fits in the big picture of the formation of the Rockies.

Folding in Shell Canyon

Megan explained how her research in the Sierra Pampeanas in Argentina could be a modern analog of the Rockies in North America. Currently the Nazca plate is being subducted under South America creating the Sierra Pampeanas. Scientists are theorizing what will happen when the mid-ocean ridge in the Nazca becomes subducted. In North America it is thought that once the Farallon plate subducted the spreading ridge, tectonic motion changed and a strike-slip fault formed. Could this be the future of South America? Chris and Eric creatively model this theory in the following video.

Click here to view the embedded video.

Shell Falls, still in its’ geologic “youthful” phase, gave us a fantastic view of joints and trilobites.

Joints

Digging through Gros Ventre shale, I played paleontologist in search of my very own trilobite fossil. Karen made the find of the day, a trilobite mold!

Shell Falls

Karen's Trilobite Mold

Driving along Highway 14 back to Sheridan, we stopped at several outcrops viewing sheer surfaces in the five springs fault zone and folding in the Amsden Fm.

The crew looking at folding in the Amsted Fm.

The fourth and final day on my structural tour of the Bighorns started out slow. After several hours of following dirt roads just to meet barred gates and private land, the crew settled for lunch overlooking the Piney Creek salient.

Locked gate...NOOOOOOOO

From there we parted ways: Eric and his grad students headed back to Laramie and Chris, Megan, Tyler, Will and I back to Sheridan for a couple days of “rest”

before the storm of seismic installations.

Overall the journey was a success. I was able to follow 2.9 billion years in the life of the Bighorns in only 4 days. What a trip!

“And be on guard, I saw five or six rattlesnakes last time I was here.”

Chris and Eric at the anticline

Not only were we walking blindly on train tracks into the canyon, now with the prospect of snakes this venture became ten times more exciting! Eric was in the lead, narrating the way through Sheep Mountain where he and his students have done research previously. One of the most well known anticlines in North America, the Sheep Mountain anticline is a structural beauty. Along the hike we identified collapsed caves, stylolites (an irregular, suture like contact produced by pressure dissolution of rock under deep burial conditions), small thrust faults (perpendicular to the anticline hinge), slicks along bedding planes and witnessed a spectacular view of the Sheep Mountain anticline. After exiting the canyon, Eric surprised us with a group activity: an exercise formerly prepared for his Exxon Mobil cronies. Pairing up with Megan, I set out to complete a geologic cross section through Sheep Mountain.  After all submissions from the 4 teams were in, it was the geophysicists who reined supreme, our cross section most resembling Eric’s (Tyler coming in a close second). A rough sketch of Eric’s proposed model can be seen below.

Me at the Sheep Mountain Anticline

Proposed model for the Sheep Mountain Anticline

I was able to witness science at its best as Eric and Chris discussed alternate theories regarding the faulting under Sheep Mountain. Using the sketch as a guide, the first question discussed was which fault was the main fault and which the backthrust (#1 and #2). After Eric identified fault #1 as the main fault and fault #2 the backthrust, Megan raised the question, why would you have a new fault break through an existing fault instead of reinforcing the existing fault? Eric’s hypothesized that the gravitational load of the rising mountain changes the stress state and as such forces faulting west. Chris took a different angle suggesting that the original fault could be reactivating pre-existing structure. It was great to witness active questioning of current models firsthand!

Tearing through dirt roads in our big ole’ SUVs, we made our way to the north-plunging nose of the anticline. We walked into a gully where we could “look down- plunge,” and viewed wild folding at the apex of the Gypsum Springs Formation and identified Z, M, and S folds.

Z, M and S folds

We slowly made our way up the mountain stopping here and there looking at fracture patterns, questioning whether they were the result of local, Laramide or younger deformation. The view from the top gave us an exquisite view of the backlimb tightening zone.

Backlimb tightening zone

Our last stop of the day was Dirty Annie’s diner, Shell’s hidden gem. As a malt connoisseur, I am quite the critic but I must say Dirty Annie’s makes mean malts. A dessert coma made the perfect end to a perfect day!

I joined EarthScope investigators Christine Siddoway and Megan Anderson (Colorado College) and Eric Erslev (University of Wyoming) on a 4-day structural geology tour of the Bighorn Mountains in northern Wyoming. A group to be led by Megan will install seismic stations throughout the range this summer as part of the Foreland Basement Arch Seismic Experiment. Christine and Eric have students starting fieldwork in the Bighorns for structural geology theses. For all who are involved in the work this summer, the field trip provided an introduction to the regional geology of the Bighorns basement arch.

6/22/09

Out route through the Bighorns

Out of Buffalo we followed Highway 16 to view the Clear Creek Fault, the structure that bounds the Bighorns Arch on the east. Before exiting the vehicles, Eric equipped us with a sheet showing the stratigraphic column of the Bighorns, which became tattered and discolored after a day of constant referral. Resiliently (and to my dismay) we trudged up the highway into a strong head wind evading zooming vehicles along the way. Beginning with the Madison Limestone, each rock formation was marked by a sign (common throughout the Bighorns). In order followed the Bighorn Dolomite (Ordovician) and Gros Ventre Shale Formations (Cambrian).

The crew inspecting the Gros Ventre Fm.

Following the structural geologists’ lead, I oh and ah’ed over the formations and watched each of them scribble ardently in their notebooks, fuss with compasses and hammer at the rock, BUT I couldn’t help thinking, what’s all the fuss about? Boy was I enlightened. The lesson of the day, and essentially the credo of geologists is that while we enjoy the landscape that surrounds us, it’s what lies beneath the face value, rather the architecture of the rock formations and the evolution of forces that shape the surface of the land that are worth the ohs and ahs.

Let me begin.

From the Madison through the Gros Ventre, the bedding is almost completely vertical. Also, with increasing elevation, the formations become older. So what occurred in geologic history that caused this deformation? Fault bend folding!

Eric explains the basic principles of folding in the following video.

Click here to view the embedded video.

Leaving the road cuts behind we made our way into the heart of the Bighorns and ate lunch with a view.

At our next stop in the Bighorn Stratigra-tour, the Cambrian Flathead Sandstone, we stumbled upon the Great Unconformity! Entering the canyon, the backlimb of the fold came into view and we were able to identify the majority of the stratigraphic column through the Paleozoic. We also explored Eric’s model explaining the deformation of the Bighorn range coined rotational fault bend folding. This hypothesis explains the features observed in the crust better than prior hypotheses such as kink bend folding (as Eric explains in video #2).

Click here to view the embedded video.

Chris's "Orientator"

The Earthscope Bighorn Mountains project aims to unify this model of upper crustal structures with lithospheric scale structures through the Foreland Basement Arch Seismic Experiment.