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Field Trip: Petrology

Weekend trip, Fall 2015

Dr. Flood’s Petrology class (GEOL 320) set out to the north-central part of Wisconsin to study the state’s geologic history, beginning with the Archean (2.5 billion years ago). This weekend trip gave students the opportunity to understand the area’s bedrock geology and see features such as metamorphosed stromatolites, evidence of glaciers, and the suture zone between the Superior Craton and Marshfield Terrane (also known as the Northern and Southern Continents).

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The first stop of the trip was along the Wolf River. This river represents the boundary between the island arc of the Pembine-Wausau terrane and the Wolf River Batholith.

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Granites associated with the Wolf River Batholith tend to have large, euhedral (near perfect) crystals; as seen in the above right photo. Stormy Schreiber ’17 (left) and Giovanni Bisi ’18 (right) consult on giving this granite a more specific name based on the percent mineral composition. A helpful trick is to wet the rock to see more defined crystals.

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The class took a detour to a gravel pit just outside of Wausau, Wis., to examine an exposed pile of sand, silt, clay, and coarser materials. This mound (conquered by Angelina Pankow '17 and Hannah McTavish '17) demonstrated what glacial till would look like if completely exposed. It is this type of mixture that is found throughout Wisconsin, excluding the Driftless Area.

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At the Dells of Eau Claire, the first features students noticed were the blocky fractures of the rock and the scattered potholes (top left). Taking a closer look, the students identified the peculiar shape of the mineral grains within the granite (top right). These grains were elongated and looked as if they had been smeared, while the surface of the granite was smooth. This area is the suture zone of the Southern Continent and the island arc.

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Wausau is home to the famous Wausau Syenite. Syenite is a type of granite that has little to no trace of quartz, for this granite is silica-deficient and potassium-rich, containing gray-colored orthoclase. Dr. Flood pointed out the mineral riebeckite, a sodic-rich amphibole and polymorph of hornblende (bottom left). At this outcrop, students were able to collect their own samples of syenite, and if they were lucky, find a nice riebeckite crystal.

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Here at Rib Mountain on Granite Peak, ripple marks in a large piece of quartzite were seen, as pointed out by Riley Hacker ’17. The quartzite formed as a result of extreme chemical weathering (hydrolysis) on pure quartz sandstone. Because the surrounding syenite rock weathers faster than quartzite, the quartzite is described as holding up the hill. As mentioned, the surrounding rock is syenite, not granite, yet the peak is called Granite Peak. This calls for a name change… for geologist’s sake.

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After returning from the field, Dr. Flood prepared his famous spaghetti dinner and closed the night with tunes around the campfire.

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Day Two plans were more focused on exploring and discussing the Northern Continent.

The first site was an example of a glacial feature, a kettle lake. Metonga Lake formed from the weight of ice, which created a depression in the ground that eventually filled with water. This site also houses a massive sulfide deposit with minerals like sphalerite and galena; however, the deposit is difficult to reach.

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Dr. Flood knew of a complex pegmatite tucked back in the woods along the Pine River. The pegmatite formed from partial melting of pre-existing continental granite materials, and consists of pink tourmaline, spodumene, muscovite, and quartz. The group trekked through the brush for some time. Unfortunately, the search was unsuccessful.

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Even though the students came out empty-handed with the pegmatite, they had a chance to collect phyllite from the Northern Continent in Iron Mountain, MI. Phyllite is a metamorphic rock, with its parent rock being a mudstone. Originally deposited as mud (approximately 2.2-1.4 billion years ago), the sediment experienced low-grade metamorphism due to a mountain-building event known as the Penokean Orogeny. These phyllites were moved up as low angle panels onto the continent.

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Metamorphosed stromatolites are located at Fumee Falls. Stromatolites are defined as calcareous mounds built up by cyanobacteria and layered, trapped sediment. Having these stromatolites preserved during the process of metamorphism is rare, so our students were fortunate to see these amazing fossils.

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To close the weekend, the final stop was in Niagara, WI, along the Menominee River. The river marks the suture zone between the Northern and Southern Continents. Over the course of the weekend, the Petrology students pieced together Wisconsin’s 2.5 billion year history up to the time of the glaciers, while adding more rocks to their collections. Standing on the Marshfield Terrane and looking across the river at the Superior Craton was the perfect ending to the Intercontinental Trip.

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