Day #40 Deskcrop: Orbicular Diorite

Today’s deskcrop displays a rather uncommon igneous texture: orbicular comb layering. I first heard about this remarkable texture in a talk at Colgate University by D. Brooks McKinney of Hobart and William Smith College. I didn’t get out to the site to observe these remarkable structures in their natural habitat until a couple years later in grad school, but the hike was well worth it for both the rocks and the view.

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Orbicular Diorite

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Day #39 Deskcrop: Oolitic/Pisolitic Grainstone

We’ll begin the week where we left off last week – at a roadcut on the south side of Douglas Pass in western Colorado. In fact, today’s deskcrop was formed in pretty much the same depositional environment within the Eocene Green River Formation’s lacustrine facies as last Friday’s. Today’s displays a remarkable texture (best viewed large in Flickr) with plenty of primary porosity in the interstices between the oolites/pisolites that make up the bulk of this rock. It sure would’ve been interesting to frolic in the shallows of this lake…

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Oolitic/Pisolitic Grainstone

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Day #38 Outcrop: Diorite, Laccolith, Henry Mountains

I’ve never had enough time to truly explore the Henry Mountains of Utah, but I have had the opportunity to pass through their neighborhood long enough to get a glimpse of the distal edge of a few of the laccoliths made famous by G.K. Gilbert’s geologic expedition of 1875. Here’s hoping I can get back there sometime soon and do some proper exploring.

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Laccolith, Henry Mountains

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Diorite, Henry Mountains

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Day #37 Outcrop: Varved Clays, Whitefish Bay

A former student reminded me of the spectacular varved clays we did fieldwork on during my time teaching at Lake Superior State University. Notice in the Google Earth view of this locality that the exposure is located at the toe of a landslide along the south shore of Whitefish Bay at the east end of Lake Superior. The varves are some of the best I’ve ever seen, but let me tell you it was no fun to trudge around in the slippery, sucking clay of the shallows.

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Varved Clays, Whitefish Bay

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Students Record Field Observations of the Landslide Toe

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Day #36 Deskcrop: Algal Stromatolite

Sticking with the sedimentary theme, today’s deskcrop is another biochemical limestone – this time, an algal stromatolite from the Eocene Green River Formation at Douglas Pass, in western Colorado. Algal stromatolites are some of the oldest fossil life forms preserved in the rock record, but they’re not limited to the Precambrian. This sample is one of a couple of very nice textural samples of carbonate rocks I’ve collected from the lacustrine deposits on the southern approach to Douglas Pass. Look for another one from this locality with a very different texture next week!

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Algal Stromatolite

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Day #35 Deskcrop: Fossiliferous Packstone

After a string of igneous rocks it’s time to mix things up a bit and go sedimentary on you for a change. Today’s featured deskcrop is a fossiliferous limestone that I collected at the base of the Mississippian Muleshoe Mound in the Sacramento Mountains near Alamagordo, New Mexico during a spring break field trip led by University of Wisconsin carbonate sedimentologists Lloyd Pray and Toni Simo during my first year as a grad student at UW-Madison. I’m invoking the Dunham classification scheme to label this a “fossiliferous packstone” but I’d be glad to be corrected if any of my more sedimetarily inclined colleagues sees fit to do so. I can tell you that the most prominent fossils in this sample are the cyllindrical crinoid stems that appear throughout the muddy matrix of this rock.

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Fossiliferous Packstone

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Day #34 Deskcrop: Mantle Xenolith in Scoria

Today’s deskcrop has as deep an origin as any I’ve yet featured. This is a mantle xenolith (peridotite) in scoria (a type of vesicular basalt). This particular sample comes from Dish Hill in the Mojave Desert. It’s one I collected with Jean Morrison and Lawford Anderson on a prospective graduate student visit to USC during my senior year at Colgate University. That was a really memorable trip, and I’ve gotten a lot of teaching mileage out of this sample over the years.

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Mantle Xenolith (green, at left) in Scoria (black, at right)

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Day #33 Deskcrop: Keweenawan Amygdaloidal Basalt

Basaltic lava flows are commonly vesicular near their tops and bases as the lava there chills quickly enough to trap gas bubbles. Over time, as these basalts are exposed to alteration by hydrothermal fluids, new minerals may be deposited in these void spaces to form amygdules. This sample of amygdaloidal basalt is from the 1.1 Ga Keweenawan Rift series currently outcropping along the east shore of Lake Superior and exposed beautifully in roadcuts along the Trans Canada Highway in the Batchewana Bay region of Ontario. The rounded amygdules of this particular piece of basalt are filled primarily with chlorite and calcite; elsewhere in this unit epidote is a common amygdular mineral. Since these minerals are not commonly primary in basalts there is little chance of mistaking these amygdules for phenocrysts, which can also be seen in this rock (the small white laths of plagioclase).

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Keweenawan Amygdaloidal Basalt

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Day #32 Deskcrop: Half Dome Granodiorite

You’ve seen what it looks like after crushing, now see it in its whole form: the Half Dome Granodiorite. This sample is from the equigranular phase of that intrusion (there’s also a porphyritic phase). This sample is from near Olmstead Point along the Tioga Highway. The Half Dome Granodiorite is one of the larger units within the Tuolumne Intrusive Suite in Yosemite National Park.

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Half Dome Granodiorite

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