Day #105 Deskcrop: More Black Hills Staurolite Schist

Mmmm, staurolite! One of the loveliest porphyroblasts – particularly when twinned (see the left end of today’s deskcrop, below), staurolite is the metamorphic index mineral above garnet in the progressive metamorphic sequence in metapelites. This particular sample has a more schistose texture than the staurolite schist I previously featured, and comes from a bit more remote locality.

When I collected this one back in 2003 web-based mapping tools were still in their infancy. Nonetheless, I have a distinct memory of planning the search for this locality with them and printing out a bunch of satellite images of this area of the Black Hills to aid my back-roads navigation. Today I can do all of that on the fly, in the field, and in real time on my Motorola/Verizon Droid cellphone. Oh, how times have changed!

The kyanite zone is up next, with a deskcrop from Wisconsin…

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Staurolite Schist, Black Hills

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Day #104 Deskcrop: Garnet Schist, Black Hills

Got garnets? Oh, have we got garnets!

Look at all the lovely garnet porphyroblasts in this lovely piece of schist from the Black Hills of South Dakota. In fact this outcrop is just up the road from another one featured in this series back in March. This particular bed was just chucky-jam full of little 2-3mm diameter dodecahedrons of joy. Boy, would I like to see what this one looks like in thin section.

Having visited the garnet zone, the next scheduled stop is the staurolite zone. I have a feeling we’ll be revisiting the Black Hills on Tax Day…

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Garnet Schist, Black Hills

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Day #103 Deskcrop: Biotite Phyllite, Walloomsac Fm.

We’ve arrived in the biotite zone (just barely). Today’s deskcrop comes from the Middle Ordovician Wolloomsac Formation in Dutchess County, New York. This deskcrop was metamorphosed during the Taconic and Acadian Orogenies and is just inside the mapped biotite isograd. There’s supposed to be macroscopically visible porphyroblasts of biotite, but I’m hard pressed to recognizing them, myself.

Tomorrow we’ll move on to the garnet zone…

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Biotite Phyllite, Walloomsac Fm.

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Day #102 Deskcrop: Ellsworth Schist

Tax week has been a pretty busy one for me with grading and other meatspace commitments. As I catch up on Deskcrops for the week there’s a theme that geologists will recognize that relates to an earlier blog post. Nuf said. ;-)

I’m kicking the week off in the chlorite zone. Today’s deskcrop is Ellsworth Schist, a Cambro-Ordovician rock unit cropping out in coastal Maine near Mount Desert Island. This rock unit has experienced quite a bit of deformation but appears to have only been heated to a point such that chlorite is the highest grade metamorphic index mineral present.

Tomorrow we’ll visit the biotite zone…

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Ellsworth Schist – Chlorite Zone

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Where on (Google) Earth #198?

I greatly enjoyed finding Péter’s lherzolite locality in the French Pyrenees, though it took me a week searching Switzerland before I read his clue carefully and considered the possibility that such gorgeous alpine geology was not hidden in some nook in the Gruyère.

I especially enjoyed the geological aspect of Péter’s challenge – so much so that it has inspired (indirectly) the locality I’ve chosen here. As always, the object of Where on (Google) Earth is to identify the locality of the image below (latitude and longitude will generally suffice), but also to explain the geological significance of the site. Quite often that is simply a matter of interpreting the landforms that can be readily identified in the GE image. However, the landforms seen here are merely the key that will help you unlock the deeper geological significance of the site. I would ask that you refrain from identifying the locality until such time as you are prepared to map out a sufficiently detailed geological explanation. This may be frustrating if you find the locality quickly (a distinct possibility, in this case), but I hope that you’ll find the challenge of unearthing the geological significance of the region a worthwhile quest in its own right.

I think WoGE #198 will be relatively easy to locate, so I’m choosing to invoke the Schott Rule – wait an hour for each WoGE win before answering, please. Post time: 3/13/2010, 23:37 Central Standard Time (USA) – 3/14/2010, 4:37 GMT.

Where on (Google) Earth #198.

Dig in!

Day #101 Outcrop: Differentially Weathered Basaltic Dike

Today’s outcrop is a little north of yesterday’s baked angular unconformity in the Paradise Valley between Gardiner and Livingston, Montana. It is an outcrop of a vertical basaltic(?) dike intruded into volcaniclastic conglomeratic sediments. The dike stands up in positive relief because of differential weathering. There are also a number of small plugs of the same rock just to the northeast of the dike that presumably share its origin. Upon closer inspection, the knobs just to the northeast of the dike appear to be more consolidated deposits of the volcaniclastic conglomerates into which the dike is intruded. None of the field guides of the area I have indicate the age of this dike, but I would presume it is geologically young given the apparently unconsolidated nature of the volcaniclastics that surround it. Note also the center pivot irrigation system in the field in the foreground to get a sense of the scale of this dike. Measuring in Google Earth I estimate it to be about 11 meters wide.

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Approaching from the South

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On Strike View

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Day #100 Outcrop: Baked Angular Unconformity

During the course of the last week I’ve been busy geotagging, describing, and uploading to Flickr field photos from my trip to the northwest USA last summer. In so doing, I think I’ve discovered a fitting photo for my hundredth daily Deskcrop/Outcrop post. (Kudos to Ian Stimpson for reaching the 100th daily rock post on his Posterous blog, too!)

Today’s outcrop is located near the northern entrance to Yellowstone National Park and just across the Gardiner River Valley from Mammoth Hot Springs. Geologically it features a textbook angular unconformity, with southward dipping 2.1 Ma Huckleberry Ridge Tuff unconformably overlying northward dipping Paleozoic sedimentary rocks. That much I realized when I shot the photos and GigaPan below last summer. What didn’t dawn on me in the field, but jumped right out at me after our recent geomeme of baked contacts, was that it’s also a great example of a baked contact in its own right. How’s that for a concrete example of how blogging advances one’s understanding and appreciation of geology?

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Huckleberry Ridge Tuff in Angular Unconformity Over Paleozoic Sedimentary Rocks



Baked Angular Unconformity – Huckleberry Ridge Tuff Unconformably Overlying Paleozoic Sedimentary Rocks
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Let’s take a closer look at two snapshots from this GigaPan. You can see that the Paleozoic rocks immediately below the contact have a reddish tint that fades with distance from the contact. I interpret this to be oxidation caused when the thick Huckleberry Ridge Tuff erupted 2.1 million years ago and blanketed and baked the rocks below it.

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Baked Contact at the Angular Unconformity

Note that the baked red rocks are not merely talus coming from the Huckleberry Ridge Tuff. In the snapshot below you can clearly see talus from the HRT above the contact that is not colored red. Thus, the rusty color of the contact metamorphic aureole clearly originates within the Paleozoic rocks beneath the contact.

Distinction of Talus Origins
Clear proof that the rusty color is the contact aureole, not HRT talus.

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Day #99 Deskcrop: Mirrorlike Slickensides in Slate

Without a doubt this is one of my favorite deskcrops. It’s a piece of maroon slate of the Metawee Formation that has a slickensided surface that is polished to a high, mirrorlike shine. Rarely have I found such a naturally polished rock surface – even less commonly as the result of a structural process.

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Slickensided Surface in Slate – Oblique View

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Light Reflects Off the Mirrorlike Slickensided Surface

Once again, I’m uncertain of the exact slate quarry where I collected this one during undergrad field camp, so I’m locating the placemark in a random quarry in the general field area that I mapped.

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Day #98 Deskcrop: Slickensided Vein in Slate

As promised I’m following up today with another slickensided deskcrop. Whereas yesterdays slickensides (on a conglomerate) were just a polished surface, today’s slickensides are developed on a vein surface in slate. The slickensided surface (top photo) bears the chlorite green color of the host slate. When we turn the sample (center) for an edge view (bottom) one can see that these slickensides have developed on a quartz vein, with what appears to be some sort of augen(?) or boudin(?) of slate caught up in the vein. I’d welcome it if a structural geologist would care to offer a more refined explanation.

I don’t know the exact locality of this sample, so I’ll again throw the placemark in a random slate quarry in the region where I collected this sample.

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Slickensided Surface

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Oblique View – Slickensides on Top and Quartz Vein Filling

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Edge-On View of the Vein and Slate “Augen”(?)

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Day #97 Deskcrop: Conglomerate with Slickensides

Today’s deskcrop comes from the Cedar Mountain area of the northern San Rafael Swell in Utah. This rock is from a thin unit that is found at the contact between the Jurassic Entrada and Summerville (Curtis) formations. The conglomerate contains a variety of pebble types including limeclasts, suggesting an arid depositional environment. This particular piece also has some nice slickensides developed on one side (below). I’ll follow up with some more slickensided deskcrops over the next couple of days.

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Conglomerate…

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… with Slickensides

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