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.

Approaching from the South

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?

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.

Baked Angular Unconformity
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.

Slickensided Surface in Slate – Oblique View

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.

Slickensided Surface

Oblique View – Slickensides on Top and Quartz Vein Filling

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.


… with Slickensides

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Day #96 Deskcrop: Granodiorite of Wagy Flat

During the course of my M.S. and Ph.D. research I collected a great many samples of the granitoids of the southern Sierra Nevada Batholith as reference samples of the plutons that may have been parental to the conglomerate clasts from the Gualala basin whose provenance I was attempting to ascertain. Today’s deskcrop is from one of those southern Sierran plutons. I’m guessing a bit at the exact locality of the sample, as my field notes are shamefully thin on detail. I really need to get back to this area and reacquaint myself with the details of its geology…

Granodiorite of Wagy Flat

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Day #95 Deskcrop: Charnockite

A couple of weeks back Callan kicked off his transect of the Virginia Appalachians with a great post on charnockite. Well, I did a little digging and I’ve unearthed a deskcrop of charnockite of my own. Mine is from the Adirondacks, most likely in the neighborhood of the Canada Lakes. My piece is pretty fresh so you see the green color of the orthopyroxene but not any of its dun weathering color. My deskcrop also shows a bit more strain than Callan’s. See my discussion in Callan’s comments for more geologic background on the Adirondack charnockites.


I’m not certain, but that may be a slightly larger crystal of andesine plagioclase showing a bit of labradorescence near the right side of this deskcrop.

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Day #93 Outcrop: Baked Contact(?), Green Sand Beach

I’ve been trying to find a nice example of a baked contact so that I could jump on the geomeme that Callan and Silver Fox started earlier this week. I thought about maybe using a skarn contact (I think I’ve got pictures of one somewhere), but that’s not exactly aligned with this meme. What I did find I’m not 100% certain qualifies as a baked contact, though I certainly think it might.

These shots are from a ledge above the famous Green Sand Beach near Ka Lae (South Point), Hawaii. My interpretation is that what we’re seeing here is a basaltic lava flow capping the littoral deposits that form the source of the eponymous green sand. This is one of those situations where I took the photos because there appeared to be something geologically interesting going on, even though I wasn’t exactly sure what it was. I’m not certain whether the rusty layer at the contact represents contact metamorphic baking from the lava flow, or later hydrothermal(?) alteration by groundwater, but in any case I think it’s the best I’m likely to do to keep the current geomeme going. Perhaps some more familiar with this locality or these sorts of processes can fill in the details or set the record straight.

Baked Contact(?), Green Sand Beach

A Closer Look at the Contact

And of course, the main attraction of this day’s hike was beckoning me to turn around and descend to collect a deskcrop. It’s not easy to linger on an undocumented curiosity when the siren below is singing her song…

The Green Sand Beach – Eroding Littoral Cone

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Day #92 Deskcrop: Favosites

Why don’t we close out the work week with a Fossil Friday, where I get to display my general ignorance of paleontology (which is why Fossil Fridays are likely to be few and far between). Today’s deskcrop is from the same locality as the deskcrop of Petoskey Stone I featured back in early February. As best I can fathom, this is a fossil of some flavor of Favosites. If a qualified fossil fancier (paleontologist) would care to proffer a refined identification, I’d find that useful.


This post is brought to you by the letter “F”.

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