Day #116 Deskcrop: Half Dome Granodiorite (Thin Section)

I’m kicking this week of thin section deskcrops with a rock you’ll already be quite familiar with if you’ve been following my blog this year – that’s right, it’s another look at the Half Dome Granodiorite! We’ve previously seen it pulverized, in deskcrop, and outcrop. Today’s views are of thin sections (30 micrometer thick slices of the rock) as seen under a petrographic microscope (or technically, a petrographic macroscope, in this case).

The plane light view is nothing spectacular, since the minerals that make up the bulk of this rock (quartz and plagioclase) are low relief, uncolored minerals in plane light. Near the center of the plane light view there is a grain of biotite with secondary chlorite oriented parallel to cleavage in a NE-SW orientation in this grain. Just to the right of it is a large green crystal of hornblende. Both the biotite and hornblende are pleochroic (they change colors as the stage is rotated in plane light), but I’ve only photographed this orientation.

Half Dome Granodiorite (plane light)

In cross polarized light the plagioclase grains really steal the show. There are prominent grains that show both Carlsbad and albite twinning, as well as a couple that show concentric compositional zoning (check out the NE quadrant). To get a closer look at any of these photomicrographs click through to the image on Flickr and then click the link to “All Sizes” above the photo there.

Half Dome Granodiorite (X-polarized light)

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Day #115 Outcrop: Putnam Station Nonconformity

Today’s “outcrop” is, in fact, yet another roadcut – it seems all the best “outcrops” in the northeast are roadcuts, but who’s quibbling. This time we’re on the eastern margin of the Adirondacks near Putnam Station, NY. The contact here, between Proterozoic gneisses (below) and Cambrian Potsdam Sandstone (above), is extremely well preserved near the top of this small cut. The Potsdam even has a thin conglomerate bed just above the contact before grading upwards into more common sandstone layers. Zoom in and you’ll note that my field assistants came out to examine this exposure, as well. Berti is waving at you with his feet planted firmly on the Great Unconformity and Edi is below him examining the vertically foliated gneisses. Needless to say, a good time was had by all.


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Day #114 Outcrop: Alexandria Bay Nonconformity

As promised back on Tuesday, this weekend I’m going to visit a pair of outcrops (well, roadcuts, actually) on opposite sides of the Adirondack Mountains that expose the Great Unconformity where the Cambrian Potsdam Sandstone lies depositionally atop a nonconformity developed on Late Proterozoic gneisses of the Adirondacks.

We’ll begin on the west side of the Adirondacks in the St. Lawrence River Valley. Today’s “outcrop” is actually a long roadcut just northeast of Alexandria Bay, NY and just west of Cranberry Creek. The unconformity surface is near the base of this cut and there is a considerable thickness of Potsdam Sandstone overlying it. The contact itself is somewhat weathered, so it’s not the best place to put your finger on a gap in time of hundreds of millions of years, though that is what it represents. If you look carefully you’ll note there’s some paleotopography on this unconformity surface.


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Day #113 Deskcrop: Orange Mountain Basalt

The Great Falls of the Passaic River at Paterson, New Jersey cascade over the Jurassic Orange Mountain Basalt, the lowest of the basalt flows that make up the Watchung Mountains in the Triassic-Jurassic Newark Basin. The hydropower generated by these falls was important to the industrial development of the newly independent United States in the late 1700s and throughout the 1800s. Today’s deskcrop is from a low roadcut in the park just downstream of the falls. Its diabasic texture is nothing to write home about – it would be a lot more interesting to look at in thin section, but I’ve only got this hand sample for now. (I do think that some thin sections would make for a nice change of pace for the deskcrop series – sounds like a theme for next week!)

Orange Mountain Basalt

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Day #112 Deskcrop: Nepheline Syenite, Stettin Complex

I feel much more confident of the locality of today’s deskcrop. This is due in no small part to the fact that an ILSG field guide to this particular locality is online in PDF format. The sample is a piece of nepheline syenite from the southern edge of the ~1520 Ma Stettin Syenite complex, located not far to the northwest of the Wausau Syenite featured back in March. This sample is one of my favorites for teaching about differential weathering of minerals – the large crystals of nepheline (a variety of feldspathoid), abundant in this sample, weather much more rapidly than the feldspars, giving the weathered surface of the sample a distinctive pitted texture (best seen in the second photo below).

Nepheline Syenite, Stettin Complex

Nepheline Syenite – note the pitted nepheline crystals

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Day #111 Deskcrop: Mylonite, Eau Pleine Shear Zone

The Eau Pleine Shear Zone marks the boundary between the Archean Marshfield Terrane and the Proterozoic Wausau-Pembine Terrane in central Wisconsin. Today’s deskcrop is a mylonite from the Eau Pleine Shear Zone. I have only the vaguest recollection of the actual locality where this rock was collected, and it’s entirely possible I’ve badly mislocated the quarry from which it was retrieved, but the quarry near Dancy, WI where I’ve located the placemark is within the mapped boundaries of the EPSZ and seems plausible based on my weak recollection of the particular field excursion when I collected this deskcrop. I really need to spend a couple of weeks reacquainting myself with Wisconsin’s outcrops…

Mylonite, Eau Pleine Shear Zone

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Day #110 Deskcrop: Potsdam Sandstone

Today’s deskcrop is a piece of the Cambrian Potsdam Sandstone from near Alexandria Bay, NY. The Potsdam at this locality lies nonconformably over gneisses of Proterozoic age. One can see bedding laminations in this sample as well as alteration zones at the top and bottom of the sample marking where groundwater flowing along bedding planes has begun to weather the sample. I also GigaPanned the roadcut at this locality, but I’ll hold that view for one of this coming weekend’s “Outcrop” posts.


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Day #109 Deskcrop: Sillimanite Schist, Tracy Hill Road

Concluding the Barrovian metamorphic index minerals theme I began last week (as a series of clues to WoGE #198), today’s deskcrop is a sillimanite zone muscovite-rich schist from Dutchess County, New York. There is no sillimanite visible in hand sample (like most sillimanite +/- K-feldspar-zone metapelites I’ve known, but not all), though there are some bodacious garnets hidden in that sea of muscovite. The Barrovian-sequence metapelites from Dutchess County were the topic of a two-week metamorphic petrology lab in my undergrad days at Colgate University, and names like Tracy Hill and Still Road bring back memories of that lab with a fondness that undoubtedly grows with the passage of time.

Sillimanite Schist, Tracy Hill Road

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Day #108 Outcrop: Carlin Canyon Angular Unconformity

This is the second of two GigaPanned outcrops I’m posting this weekend accompanied by text from Annals of the Former World, by John McPhee, upon which I cannot improve…

Interstate 80, in its complete traverse of the North American continent, goes through much open space and three tunnels. As it happens, one tunnel passes through young rock, another through middle-aged rock, and the third through rock that is fairly old, at least with respect to the rock now on earth which has not long since been recycled. At Green River, Wyoming, the road goes under a remnant of the bed of a good-sized Cenozoic lake. The tunnel through Yerba Buena Island, in San Francisco Bay, is in sandstones and shales of the Mesozoic. And in Carlin Canyon, in Nevada, the road makes a neat pair of holes in Paleozoic rock. This all but leaves the false impression that an academic geologist chose the sites-and now, as we approached the tunnel at Carlin Canyon, Deffeyes became so evidently excited that on might have thought he had done so himself, “Yewee zink bogawa!” he said as the pickup rounded a curve and the tunnel appeared in view. I glanced at him, and then followed his gaze to the slope above the tunnel, and failed to see there in the junipers and the rubble what it was that could cause this professor to break out in such language. He did not slow up. He had been here before. He drove through the westbound tube, came out into daylight, and, pointing to the right, said, “Shazam!” He stopped on the shoulder, and we admired the scene. The Humboldt river, blue and full, was flowing toward us, with panes of white ice at its edges, sage and green meadow beside it, and dry russet uplands rising behind. I said I thought that was lovely. He said yes, it was lovely indeed, it was one of the loveliest angular unconformities I was ever likely to see.

Carlin Canyon Angular Unconformity

The river turned in our direction after bending by a wall of its canyon, and the wall had eroded so unevenly that a prominent remnant now stood on its own as a steep six-hundred-foot hill. It made a mammary silhouette against the sky. Mu mind worked its way through that image, but still I was not seeing what Deffeyes was seeing. Finally, I took it in. More junipers and rubble and minor creases of erosion had helped withhold the story from my eye. The hill, structurally, consisted of two distinct rock formation, awry to each other, awry to the gyroscope of the earth-just stuck together there like two artistic impulses in a pointedly haphazard collage. Both formations were of stratified rock, sedimentary rock, put down originally in and beside the sea, where they had lain, initially flat. But now the strata of the upper part of the hill were dipping more than sixty degrees, and the strata of the lower part of the hill were standing almost straight up on end. It was as if, through an error in demolition, one urban building had collapsed upon another. In order to account for that hillside, Deffeyes was saying, you had to build a mountain range, destroy it, and then build a second set of mountains in the same place, and for the most part destroy them. You would first have had the rock of the lower strata lying flat-a conglomerate with small bright pebbles like effervescent bubbles in a matrix red as wine. Then the forces that had compressed the region and produced mountains would have tilted the red conglomerate, not to the vertical, where it stood now, but something like forty-five degrees. That mountain range wore away-from peaks to hills to nubbins and on down to nothing much but a horizontal line, the beveled surface of slanting strata, eventually covered by a sea. In the water, the new sediment of the upper formation would have accumulated gradually upon that surface, and, later, the forces building a fresh mountain range would have shoved, lifted, and rotated the whole package to something close to its present position, with its lower strata nearly vertical and its upper strata aslant. Here in Carlin Canyon, basin-and-range faulting, when it eventually came along, had not much affected the local structure, further tilting the package only two or three degrees.

Clearly, if you were going to change a scene, and change it again and again, you would need adequate time. To make the rock of that lower formation and then tilt it up and wear it down and deposit sediment on it to form the rock above would require an immense quantity of time, and amount that was expressed in the clean, sharp line that divided the formations-the angular unconformity itself. The lower formation, called Tonka, formed in middle Mississippian time. The upper formation, called Strathearn, was deposited forty million years afterward, in the late Pennsylvanian time. — John McPhee


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Day #107 Outcrop: Eocene Auriferous Gravels

Two GigaPanned outcrops this weekend accompanied by text from Annals of the Former World, by John McPhee, upon which I cannot improve…

The dry bed of an Eocene river carries Interstate 80 past Gold Run. The roadside records the abrupt change. As if you were swinging off a riverbank and dropping into the water, you go out of the metavolcanic rock and into the auriferous gravels. We stopped, stood on the shoulder, and looked about a hundred feet up and escarpment that resembled an excavated roadcut but had not been excavated by highway engineers. It was capped by a mat of forest floor, raggedly overhanging. The forest, if you could call it that, was a narrow stand of ponderosas, above an understory of Manzanita with round fleshy leaves and dark-red bark. The auriferous gravels were russet, and were full of cobbles the size of tomatoes-large stones of long transport by a most impressive river.

I-80 Roadcut in Eocene Auriferous Gravels

To the south, across the highway, the scene dropped off into a deep mountain valley. The near end of the valley was three hundred feet below the trees above us. The far end of the valley was nearly twice as deep. A mile wide, this was a valley that had not been a valley when wagons first crossed the Sierra. All of it had been water-dug by high-pressure hoses. It was man-mad landscape on a Biblical scale. The stand of ponderosas at the northern rim was on the level of original ground.

The interstate was on a bench more than halfway up the gravel. Above, us behind the trees, were the tracks of the Southern Pacific. In the eighteen-sixties, when the railroad (then known as the Central Pacific) was about to work its way eastward across the mountains, it secured the rights to this ground before the nozzles reached it. Moores and I made our way up to the tracks, where the view to the north was over a hosed-out valley nearly as large as the one to the south, and bordered by white hydraulic cliffs. The railroad, with the interstate clinging to its hip, ran across a septum of the old terrain, an isthmus in the excavation, an unmined causeway hundreds of feet high made of gravel and gold. — John McPhee

I-80 Roadcut in Eocene Auriferous Gravels, near Gold Run, California
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