13 July 2018
Chickie’s Rock is a prominent cliff of Cambrian quartz arenite (sandstone) in Lancaster County, Pennsylvania. I visited it last month with field trippers at the eastern section meeting of the National Association of Geoscience Teachers. One of the aspects of the site is this gentle anticline with axial planar cleavage:
The yellow rectangle is my field notebook for scale. Another shot:
Happy Friday to all!
6 July 2018
Last week, I spent two perfect days camping with family at Usal Beach, in Mendocino County, California. Along the beachside cliffs there, I spotted plenty of lovely turbidites: graywacke and shale and a little bit of conglomerate that had been scraped off the subducted Farrallon Plate to help contribute to the bulk of the Franciscan complex. That accretionary process imparted some stresses on these deep-sea deposits, and in many places they show significant deformation. Here are a dozen images to highlight some of the folds I saw:
25 June 2018
As mentioned the other day, I’ve been in the Sierra Nevada this past week, which is full of geological delights. Today I’d like to show you a pretty profound unconformity in the region of Sonora Pass, with ~11 Ma volcanic rocks overlying ~89 Ma plutonic rocks.
I’d like to first examine the two major rock units, exploring their varieties, and then look at the contact between them. So our plan is:
- The granite (granodiorite) of Topaz Lake
- The overlying volcanics (lava flows and lahars)
- The unconformity that separates them
1) The granite
The granodiorite of Topaz Lake is ~89 million years old, and is distinctive on several levels.
It includes microgranular mafic enclaves (MMEs) which tend to be, as their name implies, finer-grained and darker in color than the granitoid in which they are hosted. These are interpreted to be small blobs of immiscible mafic magma within a larger felsic-composition magma body. They tend to be round, though they can be elongated by magma flow (prior to crystallization) or by post-crystallization deformation. The first would be a primary magmatic structure; the second would be a secondary tectonic structure. A few examples:
Here is one that shows internal spheroidal weathering, something I had not previously seen:
I also found a single example of a xenolith, a chunk of pre-magma host rock that was broken off and inforporated into the magma chamber prior to cooling and crystallization:
However, the most striking thing about the granodiorite of Topaz Lake are its eye-catching huge potassium feldspar megacrysts:
In places, these make up a significant volume of the rock:
In places, these huge feldspars weather out and can be collected from among the grus-like soil:
My son shows off his find, along with my friends Jason Westfall of Sonora High School, and Laura Hollister:
2.) The volcanics
The next of our two units is a sequence of mafic lava flows and lahar deposits that erupted from the Little Walker Caldera sometime around 10 or 11 million years ago. This is the caldera itself (on the east side of Sonora Pass):
The rocks which emerged as either lava or mudflows from this (today) scenic caldera look like this:
The lahars include logs of petrified wood, representing the ~10 Ma forest that used to exist here. It’s a distinctive orange color:
Lavas with flow-aligned feldspar phenocrysts:
Higher up in the sequence, I found beautiful almond-sized and -shaped amygdules:
On the banks of the stream, we found plenty of charismatic volcanic breccia cobbles – bits of lahar subsequently tumbled by the headwaters of the Stanislaus River:
3) The unconformity:
So now we can discuss the relationship between these two units:
At the right (western) edge of the Little Walker Caldera, the same situation applies: You can make out the layered volcanics directly overlying the spheroidally-weathered granite beneath:
The rocks below this surface (traced out as a black line on the annotated photos above) is an ancient erosional surface, an unconformity. The rocks below that surface cooled deep underground around 89 million years ago. The rocks above it formed on the surface of the planet as layered deposits, around 10 or 11 million years ago. This contact is a nonconformity, a type of unconformity where the rocks below the erosional surface are igneous or metamorphic (as opposed to sedimentary).
When you’re just above the unconformity surface in the lowermost volcanic strata, you can find granite cobbles included within the lahars:
As you can see, the shape of those granite cobbles varies between a bit more angular and quite well-rounded. These are interpreted to be cobbles tumbled in the 11 Ma ancestor to the Stanislaus River, the river that carved the paleo-canyon into which the Little Walker Caldera vomited its lava and lahars. Downstream (to the west), you can see some lovely well-rounded boulder gravels, some of which show clear imbrication, as Ryan Hollister demonstrates here at a roadcut on Highway 108 west of Cold Springs:
A wee bit to the west, at the gas station that marks Cold Springs, you can see these river gravels in contact with overlying lahar deposits: Here, the gravels remain in between the other two units; they have not been removed.
Overall, the thought is that with such significant (paleo-)topographic relief on the nonconformity surface in the Sonora Pass region, we’re looking here at a scene ~11 million years ago with relief not too different than today: an ancient river had cut into uplifted Sierra Nevada plutons, tumbling boulders it derived from those granites downstream. Then the Little Walker Caldera roared to life and poured all kinds of volcanic slurry down the ancient valley gradients. In the upper reaches of the (paleo-)drainage, the volcanic deposits scoured out most of the fluvial deposits, but lower down in the drainage, relief may have been gentler, or the volcanic flows had lost a lot of their scouring power, and the lahars were laid down atop the fluvial gravels. The paleo-canyon was filled up with volcanic debris. Later, erosion began anew, and cut into both rock types to carve a new canyon that partially coincided with the old one. This newer version of the canyon was enhanced by Pleistocene glaciation.
Amazingly, the story doesn’t stop there, but includes a surprising denouement: the eruption of another batch of lava into the newer version of the canyon. It was this most recent volcanism that built the Columns of the Giants, a basaltic feature about mid-way down the drainage, and the subject of Ryan Hollister’s acclaimed virtual field trip for Science Friday. The volcanism there occurred after glaciation had happened at least once, and was succeeded by at least one more episode of glaciation.
All told, I’d say the Sonora Pass region is an extraordinary example of a landscape with a beautifully repetitive series of igneous and erosional events through the past 90 million years.
23 June 2018
I’ve been lucky to spend the past week+ in California with friends, including geoscience outreach wonder duo Ryan and Laura Hollister. We spent an enjoyable 4 days on the east side of the Sierra Nevada, attending field trips through the Mono Basin Bird Chautauqua alternating with excursions to entertain our collective posse of four kids. One day, we took the kids up to Virginia Lakes to go fishing, and Laura and I were able to get in some geologizing. We hiked up to the Frog Lakes, where there is a beautiful deformed conglomerate exposed.
These rocks were very exciting to me.
Here’s a boulder: You can see the elongation of its component clasts:
Some of the clasts are themselves made of smaller clasts, and they too show internal development of the regional foliation:
Deformation here appears to have been accomplished by pressure solution, causing the clasts to warp and wrap around each other:
Here’s an example of what Chuck Bailey might call a “double-duckbill”:
Douglas, et al. (2011) used this unit (which they call a ‘breccia’) as a sedimentary record of the flavor of early arc magmatism affecting coastal paleo-California about 224 million years ago. But I was interested in it from a structural perspective: how the big, charismatic clasts serve as records of the strain accumulated by the wall rocks (or “roof pendants” in this case) during transpressional squeezing prior to (or coincident with) the intrusion of the magmas that would cool to become the Sierra Nevada batholith. This was, after all, the subject of my master’s thesis at the University of Maryland. I mapped very similar rocks south-southwest along strike from Frog Lakes. It was a delight to clamber around and check out these beautiful rocks, simultaneously showing sedimentological and structural features. It made me wish I had mapped this area back in 2003.
Here’s an example of the sort of thing that gets me excited: a boulder showing both graded bedding (fining upward, from the lower left to the upper right) and a pronounced tectonic fabric (running subhorizontal from left to right):
Here is another example, with bedding running from left to right, and foliation running horizontally from upper left to lower right, straight across the contact between a conlomerate and a mudrock:
The great thing about making the hike up to Frog Lakes was to get to see large outcrops showing the stratigraphy plainly, with a delicious tectonic fabric overprinted on it.
Some examples of what I’m talking about:
Here is Laura next to the contact between a meta-mudrock (now slate) and an overlying metaconglomerate:
Here’s another spectacular outcrop, showing a gradational upper contact with the lower conglomerate, and a crisp lower contact with the upper conglomerate, suggesting these beds are right-side-up. Additional evidence for this interpretation comes from the structural domain: the bedding in the muddy unit and the cleavage both dip to the left (~west) but the cleavage is steeper, and the bedding dips more shallowly. This is the basic relationship we would expect if the beds were upright.
Zooming in on the most informative central portion of this outcrop, where the cleavage shows a prominent deflection as it crosses from conglomerate to mudrock to conglomerate again:
Mostly the rocks here were either (meta-) conglomerates or mudrocks, without any intervening sandstone layers. I did spot one isolated “outsized” clast in the mudrock subunits:
And here is a relatively thin layer of cobbles in a rare sandy deposit adjacent to one of the conglomerates:
A hodge-podge of other images of these photogenic rocks:
Here’s an apparent flame structure (just above and to the right of the lens cap):
Zooming in on the “flame”:
A few more:
Did you notice the bonus glacial striations in that last shot (on the left)?
We’ll close it out there: you can count those striations as a small geo-lagniappe. I hope you enjoyed visiting Frog Lakes with me! I look forward to revisiting to this exciting location someday soon – hopefully it won’t take me another 15 years to return.
22 June 2018
Another guest Friday fold (keep ’em coming, folks!) – This time from Eric Pyle of James Madison University:
This is a weathered outcrop of the Connemara Marble in western Ireland, about a meter wide.
Thanks for sharing, Eric!
15 June 2018
My friend Karen Aucker from NAGT’s Eastern Section shared these images (and a video!) of the folds she glimpsed from the cable-car on her way up the Schilthorn in Switzerland. I reckon they will do for our Friday fold:
Thanks for sharing, Karen! These are great.
Happy Friday, all!
9 June 2018
Today, in Millersville, Pennsylvania, on the campus on Millersville University, I saw these contorted carbonates. They are of the Cambrian Conestoga Formation, and I saw them on a NAGT Eastern Section field trip led by Lynn Marquez of Millersville University.
This deformation is purported to be Taconian, but it looks very much like Alleghanian deformation in similar aged and composition rocks in Virginia. Interesting!
5 June 2018
When I was learning about the Archean from the perspective of Barberton Mountain Land in South Africa, I was expecting to see komatiite lava flows. Early on in Earth history, the planet was hotter: (1) it was closer to the many thermokinetic impacts that built the planet from stone-cold meteorites, and (2) there were many more unstable radionuclides around, decaying and releasing their energy into the young planet. As a result, some minerals that don’t melt readily at modern volcanoes were able then to turn to liquid and ooze out of volcanoes at temperatures much higher than modern eruptions. As they crystallized, these lava flows grew “chandeliers” of spinifex textured olivine and pyroxene crystals.
However, I was not expecting to see primary sedimentary structures in these ash deposits. Some of the ultramafic volcanoes blew up, sending ash into the Archean atmosphere, from where it rained down as itty-bitty particles. In addition to accretionary lapilli and tsunamites (with accretionary lapilli!), the komatiite ash deposits show current flow indicators such as cross-beds, indicating they were moved around by currents of either air or water prior to final deposition:
These cross-beds are concave-up (like smiley faces), and if you’re astute, you’ll be able to find a few instances here where the top of the cross-beds are truncated by an overlying bed. Here’s a GIGAmacro example of that, with cross-bedding annotated in blue, and the bottom of the overlying bed in gray:
Note also the rusty patina of these ultramafic volcanic/sedimentary rocks – the olivine therein is ready to rust, even in the arid climate of South Africa.
4 June 2018
Last week, I lost two of my colleagues, Declan De Paor and Ron Schott. Both were involved in Google Earth for On-site & Distance Education (GEODE), the 5-year project I serve as PI on. Declan had battled various forms of cancer for many years, and most recently had developed several brain tumors. His passing was preceded by knowledge of his condition, while Ron’s death was unexpected and therefore shocking. Both geologists were important to me personally, and I wanted to commemorate them here, outlining both their roles in larger geo-society, but also what their influence meant to me.
Declan taught at Old Dominion University, where he directed the planetarium and had an appointment in the department of geophysics. He was a structural geologist (as is his wife, Carol Simpson, who served as provost at ODU for many years), but he was more known in recent years for his innovation and promotion of digital geology teaching tools. Many of his collaborations through the years were with Steve Whitmeyer of James Madison University, and it was the two of them who contacted me about six years ago to start developing a proposal that became GEODE. Declan was exceptionally clear-headed and proactive as a thinker. I remember first meeting him at a GSA meeting in Pittsburgh, as he mediated (with a martini) about a way to solve a technical issue with his planetarium, and delightedly coming to a solution right before shaking my hand. He was gleeful in that moment, and keen on sharing his insight with me, and it was emblematic of his positive attitude about all things, including problems. He smiled very easily and exuded charm and sincerity. His Irish accent added to this effect. I’m grateful to Declan and Steve for bringing me into the fold of Principal Investigators, giving me an opportunity to learn project leadership. Declan also initiated the nomination process that led to my GSA fellowship last year, an honor I’m quite grateful for. Declan retired recently and moved to the island of Mallorca in the Spanish Mediterranean, and I’m glad he got to enjoy living there for some time before his passing away. My condolences to Carol and his surviving family.
Ron Schott was a giant in geoscience outreach on the internet. He was an early adopter of just about every technology you can think of: Google Earth, GigaPan, Twitter, Google+, geological apps for augmented reality. He was always pushing to innovate for the public good with these technologies, making publically-accessible “Geology Office Hours” on Google hangouts and inventing new geo-ed hashtags like #weatheringWednesday and #btgt (“Been there; GigaPanned that”). He was the king of “Where on Google Earth?” so much so that the players of that game invented “the Schott Rule” in his honor. He was kind and inclusive, encouraging and thoughtful. His omnipresence on geology Twitter was pretty much unmatched. When I announced his death there last week, the outpouring of grief was unprecedented. Ron was unlucky in work: he didn’t get tenure at Fort Hays State University in Kansas, nor at Bakersfield College, but he was absolutely dedicated to public outreach via the web and he stayed in Bakersfield anyhow. The online world was his domain, and he excelled at that work. Nobody did more to push GigaPans on the geology world. Ron had almost 1500 to his name. Though I ended up in the PI position on GEODE, Ron was the pioneer, and in many ways I was following in his footsteps. When I was a GigaPan tenderfoot, Ron connected me at a conference with Gene Cooper of Four Chambers Studio (later to branch out and become GIGAmacro), who also became a GEODE collaborator. Without that connection, I’m not sure what my digital legacy would look like: our team’s GIGAmacro images of geological subjects are one of our major contributions to digital geology. Without Ron, I wonder if it would have happened. Prior to that, I had launched a geology blog aimed at my students, but it was Ron who encouraged me to open up the comments so other people could interact with it. That was huge – if he hadn’t nudged me to have the blog be a conduit for a two-way conversation, I wonder if it ever would have developed into what it became, and I wonder how much of my personal success would never have manifested. It’s an interesting question – how much of a catalyst can one person be for another’s life path? The commemorations I’ve seen play out on Twitter the past few days have shown me that I am not alone in having been seriously and positively influenced by Ron. He has manifested hundreds of other instances of proactive awesomeness, helpfulness and growth among my peers and his followers online. My condolences to Ron’s sisters and surviving family.
Both of these geologists were great people. Both of them will be sorely missed. We are lessened by no longer basking in their glow.
Rest in peace, gentlemen.
1 June 2018
Marli Miller is a senior instructor at the University of Oregon. She is the author of Roadside Geology of Oregon and (with Darrel Cowan) Roadside Geology of Washington. She’s also a very talented geological photographer. She launched a website recently to showcase her work and make it available for instructors: Geology Pics. After chatting with Marli a bit in Flagstaff at the Rocky Mountain / Cordilleran section meetings of GSA a few weeks ago, I got her permission to share a small sample of her lovely fold photos with you here.
Similar folds (passive folding) in marble and quartzite. Rock is of the Plattenkalk Series, Crete, Greece:
Axial planar cleavage and folds in quartzite:
>Proterozoic Banded Iron Formation, folded into asymmetric anticline:
Soft-sediment deformation expressed as folded Pliocene lakebeds of the Coso Formation, SE California:
There are many more awesome fold photos to be seen at Marli’s Geology Pics website, as well as geology themed images on many other subjects. Go check it out and explore!
Thanks for sharing, Marli.