27 January 2015
Today, It’s going to get a bit coarser.
There’s a nice roadcut in Miette gritstone (coarse sandstone and mudrock with rounded pebble conglomerate) on the Trans-Canada Highway just east of the British Columbia / Alberta border. Aaron Barth and I GigaPanned it last summer:
Here’s the resulting image:
As you explore it, you will get plenty of good looks at the “grit”:
There is also a nice example or two of graded bedding, a sedimentary structure with a gradational change in grain size, bottom to top through a bed:
Elsewhere, you can see two other striking features:
- mudchip rip-up clasts (dark, angular chunks), and
- arcuate-bottomed contacts between coarser upper layers and finer lower layers:
Let’s consider the mudchips first:
Mud is made out of small particles, mainly clay. As such, it tends to remain in suspension in the water column if the water is agitated (energetic). It only gets deposited if the water is really calm. As such, these mudchips tell us about calm-water deposition.
But they are not mud layers, they are chunks of mud, and they are angular, and they are surrounded by “grit”…
Clay is more-self-cohesive than sand (dominated by quartz, say). When a pre-existing deposit of mud gets blasted by a powerful current (one capable of transporting pebbles, for instance), the mud rips up in flakey chunks. These pliable chunks then become sedimentary particles in their own right, tumbled along with the new stuff (whatever this interloper current dragged in).
When the current wanes in energy, the mudchips and pebbles drop out together, a mixed deposit:
Next, consider how the arcuate (concave-up) shape of the cross-bedding is mimicked along the contact between lower (earlier) finer-grained strata and upper (younger) coarser-grained strata, like right here:
I interpret that as evidence of scouring in the depositional environment. As with the mudchip rip-ups, the story is one of changing water current strength over time. At one point, a moderate current deposited finer sand, and then the energy of the water got jacked up (a storm, an avulsing river) and the water switched from depositing to eroding these easy-to-move smaller grains. As the current energy dropped again, the bigger stuff it was carrying filled in the hole that had been eroded in the older deposit.
In other words, these are channel forms, trough-like scours into the pre-existing sedimentary deposits, then filled with the detritus that the scouring current carried in.
An animated GIF can impart some perspective on how this arcuate contact weathers out in three dimensions:
A closer look at the bottom of the incision:
There is also evidence in the outcrop of directional currents in the depositional environment. Here are several nice examples of cross-bedding:
In this case, the water currents were moving from the right side of the photo toward the left, depositing sand in the slightly-less-energetic waters downstream of a migrating ripple (bedform).
Lastly, the Miette shows in some places the presence of “outsized clasts,” really big chunks mixed in with the relatively fine “grit.” Here’s an example, from about two miles east of the site above, right near the start of the Icefields Parkway:
It’s hard to tell a story of waxing and waning currents of water when something like that is staring you in the face.
These outsized clasts (boulders, essentially) are so much bigger than the rest of the deposit that it’s hard to imagine how they could co-exist without multiple factors influencing the deposition. They could potentially be interpreted as “dropstones,” which are iceberg-rafted chunks. Glacial ice is capable of supporting much larger particles than liquid water, and when a glacier calves, its icebergs float away with an internal sedimentary load that could include big boulders. When the iceberg melts, the boulder drops into the (water-deposited) finer sediment below. Because of its different mode of transport, it fails to “match” the character of the local sedimentary deposit.
These putative dropstones could be evidence of the Snowball Earth glaciations, which froze a significant portion of Earth’s surface during the Neoproterozoic.
23 January 2015
Elizabeth Kosters contributed this week’s Friday fold:
It’s from Rainy Cove, Nova Scotia, Canada. Click through to read Elizabeth’s post on the site.
22 January 2015
One last post from my September trip to Greece. Here’s a look north along the inside wall of the central caldera of Santorini, taken from the deck of the Santos Winery.
It’s not hard to imagine the volcanic edifice that filled the space to the left (west) prior to The Big One.
21 January 2015
Three images, working our way in from outcrop setting to hand sample:
These fine red sandstones are the Silurian-aged Bloomsburg Formation, as it crops out in Fort Valley, Virginia.
What would we see if we kept zooming in?
20 January 2015
As a follow-up to my post about the geology of the Acropolis klippe in Athens, Greece, and in the spirit of my post on the building stones of the Haghia Sophia in İstabul, Turkey, let’s turn our attention today to the various rocks that ancient Greeks used to construct the buildings of the Acropolis, such as the Parthenon.
When we went to Greece in September, we didn’t just look at the Acropolis from afar, we wandered up into it.
Behind my wife and son in the previous image you can see two distinct rock types – a lower limestone and an upper marble. The limestone has little nooks and crannies along the bedding plane, some highlighted with dark soot and dirt. The marble lacks this.
Old marble and new, as the Parthenon is repaired:
Stylolites in the limestone were preferential sites of dissolution weathering in some examples:
A close-up series of three looks at the lower limestone building stone reveals it is chock full of bivalve fossils:
The marble, on the other hand, is slightly impure, resulting in blocks like this one:
That’s chlorite-rich schist interlayered with the calcite of the marble:
The “front” side of this block is more or less the plane of foliation.
There were also exotic rock types beyond the two I’ve mentioned, such as the pillars laid out here:
The closest one to the photographer’s vantage point is the distinctive green breccia:
…I reported seeing that at the Haghia Sophia, too.
One final look, at the limestone (foreground) and the marble (background):
17 January 2015
The astronaut Sam Cristoforetti, currently aboard the International Space Station, tweeted this photo today:
It shows her view of Athens, Greece. Zooming in, you can see the dense urban sprawl of Athens filling a broad valley between several mountain ranges. Within this valley, several hills poke up like islands in a sea. How many do you count? Athens is one of many cities that claim to be built on or around seven hills, and I’d say the number is approximately seven. One of these hills is known as the Acropolis (Greek: Ακρόπολη).
You know the Acropolis – it’s that distinctive hill with the ruins of the Parthenon sitting atop it:
So what’s the geologic story here? Why is the Acropolis a hill?
Let’s turn to the geologic map of Athens by Gaïtanakis (1982), zooming in on the portion surrounding the Acropolis:
Dark green here is Cretaceous-aged Tourkovounia Formation. Light green is a mix of marl and sandstone known (counter-intutitively) as the Athens “Schist.” It is not a true schist in the sense that the term is used in modern geology – a medium-grade metamorphic rock with visible crystals (including perhaps porphyroblasts) but lacking gneissic banding. Instead, these rocks are very lightly metamorphosed sedimentary rocks. The Athens Schist is also Cretaceous in age, and perhaps parts of it are younger than that. Everything else on the map (the blues, tans, yellows, and circles) is Quarternary sediment of several varieties. I don’t care about that stuff; I’m focused on the bedrock – those two shades of green.
Now, what is the relationship between these two units? Is the contact conformable, unconformable, intrusive, structural, or what?
Consider these two cross-sections, the first Gaïtanakis’ map, and the second from a book by Higgins & Higgins (1976):
I’m really indebted to the article The geology of the Acropolis (Athens, Greece) by M. Regueiro, M. Stamatakis, & K. Laskaridis from a couple months ago for linking me in to these resources, which I would have had a much harder time finding on my own.
The limestone capping the Acropolis is the Cretaceous-aged (specifically Cenomanian-Turonian) Tourkovounia Formation. The layer beneath that, the Athens “Schist,” is Cretaceous too, but here’s the neat thing: it’s Maastrichtian to Eocene (?) in age. The age difference is estimated to be 30 million years. Consider this graphically, from the International Commission on Stratigraphy’s time scale:
In other words, the upper rock layer is older than the lower rock layer, a perversion of the principle of superposition. Note the “cataclastic limestone” (dark blue) in the Higgins & Higgins cross-section. Cataclasite is a fancy name for “fault breccia:” the crushed up rock that forms along a fault zone: this is a clue as to how superposition was subverted.
Consider this sunset shot of the hill from our hotel when we visited Athens in September, and the ensuing annotations:
The limestone capping the Acropolis is older than the marl + sandstone at the base of the hill (the “Schist”). It moved to its present position from an original position some distance away. It moved there along a break in the rock: a horizontal fault, often called a “thrust fault.” Hence, the hilltop is “allochthonous” (translation: “not from ’round these parts”). Meanwhile the base of the hill is more or less where it’s been ever since it was deposited, a condition graced with the appellation “autochthonous” (translation: “local yokel”).
Because the sandstone / marl of the Athens Schist is more susceptible to weathering in the arid climate of Greece than is the Tourkovounia limestone cap, the arrangement is expressed as a hill. The hill used to be larger, but is being nibbled away over time from the sides. It’s an erosional remnant of a much larger thrust sheet.
Here’s a Google Earth “3D buildings” view that I modified a bit in Photoshop to emphasize the topography of the hill, as well as our digs in Athens, and the take-home message of this blog post:
A klippe is an erosional remnant of a thrust sheet. Chief Mountain, Montana, is a classic example:
Of course, there are some differences between the two klippes…
(That reminds me of the Mountain Beltway banner… but older both above and below!)
Anyhow, here’s a zoomed-in look at the limestone cataclasite at the Acropolis along the fault zone:
Some big and small clasts in there. And a little guy for scale!
Tectonically, this faulting (presumed to be Eocene in age) is related to the compressional tectonics that are causing Africa and Europe to collide, closing the Mediterranean Sea (née Tethys Ocean) and building the Alps.
Here’s a look along strike of the trace of the fault zone, out across eastern Athens:
Here is the street west of our hotel, where the Athens “Schist” crops out:
What follows next are five field photos of this uninspiring unit:
One more detail: Sometime after thrust faulting, the contact (fault zone) between the Acropolis’ upper, older limestone and its lower, younger “Schist” was itself faulted. Let’s zoom into this little exposure of the contact, just below the cliff-forming limestone but above the tree line:
See how the contact there is jagged? That’s due to a number of low-offset block faults (~normal faults):
Obviously, this would have to be more recent than the thrust-faulting, since it cross-cuts the main thrust.
So, to summarize, the recipe for one Acropolis is:
- Deposit limestone in the late Cretaceous.
- 30 million years later, deposit sandstone and marl.
- In the Eocene, shove the limestone up and over the younger strata, shattering the bottom of the limestone in the process.
- Break up the fault with some small-offset normal faults.
- Differentially erode the landscape.
- Add people, who:
- build the Parthenon, and
- misname the sandstone and marl as “schist.”
Pretty cool story overall, eh?
“…Yep, dad. Pretty cool.”
Thanks to “Astro Sam” for posting the ISS photo this morning and inspiring me to finally write this post!
References utilized and cited in the preparation of this blog post:
Gaïtanakis, P. 1982. Geological Map of Greece 1:50.000. Athinai-Pireus sheet. Institute of Geology and Mineral Exploration, Greece.
Higgins, M. & Higgins, R. 1976. A Geological Companion to Greece and the Aegean. London, Gerald Duckwort & Co Ltd.
M. Regueiro, M. Stamatakis, & K. Laskaridis. 2014. “The geology of the Acropolis (Athens, Greece).” European Geologist, v. 38, November 2014.
16 January 2015
Zoltán Sylvester has contributed today’s Friday fold, an anticline in the Ross Sandstone of Ireland:
Click image to go to the source (full sized).
Thanks Zoltán! Happy Friday everyone.
13 January 2015
Check out the argillite boulder in the left midground of this GigaPan, which I’ve showed here before. It was taken at the Icefields Center parking area in Jasper National Park, Alberta:
There, you’ll find some lovely orange lichens, some iron oxide staining, some graffiti, and a fair number of sub-aligned glacial striations.
Also, at the top edge of the boulder, there’s a nice set of big hackles, running along the crest like the knobby backbone of a sea monster:
A wealth of lovely detail to be explored…
12 January 2015
Another pair of shots of the Ferrar mafic intrusives from Antarctica, courtesy of Lauren Michel…
Zooming in more…
There are some major disruptions to the strata – I wonder if the story is more complicated than my simple annotation suggests…
9 January 2015
Along Going-to-the-Sun Road in Glacier National Park, you can see this outcrop of Mesoproterozoic Grinnell Formation quartzite (former quartz sandstone) and red argillite (oxidized mud rock). These strata are part of the Belt Supergroup.
It proudly displays an asymmetric / overturned anticline.
The outer arc of the hinge of this anticline shows extensional fractures in the red argillite “coating”:
Those photos were taken by Tom Biggs (UVA) in 2013. Here’s a shot I took in 2010 of the same fold, showing its cross-sectional profile:
Note the primary sedimentary structures: the ripples (bedform) and their guts (cross-bedding), as well as evidence of fluctuating current strength (mudchip rip-up clasts “suspended” in quartzite):
I look forward to revisiting this fold in July 2015 as part of my Rockies field course (for which, by the way, applications are now open).