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6 June 2017

Loading sags in homogeneous lithologies?

I was out in the field over the weekend, and saw something new. Readers, I’d be eager to hear what you think of it.

The feature is what appears to be a series of small “ball and pillow” type loading structures (soft sediment deformation), but the thing that’s weird about them is rather than being sand sagging into mud, we have instead silt or fine sand sagging into identical sediment. Check it out; in each case I offer a plain photo followed by a version with the base of the “pillows” highlighted:

Some of these have very compelling shapes. It just seems so bizarre that they could exist in sediments where the lower layer appears to have the same grain size (and thus mechanical properties?) as the upper layer. Where’s the differential viscosity coming from? Perhaps there was a component of seismicity that induced these structures to form through preferential liquefaction of the lower layer? Does that even make sense? Could it be that there used to be a lot more mud here, but it all got squished out of the outcrop through volume loss, and so doesn’t appear in modern cross-sections? The last one I think is most compelling as a traditional case of density inversion, with a slight difference in grain size preserved through the differential weathering profile: beds crisp at the base, then grading slightly upward to the crisp base of the overlying bed. It may be that examination under a microscope would reveal a discernible grain size difference.

It may be relevant that this site is very close (just a few meters above) the mass transport deposit I showcased previously.

Thoughts? (Thanks in advance)

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5 June 2017

3D models of sedimentary structures

I’ve been busy making 3D models lately. Here are three ones united by a theme of being sand that was deposited relative to mud. In one case we have scouring to make flutes, in another case we have have localized sagging to make “ball & pillow” structures, and in the third case we have an extraordinary submarine landslide deposit. For two of them, the shale has been preferentially etched away, leaving only the sandstone. For one, the shale remains (but it’s breaking apart fast!).

In each case, you should be able to spin the models, zoom in and zoom out, and read the information in the annotations.

Here’s a 3D model of flute casts, from a slab on display on the campus of Penn State University:

Question for students: Which way was the current flowing?

A 3D model of load casts in the Hampshire Formation, Corridor H, West Virginia:

Question for students: Is this the top or the bottom of the bed?

And finally a 3D model of an outcrop of uppermost Devonian Spechty Kopf Diamictite on Corridor H, showing a mass transport deposit:

Question for students: Can you find a “jelly rolled” sandstone bed within this mess? What events are implied by this feature?

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1 June 2017

The geology of the Yesnaby coast, Orkney

Ahhhh, a raw Scottish coast.          …Let’s go there.

There’s sedimentology to be learned here, and coastal geomorphology to be ogled.

This is Yesnaby, on the west side of Orkney. The rocks there are part of “the Old Red Sandstone,” a neat package of Devonian strata, flavored here and there with an igneous dike, a fault, etc. But overall, I’d like to focus on the strata.

The strata I saw at Yesnaby last summer came in two varieties: flagstones and dune sandstones. The flagstones are alternating layers of sandstone and shale, deposited in an ancient lake. They are gently dipping:


Link 0.31 Gpx handheld GigaPan by Callan Bentley

Here’s another coastal outcrop:


Link 0.69 Gpx handheld GigaPan by Callan Bentley

Between the packages of “flags,” there is the “Sandwick Fish Bed,”  Fish fossils extracted from these rocks are Eifelian Age (Middle Devonian), and were therefore deposited 398 to 392 million years ago. The images here are from the package below the Sandwick Fish Bed, called the Lower Stromness Flags. This package crops out between the small car parking area and the goal of our visit last summer, Yesnaby Castle.

There’s beautiful sedimentary structures to be seen, like these fine examples of bioturbation – trace fossils etched into the mud and filled in with sand.

On the bedding plane, these burrows appear as semi-cylindrical wiggly-squigglies:

Here’s an example of soft sediment deformation within some of the Lower Stromness Flags’ strata:

Oscillation ripple marks indicate shallow water conditions:

These are (weathered out) mud-chip rip up clasts included within a sandstone:

They suggest a story of changing water energy: calm conditions to deposit the mud, then energetic conditions to rip up the chunks of self-cohesive mud, and bring in the sand to mix them in with.

Further down the coast is a sequence of aeolian strata that are probably the same age, though (because they are coastal dune deposits) they aren’t as fossiliferous and thus aren’t as easy to date. They crop out in dramatic sea stacks and cliffs:

Zooming in, you can see the cross-bedding:

And of course there are nice “storm beach” examples along the tops of these cliffs.

Does it count as a sea stack if there’s a tiny little 2 foot wide “bridge” connecting it to the headland?

This particular sea stack has a dramatic name — Yesnaby Castle — and it has a dramatic story to tell:

Yesnaby Castle in the medium of GigaPan, two views:


Link 0.92 Gpx GigaPan by Callan Bentley


Link 0.37 Gpx GigaPan by Callan Bentley

If you zoom in on it, you’ll find excellent examples of large-scale cross-bedding:

The implication of these structures is a series of coastal dunes adjacent to the lake wherein the Stromness Flags were deposited.

Here’s my field assistant and I setting up the GigaPan to capture this extraordinary feature:

…Thanks for joining us for this virtual day in the field!

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30 May 2017

A Field Guide to Lies, by Daniel J. Levitin

In the library the other day, this book’s title caught my eye. I grabbed it and readily consumed it over the past week. It’s a guide to exercising our best critical thinking skills during a time when our attention is awash in claims both vital and derivative, important and erroneous. How do we tell truth from fiction? Politically, the timing could hardly be more propitious for the release of this volume, but politics is only a very small part of the Field Guide‘s purview. There’s plenty about health and medicine, conspiracy thinking, statistics of risk, and the role of expertise/authority. Levitin lays the book out in three parts: quantitative, qualitative, and the practice of science. It’s clearly written, with plenty of good examples rendered quickly in succession, providing a more robust view of the techniques / ideas under discussion. I appreciated the extended discussion of ways graphics can be manipulated in order to deceive. There’s also a bite-sized primer on formal logical statements and fallacies that result from mangling them. Bayesian probability and how to calculate it also gets a lot of attention, and comes close to being a “theme” that runs through the book. A final section examines four disparate “case studies” where Levitin examines what it makes sense to think or do. Because these case studies are explored in more depth than the examples cited elsewhere in the book, this section has a very different feel to it – it’s not as fast-paced. Overall, I found the book to be a useful read, and would recommend it to anyone who’s not already well versed in critical thinking.

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26 May 2017

Friday fold: Macigno turbidites II

Samuele Papeschi contributes today’s fold photo:

These are

liquefaction-related folds in turbidites of the Macigno Formation sandstones at Cala del Leone, Livorno (Italy).

If that name sounds familiar, that’s because it’s the same formation that gave us another celebrated Friday Fold via Alan Pitts.

Great image, Samuele. Thanks for sharing.

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25 May 2017

Chert-slab conglomerate from the Fig Tree Group, Barberton

A return to the Barberton Greenstone Belt in South Africa today….

Let’s examine a fascinating chert-slab flat-pebble conglomerate in the (Archean aged) Fig Tree Group’s Mapepe Foramtion, exposed at one of the stops along the R40 “GeoTrail”:

These shots show close-up images of the texture of a single 6m thick unit that cuts into (and is overlain by) deep-water ferruginous and tuffaceous shale. It was probably deposited in a single violent moment in geologic time. It may have been triggered by a tsunami, which itself may have been triggered by an earthquake or meteorite impact.

I visited it on a terrific field trip I took prior to the International Geological Congress meeting I attended in Cape Town last summer. The field trip was led by Don Lowe, Chrisoph Heubeck, and Gary Byerly.

The clasts are slab-like in shape: roughly 1:15 axial ratios as exposed on these bedding-perpendicular outcrop surfaces. The largest slab has roughly the dimensions of a family-sized pizza!

The clasts are oriented roughly (paleo-) subhorizontally, though there’s plenty of imbrication to be observed, too.

It’s intriguing to think of the story implied by these clasts: chemical deposition of chert in calm waters below the wave base, for years and years. Then, a sudden violent event (tsunami from a distant meteorite impact??) fractures the chert and triggers a turbidity current of the unsettled debris. This tumbles its bits and pieces down a submarine channel en masse, to be dumped in a huge pile. Wow: sudden enough and violent enough to qualify as downright catastrophic!

And then, it was over, and the debris rained out, and the water cleared, and a more gradual deposition of sediment resumed in its same old uniformitarian way.

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The World’s Religions, by Huston Smith

I’ve just finished an excellent book about religion. It’s a survey of major world religions by Huston Smith, titled straightforwardly The World’s Religions. I find religion to be fascinating. It’s a distinct human phenomenon that provides structure and meaning to so many people’s lives, and yet seems entirely superfluous to my own life. That discrepancy is so strange – it motivates me to understand it better. I found this survey to be an ideal entry point to better grasping the distinct aspects of different “wisdom traditions” (Smith’s phrase) because it’s written with the best of intentions. Smith isn’t a critic of religion, nor a credulous believer with a particular ideological axe to grind. He’s a scholar, and an exceptionally articulate author. His approach is to set aside a lot of what we find spurious or objectionable or lurid about the world’s religions, and examine each in its best manifestation, its highest aspirations. Smith has a charitable perspective, and asks “what drives people to keep practicing such a belief system over time? What motivates the adherents of a particular religion? What do its practitioners get out of it?” A modicum of historical background is presented for each, but the “backstory” is weighted just right – not overwhelming or indulgent. Then he proceeds to examine key aspects of each faith, their particular emphases and varieties. These aspects and characteristics don’t necessarily line up from one tradition to the next, so this isn’t really a comparative religion exercise wherein each faith is examined in the context of a checklist of features, the sort of thing that could be organized in a table. Instead, it’s very cleanly organized into hierarchies of ideas (in easily comprehended clusters of three or four), a unique structure for each unique religion. The focus is on understanding that religion within its own context; how it sees itself, where it places its own emphasis. Reading it, I could tell Smith had a lot of experience teaching, because the book feels like a sequence of the best lectures you ever heard from a religious studies professor, ready-made for the keen college student to translate into systematic notes. The analogies used were evocative and insightful, and I found Smith’s style of posing the faith’s answers to a hypothetical question to be illuminating. Overall, I found its unassuming style to be clean, frank, and obliging. I learned a lot, but upon finishing it, I felt sad it had concluded: I felt like there was more to be understood. I may just have to read it again. Recommended.

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23 May 2017

Silurian tidal flat carbonates of the Tonoloway Formation

In the Valley & Ridge province of eastern West Virginia, my favorite formation is the Silurian-aged Tonoloway Formation. Here’s an outcrop on Corridor H, right on the Grant/Hardy county line, as it appeared a few weeks ago when Alan Pitts and I took a field trip out there:

It’s got some lovely structure, as you can see! However, today, I’d like to ignore the folds (!) and  focus instead on the original depositional setting. The Tonoloway was deposited in the passive margin setting between the late Ordovician Taconian Orogeny (lots of clastics shed into this basin) and the Devonian Acadian Orogeny (lots more clastics shed into this basin). There are no significant clastics in the Tonoloway: it’s instead composed of crystallized seawater! It consists of carbonate rock (limestone and dolostone) that shows numerous indications of shallow water deposition under arid conditions.

Much of the formation looks like this: Super thin laminations of limestone and dolostone:


Link 1.1 Gpx GIGAmacro by Callan Bentley

This may look like the same thing, but it’s got a bonus: a suite of hidden “Easter eggs” to encourage exploration and reflection.


Link 1.1 Gpx GIGAmacro by Callan Bentley

These thin laminations indicate long periods of very low energy deposition, likely in a tidal flat rather than deep water. Why should I say that? There are numerous primary sedimentary structures pointing unequivocally to being deposited at or near sea level.

For example, the carbonate mud shows many fine examples of desiccation cracks.

Here’s a slab with well-developed dessication cracks and a “pebbly” texture on the edges. When you look closely, these little bumps resolve themselves into cubes: they are halite casts, or “fossil salt!”

Another example showing the halite casts, some with a distinctive “hopper” structure, which is the corner of a cube, where the cube’s edges have grown further than the center of each face of the cube. They look like a letter Y.

Here’s another mud-cracked sample, with little oval shapes decorating its surface.

These are ostracode fossils. Ostracodes are arthropods that look something like a shrimp hiding inside two “bean” like shells.

Ostracode fossils:


Link 0.54 Gpx GIGAmacro by Callan Bentley

Snails are often found in shallow water conditions, where they graze on algal and microbial mats.

Here’s a slab showing a mix of gastropod and ostracode fossils:


Link 0.45 Gpx GIGAmacro by Callan Bentley

And then there was this:

I’m pretty sure those are gypsum casts – a different evaporite mineral with a different habit, but entirely congruent with this arid, hypersaline setting.

The views here are cross-sections of the bedding plane: so you can see that there are clear horizons with more of the gypsum casts than others. These likely represent particularly dry times in between wetter times when carbonate was deposited.

Here’s are four GIGAmacro examples of this interesting texture in three hand samples:


Link 1.43 Gpx GIGAmacro by Callan Bentley


Link 0.92 Gpx GIGAmacro by Callan Bentley


Link 0.73 Gpx GIGAmacro by Callan Bentley


Link 1.92 Gpx GIGAmacro by Callan Bentley

In a couple of places in those samples, you can see some rounded mud-chip rip-up clasts, which likely represent periodic high-energy events such as storms. But the dominant feature is that “barbed-wire” looking mass of “fossil gypsum crystals.”

Here is a good example of the gypsum casts on the bedding plane along with some nice mud cracks:


Link 0.67 Gpx GIGAmacro by Callan Bentley

These mud cracks are awesome to behold where well exposed:


Link 1.42 Gpx GigaPan by Callan Bentley

There are also stromatolitic horizons, though they are not as common, and I don’t seem to have any photos of them. Oh well: the rest of it is sufficient to make the case that the Tonoloway represents a period of extended tectonic calm, where sea level was pretty stable and the climate was pretty dry.

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13 May 2017

Slump palimpsest, Corridor H

There’s a section of my favorite road, the lovely nowhere-to-nowhere Corridor H, that seems to be having some issues with slumping. I noted this in November of 2015, and I return to the topic today. Here’s a look at the slope, with old drainage “French drains” installed, and a fresh scarp transecting it just the same:

I see at least three small scarps there.

A short distance further to the east, there’s another example of a fresh (post-treatment) scarp:

This is the one I photographed previously.

Click to enlarge

 

Particularly striking, I think, is where this scarp cuts clean across one of the downslope-oriented drainage structures:

Clearly, this slope has slipped before, then highway engineers attempted to stabilize it, and then it slipped anew.

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11 May 2017

The Seven Hills of Rome: A Geological Tour of the Eternal City, by Grant Heiken, Renato Funiciello, and Donatella de Rita

I’m preparing for some time in Italy this summer, and picked up a couple of books to bring me up to speed geologically. The first is a geological guide for Rome. It’s structured around the archetypal “seven hills” of Rome, but the story is simpler in many regards than some other seven-hilled European capitals I could name. Rome’s geology appears to consist of four major units: older sedimentary rocks (which are Pliocene and Pleistocene, so still quite young in the grand scheme of things), deposits from the ancestral form of the Tiber River which incised and back-filled in alternation, depending on where sea level was, then volcanic deposits (mainly tuff) from nearby volcanic fields, and finally an Anthropocene layer of human debris. This last is unique in my experience – it’s actually a stratum of substantial thickness in some parts of the city, such that some older buildings are now in a “hole” as the city has been slowly built up (literally UP) around them. One hill is composed of almost nothing but olive oil amphorae! Imagine that: making a landscape feature from old bottles! Other interesting tidbits: much of the quarrying of tuff for building stone took place underground, as surface real estate was too valuable for other purposes. This however has resulted in occasional cave-ins of the old emptied-out mine workings below Rome’s surface. Also: the asymmetric damage to the Colosseum that gives it such a unique “look” results from it being built astride a geologic contact between Pleistocene sedimentary rocks and unconsolidated alluvial channel fill. When shaken by an earthquake, the loose sediment amplified the shaking, and thus the damage to the overlying structure: but only the half on top of the old river deposits. Additional portions of the text address travertine, marble, historical flooding on the Tiber River, volcanic and earthquake risk, and the aqueduct system that supplies the city with fresh water. A series of field trip itineraries are described in detail at the end of the book.

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