It’s accretionary wedge time again an this month Dino Jim is asking us to ‘think outside the box’ when teaching geology. I seem to detect a food theme developing.

Here is a quick piece on using a banana as an analogue for rock deformation in general, and fault propagation folding in particular.

First take your banana and peel it.

Banana: Undeformed

Grasp an end in each hand leaving at least the central third free. Slowly move your hands towards each other.

Initially the banana will deform ductilely, and actually thicken. After the initial thickening, the banana will start to fold.

Banana: Thickening and Fold Initiation

As the fold develops into an anticline-syncline pair, note the extra compression on the inside of the folds generating buckling and extension on the outside of the folds generating tension cracking. If you look closely you can also see shearing starting to develop in the central limb between the two folds.

Banana: Folding developing

Deformation switches from ductile to brittle as shear failure through the central limb generates a thrust fault separating the hangingwall anticline from the footwall syncline.

Banana: fault propagation fold

And here is the real thing for comparison…

Broadhaven, Pembrokeshire fault propagation fold

You can see a gigapan and photosynth version of this structure in my previous blog post here.

Note: this isn’t my idea, I picked it up from Prof. Patrick James, Head of the School of Natural and Built Environments at the University of South Australia at a teaching and learning in geology conference.

 

Volcanista is hosting July’s Accretionary Wedge on what inspired one to become a geologist. For me, the reason is simple, if somewhat convoluted. I became a geologist because I can’t sing.

Back when I was in high school, my school, a ‘bog-standard’ comprehensive, reckoned it had a really good school choir. It didn’t, of course, but the important part to this story is that it thought it had. One day, the whole of my year were lined up in the school’s assembly hall and told to sing hymns. Teachers walked up and down the rows of us pupils listening to the ‘singing’. If they thought you had a modicum of singling ability they would tap you on the shoulder and you were in the choir – no arguments. As anyone who knows me well will testify, I can’t sing for toffee. My better half (who can sing very well) bans me from even trying to sing along to the radio in the car. It was no surprise then, that my shoulder remained untapped, I was barred from the school choir, and the rest, as they say, is history.

Those of us who were choristercally challenged then had to be given things to keep us occupied during the two-hour a week choir practice sessions. We were given a variety of things in six week blocks, I can remember model making, drama, and gaming – and then there was an introduction to rocks and geology. I found it moderately interesting, but I can’t say I was blown away by it at the time.

Later on, I reached the point at which I had to choose the subjects I studied for GCE ‘O-level’. Some choices were straight forward, e.g. Maths, English, Physics, Chemistry, Geography. I didn’t like biology, I was squeamish about dissections, so I didn’t choose that. For languages I chose French and German, the latter being a big regret and in hindsight I should have done Spanish as it would have been much more useful to me.

There was then one subject I had left to choose from a list of less common subjects. The one I wanted to do was Technical Drawing. This was largely because my father was an engineering draughtsman. When I handed my selection form in, my teacher told me that I should probably choose something else as, as well as being useless at singing, I was rubbish at art. While this is true I do think that technical drawing is a different skill and I like to think that I was/am reasonably good at it. It certainly helped later on with geological map making as an undergraduate and producing diagrams for my Ph.D. thesis in the pre-Corel Draw days. Anyway, I took my teacher’s advice and looked for another topic. Geology was on the list, and having done a bit of it instead of choir practice, I chose to do that.

I got on reasonably well with geology. I had my first geology field course to the Isle of Wight. However, it was combined with geography and I’m still mentally scarred from having to do Human Geography questionnaires.

Whilst geology ‘O-level’ was interesting, my main interest at this stage was Physics. After high school I went to my home-town’s sixth-form college and again was faced with choosing subjects to study, this time for GCE ‘A-level’. Again, a couple of subjects were straight forward choices – Maths and Physics. The big dilemma was between Chemistry and Geology. I had probably enjoyed chemistry as much as geology at O-level. My chemistry teacher was a pyromaniac and most lessons seemed to involve explosions. Whilst this had been fun for a teenage lad, I didn’t feel I’d actually learned that much and I was starting to struggle a bit with some of it. So, after much heart searching, I chose Geology, and it was one of the best decisions that I have ever made.

At sixth-form induction I was asked if I wanted to do four A-levels. Great, I thought, I can do both geology and chemistry. Unfortunately, what they had in mind was to do two Maths A-levels. So I ended up doing Pure Maths, Applied Maths, Physics and Geology. In Maths, it was the hardest I have ever worked. I was studying with some seriously bright people and it was really hard to keep up. Physics was hugely enjoyable and if it wasn’t for geology I would have probably become a physicist.

However, geology was truly inspirational. This was due to two reasons. First and foremost, was that we had a superb and inspiring teacher in Fran Stratton who really brought the subject alive. She even put on additional classes for some of us so we could take the more advanced ‘S-level’ as well A-level. Second, was the group of my fellow students. We were only a small cohort of nine students (three of us called Ian) and we got on very well. We were allowed to hang out in the geology laboratory at lunch times. We honed our rock identification skills by selecting a sample from the drawers at random, tossing them across the room to colleague (including an asbestos sample I remember) and asking them to identify it. I also spent many hours in the school’s geological map library, learning to read them and draw cross sections. Geological maps still fascinate me today. I had my first proper field course – to Derbyshire which also included my first (underage) pint of beer. I was hooked (on both).

So, my allegiances switched from physics to geology. When choosing a degree course to study I went for geology, but a course where I could specialise in geophysics, bringing the two threads together. Although I now teach geophysics at university, I still regard myself much more of a geologist than a physicist.

And all because I can’t sing.

 

Lockwood over at Outside The Interzone is successfully ensuring that the Acccretionary Wedge is rising phoenix-like from the ashes. The mission for this month is … “Where and when would you most like to visit to witness and analyse an event in Earth’s history?”.

I suppose that like most geologists I’d like to visit the K/Pg boundary to see if it was the volcanoes or the asteroid/comet ‘wot dun it’ but leaving aside the obvious I’d like to travel back to the Lower Cambrian to answer the question – “Why is this rock red?”

Caerfai Bay Shales

These are the Caerfai Bay Shales, from their type locality in Caerfai (pronounced care-vahy) Bay, Pembrokeshire, South Wales. The Lower Cambrian was actually known as the Caerfai until International Commission on Stratigraphy got their grubby hands on it. These siltstones and mudstones are marine, they contain bioturbation and the very occasional ostracode Indiana lentiformis fossils. So why are they red? This colour is characteristic of haematite, with oxidised iron, typical of oxidising terrestrial environments. Marine silts tend to have reduced iron and are greenish coloured.

The story I was spun twenty years ago was that the haematite was detrital, from the erosion of lateritic soils. The major problem with this is that Wales in the Lower Cambrian was at a latitude about 70°S, well out of the tropical weathering zones.

More recent work would indicate that the haematite is of biogenic origin but this in turn means that something very strange is happening to the ocean water chemistry at the time. Not only must the sea water become very iron rich but we are also at the time when there is a major change from high magnesium concentrations to low magnesium. Prior to this was an ‘Aragonite Sea’ with the primary organic carbonate precipitates being aragonite and high-magnesium calcite, subsequently it is a ‘Calcite Sea’ of low-magnesium calcite.

So, we have a major change in sea water chemistry. We also have the ‘Cambrian Explosion’ occurring at precisely that same time. Up to this point in the Cambrian we have the ‘small shelly fauna’ but it is now that the triboltes and echinoderms kick in. This surely cannot be a coincidence.

So I’d like to take my time machine back to the Lower Cambrian to find out just what is causing the chemistries of our oceans to change as it may well have had a very significant influence on life on Earth.

 

Gary Hayes at Geotripper for accretionary wedge #16 asks …

“What are the places and events that you think should all geologists should see and experience before they die? What are the places you know and love that best exemplify geological principles and processes?”

He also wants …

“a truly international list. I also want to get a list of those places that don’t always make the “must visit” lists. And why should this place be included?”

I also threatened to produce a list for 10 sites in the UK that I think are better than a pile of cracked basalt that is a UNESCO world heritage site.

Here are 10 UK sites with some brief reasons that I think are worthy of consideration and I intend to produce lengthier posts on some of these in the near future.

1. The Pembrokeshire Coastline, West Wales
This is a superb place to teach geology, where the Caledonian and Variscan orogenies meet. There is a pretty much complete section from Upper Precambrian to Upper Carboniferous. So many places to choose from but as a taster here is probably the best fault propagation fold in the world at Broadhaven.

scans006

2. The Jurassic Coast.

The UK’s other World Heritage Site and probably far more deserving than the Giant’s Causeway. The Dorset to East Devon coast actually covers Triassic through to Cretaceous. Again so many places to chose here including Lyme Regis where Mary Anning, ‘the greatest fossilist the world ever knew’, went ichthyosaur hunting. However, as a structural geologist I’d have to go for the Alpine deformation at Lulworth Cove, Stair Hole and the nearby fossil forest and broken beds (pictured).

scans014

3. Siccar Point, Berwickshire, Scotland
One of the most important sites in British historical geology. The angular unconformity between Devonian Old Red Sandstone and Silurian greywackes was visited by James Hutton in 1788. John Playfair later commented about the experience, “the mind seemed to grow giddy by looking so far into the abyss of time”. This unconformity, together with other Hutton’s unconformities at Newton Point, Arran and at Inchbonny, Jedburgh convinced Hutton of the concept of deep geological time.

4. Eriboll, Sutherland, Scotland
The Moine Thrust at Eriboll is where the term ‘thrust’ was first coined by Geikie in 1884. Here are excellent exposures of thrusts and related folding. Late Proterozoic Moine metasediments together with their basement of Lewisian gneisses are thrust over Cambro-Ordovician sediments that are highly imbricated.

5. Glen Sligachan, Skye, Scotland
The walk from Camasunary, over the ‘bad step’ above the sea, up Glen Sligachan, climbing Harker’s Gully to the top of Marscoe is simply one of the greatest geological walks anywhere . It talks one from the cumulates at the bottom of the magma chamber, past the feeder dykes with evidence of magma mixing and into the volcanics at the top.

6. The Ercall, Shropshire, England
In the heart of England lies Ercall quarries which exposes the Precambrian-Lower Cambrian unconformity. Uriconian volcanics are intruded by dolerite dykes the Ercall Granophyre at the end of the Precambrian. The granophyre shows a weathered upper surface and is overlain by a beach breccio-conglomerate, shallow marine quartz-arenite and glauconitic siltstone as sea-level rises. [A very similar sequence is is seen in Pembrokeshire - see #1]

scans040

7. Kilve, Somerset, England
The beach at Kilve is cut by a series of normal faults with displacements varying strongly along their length which causes folding of the strata. A brick-built oil retort house is believed to be the world’s first structure built for the extraction of oil from shale.

grgegbgh007

8. Wren’s Nest Dudley, West Midlands, England
The UK’s first ever geological National Nature Reserve. An area of formerly quarried and underground mined Silurian limestones. Exceptional fossil fauna including the ‘Dudley Bug’ (Calymene blumenbachi).

9. Isle of Mull, Scotland
2km of lavas intruded by both acid and basic intrusions. The Loch Ba Ring dyke has been described as ‘the finest ring dyke known to science’ and includes the Loch Ba Felsite which is a classic example of a mixed intrusion with acid igneous rock containing about 15% basic inclusions.

mull

10. West Runton, Norfolk, England.
Ice deformed glacial sediments including thrust rafts of basement Chalk. Glaciers can produce thrust tectonics too.

And apologies to all the great geology of the British Isles I’ve omitted, from the Shetlands in the north to Cornwall in the south. So much to choose from. I’m sure I’ve missed something important…

Google Earth KMZ file of localities here.


View Accretionary wedge #16 in a larger map

 

What is your favourite place to do field work?” is the question Dave Schumaker at Geology News is asking as last this month’s Accretionary Wedge.

I am extremely lucky having a job that allows me out into the field occasionally, even if at the minute it is just down the road. In my top five I would have to include the Atacama Desert of Chile, Iceland, Colorado and the Alps but at number one has to be the Spanish Pyrenees.


View Larger Map

I’ve been many times, as a postgraduate demonstrator and lecturer on undergraduate field courses and twice as a field assistant to a Ph.D. student. However, I’ve not been back in a long time, so apologies in advance for the scans of twenty year old slides.

The Spanish Pyrenees is a classic place to teach geology. The Spanish side (unlike the French side) is arid so there is excellent exposure, and, unlike the Alps, they are not too high and much of the geology is accessible from the roadside (with a suitable loose definition of road).

The geology shows a superb interrelationship of sedimentation and tectonics with the sediments eroded from the high Pyrenees being progressively deformed as the mountain front advances.

Riglos

Riglos 2

Turbidites

The variety of geology is also stunning from vertical bedding to trace fossils.

Vertical bedding

trace fossils

The local cuisine is excellent with chorizo, pyrenean mountain cheese and local wild boar washed down vino tinto. The only exception to this is breakfast (sweet cake is not my favourite at the best of times) but we did manage to train a cafe owner in Jaca in the art of bacon and egg butty making.

Of all the places in the Spanish Pyrenees I think my favourite has to be the Ordesa National Park. From the olistostromes at Torla to the climb up to Monte Perdido is the most spectacular walk I have ever done. The views from the top are absolutely breathtaking.

cylindro

ordesa

One day I shall return.

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