Geoscience, serendipity & whimsy from Oxford Geology Group…

Curious Nodules

Porth Ceiriad4 Porth Ceiriad6

 

As part of the OGG residential field trip to the Llyn Peninsula we visited the beach at Porth Ceiriad, near Abersoch. We found these interesting looking nodules in the lower Maentwrog Beds  (early Upper Cambrian in age) in a slab at the foot of the cliff.  The nodules appear in grey micaceous sands and silts in which cross-bedding, ripple marks and various trace fossils suggest fairly shallow water at the time of deposition.  A great debate ensued about how the nodules may have formed and we finally came to the conclusion that we were looking at diagenetic concretions, possibly carbonate (Siderite FeCO3).  Unless of course,  dear reader,  you know differently…

 

 

The OGG Facebook team have been very active promoting the Oxford Geology Group activities and the geosciences in general.  We’ve had fun with some regular postings: mystery minerals, thin section teasers and where in the world?  We’ve posted links to journal articles, our own briefings on landslides, fracking, volcanic activity and earthquakes.  The response has been great with some very favourable numbers and marketing metrics for our team to glow with pride about.

One of the nicest things we have been able to do is help high school students with their projects, reports and investigations.  We don’t answer questions for them – this wouldn’t be at all productive.  No, we try to produce some nice graphics, give them some background reading or recommend things they may want to search the internet about.

Here’s three graphics that we have produced recently and they are available to download free of charge at the Oxford Geology Group Facebook page.

This one is on the metamorphosis of shales:

Metamorphic grades.001

This one is on the classification of igneous rocks:

Classification of Igneous Rocks (2).001

and finally this one is all about Bowen’s Reaction Series:

Bowen's Reaction Series.001

 

Obviously we can’t help to this extent with very single request that comes in, but we do our best and have received some delightful messages of thanks.

Add to that a tantalising mix of … gas hydrates, biomineralising algae, stable isotopes and lung disease, Lizardfishes, landslides, Santorini,  seamounts and Stuart McKerrow.

Some explanations –

Stuart McKerrow, known as Mac to his students, was a renowned geologist and palaeontologist who lectured in the Earth Sciences Department of Oxford University from 1947 to 1989, and was often to be found there long after his retirement. He was a founder member and President of the Palaeontological Association and wrote his doctoral thesis on Terebratulacea (Brachiopoda) of the Middle Jurassic.

The McKerrow Cup, named in his honour, with a cheque for £200, was awarded to the winner of the Oxford Young Geoscientist competition held on Saturday 8th March 2014 as part of the Oxford Colloquium Fringe.

Ten doctoral and fourth-year undergraduate students from the Earth Sciences Department competed hard for the cup, each giving a 20-minute presentation of their research project – encompassing the wide range of subjects mentioned above. Their brief was to present their work to an educated but non-specialist audience.

Tim Hedgeland (Student Secretary of OGG) chaired the proceedings and the two judges were Kathryn Kavanagh and Jeff Davies (Chairman of OGG).

Presentations were judged on the following criteria:

Concise, informative and engaging title

Quality, style and legibility of figures

Presence and demeanour of the presenter

Engagement with the audience

Structure and balance of the presentation

Level of the presentation

Content of the presentation

Ability to inspire the audience to want to learn more

Timing of the talk

Having informed and entertained the small but attentive audience, which included Andrew and Graham McKerrow, two of Stuart’s sons, the students waited impatiently while the judges conferred. The high quality of all the talks, as well as the extensive range of subjects, gave the judges a difficult task.

Finally, everyone reassembled, the judges explained the criteria they had used in reaching their decision and made a few general comments, and then Graham McKerrow announced the winner: Harriet Rawson, for her presentation entitled ‘Reconstructing the explosive history of Mocho-Choshuenco volcano, Southern Chile’. Andrew and Graham then presented her with the McKerrow Cup.

Many congratulations, Harriet!

SAM_1641 (1)

Island of Fire

Oxford Geology Group is pleased to announce that Alexandra Witze, Earth and planetary sciences correspondent for Nature, will be talking to Oxford Geology Group on Thursday April 24 about the book she co-authored with Jeff Kanipe: Island on Fire  The extraordinary story of Laki, the volcano that turned Europe dark.

The 1783 eruption of Laki is one of history’s great untold natural disasters. The Icelandic volcano spewed out a poisonous fog for eight months, but its effects lingered across Europe for years. It caused the deaths of people as far away as the Nile, and created famine that may have triggered the French revolution of 1789.

laki-cover-cropped

Island on Fire is the story not only of a volcano but also of the people whose lives it changed, such as the pastor Jón Steingrímsson who witnessed and recorded the events in Iceland. It is the story, too, of modern volcanology and the history and potential of supervolcanoes around the world. And in the wake of the 2010 eruption of Eyjafjallajökull, which closed European air space, it looks at how events might transpire should Laki erupt again in our own time.

“A volcanic tour de force; terrific story-telling,” says British explorer Nick Crane. Like the sound of that?

Tea-Time at Crackington Haven

Crack 1

OGG member, Graham Cunningham’s personal account of a visit he made to Crackington Haven, Cornwall, UK.

We took a one-hour  break during a stroll on the coast of North Cornwall last May.  Crackington Haven is  a delightful spot between Boscastle and Bude  where the tea-shop provides blankets as well as   cakes – both are life-savers.  Now, since I see  geology as essentially rather like beach-combing,  I decided to take a look…

Crack 2

As you can see in the picture facing out-to-sea,  which shows the southern side of the beach,  the rocks  are bedded, and they dip quite steeply to the right,  which is North.  They’re Upper Carboniferous black shales and greywackes – no Coal Measures here.  The greywackes resist erosion better then the shales,  so they form most of what you see.  But what’s this…?

Crack 3

(Limpets for scale).  These are flute-casts,  where small pebbles have scoured out channels  (narrowing to the top left,  so the  current ran in that direction), which are then filled-in by the next sediment to be laid down.  But these flute-casts are upside-down!  The pool, top left, gives an indication of where up is.  The scoured  shale (scoured in the Carboniferous, that is) has been eroded away (Well, not completely.  The lower right of the slab is made of surviving shale, so the flute-casts are not exposed there),  uncovering the later bed  from beneath it.  All these rocks are upside-down! We need to look around.

We’ll turn  180 degrees to face more-or-less East.  It’s an iffy day, so the exposures at the back of the beach  (the shore end of the big headland on the right in the air photo) are not draped with towels/tourists,  so let’s have a look…

Crack 4

Ok,  these are the same black shale and pale quartz-rich greywacke we saw before,  but  the bedding is more or less horizontal, with  little  faults running diagonally lower-left to upper-right. The fault   across the centre is normal  (down-to-the-left).  You can see  the  ‘dragging’ of the greywacke bed at the lower left,   and the same bed re-appears centre right,  assuming that’s the same thin greywacke bed  just below it. Slip on the fault is about a metre.  Now, the top of the  bed I just mentioned  interests me – it   seems to be overlain by  sizeable pebbles, at least the bit on the lower left does.

Crack 4a

Here it is,  a few metres to the right.  Well, they’re  not pebbles. You see the concentric growth on the two nice big ones on  the right? They appear to be manganese nodules.  We’re in very deep water here,  with reducing conditions  encouraging the nodules to grow,  slow deposition of  fine mud (the black shale) and occasional whooshes of  clastic-sediment-transporting turbidity currents to form the sandy greywackes.  So there’s a source of coarser sediment not too far away.  See how the nodules sink slightly into the greywacke, distorting the bedding?

Above the nodules there’s the beginnings of another greywacke,  but it quickly grades upwards from sandy white to black shale.  Graded bedding is an excellent way-up indicator,  with finer sediment always increasing upwards.  And do you see the squiggly  white structures dropping into the spaces between the nodules,  like servings of Mr Whippy ice-cream?  These are called flame structures, and they develop while the sediments are still soft and behave as fluids.  A density inversion occurs when the upper, newly-arrived and  denser, layer tries to sink into the lower one, in this case the mud between the nodules.

So three things – the sinking nodules, graded bedding and the flame structures  –  show us these beds are clearly the right way up.

Crack 5

Further right again,  and we can see some folding.  There are three little anticlines, best  one in the middle,  which are asymmetric with a steep right-hand limb and gentle left-hand.  Two things suggest they are not formed by soft-sediment slumping. First,  the nodules ‘track’ round the fold in the centre – one has been turned to the vertical above the right limb of this fold.  Now nodules need time to develop, but slumping should have happened quite quickly after the greywacke was deposited – contradiction.

Also,  I think I can see a weak fracture-cleavage dipping steeply left and maybe best-developed in the axis of this central fold,  but visible elsewhere  across the same bed.  It may be a bit clearer if you look at the two limbs of the folds and try to imagine the angle-bisector between them.  It won’t be vertical, because the folds are asymmetrical.  The cleavage is an axial-plane  cleavage,  parallel to this direction.  Ignore for the moment the quartz-veins.

 Now,  if these folds  are tectonic,  their asymmetry suggests they may be minor folds on the limb of a larger one.  They can be said  to ‘face’ right,  because their steep limbs are to the right,  and the axial-plane cleavage ‘leans’ that way.  So there would be an anticline to the right.

Crack 6

Slightly fraudulent, that, since all it took was a step or two backwards.  And here’s the larger fold.  The right limb is overturned,  so the teacup scale rests on the bottom of the bed,  and the thin greywacke is no longer underneath, but on top of it.  I reckon you can see at least one mirror-image of the minor folds just under the tea-cup,  and the facing is also upside-down and towards the axis of the fold.  Turn your computer over and see  if you agree.

The shale inside the fold shows nice colour-change bedding layers,  but also, perpendicular to bedding at the fold axis, a well-marked axial-plane cleavage.  Shales do tend to develop a cleavage much more clearly than coarser-grained greywackes.  By the way,  some but not all quartz veins agree with the cleavage to some extent,  again particularly close to the fold axis,  slightly above and right of centre.  Quartz veins can be useful sometimes,  but they tend to do what’s easy, for example following a pre-existing cleavage,  so they need to be treated with care.

 Of course all this tectonic stuff happened later than those sedimentary structures,  but you can see how useful the latter are,  quite apart from telling us a lot about the sedimentary environment in which the beds were formed, in showing us which way is up!  This coast is absolutely classic as a place to find these kind of relationships.

Another step back,  and a sip of tea,  and here’s a fault decapitating the fold, on the right.  Matching across the fault is quite easy here, with the fold as a guide.  Slip again about a metre.

Crack 7

At the bottom centre,  underneath the next greywacke up the sequence,  you can see directional  bottom-structures a bit like the flute casts we saw earlier,  and  these are repeated across the fault  by that little grey boulder on the right.

So the situation we saw at first on the beach is clear.  The upside-down beds over there form the overturned limb of a fold like this,  the horizontal limb having been eroded off.  You can see that since we have only seen two bedding orientations,  one horizontal and one steeply-overturned,  the ‘style’ of this folding is zig-zag,  with only the hinges at all rounded.  This style can be found all along this coast at places like Millook, Bude and Boscastle.

So there we have it –  Devonian and Carboniferous pulling-apart of  continental crust to form a deep sedimentary basin (this one known as the Culm basin),  filling (with interesting sedimentary structures) then closure ( marking the final assembly of the supercontinent Pangaea), resulting in folding,  and finally faulting.  The Variscan orogeny in a few metres of beach.

And tea.

Over the rocks I toiled

A former chairman and current Vice President of Oxford Geology Group, Michael Winterbottom,  tells us how he got involved with OGG.

A few years ago I retired from teaching classics, and looked around for new interests. I was a keen hill walker, and had often felt I should know something of the rocks over which I toiled, and something too of how the beautiful scenery I explored had come into being. I turned first to the University Museum of Natural History, where I found a fine display on Oxfordshire geology. I next found, via the Web, that there was an active Oxford Geology Group. I joined it, and have enjoyed every aspect of its activities ever since.

I tell this personal story to make it clear that the Group is not only for the expert geologist. I have no scientific background whatever, and can contribute only my enthusiasm. But for all my deficiencies the group gave me a heartening welcome. Anyone who is interested in geology, at whatever level, is welcome too.

The Group plays both at home and away. In Oxford it arranges regular talks in the darker months, held in the lecture  theatre   of    the   University   Earth    Sciences Department in South Parks Road. The atmosphere is informal, questions are welcomed, and members are encouraged to talk to the speaker afterwards. Topics are very varied: we have recently learned, for example, about a meteorite strike in Greenland, new mineral species, and dinosaur footprints in Oxfordshire. A wonderful Christmas Lecture is held in the Museum just before Christmas, and a party follows, with food and drink enhanced by the evocative surroundings: to be able to stroll around this wonderful space outside opening hours, with a glass of wine and a slice of Audrey Hayes’ cake, is in itself worth the modest membership fee.

Outside Oxford, we have field trips all year round. These are usually to quarries, stretches of wild coast or other sites of interest within fairly easy reach of the city, but day trips further afield by minibus are regularly arranged. We also promote longer excursions, a weekend or several days; in recent years we have been to the Lake District, Cornwall and Dorset.  Trips abroad are also part of our programme and OGG parties have visited Sicilly, Tenerife, The Auvergne and Ireland. The trips are often attractive to fossil and mineral hunters, but those keen to see new rocks and exciting strata are well catered for. These are friendly and informal occasions; experts are always on hand to answer questions and identify finds.

The Group’s web site (www.ogg.uk.com) will tell you more. It carries accounts of past events and announcements of future ones, as well as contact details for committee members and information on how to join. We hope in particular to attract more young members (one on a recent field trip made far the best haul of fossils); but age, as I have found, is no bar. Come to a meeting or join us on a trip, and try us out!

 

Michael Winterbottom examines the famous Filey Erratic

Michael Winterbottom examines the famous Filey Erratic

Back in Africa

OGG’s Kathy Kavanagh is back in the Democratic Republic of the Congo working as a field geologist.  She takes some time out from geology to file this Despatch about local life…

Ten of us arrived back at our camp in the DR Congo on Tuesday night to find an electrical problem and no lights in our accommodation block or in the dining room, no access to the locked food store or to the inside kitchen, no cook, and everything covered with dust, black flecks from a grass fire and sundry dead insects. Not the warm welcome we would have wished. However, all turned out well and it gives me an opportunity to write a little about local life rather than geology.

Our camp in the northeastern DR Congo

Our camp in the northeastern DR Congo

We were made aware only when we crossed the Uganda–Congo border that there were no kitchen or camp staff on site, only the security guards. We bought ourselves lunch in a local cafe (rice and fish) and some emergency rations for the evening (tinned corned beef, sardines and bread). There are no other options and certainly no M&S meal deals here! On arrival at camp our bulldozer operator sorted out the electrics so we could at least eat in the light and our storeman found us brand new bedding as well as the keys to the showers.

Food, and coffee, are very important to geologists, as you will know from all OGG field trips and georambles. Fortunately, a small jar of coffee had been left in the dining room over the holiday, as well as five teabags and so this, with some leftover bread, served as breakfast. With a mere USD 10 to spend, courtesy of our accounts clerk, some of us went up to Moku, about 15 km away, to buy provisions on the market. Our priority was to be able to make a proper meal for ten increasingly hungry people and to buy more tea and some fruit. In fact by the end of the day there were 17 of us as some of the local staff started drifting in.

Moku market

Moku market

K3

Moku market: vegetable stall

Moku market: buying cooking oil – measured out with a cup and tomato paste tin into a Coca Cola bottle before being poured into a plastic bag

Moku market: buying cooking oil – measured out with a cup and tomato paste tin into a Coca Cola bottle before being poured into a plastic bag

Before setting out we had found some meat in the freezer – after picking the lock! So the menu for the day was:

Before setting out we had found some meat in the freezer – after picking the lock! So the menu for the day was:

K7

Ugali is maize flour cooked with water to make a stiff, sticky paste and fashioned into a ball. A small piece is broken off, rounded, dipped in a sauce and eaten with the fingers. Pondu is made of pounded cassava (manioc) leaves, with garlic, salt and palm oil added.

After we returned from Moku everyone was asked to help with the cleaning or cooking. The three (male) junior geologists cleaned the dining room and a geotechnician set the tables, I cleaned the outside kitchen and washed the dusty dishes and utensils, our other senior geologist managed the urn to provide us with a regular supply of hot water and carried dishes to and fro, and Jeanne, the accounts clerk, began preparing the food. Two local staff pounded the cassava leaves for the pondu while another cleaned our rooms.

Pounding cassava leaves for pondu; Jeanne adds the garlic

Pounding cassava leaves for pondu; Jeanne adds the garlic

Jeanne preparing the goat stew; the rice is already cooking on the second brazier

Jeanne preparing the goat stew; the rice is already cooking on the second brazier

By 4.30 pm a wonderful aroma drew everybody to the dining room. The normal hierarchical seating arrangements were abandoned, plates were piled high and Jeanne’s cooking highly complimented. Everyone cleared away their own plate and some kind people cleaned the tables and laid them for breakfast while several of us very cheerfully washed up.

In the evening another grass fire in the adjacent field narrowly missed our camp, and the fuel supply. It is the dry season and grass and bush burn rapidly. Our firebreak will be widened! At 10.30pm a food delivery truck arrived from Uganda, with the restaurant manager and one of our cooks. So today, unfortunately, they prepared the food and the rest of us had to go back to our computers and regular jobs.

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