Dotternhausen and the Posidonia Schieffer

While on my third year residential fieldtrip to Southern Germany we visited to early Jurassic strata known as the Posidonia schieffer (shale). The rock here is somewhat akin to the Blue Lias of Lyme Regis and Charmouth, however the rock here is much more uniform and does not feature Milankovitch Cycles. This particular outcrop was in a quarry just outside Dotternhausen.

Everyone hard at work counting ammonites, you can see the
enthusiasm in the picture.

The shale was deposited under anoxic marine conditions, the sea floor would have been a soupy mud that supported no benthic fauna. This is perfect for exceptional preservation. Ammonites, crinoids, ichthyosaurs, pterosaurs, sharks and fish all would fall into this mud and sink. Due to the lack of oxygen aerobic bacteria would not be present and therefore decay would be inhibited. This leads to the preservation and discovery of fossil logs with crinoids attached and ichthyosaurs with skin outlines and embryos in the womb, just to name some examples.

The morning was spent at the Werkforum Museum at the cement works in Dotternhausen. Here we got a brief background to the fossils found in the quarry and what the environment would have been like 185 Ma.

So while visiting the Dotternhausen Quarry it was to be expected that as a cohort of 20 students we should find something between us.

Dactylioceras in one of many sheets of split shale.

Phylloceras from yet another sheet of shale.

Our first task was to collect and tabulate the number of ammonites with epibionts living on them. Myself and three of my close colleagues set to work splitting shale "sheets" and counting every ammonite in sight. Here I had my first find, a beautifully preserved fish fin, encircled by the disarticulated 'horseshore' structure of a Lytoceras ammonite.

The well preserved fish fin with the Lytoceras horseshoe
in the top of the picture.

I would have been happy to come away from the entire field trip with just this one find. But a few layers down, I come across a very small bone, about a centimetre or so in diameter. What we know is this is an Ichthyosaur vertebra. What we believe is that it is a tail vertebra of a juvenile because of how small it is. Unfortunately this was an isolated bone. Fortunately, it doesn't need any mechanical preparation as it is already well presented on the slab of shale it came from. Already I have had more success here in an hour than I have in three years of fossil hunting across numerous sites on the south coast of England.

The small tail vertebra from the ammonite exercise.

The small Ichthyosaur vertebra, in need of a
little treatment to protect it.

An hour or two after we arrived to the quarry we had all just about finished the exercise, and not a moment too soon with the day only getting hotter and hotter! And so, true to form with this class, we enthusiastically scrabbled over freshly blasted rock from the quarry wall in search of the fossils we had seen in the museum that morning.

I chose to split larger blocks bit by bit in the hope that there would be a bone or two preserved inside. A few blocks in, I start on one particular piece and put it on its side and begin hammering. It split a little too easy and at an odd angle, revealing a line of bones in the piece that had come away. Turning it over, I found that there was a line of vertebrae, criss-crossed by slender ribs. This was a disarticulated Ichthyosaur. Just on this block there was around 15 vertebrae. Definitely not complete but certainly exciting to find! It took three of us to move this block out so that the rest of the class could see what had been found.

The first block to be found in the quarry, with
some of the offcuts to the right.

The two main blocks and offcuts that we
managed to find and bring back to the
UK for preparation.

Meanwhile, the search continued for the rest of the animal. After shifting some rock, another block with a single vertebra and some definition of ribs was found. Still not complete but unfortunately there was no more of the Ichthyosaur to find. At first, I thought that the museum, or at least the cement works, would want this find for themselves. Some quarry owners in the UK confiscate fossils and sell them as profit, I assumed this was the same here. But, I was told that I would be allowed to keep them and bring them back to the UK to prepare the bones myself.

Side view of one offcut that thankfully fits back onto the
rounded block quite nicely

The two blocks are jam packed with bones, still not complete unfortunately. I believe that the centre of the skeleton is preserved, the tail and head unfortunately missing, possibly eaten by a much larger Ichthyosaur.

There's always one piece left over, no idea how this fits onto
either block. But it'll still make a nice addition to my collection.

I spoke to Professor Dave Martill about why the bones are scattered in the block as opposed to being articulated like the specimens we have seen before. He said that this is probably due to the final resting position of the animal in the Jurassic. It may well have come to rest on its ventral side and not be completely buried in the sediment, therefore decay would have taken place. Therefore you know have vertebrae that are elevated above the sediment and becoming loose due to the decay process. They will begin to fall out and land on the sediment in seemingly more random orientations. The same is true for the ribs.

The local museum has very kindly allowed me to use their equipment to prepare this find. Work will begin on the 27th June. I plan to upload nightly on the progress of the day even if it is just a photograph of what has been revealed so far. Needless to say, I'm very excited!

Kimmeridge Bay and Etches' Collection

In this series I want to share three residential fieldtrips that I went on during my time at University. In these I will discuss the geology and palaeontology of the site and also if anything interesting was found. I won't be discussing every site we visited either because there were a lot of small ones or there just isn't too much to say about a particular site. Any information has come directly from my notes made in the field.

At the start of the second year we spent a week based in Swanage, Dorset. Here we were being trained to construct accurate geological maps of the coastline between Bat's Head and Mupe Bay, this included the world famous Durdle Door and Lulworth Cove. We also had the chance to visit a number of sites that yield some particularly interesting fossils.

Unfortunately I haven't got any photographs of the localities (any photographs will be credited in the caption). The finds were a little scarce, I managed to collect an echinoid spine, bivalve and Perisphinctid ammonite from Black Head, near Osmington Mills (I'll photograph these for when I do a piece on my collection). There was a single fragment of dinosaur bone found in an outcrop of the Wessex Formation at Dungy Head and some large ammonite fragments from the uplifted Kimmeridge Clay at the same locality. Also 2 Lepidotes scales were found in a mudslide in the Wessex Formation at Lulworth Cove.

Kimmeridge Bay and the Etches Collection:

Map of East Dorset showing the location of Kimmeridge Bay (Source: Google Maps)

The wave cut platform at Kimmeridge Bay showing the
 localised thrust structures, these are found in the east
of the bay where the cliff debris and beach material has been
removed. (Source:
https://www.geoexpro.com/articles/2009/06/
where-does-it-all-come-from)

The first visit of this fieldtrip was Kimmeridge Bay. This is a Jurassic locality, dating back to the Kimmeridgian age of the Late Jurassic (~157 to 152 Ma). Unfortunately the only fossils that were found here were flattened and very delicate ammonites, what was more interesting was the sedimentology.

View of the cliff at Kimmeridge Bay. The anticline peaks at
this point in the succession, making it suitable for hydrocarbon
exploitation. (Source:
http://www.discoveringfossils.co.uk/kimmeridge_fossils.htm)

When visiting Kimmeridge Bay you will notice immediately that there are two types of bed. One that is a dark bluish colour and another that is a rusty orange colour. The blue rock is a friable clay and the orange rock is a clay that has been cemented with calcium carbonate. This alternation is caused by Milankovitch Cyclicity. In essence this the variations in environmental and/or astronomical conditions that causes a repeating succession of rock.

At the top of the cliff you will see that one of these beds is very prominent (middle right), making it a useful marker bed when constructing field sketches and making observations. If you follow this marker bed with your eyes to the west of the bay you will notice that it drops down (roughly where the MOD flag is at the top of the cliff). This is a fault, a weakness in the rock caused by the upwards thrusting of the Kimmeridge Clay within the bay itself. The structure that this marker bed highlights is called an anticline, a gentle fold in the rock.

The Etches Collection Museum, well worth a visit to
appreciate the magnificent finds that Steve Etches has
collected himself. (Source: http://www.dorsetlife.co.uk/2017/06/
from-beach-to-museum/)

In the west of the bay there is a wave cut platform made of a pale rock (above right). This platform is covered in raised structures, these are localised thrusts. This was caused by expansion. At the time of deposition of this particular bed the waters had an elevated level of magnesium, this reacted with the calcium carbonate in the rock thus forming dolomite. The rock now has now increased its volume by 10%, forcing it to fracture and overlap itself.

Kimmeridge Bay is not only known for its exceptional fossil and geological record but it is also a source of hydrocarbons. The Kimmeridge Well has been pumping oil since the 1950's and continues to this day. The Kimmeridge Clay is not the rock that is producing the oil however, the oil is coming up in fractures in the underlying Oxford Clay, however this is not the source. Although, the source of the oil is not completely clear it is likely to be migrating from the Blue Lias (the rock that outcrops at Lyme Regis and Charmouth to the West). The reservoir does not appear to be slowing down on production, hinting that maybe it is being replenished by a source deeper than the Lias.

The presence of hydrocarbons aids us in the palaeoenvironmental analysis of the area. Oil forms when organic matter is preserved and broken down by anaerobic bacteria, after diagenesis this becomes Kerogen, another process, catagenesis, turns this into oil. Should temperature and pressure continue to increase a process called metagenesis will take place forming gas.

We also had the privilege of viewing the private collection of Steve Etches before it was taken to his new museum in the village of Kimmeridge (below right). Etches is a local fossil hunter who has tirelessly devoted himself to finding Kimmeridge fossils. He has found everything, from a set of giant pliosaur jaws to complete Ichthyosaurs and Pterosaurs, even dinosaur bones from large sauropods that would have been washed out to sea during the Jurassic. His collection is something to be marvelled at, words cannot justify the significance of his finds. The Etches Collection Museum is now open and I highly recommend a visit if fossils are an interest.