Bat's Head to Mupe Bay Mapping

In the final part of the Dorset fieldtrip I would like to discuss the section of coast between Bat's Head and Mupe Bay. This will include Durdle Door, Lulworth Cove and Stair Hole. This part of the fieldtrip was the main mapping task and therefore took two and a half days to complete.

This is an odd bit of the Dorset coast as it is nestled between two large outcrops of upper Jurassic Kimmeridge Clay, with Osmington Mills to the West and Kimmeridge Bay to the East. The geology between Bat's Head and Mupe Bay are latest Jurassic to Upper Cretaceous, the only explanation as to why this area is now surrounded by older rock is faulting and uplifting. This area is heavily faulted, as we will see, and therefore the older Kimmeridge Clay has been thrown down preserving the rock above.

Bat's Head to Mupe Bay

Map of east Dorset showing the location of Lulworth Cove. (Source: 
Google Maps)

Map of the stretch of coastline that was mapped between Bat's Head and Mupe
Bay (Source: Google Maps)

Durdle Door viewed from the Chalk ridge to the north.
(Source: Saffron Blaze www.mackenzie.co)

The basic geology of this section of coastline is relatively simple. The geological boundaries typically run west-east. The oldest unit is the limestone closest to the sea is the Portland Limestone and is only really accessible at Durdle Door and Lulworth Cove. Resting on this is the Lulworth Formation, another limestone unit, forming the lower part of the Purbeck Group. The upper part of the Purbeck Group, Durlston Formation, is separated by the Cinder Bed. This is recognisable from the bluish purple colour of the mud matrix that holds thousands of small bivalve shells. It is in the Purbeck Group that we find the Jurassic-Cretaceous Boundary.

The Lulworth Crumple seen from the west end of
Stair Hole. (Source: Stuart Chettleburgh
http://www.bournemouthweather.co.uk/gallery.php?
image=2010-07-01-095124Stair%20hol
e,Lulworth%20Cove.jpg&caption=
Stair%20hole,%20Lulworth%20Cove%20-
%20Taken%20by%20Stuart%20Chettlebu
rgh&curPage=2&id=68&rating=4.3&
totalratings=14)

Moving above this is the Wealden Group, which includes the dinosaur bearing Wessex Formation. You can quickly identify this unit by the orange sands and clays that typically form the topographical lows of this length of coast, being the softest of the units. There is another unit that is rarely seen, this is one outcrop at St Oswald's Bay. This is the Gault Clay, a soft black clay that is faulted out in most of the succession. This unit marks an intermediate stage of a marine transgression between the river facies of the Wealden Group and the marine facies of the Greensand and Chalk.

This brings us onto the Greensand, which like its name suggests is green and sandy. This is usually a thin unit at most exposures, the largest being St Oswald's Bay due to the angle of the erosion of the bay.

Lulworth Cove viewed from the viewpoint to the west.
(Source: Gregg M. Erickson)

The most obvious and youngest unit the outcrops here is the Chalk. This forms the back wall of the bays that are dotted along the coast and also forms the large ridge that runs across east Dorset to north of Swanage.

Mupe Bay viewed from the chalk ridge to the north, with
Mupe Ledge and Mupe Rocks in the distance. (Source:
https://www.geograph.org.uk/photo/1707606)

When you look at the succession, the best place to do this is either Durdle Door or Lulworth Cove, you will see that the angle that the rock dips changes from south to north. The Portland and Purbeck limestones dip approximately 50 to 60 degrees to the north. Moving to the Wealden Group the beds are near vertical, so the dip is getting steeper and steeper to the north. This dip continue to steepen until the chalk becomes overturned and begins dipping to the south.

This deformation is best seen at Stair Hole with the Lulworth Crumple. This is the folding of the Purbeck Group. The folds are more dramatic here due to the faulting of the beds. Deformation along this coastline was the result of the collision of the African plate into the Eurasian plate, the same collision that formed the Alps in Southern Europe.

The chalk cliffs in the north of Mupe Bay viewed from the
south. (Source: Jim Champion)

At Mupe Bay there is the opportunity to see the hydrocarbon potential of the Wealden Group. The sands of the Wealden Group, are stained black with oil. This has seeped up from much lower down in the Jurassic, possibly the Blue Lias, and been stored in the porous sands.

Lyme Regis (and a passing visit to Charmouth)

In this second part to the Dorset Fieldwork I will show you all the world famous Lyme Regis Lias outcrops. These were incredibly interesting both geologically and palaeontologically. Our trip leader, Professor David Martill found something very exciting when making our way back to the coach.

We visited the beach to the west of the town of Lyme Regis, Monmouth Beach.

Lyme Regis (with a quick stop at Charmouth):

Map of West Dorset showing the location of Monmouth Beach.
(Source: Google Maps)

The alternating succession of shales and argillaceous
limestones at Monmouth Beach. (Source:
https://www.shutterstock.com/video/clip-5416676-stock-
footage-the-blue-lias-beds-and-limestone-pavement-on-
monmouth-beach-cliffs-lyme-regis-on-the-jurassic.html)

The limestone ammonite pavement, or ammonite
graveyard. (Source: https://chandlerscottage.co.uk/1
125x750-lyme-regis-0772/)

In the morning after a rather lengthy drive from Swanage, we arrived in Charmouth. Although we didn't actually go onto the beach to see the cliffs up close, we got a pretty good view of the geology. I have also visited this site a number of times and will post the fossils I found in my collection series.

When standing at the Charmouth Heritage Centre, if you were to look to the west towards Lyme Regis those dark clay cliffs are called Black Ven. To the east you have Stonebarrow and the Golden Cap.

Charmouth is a Jurassic succession being deposited during the Sinnemurian to Pliensbachian ages of the lower Jurassic, approximately 190 Ma.

Black Ven is highly fossiliferous with ammonites being the most common fossil you will find. If you are very lucky you can also find crinoids, Ichthyosaur and Plesiosaur remains. A complete dinosaur, Scelidosaurus, has also been found here and is on display in the Charmouth Heritage Centre. Another must see is in the fossil shop, where they have on display a large Temnodontosaurus skull.

This is also one of the sites that Mary Anning collected from, finding marine reptiles and the pterosaur, Dimorphodon macronyx, later described by Sir Richard Owen.

After this brief talk we then drove a bit further west into Lyme Regis and walked down onto Monmouth Beach. Here you will notice, similar to Kimmeridge Bay, a repeating pattern in the cliffs between shales and argillaceous limestones, another example of Milankovitch Cyclicity.

But the really interesting thing to see here is in the wave cut platform that is formed by one of these bands of limestone. It is covered in hundreds of ammonites, the majority are of the genus Coroniceras. This was a single event where all of these ammonites died at the same time, this was gradual, possibly they returned to the same site seasonally. Ammonites won't be the only fossil you find here, you can also spot nautiloids, crinoids and gastropods. A complete Plesiosaur was even found in the pavement some years back.

If you wet the limestone around an ammonite you will find dark marks in the rock. These are fossilised burrows. Some are branching 'Y' shapes, this is formed by a shrimp, the name given to these burrows is Thalassinoides. Others will be two circles next to each other (Diplocraterion), straight burrows (Planolites), and some very fine burrows. These very small burrows are called Chondrites and show that the water had become anoxic for a period.

On the walk to the next locality, Professor Andy Gale found a large nautilus, preserved in three dimensions, a very nice find indeed. It was picked up by a course mate on the way back to the coach.

The next site was geologically of interest. This was the boundary between the limestone dominated White Lias, to the shale of the Blue Lias. The interesting part is, lithologically speaking the boundary is very clear, you can see a pale limestone become a dark shale. You would be forgiven for thinking that this is simply the boundary between the Rhaetian of the Triassic and the Hettangian of the Jurassic.

But to date the rock to confirm this is very difficult. The use of lavas of similar ages are regularly used to radiometrically date rock, however, there are no lavas to use. So dating could be done with the fossils, using relative dating. The change in the ammonites present between the White and Blue Lias represents a biological boundary. This is higher in the succession than the lithological boundary.

On the way back, Professor Dave Martill came across some Ichthyosaur remains in a limestone band of the Blue Lias. Unfortunately I did not see it but there was enough articulated material to warrant him going back to recover it.

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.