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?
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%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.

The Big Five Mass Extinctions

Mounted Brachiosaurus skeleton at the
Naturkunde Museum in Berlin, Germany. The dinosaurs
were the major casualties of the K/T Extinction. Image
credit: Bill Sellers

As I said in my Devonian post yesterday, I am going to cover the Five biggest mass extinctions in the Earth's history. We will look at these from the one with the least impact on global species to the one with the most impact. You will notice that when reading the statistics for what went extinct that the number of species will be higher than the number of families or genera that were lost, this is because you can have vast numbers of species die out without losing a genera if some species are to survive. For instance, the Sarcopterygii that was discussed in the Devonian post largely have died out but the whole class was not lost as it survives with the lungfish and Coelacanth. I am also using marine life families as some of the extinctions took place when there was little or no life on land.

#5: The Cretaceous Tertiary Extinction 
Also known as the K/T Extinction or K/Pg Extinction, Pg being the Palaeogene period that followed the Cretaceous, is famed for being the extinction that wiped out the dinosaurs and closed the Age of Reptiles. Extensive studies, including the drilling of boreholes, of the Chicxulub crater off the coast of the Yucatan Peninsula in Mexico have given a clearer picture of what happened 66 Ma. An asteroid nearly six miles wide smashed into the earth, this caused a chain reaction of events that doomed the dinosaurs. After the impact, mass volcanism, particularly around the Deccan Traps area in India, polluted the atmosphere and caused the Earth to heat up. Sea levels also fell by 150 metres which could be attributed to the loss of the marine life as many would have depended on shallower seas to survive. 
Marine Families Lost: 16%
Genera Lost: 67%
Species Lost: Approximately 76% including non avian dinosaurs, marine reptiles, pterosaurs and ammonites.

Artist's impression of the Chicxulub Crater. Image credit:
Detlev van Ravenswaay

#4: End Triassic Extinction
Little is known about the causes of this extinction that took place 201 Ma, but we do see evidence of falling sea levels which is potentially the reason behind a larger loss of ocean species, including some marine reptiles. There is also sedimentary evidence for volcanic rifting that took place as Pangaea broke apart. Taking place over where North America, Europe and Africa would have been on the supercontinent, the air would have been toxic to nearby ecosystems, another possible reason for an extinction event. 
Marine Families Lost: 22%
Genera Lost: 53%
Species Lost: Approximately 80% including most mammal-like reptiles and large amphibians, surprisingly plants made it through largely unscathed. 

#3: Devonian Period's Two Extinction Events
There were two separate events of extinction in the Devonian; the first being the Kellwasser, in the late middle Devonian, which pushed corals and jawless fish to extinction as well as reducing trilobite species. The second was the Hangeberg, which took place on the Devonian-Carboniferous boundary, was responsible for the extinction of Placoderms and many early ammonite species. The Hangeberg extinction event is believed to have been the result of global cooling due to increased volcanic activity. There is also evidence of eutrophication in the shallow seas. This is where there is an excess of nutrients, usually caused by the run off water from the land, this causes algal blooms. These blooms are disastrous for marine life as it prevents sunlight from penetrating the water, thus there is no replenishment of the oxygen that the algae removes from the water, which in turn kills the marine life that depend on the oxygen rich waters. The result of this is the coral reefs not making a return for another 100 million years.
Marine Families Lost: 22%
Genera Lost: 57%
Species Lost: Approximately 83% 

#2: End Ordovician Extinction
Taking place between 445 and 440 Ma, it is believed that an intense global ice age was the trigger for the event. As the supercontinent of Gondwana moved further South to take it's place over the South Pole, vast glaciers spawned gradually lowering global temperatures and therefore causing a fall in sea levels. It is believed that the sea levels fell by between 70 and 100 metres. 
Marine Families Lost: 26%
Genera Lost: 60%
Species Lost: 85%

#1: End Permian Extinction
The largest extinction event in Earth's history took place at the end of the Permian period and the end of the Palaeozoic Era around 252 Ma. Also known as the "Great Dying", the causes are unknown for certain, however, we already know that Pangaea was incredibly hot and dry which would push a great many of species to the brink. But we also see a spike in greenhouse gases such as Carbon Dioxide and Methane that would have been released from frozen stores in the oceans. Combined with one of the biggest volcanic eruptions ever and we can start to see why so many species were lost at the close of the Permian. 97% of all species that were present on Earth in the Permian were wiped out 252 Ma, all life that followed, the majesty of the dinosaurs, the radiation of mammals and eventually the success of Homo sapiens can all be traced back to the 3% of species that clung to survival at the end of the Palaeozoic Era.
Marine Families Lost: 51%
Genera Lost: 82%
Species Lost: 97%

I hope you find this helpful and interesting, let me know what you think in the comments.

Palaeoart: Super Croc Ambush Part 2

Further to what I said in the previous post about crocodiles generally being subdued by the dinosaurs. In North America at the end of the Cretaceous period we find another leviathan; Deinosuchus. Here it is depicted, again by the incredibly talented Raul Martin, launching what can only be described as a surprise attack on an unsuspecting Albertosaurus. This shows once again that perhaps dinosaurs were destined for their downfall before Mother Nature took control of their fate. Was evolution kicking other groups of animals into overdrive in order to combat the dinosaurs' reign.

Let me know what you think in the comments section below, it would be interesting to see what you think.

Paleoart: Super Croc Ambush

Raul Martin is a well known paleoartist, creating some true masterpieces over the years, I've chosen this one because it shows two key concepts that I will discuss on this post.

This scene depicts a confrontation between two African giants. Two different animals sharing the same lifestyle. The first concept I like about this is the fact that dinosaurs, even large predators such as Suchomimus tenerensis, are threatened by crocodiles that elsewhere is the world are subdued by the dinosaurs, with the exception of Deinosuchus at the end of the Cretaceous period. As the spinosaur is believed to have primarily fed on fish these two massive predators would have come in to regular contact, making confrontations frequent. Sarcosuchus imperator would have posed a genuine threat to even the biggest of African dinosaurs, with the stealth of modern crocodiles and massive, powerful jaws, the odds of surviving would be very little. This shows that in some places around the world, the dinosaurs did not have absolute rule over their territory.

Secondly, we are seeing an ecological struggle. Suchomimus has evolved to feed on fish, a niche that few other theropod dinosaurs had filled. However, this created competition with the aquatic predators such as Sarcosuchus. So despite escaping competition with other dinosaurs, Suchomimus is no better off as the super croc has grown to titanic proportions in order to reign supreme over the waterways of Africa.

To summarise, the most important thing to take from this is that the dinosaurs were not always at the top of the food chain and that even the largest most specialised theropods could become prey.

Palaeoart: North Africa in the Cretaceous

Today, I have chosen artwork from Mohamad Haghani. A depiction of North Africa in the late Cretaceous featuring two of the world's largest predators.

I like this piece because we are seeing two different lifestyles of Cretaceous predatory dinosaurs. The two Carcharodontosaurus' are shown feeding on an Ornithopod, whilst Spinosaurus is in the river catching the giant sawfish, Onchopristis. Spinosaur dinosaurs are generally regarded as ichthyophagous now due to their crocodilian features, thus it would be rare, but not impossible, to find Spinosaurus hunting large prey when fish is plentiful as this was the prey they were best adapted to hunting. However, having said this there is evidence for other members of the Spinosaur family hunting prey other than fish, an Irritator was found in a pterosaur vertebrate, whilst evidence of Baryonyx eating Iguanodon has been found in England. Carcharodontosaurus on the other hand has very large blade like teeth that are perfect for cutting through flesh. Studies revealed that the skull of Carcharodontosaurus could not withstand a great deal of strain, and therefore not wrestle large prey to the ground, the dinosaur would thus have to rely on dealing a devastating wound and simply wait for the animal to die of shock and blood loss.

Although this is a very nice piece of art, I do have some problems with it. Before I get to that I understand that this art is now getting old and that our theories on dinosaurs are changing rapidly, so these are not faults per say but just areas that I want to highlight. Firstly there is the Spinosaur itself. Spinosaurus is the world's first semi aquatic dinosaur, being adapted for a life in the water and not stalking the African plains in search of food as previously thought. We now also know that the animal was quadrupedal not bipedal as shown above. But this was of course discovered after the creation of the art. Also these two giants would not come into close contact very often. Spinosaurus was best suited to the swampy deltas, whereas Carcharodontosaurus was adapted to actively hunting. Both animals would need a massive territory to sate their apetites. It was only in times of hardship, for instance drought, that the two would be in direct competition with each other for food.

But I respect the artist's decision to put the dinosaurs in the same location, it highlights that despite being the top predators and two of the largest carnivorous dinosaurs to walk the Earth, they had very different lifestyles. Spinosaurus being water based, relying on swarms of fish to hunt and Carcharodontosaurus depending on herds of herbivorous dinosaurs to feed. That, I think, is probably the most interesting thing about this artwork despite being inaccurate.

A quick visit to Hastings

The mystery modern bone fragment

Hastings has always been a place I'd like to visit, both for the history and the geology. Hastings is the only coastal outcrop of Wessex Formation on the mainland of the UK, the most famous outcrops being the Isle of Wight. 

This location is definitely an interesting one, both from a palaeontological and a geological perspective. Palaeontologically, the range of fossils to find is rather wide, anything from Hybodus sharks teeth and fin spines to fossilised bone of Iguanodon dinosaurs can be found. Unfortunately, being the summer and scouring tides are few and far between I found neither, therefore I plan to revisit in the winter after some bad storms. This is the case with most locations, erosion in the summer is very little compared to the winter so fossil discoveries are scarce, although I did pick up a modern bone fragment. I keep these despite not being palaeontologically valuable as it gives me something to compare to the fossils of extinct fossils that I find. 

Straight ripple marks in a boulder at Hastings.
50 pence piece for scale.

But what I found interesting about this site is the geology. There were many geological wonders. The most fascinating that I haven't yet seen on the Isle of Wight are the straight ripple marks in a number of boulders. Straight basically means that they were formed under a unidirectional flow, for instance a river. Taking into account that the Wessex Formation was deposited in the Cretaceous period, at which time the South of England was a lagoon environment. These ripple marks are evidence for small rivers and streams flowing into said lagoon, on the Isle of Wight there are casts of smaller streams that have retained their characteristic channel shape, with shark and fish fossils in the bottom. I will do a separate post on the streams later on. Further evidence that these are indeed the unidirectional straight ripple marks is the cross bedding on many boulders. Straight ripple marks create cross laminae that dip in the same direction, these can be found in the cross section of many boulders. Cross bedding is formed by the migration of ripples downstream, This is caused by the build up of deposited sediment on the stoss (upstream) upstream side of the ripple, this continues until the crest is too heavy to hold more sediment and thus collapses down the lee (downstream) side of the ripple. These avalanches continue down river and create unidirectional cross bedding. This cross bedding is a good indicator for the palaeocurrent, the dip will always face downstream, this is also a good indicator, if the bedding is in situ, as to whether the rock has been rotated since deposition.

Unidirectional cross bedding. 50 pence piece for scale.

Also, a monospecific assemblage of bivalves was found in another boulder. These are most likely brackish water bivalves, tolerable to more stagnant water, as the Wessex Formation is a lagoon environment. The photograph I took shows numerous empty burrows, and some with the bivalves still inside. This could be an indicator to a storm that removed the bivalves from their shallow burrows and deposited them elsewhere, hence the empty burrows. Their preservation shows that they were filled in and fossilised at the time of the storm, a good palaeoenvironmental indicator. The climate would have had to have been much warmer than today to generate storms of such magnitude that they can reach down to the bottom of the lagoon and rework these bivalves and deposit sediment into their burrows. 

Burrows of brackish bivalves, some bivalves
still present. Hand for scale.

All in all, this was an interesting visit despite the lack of fossils, definitely worth a trip whether you are interested in geology or palaeontology as there is plenty there for both fields, although I recommend palaeontologists visit in the winter if they are hoping to find something.

If you have visited the Hastings Wessex Formation do share what you thought of it or what you have found there in the comments section.