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.
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.
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.
No comments:
Post a Comment