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.