After completing my dissertation fieldwork at Whitecliff Bay on the Isle of Wight, I will cover the sedimentology and palaeontology of the Thames and Bracklesham groups, starting with the Thames Group. All images are my own. rock is older moving to the right in images.
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| The Thames Group exposed at Whitecliff Bay. The Harwich and Reading formations are located out of frame to the left. Approximately 100m of strata shown. |
At Whitecliff Bay the older rocks are found in the South, as is the general trend for all the geology of the Isle of Wight. The soft clays of the Eocene rest unconformably on the Upper Cretaceous Chalk of Culver Down, the succession then stretches North until it reaches the earliest Oligocene with the exposure of the Bembridge Marls. The stratigraphy here is vertical in the south of the bay, this makes it very easy to log as all of the strata is at beach level. The strata in the north of the bay becomes more horizontal, to approximately a 5 degree dip angle at the top of the Solent Group. The reason for the geology being this way is that during the Miocene, the African plate collided with the Eurasian plate causing the Alpine orogeny, the impact was felt in the UK, with the chalk and clays buckling and being forced vertical.
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| The clasts in the Harwich Formation approximately 50cm above the base. |
Above the Chalk is the Reading Formation. This is a very soft red clay, and is almost always found to be slipped both at Whitecliff Bay and Alum Bay in the West of the Island. This is a completely unfossiliferous clay and was deposited as a result of a river flowing from the North. In other exposures around the London Basin, lignite and fossilised plant material can be found.
The upper boundary of the Reading Formation is marked by a brown clay with large clasts supported by the grains. This conglomerate also contains small red clay clasts that have been eroded and reworked into the conglomerate, this is evidence that this bed is a transgressive surface. As the sea level rose in the Hampshire Basin 56 Ma the soft clay was eroded and lithified as clasts. This is the Harwich Formation and is only approximately 2 to 3 metres thick but marks the start of the Thames Group.
The next 130m of cliff are part of the London Clay Formation. this is a sequence of alternating sands and clays marking the transition from high to low energy and the gradual shallowing and deepening of the marine environment.
This part of the succession is heavily bioturbated, this is where burrowing organisms destroy sedimentary structures and oxygenate the sea bed. This is also evidence for the oxygenation of the sea bed during the Eocene. This can be further supported by the presence of microfossils called foraminifera, a group of single celled organisms that are both benthic and planktic, with infaunal and epifaunal species as well.
The assemblages of foraminifera in the London Clay Formation's clay beds exhibit various morphologies. A study in 1988 by Corliss and Chen on modern foraminifera showed that foraminifera can be divided into 9 morphogroups based on the shape of their test. It was established that those with elongated tests are infaunal and the rounded trochospiral foraminifera are those of epifaunal. Note that this only applies to small benthic foraminifera.
In the brownish clays of the London Clay Formation the elongate textulariid foraminifera are more abundant, notably Bolivinopsis adamsi and Textularia agglutinans. This is evidence for the oxygen levels in the sediment being relatively oxygenated, oxic to dysoxic conditions within the first few centimetres of the sediment and just above the sediment-water interface. The sediment can also be referred to as being mesotrophic, meaning that the nutrients available are of a moderate level. It would be expected to see more of the epifaunal trochospiral forms if the sediment was oligotrophic, allowing animal life to thrive due to the reduced plant material.
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| The diagonal structures in these sand beds are indicative of cross lamination showing a gentle flow to the south when the strata was horizontal. Approximately 4m of strata shown. |
At approximately 40m above the base of the Harwich Formation the strata disappears beneath beach level. It was here that the Planktonic Foraminiferid Datum Point was discovered. This was where the marine environment deepened, upwelling here provided the habitat for planktic foraminifera. This brief occurrence of planktic foraminifera shows us that there is an abundance of nutrients being bought up into the water column through the upwelling.
At the top of the London Clay Formation, there is a division that is made up entirely of sands. Fossils are not regularly found here. But interesting sedimentary structures can be seen in the cliff. The cross lamination of some of these sands provides us with evidence of flowing water, albeit of a very low velocity. This water was flowing gently to the south and was the result of a river to the north bringing sediment into the basin. The freshwater input made it difficult for organisms such as foraminifera to survive. The grian size of this strata is coarser than that of the underlying clays, showing an increase in velocity of flow. A higher velocity is needed to transport the larger and thus heavier grains. This is further evidence for the presence of a river to the north.
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| The top of the London Clay Formation and the base of the Wittering Formation, separated by the thin conglomerate. Approximately 10m of strata shown. |
The top of the formation is marked by a thin 30cm conglomerate. The large pebbles in the bed are evidence that the succession has reached beach level. In the foreshore exposures there are a number of conglomerates that aren't found in the cliff, showing that the sea had regressed before this point, however, this conglomerate is the most prominent. It is also made more evident by the yellowish orange sands below it and the purplish blue sandy clays above in the Wittering Formation.
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| Close up of the conglomerate at the top of the London Clay Formation |
