fossil Journeys

The brick pits of Kings Dyke, Whittlesey will always be a special place to me, it was one of the very first formations I hunted for fossils in, and I have returned to it many times.

The Oxford Clay itself is marine strata from the late Jurassic, the same time dinosaurs such as Stegosaurus were roaming the land. The formation itself stretches from the Midlands all the way down to the south where it is known as the Kimmeridge Clay. The Oxford Clay was originally subdivided into the Lower, Middle and Upper. These are now known today as the Peterborough, Stewarty and Weymouth Members.

The Oxford Clay is known in the past for its bricks. It was once a major source for brickmaking clay, 33 per cent of the total national production used this raw material. The Peterborough region includes the second most important complex of brickworks. Peterborough once had six operational works with only one active brick pit left today which is Kings Dyke. The Peterborough Member of the Oxford Clay has the ideal characteristics for brick clay, including having the right lime content as well as being dominated by clay mica minerals, along with having few impurities.

The Ecology and Paleogeography

The ecology of the Lower (Peterborough) member of the Oxford Clay is thought to have accumulated in a wide, shallow sea at the early stage of a major transgression (a rise in sea level), with water depths around 10-50m.

The nutrients within the sea were thought to be abundant, leading to many creatures thriving within the sea. The source of these nutrients must have been from the nearby land, which was well vegetated. The sea floor accumulated fine grain organic sediment that formed a ‘soupy mixture’ that was maintained by an abundance of burrowing and deposit feeders.

It is thought the sea floor was low in oxygen levels but rarely anoxic due to organisms living on the sea floor. Fossils of invertebrates and vertebrates are abundant throughout the Peterborough Member. This is due to high sedimentation rates. Evidence such as shell beds indicates there was a presence of large storms. Oxygen isotope ratios of well-preserved ammonites have been used to calculate a temperature of around 20 degrees within the water column.

Less detailed work has been carried out on the ecology of the Middle and Upper Oxford Clay, but in short, the substrate was firmer with an increased importance of shallow-burrowing and suspension feeding bivalves. Layers of large Gryphaea are common.

Fossil Preservation Within the Oxford Clay

The Oxford Clay can be considered as a Konservat-Lagerstätten (a site with a high abundance of fossils) for the main part, as it yields a high quantity of fossils. In places it can also be considered as a Konzentrat–Lagerstätten (sites of exceptional preservation) for the soft part preservation in cephalopods and in the stomach contents of ichthyosaurs. Some of the main factors that make the Oxford Clay a good site for preservation is the lack of deep burial and the richness in fine-grained organic material. because of the fine-grained material, water cannot seep into it.

Aragonite is a mineral made up of calcium carbonate, which usually dissolves easily during diagenesis or even pre-burial. Aragonite is preserved in the Oxford Clay, mainly within the 2D ammonites, belemnite phragmacones (moulds of the internal chamber) and some bivalves of the Peterborough Member. The aragonite fossils allow microstructural detail to be perfectly preserved and leave original chemical composition intact. This type of preservation is not as common in the Middle and Upper layers as it dissolved in early diagenesis.

Fossils such as Gryphaea, belemnites and brachiopods are also commonly preserved in calcite, and these tend to be more robust than the aragonite ones. Fossils found in the Peterborough Member have suffered compaction much more than the Middle and Upper Layers. In this layer, flattened aragonite ammonites are very abundant.  The secondary minerals that form within the clay include pyrite and calcite. A common result of pyrite is forming internal moulds of creatures such as ammonites. This is common in the Middle and Upper Oxford Clay.

Preservation of Vertebrates

Fully articulated specimens of marine reptiles can be found, along with isolated remains, with teeth being the most common. The teeth most likely fell out during life, with isolated bones falling from floating carcasses. There is also lots of evidence of scavenging and predation from disarticulated specimens. In the past, specimens have been found surrounded by shark teeth. Fully articulated skeletons are thought to be preserved either by low oxygen levels at depth, preventing scavenging, or quick burial by sinking in the soupy sediment.

Sources

Horton A. (1989). Geology of the Peterborough District. British Geological Survey.

Hudson J. D. (1994). Oxford Clay studies. Journal of the Geological Society, London, 151, 111-112. https://jgs.lyellcollection.org/content/151/1/111

Martill D. M, Hudson J. D. (1991) Fossils of the Oxford Clay. The Palaeontological Association Field Guide to Fossils: Number 4, London.

Tang C. M. (2002). 17 Oxford Clay: England’s Jurassic Marine Park. Bottjer D. J, Etter W, Hagadorm J. W, Tang C. M. Exceptional Fossil Preservation: A Unique view on the Evolution of Marine Life, 307-325. Colubia University Press, New York.