Thursday, 30 September 2010
An Introduction to the Oxford Clay - Part 1
Having written so many blog entries regarding the Oxford clay, I figured it was time to give a brief description and introduction to this important geological unit that has provided so many wonderful vertebrate specimens over the years and continues to do so. This is the first of a couple of blog entries describing the geology, palaeoecology and fossils of this important unit.
165 million years ago, at the end of the mid-Jurassic, the continents, as we like to think of them now, were part of the super-continent known as Pangaea. This enormous land mass was almost crescent shaped and the northern and southern arms were separated by the Tethys Ocean and it was here, in this shallow warm sea, that the Oxford Clay was deposited.
The Oxford Clay is a succession of mud rocks that lie above the sand dominated Kellaways formation. These clays are occasionally intersected by horizons of carbonate concretions and are overlain by various facies that are collectively known as the Corallian Beds. Quite often, and in many places, the upper limit of the clay is a disconformity. All of these mud rocks, and the fossils they contain, are typical of a warm shallow marine environment and, indeed, this is exactly the ecosystem that we find in locations such as Quarry 4.
This extraordinarily rich ecosystem was powered by the sun. Both free floating phytoplankton and bottom dwelling benthic microflora converted the sun’s energy into organic matter via photosynthesis and, indeed, the same process more or less continues in the shallow marine ecosystems of today. Various biological groups were represented by these micro organisms which included various types of bacteria and algae. They were incredibly abundant and their fossil remains are best observed in the sediment under the electron microscope. The fact that benthic microflora are here at all is clear evidence of shallow water since they must receive sunlight to enable them to utilise the process of photosynthesis.
From the smallest to the largest - the big carnivores of this shallow sea. Marine reptiles dominated and animals such as ichthyosaurs, plesiosaurs and marine crocodiles were the rulers of the Oxford Clay ecosystem. Linking these groups together are the other threads of this web of life including the fish and a plethora of invertebrates. When palaeontologists were piecing this astonishingly diverse ecosystem together, it became apparent that there were two distinct food webs. The first was in the upper reaches of the water column dominated by phytoplankton who were the primary food producers. The second was in the lower depths of the water column where the sea floor was covered by the benthic microflora and these were the primary food producers at this level. And, of course, an additional and an obviously major source of energy at this level came from dead organic matter that fell from the upper reaches of the sea.
Sunlight alone cannot maintain a healthy and complex web of life and this was supplemented by a rich source of nutrients. But where did this come from? Throughout the Oxford Clay you find fossils of plants and animals from the land. These fossils include an array of different plant species and trees and there have been a surprising amount of dinosaur bones recovered – remnants of bloated carcasses washed out to sea. Indeed the most complete theropod found in the UK (Eustreptospondylus) was recovered from the clay. Obviously this means that ancient shorelines were nearby and rivers would have provided a continual source of nutrients into the sea. And it is again worth pointing out that many fossils in the clay are from shallow water species.
Another source of nutrients is derived from recycling those nutrients that became available when other organisms died. Anybody familiar with the clay is aware how very productive this ecosystem really was and this is very evident from the amount of organic carbon residing within the sediment as a direct result of intense biological activity. It is this high organic volume material in the Oxford clay which made it so perfect for the brick making process. And it was the brick making industry that led to the uncovering of some of the best marine reptile fossils found anywhere in the world.
In part two, I’ll go into some more details about these animals, the quarries that they have been found in and some of the people involved with their discovery.
Calloman, J.H. 1968 The Kellaways Beds and the Oxford Clay; pp.264-290 in P. Sylvester Bradley and T.D. Ford (eds.), The Geology of the East Midlands. Leicester University Press, Leicester, UK.
Calloman, J.H., Dietl, G and Page, K.N. 1989. On the ammonite faunal horizons and standard zonation of the Lower Callovian stage in Europe; pp.359-376 in the 2nd International Symposium on Jurassic Statigraphy, Lisboa, 1988.
Cope, J.C.W., Duff, K.L., Parsons, C.F., Torrens, H.S., Wimbledon W.A. & Wright, J.K. 1980a. A correlation of Jurassic rocks in the British Isles, pt2, Middle and Upper Jurassic. Geological Society of London, Special Report, 15:1-109.
Martill, D.M & Hudson, J.D 1991. Introduction. Fossils of the Oxford Clay. The Palaeontological Association, London, UK, 1, 11-34.
Page, K. 1989 A stratigraphical revision for the English Lower Callovian. Proceedings of the Geologists’ Association, 100, 363-382.