Middle Jurassic

Towards the end of the Lower Jurassic the shallow marine environments began to be replaced from north to south by increasingly variable fluviatile, deltaic and lagoonal conditions. These changing European environments resulted from local uplift round the central North Sea where rifting was occurring: a phase of earth movements which was associated with local volcanism (Sellwood and Hallam, 1975). Prior to rifting, uplift took place and clastic-sediments were derived from the east as powerful rivers rapidly stripped the new land surfaces and enveloped northern and eastern areas of Britain in fluviatile, and deltaic sediments. Generally, away from this major source of sediment, the influence of coarse clastic detritus decreases, but the presence of major sources of fresh water had a dramatic effect upon the faunas. A complex of lagoons were established as margins to the deltaic environments, and these gave way, as the distance from the clastic sources increased, to a broad shallow shelf upon which a variety of carbonate sediments were laid down. It was only in the far west of Britain and over northern France that unrestricted marine conditions remained (Hallam and Sellwood, 1976).

The deltaic sands are poor in fossils, but driftwood is abundant and some beds include coals showing the roots and stems in their positions of growth. Freshwater ostracodes and bivalves are present in places and when they occur they are often abundant. Some of the most exciting finds are sun-cracked bedding surfaces covered with dinosaur footprints. Westwards and southwards from the area of Yorkshire deltas, euryhaline faunas are dominant in clay-rich sediments that accumulated in the complex lagoonal environments. Further towards the south, in the Cotswold Hills, there are many limestones and clays with more diverse marine faunas, though stenohaline organisms like belemnites and ammonites are still extremely rare. These limestones often acted as barriers between the lagoonal clays to the north and the more open marine environments to the south.

The deltaic sands in the north of England and the more fully marine clays in the south are much thicker than the limestones and lagoonal clays of the English Midlands. These different thicknesses suggest that differential subsidence in the underlying Palaeozoic floor was a major control on sedimentary facies. In general the lagoons and barriers of the English Midlands are sited in the region of least subsidence, and several minor local changes in environments are also linked with local swells and basins.

The various interactions of sedimentation rate, sediment type, salinity and depth of water produced a great variety of habitats. No single factor is the primary control in the development of a particular faunal assemblage, but three major environmental associations can be recognized:

1 The fluviatile and lagoonal association, which included freshwater and euryhaline animals, notably ostracodes, oysters and certain other molluscs.

2 The carbonate areas in which clear water conditions allowed the development of a relatively diverse bottom-dwelling fauna, including mollusca, corals, brachiopods and echinoderms. Locally within these areas, the diversity was reduced drastically by decrease in salinity or by the development of highly mobile substrates (for instance oolite banks or sands).

3 The open marine environments of southern England and northern France often had diverse faunas, but the diversity of the benthos was often restricted by the depth of water above the muddy sea floor. It is only in these Middle Jurassic environments that communities similar to those in the British Lias are developed.

In the Middle Jurassic, some ammonite genera began to show well-developed sexual dimorphism, a single species being represented by two forms: a small male shell (microconch) and a larger female shell (macroconch). The shells of the males have projections on their apertural margins (lappets) which are thought to have assisted attachment to the females during mating. The shells of females often had an aperture equal in size to the total diameter of the male shell but they never carried lappets.

Both epifaunal and infaunal bivalves are often diverse in the Middle Jurassic, reflecting a greater variety of marine and lagoonal environments, but restriction in the diversity of bivalves occurred when the salinity was reduced in brackish environments. In general, oysters, lucinoids and Astarte occurred in mixed marine environments which had the lowest salinity and were adjacent to areas of fresh water.

Fig. 73 Freshwater Communitie a dragonfly (Arthropoda: Hexapoda — insect) Unio (Mollusca: Bivalvia: Unionida)

Viviparus (Mollusca: Gastropoda: Mesogastropoda Valvata (Mollusca: Gastropoda: Mesogastropoda Chara (Algae — characeae) Equisetites (Pteridophyta: Calamites — horsetails) buried roots and stems of horsetails (Pteridophyta: Calamites)

dinosaur footprints (trace-fossil)

small insects (Arthropoda: Hexapoda)

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