Middle Jurassic Environments In Europe And North America

Southwards from Britain into western Europe, the faunal diversity gradually increased, particularly among the stenohaline groups of organisms. In France, ammonites and belemnites are found, accompanied by many other invertebrate genera seldom abundant in Britain. In general they occur in shallow water limestones with some sands and shales. In southern Europe (Fig. n, p. 206), however, the picture is totally different, fine grained pelagic facies being dominant and containing Radiolaria, planktonic foramini-ferida, Bositra and ammonite aptychi.

These fine grained facies represent sediments that, for the most part, accumulated in relatively deep water upon collapsed contin-tal margins bordering the edges of the opening Tethys Ocean. Not all the sediments are basinal deposits, and at various times parts of the sea floor in this offshore area were uplifted above the level of the surrounding basins; under these conditions condensed and nodular ammonite-rich limestones formed. These condensed ammonite-limestones accumulated slowly upon starved submarine 'swells' and the rock produced is sometimes termed 'Kondensationskalk'.

The collapse of the Lower Jurassic carbonate platforms was not complete everywhere and isolated platforms sometimes released oolitic and other shallow water carbonate components. Periodically these were carried down by turbidity currents into the adjacent basins. In other areas, such as Spain and Sicily, the Middle Jurassic was marked by the eruption of submarine volcanoes.

The fauna of southern Europe differed from that of the north in being far more diverse. The ammonites show the greatest increase, in diversity, while many other invertebrate groups, including foraminifera, sponges, corals and coccolithophorids, also show a greater variety of species.

The northern part of the northern hemisphere, which included Britain and northern Europe with their relatively restricted faunas, was included in the Boreal Realm while the Tethyan faunas occupied the rest of the world (Hallam, 1972; 1975). The recognition of these major realms was based initially upon the differences between ammonites; subsequent work has shown that within other groups relevant differences also existed. Within these realms provincial differences can be seen but are sometimes subtle. Throughout the Jurassic, the dissimilarities between Tethyan and Boreal communities increased, so much so that Waagen wrote in 1864 almost in despair: 'The higher we climb in the Jurassic series, the greater become the difficulties, either of recognizing or separating individual beds, or of correlating' (Arkell, 1956).

For nearly a century opinions have differed, first over the recognition of 'Realms', and second over their cause. Arkell, in his masterly review (1956) did not actually put forward an explanation for faunal realms, but one is left with the impression that he favoured the theory of a broadly 'climatic' control. At that time the idea that continents were mobile was quite unfashionable and all his arguments and reviews of evidence indicating Jurassic climatic belts were based upon present-day continental distributions.

Today, knowing more about continental motions and the complex variables that control the distribution of plants and animals, we are too cautious to assume that a single environmental parameter, such as temperature, could be the primary controlling factor in faunal realm development and perpetuation. Climate was very relevant but other factors, such as the variability in the entire environment, are likely to have been as important, or even more so (Hallam, 1975).

Arkell realized that the gradual decrease in the amount of limestones in sediments from south to north across Europe was probably related to decreasing temperatures. He also noted that coniferous plants, at least in the north, showed well developed growth rings which could only be brought about by seasonal changes. However, as we stated earlier, no evidence has yet come to light of Jurassic glacial sediments on the earth even in the areas that are assumed, from modern geophysical studies, to have lain over the Jurassic-poles (Fig. m, p. 205).

Modern ecological ideas suggest that environmental stability is a key factor in establishing a diverse population (Valentine, 1973). Where there are strong seasonal differences and high environmental stresses, as in shallow shelf seas, diversities are relatively low in comparison with those in adjacent oceanic areas where seasonal variations tend to have less effect. This may help to explain the diversity differences that occurred during the Jurassic between the Boreal faunas, which were essentially shelf faunas, and the Tethyan faunas that lived in the Tethys and the Super Pacific Oceans and on their margins.

In North America (Fig. m), the sea renewed its advance during the Middle Jurassic and spread over large areas of the Western Interior, probably from the north (Imlay, 1957). This seaway was partially enclosed, and at certain times dolomitic and gypsiferous beds were laid down as a result of evaporation. At the margin of the basin, the lagoonal and restricted sediments passed laterally into red beds that represent subaerial deposits, probably in arid and semi-arid continental areas. All the Middle Jurassic sediments of the Western Interior seem to have been deposited in very shallow water, and many of the limestone beds contain abundant molluscan faunas including Liostrea, Gryphaea and nerineid gastropods. Corals sometimes occur and in many cases the faunal assemblages seem to be comparable to those of the Middle Jurassic of Britain. The beds with evaporites tend to have extremely low diversities or, more often, no faunas at all.

During the Middle Jurassic, North American ammonite faunas became markedly differentiated into Boreal and Tethyan groups. As in Europe, the dominantly Boreal faunas occurred in the north while those with more Tethyan affinities occurred in the south (Imlay, 1965).

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