Upper Jurassic Communities In Europe And North America

Through the Jurassic, facies distributions were controlled by the interplay between two dominant factors: first a eustatic rise in sea levels, and second the regional uplift or down-warp of certain areas of the earth's crust. In the Upper Jurassic, continuing eustatic rise and a diminution in the supply of coarse clastic sediments from an ever-reducing land area led to the spread of calcareous deposits northwards over Europe. Terriginous .clastic material deposited in Britain and in the North Sea was derived only from local and ephemeral island areas (Fig. n, p. 206).

In Northern Europe the later Upper Jurassic (Kimmeridgian) is represented by an extremely widespread and uniform facies of shales that are often bituminous and appear, in the North Sea at any rate, to be the main source rocks for the 'North Sea oil' being piped ashore at the moment. These fine laminated shales accumulated in anaerobic basins that were probably not very deep but were bounded by active faults. Great slides of debris occurred periodically, bringing terriginous clastic material, and sometimes shallow marine shelly communities, into the slightly deeper areas.

The late Jurassic was a time of sponge reef development in Bavaria, barrier and back reef lagoonal environments becoming established there. Coral and hydrozoan reefs also developed at this time, and coccoliths became more diverse. The development of coccoliths caused some bizarre facies like that of the Solnhofen Limestone, which formed in some inter-reef areas as very finegrained and laminated micrites. The Solnhofen fauna is sparse, but diverse, and it includes some of the most spectacularly preserved fossils ever found. These include ammonites, insects, dinosaurs, shrimps, and of course the earliest fossil bird (Archaeopteryx). It appears that the Solnhofen Limestone was another type of anaerobic sea bottom; this would account for the splendid preservation of fossils and laminations alike. It seems that the deaths of at least some of the swimming organisms were caused by coccolithophorid and dinoflagellate blooms poisoning the water. Rapid deposition helps to account for the good preservation of very active animals like the dinosaurs and arthropods.

Further to the south in Europe, the Upper Jurassic was characterized by a series of pelagic deposits that would have been similar to our modern-day 'oozes' when they were first laid down. Now they have become massive and bioturbated muddy or chalky limestones that sometimes contain irregular beds of chert. In many parts of the 'Alpine Belt', late Jurassic pelagic deposits of a different type occur; these are nodular, sometimes red, limestones full of corroded ammonites. In Italy this rock type is known as the Ammonitico Rosso and the term has come to be applied to the facies wherever (and, often erroneously, whenever) it appears throughout the belt. These ammonite-rich beds are often extremely condensed, and rich in the bivalve Bositra. They seem to represent isolated sea mount deposits where current activity was too strong to allow the accretion of coccolithic material and only cephalopod shells and specialized bivalves could accumulate in any number.

In the Western Interior basin of the United States, the sea retreated at the very end of the Jurassic from a huge area of Colorado, Utah, Wyoming, Montana, New Mexico and Arizona. Here the sediments consist of sandstones, with interbedded green clays (bentonites) containing an entirely terrestrial or freshwater fauna famous for its dinosaurs, and containing also turtles and mammals. These beds occur almost at the top of the Jurassic sequence. They were deposited during a time of world-wide facies changes taking place towards the end of Jurassic time. Until the Oxfordian-Kimmeridgian period, world-wide sea levels had been rising, but the Jurassic was brought to a close by an important phase of regression that was associated with a major series of earth movements. The terrestrial Morrison Formation with its impressive dinosaur faunas succeeds a shallow marine phase of Upper Jurassic deposits in which beds contain many ammonites similar at the generic level to those in northern Europe.

The late Jurassic was also a time when the marine faunas of the world showed striking provincialism in that the division between the two major marine 'Realms' of the Tethyan and Boreal persisted. The Tethyan Realm included the ancestral Pacific margins as well as the traditionally Tethyan Alpine-Himalayan Belt, while the Boreal Realm mostly included the areas of the inland or epeiric seas covering the great northern continent (Fig. m, p. 205). A most likely explanation of this provinciality is that the shelf seas provided less stable habitats than the oceanic areas, and thus supported less diverse assemblages of more tolerant organisms. This may explain the great dominance of bivalves in the Boreal benthos, and the reduced diversity of groups like the brachiopods (Hallam, 1975).

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