Lower Jurassic Communities In Europe And North America

All the Jurassic communities from Britain which have been described lived in the shallow sea which spread over parts of northwest Europe during the early Jurassic.

In western North America, the Lower Jurassic commenced with a major phase of marine transgression and in general the western states (California, Oregon and Nevada) remained domin-antly marine. The seaway extended northwards through western Canada into southern Alaska and southwards into north-west Mexico (Fig. m). Eastwards from Nevada and Oregon, the marine facies pass into dune-bedded continental sands and lacustrine sediments which contain remains of terrestrial dinosaur faunas. The shallow seas immediately to the west of these areas contain sediment and faunas similar in many respects to those of northern Europe. However, the bivalves Gryphaea and Oxytoma are latecomers in North America, arriving in the late Lower Jurassic via

Canada. Conversely, the bivalves Trigonia and Myophorella appear in the early Jurassic of North America, well before their arrival in Europe during the early Middle Jurassic. These latter two genera appear to have evolved in the Pacific during the Trias and spread thence through western South America, North America and on to Europe.

In North America, similar shallow marine facies often contain identical genera to their European counterparts. For example, the coral Styllophyllopsis occurs both in the shallow marine limestones of the Sunrise Formation in Nevada and in the similarly deposited calcarenites of South Wales (Fig.70), (Hallam, 1965). However, the great suites of volcanic rocks which occur to the west of the shallow marine areas have no counterparts in northern Europe, but represent island-arc and marginal oceanic environments on the margins of the Jurassic Pacific Ocean (Stanley et al. 1971).

In Europe, there is a transition from northern areas (like Britain) which were mostly receiving a continuous supply of clastic sediments from adjacent landmasses, to southern areas where little land-derived material was deposited. The shallow water sediments in these southern areas were mostly carbonates (limestones and dolomites), with some salt deposits occurring in the more isolated saline areas. Some of these carbonates indicate a warm sea, as well as a sea free from land-derived detritus. Palaeo-magnetic evidence suggests that southern England was at a latitude of about 40° north at this time, so southern Europe was in the tropics during the Lower Jurassic. To the south of the shallow water carbonates lay the Tethys Ocean (though some of the carbonates may originally have been formed to the south of the Tethys); deep water deposits laid down on the floor of the Tethys, with only pelagic faunas and floras, are found today in Mediterranean areas.

Highly continental climates are those with the largest temperature unevenness, and with the widest variety of seasonal conditions creating corresponding fluctuations in primary productivity. The most diverse marine communities should occur on shelves in low latitudes with the most stable habitats (Valentine, 1973). Communities with lower diversities would be expected in more continental or epicontinental situations.

The shallow carbonate areas of southern Europe are best seen in the Alps and some areas of southern Spain, central Italy and the Balkans. Early Jurassic environments in this region were similar to those existing today in Florida and the Bahamas. Locally the limestone beds are very thick, and throughout the late Trias and early Jurassic high rates of organic productivity caused limestone deposition to keep pace with the rapid subsidence, and the sea was seldom very deep. The fauna of these limestones is diverse compared with that of the clastic areas. Grazing gastropods are dominant; many of them fed on the rich algal material. Stromatolites are often abundant; brachiopods, bivalves, ostracodes, foramin-fers and echinoderms are all present, many of the genera being confined to these environments and absent in the northern clastic areas. Ammonites, however, were only rarely present, usually preferring more open water habitats.

The diversity of the fauna in these massive limestones is very similar to that found in Florida Bay today where more than 100 genera of gastropods and bivalves are present. Water depths are seldom greater than 3m over an area of 15,000 square km. Within this large area, however, large seasonal fluctuations in temperature, rainfall (and therefore salinity) and turbulence (periodic storms) produce high environmental stresses which eliminate less tolerant species. Thus the more open water habitats contain the most diverse communities and this seems to have been the case on the margins of Tethys.

Towards the end of the Lower Jurassic time, and apparently synchronously along much of the Alpine-Mediterranean region, rates of subsidence increased so that carbonate production could no longer keep pace with the deepening sea (Bernoulli and Jenkyns, 1974). This is seen in the sudden change from shallow water to deep water sediments. The sea became too deep to be populated by abundant bottom-dwelling animals, but there is a rich pelagic fauna of ammonites and radiolaria, which sank to the sea floor after death.

Three sedimentary facies are common. On the more elevated parts of this deep sea, condensed red limestones accumulated, rich in floating and swimming organisms (ammonites, belemnites, and Bositra). Around these 'swells', deeper regions received thicker deposits of bioturbated marls with rare brachiopods (Pygope), belemnites, and ammonite aptychi. Aptychi are the calcite jaw apparatus of some ammonites. It is known that at great depths there is a solution of calcium carbonate; it is possible that, at the depths represented by the marls, the aragonite shells were dissolved but not the calcite shells, hence the preservation of the aptychi without the main part of the ammonite shells. At greater depths still, no carbonate fossils occur at all. On the ocean floor, basaltic rocks (which underlie all oceans), were interbedded with radiolarian cherts.

During the phases of subsidence, the surface of the carbonate platforms were extensively colonized by thick-shelled bivalves with strong hinge structures (Opisoma, Pachymegalodon and Pachymytilus), brachiopods (Hesperithyris and other rhynchonel-lids) and crinoids. Lenses of crinoid debris occur. These fragments sometimes drifted into crinoidal sand waves on the surface of collapsing Tethyan sea mounts (Jenkyns, 1971).

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