Jurassic

Jurassic The splendidly fossiliferous limestones and shales of the Jura Mountains of Switzerland attracted the attention of the early scientists of Europe long before their true nature was realized. They eventually drew the attention of the traveller and scientist, Alexander von Humboldt (1769–1859) who in 1795 had written about them, using the term Jura-Kalkstein. Somewhat later, when Leopold von Buch came to study them in the 1830s, their correlation with similar fossiliferous beds in England and Germany became apparent to him and he used Humboldt's term in establishing a formal system of rocks for all three areas (1839). The name was immediately used by the many amateur and professional geologists who then were beginning to make maps of the fossilerous formations in the various parts of Europe. The ammonite fossils were everywhere an immediate clue to the age and correlation of the rocks of the Jurassic system. They are distinct from those of the marine Triassic rocks beneath and from the ammonites of the Cretaceous system above.

The Jurassic is the second of the three Mesozoic systems, and was formed over a span of about 70 million years prior to the commencement of the Cretaceous period (135 Ma). The system is divided into 11 stages and over 70 ammonite zones in Britain. Virtually all the stages are named after localities in western Europe where the system is extensively developed in neritic (shallow-water) facies. The German palaeontologist Albert Oppel realized in the mid-nineteenth century that ammonites give by far the best means of correlation within the Jurassic. They are widespread and evolved rapidly; only within a few nearshore or brackish-water facies are they rare or absent. Oppel's detailed zonal scheme for the Jurassic has provided the basis for our modern zonation. Moreover, subzones can be defined within many of the Jurassic ammonite zones, and even smaller biostratigraphic units (horizons) characterized by the presence of a short-lived species have been identified. There are some 44 of these in the Middle Jurassic Aalenian and Bajocian Stages in western Europe. This is biostratigraphy on a very detailed scale. Where alternative zonal schemes have been erected—using foraminifera or coccolith microfossils, for example—the species used have longer time ranges than the ammonites. None of the Triassic superfamilies of ammonites lived on into the Jurassic, but three new superfamilies appeared in the Early Jurassic. Significant extinction events of ammonites occurred in the late Early Jurassic, at the Middle–Late Jurassic boundary, and at the end of the period. Such was the palaeogeography of the Jurassic that the biota was distributed across a number of palaeoecological provinces; and this provinciality poses a number of problems. This is apparent in the uppermost stage, known variously as the Tithonian, Portlandian, and Volgian in different parts of the Eurasia. They are not exactly equivalent, and the Volgian has a boreal fauna, different from the Tethyan (Alpine–Himalayan) faunas of the other two. In the absence of ammonites, the dinoflagellates may provide marine correlation, while spores and pollens serve best in non-marine facies.

Many stratigraphers have regarded this system in western Europe as reflecting a full cycle of marine deposition, beginning with a marine transgression and ending with a sharp regression. Detailed study, however, has shown that this is an oversimplification: several important transgressions follow on from phases of uplift and erosion, despite the then relative stability of much of the western European continental area.

The palaeogeography of the Jurassic period was one of change, with the break-up and dispersal of the rifted fragments of Pangaea now well under way (Fig. 1). The two principal continental masses, Laurasia and Gondwanaland, became increasingly separated by the equatorial Tethyan ocean. During the latter half of the period, north-western Africa parted from North America to create a narrow ocean, the forerunner of the Atlantic. Somewhat later the separation of South America, Africa, and Antarctica began, while ocean floor spreading in the proto-Pacific Ocean began to move small land masses on to the margins of Asia and the Americas. Plate-tectonic motions thus appear in the Jurassic period to have been more important in bringing about geographical change, with accompanying rise in sea level and climatic amelioration, than they had been in the Triassic period. Volcanic activity took place on an enormous scale in various parts of the world. Dyke-swarms were intruded in many regions of Africa. The lavas of the Karroo in southern Africa covered perhaps 2 000 000 km² to a thickness approaching 9 km. Lavas are also present in a zone running across the centre of Antarctica.

One of the most tectonically active regions of the Mesozoic world was the great belt of island arcs, deep troughs, and volcanoes stretching along the western margin of the Americas from Alaska to Panama and southwards to southern Chile. There was almost continuous crustal unrest and volcanism here, and at depth beneath the newly rising mountains great batholiths of granite and granodiorite were intruded. Volcanic rocks of enormous thickness and deep-seated igneous intrusions from this period have created much of the geology of the Peruvian Andes. Subduction of the Pacific crustal plates beneath the margins of the North American and South American plates was responsible, and the process has continued ever since.

It was a time of slow, impersistent but inevitable rise in sea level, with up to a quarter of the present continental area flooded in Late Jurassic time. The period began with sea level at a relatively low stand at around 14 per cent of the continental area under water and rather less at the end. Many geologists see this period as exhibiting a great eustatic cycle within which many smaller cycles developed. Only towards the very end of the period did sea level begin to decline rather sharply. This Jurassic inundation was very extensive in Laurasia, but most of North America and Gondwanaland was not greatly affected. Nevertheless, a narrow sea spread down the easthern flank of the present Rockies during the mid-part of the period. Much of eastern Asia persisted as an upland area. Palaeomagnetic polarity was very mixed throughout the Jurassic, but with an interval of relatively steady normal polarity in mid-period.

The climate seems to have been warm and equable; with no polar icecaps and with an equatorial ocean, conditions existed to give tropical climates well up into the present north temperate latitudes. The continental interiors, distant from the sea, were generally low-lying and arid.

Marine life was prolific with a widespread development of both calcareous benthonic organisms and vigorous free-moving forms. Reefs of corals, algae, bryozoa, and sponges are known, while the bivalvia were the almost universally dominant stock of the benthos elswhere. Gastropods, belemnites, echinoids, foraminifera, and ostracods were important in many deposits. A complex picture of developing faunal provinces emerges from studies of the Jurassic shallow-water facies. It appears to depend upon many factors, including climate and water circulation. Many areas of very strong salinity have been demonstrated. Open-water planktonic and nektonic faunas were dominated by the cephalopods. The ammonites were by far the most prolific, with many fast-evolving lineages. They have proved to be most useful in biostratigraphy because of their robust preservation, rapid evolution, and widespread distribution. This serves to illustrate the remarkable change in cephalopod fortunes following the late Triassic extinction, when only one family survived into the Jurassic. Marine vertebrates included the large dolphin-like ichthyosaurs and the long-necked plesiosaurs, marine crocodiles, turtles, and other reptiles. So high was local organic productivity during Jurassic times that many sedimentary basins became rich in petroleum hydrocarbons.

On land, there was a rich flora in the more rainy regions to provide swamp and forest coverage. Gymnosperms, including conifers, gingkoes, and cycads, were very numerous, as were ferns and horsetails. From the Jurassic forests are derived extensive coal deposits, some of the seams being 5 m or more in thickness. They are largely confined to the eastern parts of Laurasia and Gondwanaland, where a climate for long-continuing forest growth persisted into the Cretaceous. Palaeobotanical studies suggest that the temperature gradient from low to high latitudes was significantly less than today in the northern hemisphere.

Tetrapod populations were overwhelmingly of reptiles. The Triassic thecodonts had given rise to the dinosaurs and pterosaurs in the late Triassic, and these now gave rise to an extraordinary range of adaptations to new habitats. Bipedal and quadrupedal herbivors achieved considerable size, while predatory types also are well represented by active bipedal forms. By late Jurassic time the pigeon-sized Archaeopteryx had evolved and true birds were soon to follow. The mammals remained small and relatively subordinate, but the insectivors and omnivors were undoubtedly diverse and numerous. Among the fishes the holosteans, with their thick enamelled scales, were the most prolific. Some were of very large size. The sharks included large species that preyed upon the schools of cephalopods, and shell-feeding forms that snatched large bivalves from the sea floor.

The natural resources of the Jurassic system are very varied and extensive. They include all manner of construction materials, mineral ores, and non-metallics associated with igneous and sedimentary formations, fossil fuels, and other strategic materials.

D. L. Dineley

Bibliography

Arkell, W. J. (1956) Jurassic geology of the world. Oliver and Boyd, Edinburgh.
Hallam, A. (1975) Jurassic environments. Cambridge University Press.
Hallam, A. (1984) Continental humid and arid zones in the Jurassic and Cretaceous. Palaegeography, Palaeoclimatology, Palaeoecology, 46, 195–223.
Sellwood, B. W. (1978) Jurassic. In McKerrow, W. S. (ed.) The ecology of fossils. Duckworth, London.