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Jurassic

The system of rocks deposited during the middle part of the Mesozoic Era, and encompassing an interval of time between about 200 and 142 million years ago, based on radiometric dating. It takes its name from the Jura Mountains of Switzerland. Its rich marine invertebrate faunas in western Europe have been the subject of intensive study since the pioneering days of geology in the early nineteenth century, and provided the basis for the fundamental stratigraphic concepts of stages and biozones. See also: Dating methods

Subdivisions

The Jurassic System is subdivided into 11 stages which, with the exception of the Tithonian, are named from localities in England, France, and Germany (Fig. 1). These, and the much greater number of zones, are based upon ammonites, which are by far the most valuable fossils biostratigraphically because of their high rate of species turnover in time due to rapid evolution and extinction. The most refined stratigraphic subdivisions have been made in the British Isles, with 54 zones and 176 subzones. Because of biogeographic provinciality, with different ammonite taxa inhabiting Boreal and Tethyan realms, difficulties of correlation can occur for younger Jurassic strata; and the youngest stage in the Tethyan Realm, the Tithonian, embracing most of the world, is equivalent to the Volgian stage of the Boreal Realm, extending from northern Eurasia to northern North America. The ammonites of the Volgian are quite different from those of the stratigraphically equivalent Tithonian. In the absence of ammonites, dinoflagellates are the most useful marine fossils for correlation, but in nonmarine strata problems of correlation are considerable, and stratigraphically less satisfactory pollen and spores have to be used. See also: Stratigraphy

Fig. 1 Succession of Jurassic stages, with estimated radiometric ages in millions of years. (After J. Palfy et al., A U-Pb and 40Ar/39Ar time scale for the Jurassic, Can. J. Earth Sci., 37:923–944, 2000)

Paleogeography and sea level

The main continental masses were grouped together as the supercontinent Pangaea, with a northern component, Laurasia, separated from a southern component, Gondwana, by a major seaway, Tethys, which expanded in width eastward (Fig. 2). From about Middle Jurassic times onward, this supercontinent began to split up, with a narrow ocean being created between eastern North America and northwestern Africa, corresponding to the central sector of the present Atlantic Ocean. At about the same time, and continuing into the Late Jurassic, separation began between the continents that now surround the Indian Ocean, namely Africa, India, Australia, and Antarctica. As North America moved westward, it collided with a number of oceanic islands in the eastern part of the PaleoPacific. Because the impingement was an oblique one, there was a general tendency for these accreted landmasses to be displaced northward along the cordilleran zone of the subcontinent. Other examples of so-called displaced terranes are known on the Asian side of the North Pacific, and some of the accretion of oceanic islands took place in Jurassic times.

Fig. 2 Approximate distribution of land and sea in the Oxfordian stage. Small islands are excluded, but boundaries of modern continents are included as a reference.

There were also important paleogeographic changes later in the period involving the Tethys zone. An older, so-called Palaeotethys was progressively closed as an extensive, narrow continent known as Cimmeria, extending east-west, and collided with the southern margin of Eurasia. The name comes from the Crimean Peninsula of Russia, where there is well-displayed evidence of an intra-Jurassic orogenic disturbance indicative of continental collision. See also: Orogeny; Paleogeography

Sea level rose progressively through the period, with a corresponding flooding of the continents by shallow epeiric seas, that is, shallow seas that covered part of the continents but remained connected to the ocean. At the beginning, such seas covered less than 5% of the continents, but near the end, in Oxfordian and Kimmeridgian times, they covered approximately 25% (Fig. 2). The Jurassic sea-level curve also shows a succession of smaller-scale changes, of a duration of a few million years. Some of these, such as the Early Toarcian sea-level rise, are clearly global or eustatic, but others are more controversial and may reflect regional tectonic activity rather than truly global phenomena. It is uncertain by how much the sea level rose during the course of the period; but by using a hypsometric method, an estimate of between 330 and 500 ft (100 and 150 m) can be made. See also: Paleoceanography

Climate

The climate of Jurassic times was clearly more equable than at present, as indicated by two sets of facts. The first concerns the distribution of fossil organisms. Thus a number of ferns whose living relatives cannot tolerate frost are distributed over a wide range of paleolatitudes, sometimes as far as 60° N and S. Similarly, coral reefs, which are at present confined to the tropics, occur in Jurassic strata in western and central Europe, beyond the paleotropical zone. Many other groups of organisms had wide latitudinal distribution, and there was much less endemism (restriction to a particular area) with respect to latitude than there is today. The second set of facts concerns the lack of evidence for polar icecaps, such as extensive tillites or striated pavements.

However, there must have been strong seasonal contrasts of temperature within the Pangean supercontinent, and climatic modeling suggests winter temperatures at zero Celsius at or close to the paleopoles. A limited amount of evidence from northern Siberia and arctic North America, in the form of apparent glacial dropstones and glendonites, suggests the possibility of some ice, but this ice is likely to have been seasonally transient and small in volume.

There is no evidence of any significant change in the temperature regime through the Jurassic, but there are indications of a change in the humidity-aridity spectrum. Unlike the present, there were no tropical rainforests. Instead, a large area of western Pangea experienced an arid to semiarid climate in low latitudes, especially at some distance from the ocean. Precipitation is likely to have been dominantly monsoonal rather than zonal, a pattern unlike that of today. For most of the period, the continental area represented today by Eurasia had a comparatively humid climate, as indicated in nonmarine sediments by coals and the abundance of the clay mineral kaolinite. Toward the end of the Jurassic, however, there was a change to a more arid climate, indicated by the disappearance of coals and kaolinite and the occurrence of evaporites such as rock salt and gypsum. The reason for this change is unclear, but it may be bound up with a rainshadow effect created by the collision of the Cimmerian continent. See also: Paleoclimatology; Saline evaporites

Tectonics and volcanicity

Most of Pangea experienced tensional tectonics as the supercontinent began to break up. This is manifested by graben and half-graben structures, with associated alkaline volcanicity. By far the largest flood basalt province is that of the Karoo in South Africa, most of the basalts and associated igneous rocks being erupted in the Early Jurassic, prior to the breakup of Africa, Madagascar, and India. The Middle Jurassic Ferrar dolerites of Victoria Land, Antarctica, and the contemporaneous Tasmanian dolerites are further manifestations of tensional tectonics, as are earliest Jurassic basalts in eastern North America and Morocco, again signifying tension prior to the Atlantic opening. The North Sea region of western Europe is in effect an aborted oceanic rift, with a major phase of tensional activity and associated volcanicity in the Middle and Late Jurassic. This did not lead, however, to the creation of true ocean. See also: Basalt; Graben

Compressional tectonics associated with subduction of ocean floor took place in many parts of the Pacific margins, with associated calc-alkaline volcanicity. An excellent example is the Andes. The North Pacific margins were also associated with significant strike-slip faulting bound up with the accretion of displaced terranes. The other important zone of compressional tectonics was along the southern margin of Eurasia, and is involved with the collision of the Cimmerian continent. See also: Fault and fault structures

Since it is not plausible to invoke the melting and freezing of polar ice caps to account for Jurassic sea-level change, this change must be bound up with tectonic activity. The most plausible mechanism for accounting for long-term sea-level rise is the growth of oceanic ridges, displacing seawater onto the continents, but the cause of short-term sea-level changes is more obscure and remains controversial. See also: Continents, evolution of; Geosyncline; Mid-Oceanic Ridge; Plate tectonics; Subduction zones

Vertebrate fauna

The vertebrate terrestrial life of the Jurassic Period was dominated by the reptiles. The dinosaurs had first appeared late in the Triassic from a thecodont stock, which also gave rise to pterosaurs and, later, birds. From small bipedal animals such as Coelophysis, there evolved huge, spectacular creatures. These include the herbivorous Apatosaurus, Brontosaurus, Brachiosaurus, Diplodocus, and Stegosaurus as well as the carnivorous, bipedal Allosaurus. Only two rich dinosaur faunas are known from Jurassic deposits, the Morrison Formation of the United States Western Interior and the approximately contemporary Tendaguru Beds of Tanzania. The two faunas are strikingly similar at family and generic level, which strongly suggests that free land communications existed between western North America and East Africa until quite late in the period, a fact that is not easy to reconcile with some paleogeographic reconstructions. See also: Dinosauria

Flying animals include the truly reptilian pterosaurs and the first animals that could be called birds as distinct from reptiles, as represented by the pigeon-sized Archaeopteryx. There were two important groups of reptiles that lived in the sea, the dolphinlike ichthyosaurs and the long-necked plesiosaurs. Both of these groups had streamlined bodies and limbs beautifully adapted to marine life. Turtles and crocodiles are also found as fossils in Jurassic deposits. See also: Archaeopteryx; Pterosauria

Jurassic mammals, known mainly from their teeth alone, were small and obviously did not compete directly with the dinosaurs. They included a number of biologically primitive groups such as the triconodonts, docodonts and multituberculates. The fish faunas were dominated by the holosteans, characterized by heavy rhombic scales. Their evolutionary successors, the teleosts, probably appeared shortly before the end of the period. See also: Docodonta; Holostei; Multituberculata; Teleostei; Eutriconodonta (Triconodonta)

Invertebrate fauna

Because they are far more abundant, the invertebrate fossil faunas of the sea are of more importance to stratigraphers and paleoecologists than are the vertebrates. By far the most useful for stratigraphic correlation are the ammonites, a group of fossil mollusks related to squids. They were swimmers that lived in the open sea, only rarely braving the fluctuating salinity and temperature of inshore waters. They are characteristically more abundant in marine shales and associated fine-grained limestones. From a solitary family that recovered from near extinction at the close of the Triassic, there radiated an enormous diversity of genera. Many of these were worldwide in distribution, but increasingly throughout the period these was a geographic differentiation into two major realms. The Boreal Realm occupied a northern region embracing the Arctic, northern Europe, and northern North America. The Tethyan Realm, with more diverse faunas, occupied the rest of the world. See also: Limestone; Shale

In most facies the bivalves, which flourished in and on shallow, muddy sea bottoms, are the most abundant and diverse of the macrofauna. They included many cemented forms such as Ostrea, recliners such as Gryphaea, swimmers such as the pectinids and limids, and rock borers such as Lithophaga. However, the majority were burrowers: either relatively mobile, shallow burrowers or forms occupying deep permanent burrows and normally still found in their positions of growth. See also: Bivalvia; Facies (geology)

Brachiopods were much more abundant and diverse than they are today. The range of depths below the sea surface that they occupied is far wider than for the bivalves, and a definite depth zonation can be established in Europe, just as with the ammonites. See also: Brachiopoda

Echinoderms are best represented as fossils by the crinoids and echinoids, and were all inhabitants of shallow seas, unlike some of the modern representatives of this class. The echinoids include both primitive regular forms, such as the cidaroids, and irregular forms, such as Clypeus and Pygaster. See also: Echinodermata; Pygasteroida

Corals belonged to the still extant Scleractinia group and included reef builders such as Isastrea and Thamnasteria. Calcareous and siliceous sponges are also common locally, even forming reefs. It seems likely that the siliceous sponges inhabited somewhat deeper water than the corals. See also: Scleractinia; Sclerosponge

The invertebrate microfaunas are represented by abundant foraminifera, ostracods, and radiolaria. Foraminifera and ostracods are of great value to oil companies in correlation studies. See also: Ostracoda; Radiolaria

Not all Jurassic invertebrates lived in the sea. Some lived in continental environments such as lakes and rivers; they include a few genera of bivalves, gastropods, and arthropods. These faunas are far less diverse than their marine counterparts. See also: Arthropoda; Gastropoda; Paleontology

Flora

With regard to the plant kingdom, the Jurassic might well be called the age of gymnosperms, the nonflowering “naked seed” plants, forests of which covered much of the land. They included the conifers, gingkos, and their relatives, the cycads. Ferns and horsetails made up much of the remainder of the land flora. These and others of the Jurassic flora are still extant in much the same forms. See also: Cycadales; Ginkgoales

Remains of calcareous algae are widely preserved in limestone. Besides the laminated sedimentary structures produced by what have traditionally been regarded as blue-green algae but are actually cyanobacteria, and known as oncolites and stromatolites, there are skeletal secretions of other groups. Some of these are benthic forms, but many pelagic limestones are seen under the electron microscope to be composed largely of tiny plates of calcite, known as coccoliths, which are secreted by certain planktonic algae also called coccoliths. See also: Algae; Cyanobacteria; Stromatolite

It seems likely that the Late Jurassic saw the emergence of the flowering plants, the angiosperms, since well-developed forms of this group existed in the Early Cretaceous. However, it is not quite understood how they emerged, and a satisfactory direct evolutionary ancestor has yet to be identified with certainty.

Economic geology

Jurassic source rocks in the form of organic-rich marine shale and associated rocks contain a significant proportion of the world's petroleum reserves. A familiar example is the Upper Jurassic Kimmeridge Clay of the North Sea, and its stratigraphic equivalents in western Siberia. Some of the source rocks of the greatest petroleum field of all, in the Middle East, are also of Late Jurassic age. See also: Mesozoic; Petroleum geology

A. Hallam

Bibliography

W. J. Arkell, Jurassic Geology of the World, 1956

J. W. C. Cope et al., Jurassic, pts. 1 and 2, Geol. Soc. Lond. Spec. Rep. 14 and 15, 1980
A. Hallam, Jurassic climates as inferred from the sedimentary and fossil record, Phil. Trans. Roy. Soc. Lond., B 341:287–296, 1993
A. Hallam, Jurassic Environments, 1975
A. Hallam, A review of the broad pattern of Jurassic sea-level changes and their possible causes in the light of current knowledge, Palaeogeog., Palaeoclimatol., Palaeoecol., 167:23–37, 2001

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