Jurassic

On the geologic time scale, the Jurassic is the middle period of the Mesozoic era; the name was introduced in 1795 by A. von Humboldt, for whom, however, the Jurassic soils were included between the Permian and the Buntsandstein (lower Trias); later A. Boué placed the Jurassic exactly between Trias and Cretaceous, and A. Brongniart defined its main features.

The absolute duration of the Jurassic varies, according to the authors, from 25 to 45 million years (according to the absolute dating of Kulp the Jurassic goes from 180 to 135 million years ago, according to that of Holmes from 152 to 127 million years ago). In Europe, the Jurassic is commonly divided into three subperiods: Lias, Dogger and Malm, which correspond in principle to the Lower, Middle and Upper Jurassic. In Germany, however, is also adopted a tripartition, Black Jura, Brown Jura and White Jura, based on the color of the formations . While for the lower limit of the Jurassic an agreement has been reached (Luxembourg Jurassic colloquium, 1962), placing the Rhaetian in the Trias and considering the Hettangian as the basal plane of the Jurassic, for the upper limit the discussion is still open.

In the northern area (boreal region) the Jurassic ends with a regression: in this regard, some authors distinguish a Portlandian plane, still marine, and a Purbeckyan plane, brackish and lacustrine, to be considered rather as regressive facies of the Portlandian.

In the Mediterranean area, interested by the alpine geosyncline (Mediterranean province), the Jurassic is in stratigraphic continuity at the base with the Trias and at the top with the Lower Cretaceous and often it is not easy to distinguish between the Titonic, as the summit plane is indicated, even if it does not have the necessary requirements to be officially accepted as a plane both for the lack of a type series that defines it and for the inadequacy of the name, and the Neocomian.

Finally, for the eastern area of the boreal province, Russian scholars define the Volgian as the Jurassic summit plane, due to the presence of an ammonitic fauna quite different from that typical of the western area and that of the Mediterranean province.

The Jurassic lands are characterized by a remarkable richness of Ammonite faunas to the point that, taking advantage of the limited duration of most species, it was possible to divide them into numerous “zones”, each characterized by the presence of one or more significant species of these Cephalopods.

Orogenesis

As far as orogenic phenomena are concerned, while Europe and Asia go through a period of quiet, in America there are impressive orogenic phases, Andean phase (between Jurassic and Cretaceous) in the Andes and Nevadian (Upper Jurassic) and Diablean (between Jurassic and Cretaceous) phases in the North American cordilleras. Magmatic phenomena are of essentially basic type as evidenced by basal eruptions of femic and ultrafemic magmas in the Tethys geosyncline (ophiolites or greenstones), connected with the embryonic phases of the Alpine orogeny, and basaltic eruptions in the Karroo (South Africa) and Paraná, while large porphyritic flows were effused in the Andes.

Paleogeography

In the Jurassic the palaeogeography does not present appreciable differences with that of the preceding period; as far as Europe is concerned, the contrast between the vast mesogean sea, the Tethys, and the North Atlantic continent persists: in the Mediterranean province deep seas with irregular bottom dominated, interested by cordilleras (partially emerging) constituting the embryos of some of the major tectonic units that later will be part of the alpidic chains. In continental Europe, on the other hand, marine transgression at the end of the Rhaetian period led to the formation of epicontinental seas from which emerged the “hercynian massifs” or “ancient massifs”, which separated some subsiding internal basins such as the Anglo-Paris, Aquitaine, Rhone and Swabian.

At the end of the Jurassic a wide arm of sea separated, crossing Russia, the Angara continent from the North Atlantic continent (the Atlantic Ocean was not yet identified), here and there covered by epicontinental seas: the Scandinavian shield, Iceland, Scotland, most of England, Canada and the central regions of the United States emerged. Jurassic deposits reappear instead within the Pacific domain (Rocky Mountains, Mexico, Andean region). The continent of Gondwana is now reduced to the South America-Africa block, and therefore also the South Atlantic has not yet formed; the dismemberment of the Australo-Indo-Malagasy block (Lemuria) is in progress with the setting of the Indian Ocean.

Climate, flora and fauna

On the basis of the fauna it would seem that in the Jurassic the climates were differentiated so much to characterize a boreal zone and a tropical one separated by a belt of transition, but of this there is not complete confirmation in the flora that presents instead a sensible uniformity on all the surface of the globe, with predominance of the Cycadines and the Bennettitali, followed by the Conifers and the GinkgofiteOf remarkable interest is the appearance of the Caytonali, presumed group progenitor of the Angiosperms.

The fauna is very rich and often characteristic: all classes of the animal kingdom are represented, since they make their appearance also the Birds. Among the Protozoa prevail the Radiolarians, but of insignificant stratigraphic value unlike the Foraminifera (Lituolidae, Nodosaridae) and Tintinnidae (Calpionella alpina). Siliceous sponges are widespread; Corals are abundant in the middle and upper Jurassic. Widespread are the Echinoderms (Crinoids and Echinoids), Brachiopods and Bivalves, unlike Gastropods. By far the most important group of invertebrates, however, is that of the Ammonoidea, which has a large number of genera and species on which the stratigraphy of this period is based.

Significant in a stratigraphic sense are also the haptics (perhaps opercula of Ammonites) particularly abundant in some levels: “haptic layers” of Dogger and Malm. Widespread but scarcely significant are instead the Belemmites. Among the vertebrates we know many species of fish, but the most important class is that of Reptiles, which presents a development of number, variety and size truly exceptional. Among the aquatic reptiles the most characteristic are the Ichthyosaurs and the Plesiosaurs; the Pterosaurs, flying reptiles, are represented by the genera Pterodactylus and Ramphorhynchus. The terrestrial reptiles reach enormous dimensions with some Sauropods (Diplodochi and Brontosaurus), herbivorous dinosaurs of the upper Jurassic that exceed 20 m in length. The Jurassic saw the appearance of the first Birds (Archaeopteryx); Mammals are still represented by primitive forms of modest size.

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