Cretaceous

The chalk in the popular scientific literature often Cretaceous (Latin Cretaceum, derived from cretaceous, usually shortened Cretaceous: chalk time, the Cretaceous period or in accordance with ancient rock formations on ) is a period of Earth's history. Within the Mesozoic era ( Mesozoic ), it is the youngest and 80 million years, the longest duration chronostratigraphic system ( or period in geochronology ). It began about 145 million years to the end of the Jurassic and ended about 66 million years ago with the beginning of the Paleogene, the oldest chronostratigraphic system of the Cenozoic.

History and naming

The name Cretaceous was in 1822, named by Belgian geologist Jean Baptiste Julien d' Omalius d' Halloy by the strong calcium -containing fossils of crustaceans, corals, clams, snails, and protozoa that are found in the rocks of this system. The more commonly " chalk " said rock is a special form of limestone. However, sedimentary rocks of the Cretaceous period can be made up of entirely different minerals, such as the predominantly of quartz grains existing and frequently occurring " Cretaceous sandstones ", whose name refers exclusively to their time of origin.

Definition and GSSP

The exact beginning of the Cretaceous system, and thus a GSSP has not yet been finally determined. Presumably, the lower limit of the Cretaceous ( and the Lower Cretaceous series and the Berriasian stage) will be defined by the first appearance of the ammonite species Berriasella jacobi. The end of the Cretaceous is very well defined with the iridium anomaly of the Cretaceous - Tertiary boundary and the extinction of many vertebrate and invertebrate groups.

Outline of the Cretaceous

The Cretaceous system is now divided into two series and 12 steps:

• System: Cretaceous ( 145-66 Ma) Series: Upper Cretaceous ( 100.5 to 66 Ma) Level: Maastrichtian ( 72-66 Ma)
• Level: Campanian ( 83.6 to 72 Ma)
• Level: Santonian ( 86.3 to 83.6 Ma)
• Level: Coniacian ( 89.7 to 86.3 Ma)
• Level: Turonian ( 93.9 to 89.7 Ma)
• Level: Cenomanian ( from 100.5 to 93.9 Ma)

• Level: Albian ( 112.9 to 100.5 Ma)
• Level: Aptian ( 126.3 to 112.9 Ma)
• Level: Barremian ( 130.7 to 126.3 Ma)
• Level: Hauterivian ( 133.9 to 130.7 Ma)
• Level: Valanginian ( 139.3 to 133.9 Ma)
• Level: Berriasian ( 145 to 139.3 Ma)

Parent, but now outdated stages designations are: Neocomian ( Lower Cretaceous ), Gault (upper Lower Cretaceous ), Emscher (now Coniacian and Santonian ) and Senonium (now Santonian, Campanian and Maastricht).

Paleogeography

The disintegration of Gondwana, which had already started in the Jura, is continued in the Cretaceous. It comes to the separation of the more cohesive Australia / Antarctica and the beginning of the Cretaceous also still coherent Africa / South America, and India split off. In the Lower Cretaceous initially the southern South Atlantic Ocean begins to open. This opening will come during the Lower Cretaceous continued north. In the Turonian then a continuous connection was created to the North Atlantic. In the North Atlantic Ocean spreading begun in the Jura Mountains between North Africa and the North American East Coast proceeds northward. During the Cretaceous, the section between the Iberian Peninsula and Newfoundland forms. In the higher Cretaceous and the deeper Upper Cretaceous and the Bay of Biscay, the extension ranges in the Pyrenees area spreads. In the Upper Cretaceous west of Ireland formed a triple - a branch ends in a grave system between North America and Greenland, the other expands in the Upper Cretaceous and Cenozoic to the present in the northern North Atlantic. In the Alps occur first collisions ( " vorgosauische orogeny ").

Climate

The climate in the Cretaceous was generally warm and balanced. It allowed some dinosaurs, at least in the summer months to penetrate into high southern and northern latitudes. The poles were free of ice and the sea level was accordingly very high, it reached its maximum value in Unterturon. Only at the end of the Cretaceous in the Maastrichtian there was a cooling and to a marked regression.

Development of the fauna

From the Cretaceous period is known in Germany numerous footprints and skeletal remains of dinosaurs. In Münchehagen at Rehburg -Loccum was discovered among other things, the almost 30 -meter-long trail of a elefantenfüßigen " Thunder Lizard " ( Elephantopoides muenchehagensis ). Are particularly frequent footprints (upper churches, Münchehagen ) and skeletal remains ( Nehden in Brilon in the Sauerland) of herbivorous dinosaur Iguanodon, which reached a height up to 5 meters. Fossils prove the existence of predatory dinosaurs such as Megalosaurus and of marine reptiles - such as mosasaur - and crocodiles.

In the Cretaceous ocean was inhabited by giant ammonites. With a case diameter of about 1.80 meters is a copy of Parapuzosia seppenradensis from the Westphalian Bay in Munsterland the previously largest known ammonite.

Recent findings from the northeast China Jiulongshan Mountains ( Inner Mongolia Ningcheng Province, Daohugou ) and especially from the also located in Inner Mongolia Jehol Group show that the mammals in the Cretaceous not only, as previously believed, consisted of insectivorous small forms. A wide variety of fully developed mammals in the Cretaceous was preceded by the separation in different lines of development already in the Jura. The mammals of the Jurassic and Cretaceous already occupied the same ecological niche, fill in the today's mammals, and developed very similar forms of adaptation. There have been small mammal predators that specialized in reptiles and equipped with a waterproof fur, floating mammals that occupied the ecological niche of today's otters.

Development of Flora

In the Lower Cretaceous were still Bärlapppflanzen ( Nathorstiana aborea ), ferns ( Weichselia, Hausmannia ), tree ferns, Ginkos ( Baiera ) Bennettitales, and conifers, the dominant plants. From this period the coal seams of the Wealdenkohle in the Weser-Ems region originate at the edge of the Teutoburg Forest. During the Cretaceous, the first shrubby flowering plants developed. The first genus of deciduous woody plants was Credneria with tricuspid sheets ( finds from the resin). In the Upper Cretaceous already competed many deciduous trees such as maple, oak or walnut with conifers such as Sequoia and Geinitzia ( from the Aachen layers, Upper Santonian ). Grasses spread out on the mainland and heavily modified the erosion behavior.

The chalk in Central Europe

Rocks of the Cretaceous are the Hannover area, north of the resin, in the Teutoburg Forest in the Extern Stones, in the Westphalian Bay and in the space of Aachen to Liège to. Famous are the chalk cliffs in the Jasmund National Park Ruegen. Furthermore, deposits found from the Cretaceous east of the Frankish Alb and the Alpine - northern border, in the vicinity of Dresden and Decin ( Elbe Sandstone Mountains ), in large parts of the Czech Republic as well as in the Carpathian foothills and between Kielce and Krakow.

Special events during the Cretaceous

One of the outstanding events of the mid-Cretaceous period between approximately 120 million and 80 million years ago a huge Superplume activity belongs in the area of the western Pacific. Of the 40 million years of persistent volcanism huge extent on the Pacific Ocean floor had global consequences.

Despite prevailing during the Cretaceous comparatively very warm temperatures, there is evidence of glaciation phase in the region of the poles in front of about 91.2 million years.

At the end of the Cretaceous period, there was a global mass extinction, the captured almost all animal groups and many groups of plants. There are various ideas about the causes; the most common theory is a meteorite impact in the Yucatan ( Chicxulub crater ). But the enormous volcanic activity in the formation of the Deccan Plateau basalts at the end of the Cretaceous period could well have played a decisive role.