Chalk (geology)

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< Jura | Chalk | Paleogene >
145–66 million years ago
Atmospheric O 2 share
(average over period) 		approx. 30% by volume
(150% of today's level)
Atmospheric CO 2 share
(average over period) 		approx. 1700 ppm
(4 times today's level)
Floor temperature (average over period) approx. 23 ° C
(8.5 ° C above today's level)
system series step ≈ age ( mya )
higher higher higher younger
chalk Upper Chalk Maastrichtium 66
⬍
72
Campanium 72
⬍
83.6
Santonium 83.6
⬍
86.3
Coniacium 86.3
⬍
89.7
Turonium 89.7
⬍
93.9
Cenomanium 93.9
⬍
100.5
Lower Cretaceous Albium 100.5
⬍
112.9
Aptium 112.9
⬍
126.3
Barremium 126.3
⬍
130.7
Skin rivium 130.7
⬍
133.9
Valanginium 133.9
⬍
139.3
Berriasium 139.3
⬍
145
deeper deeper deeper older

The Cretaceous , often also Cretaceous in popular scientific literature (Latin Cretaceum , derived from Cretaceous , mostly shortened to Cretaceous : Cretaceous, relating to the Cretaceous or correspondingly old rock formations ), is a period in the history of the earth . Within the Mesozoic Era ( Mesozoic Era) it is the youngest and with 80 million years the longest chronostratigraphic system (or period in geochronology ). It began around 145 million years ago with the end of the Jurassic and ended around 66 million years ago with the beginning of the Paleogene , the oldest chronostratigraphic system of the Cenozoic .

History and naming

The name chalk was named in 1822 by the Belgian geologist Jean Baptiste Julien d'Omalius d'Halloy after the high calcium carbonate fossils of crustaceans , corals , mussels , snails and protozoa that occur in the rocks of this system. The rock, more commonly known as “ chalk ”, is a special form of limestone . However, sedimentary rocks of the Cretaceous Period can be made up of completely different minerals, such as the frequently occurring "chalk sandstones", which predominantly consist of quartz grains and whose name only refers to their time of origin.

Definition and GSSP

The exact beginning of the chalk system and thus a GSSP has not yet been finally determined. The lower limit of the Cretaceous (and the Lower Cretaceous series as well as the Berriasium stage) will presumably be defined by the first appearance of the ammonite species Berriasella jacobi . The Cretaceous end is very well defined with the Iridium anomaly of the Cretaceous-Paleogene boundary as well as the extinction of numerous vertebrate and invertebrate groups.

Outline of chalk

The chalk system is now divided into two series and 12 levels:

Superordinate, but now outdated, level names are: Neokom (lower Lower Cretaceous), Gault (upper Lower Cretaceous), Emscher (now Coniac and Santon) and Senonium (now Santon, Campan and Maastricht).

In the literature, the period from Alb to Turon (sometimes even from Barrême to Santon), in which important geological events took place (extreme greenhouse climate , very high plate tectonic activity , particularly high global sea levels , several oceanic anoxia ), has been increasing since the 1970s the informal term " middle chalk " is used. In the meantime there are efforts to formally replace the traditional two-fold structure of the chalk with a threefold division into lower, middle and upper chalk.

Paleogeography

The disintegration of Gondwana , which had already started in the Jura , continued in the Cretaceous. Australia / Antarctica, which was still connected, and Africa / South America, which was also connected at the beginning of the Cretaceous, separated, and India also split off. In the Lower Cretaceous the southern South Atlantic began to open up, and this continued further north. In Turonium a continuous connection was then developed to the North Atlantic. In the North Atlantic, the ocean spreading between North Africa and the North American east coast, which had already begun in the Jura, advanced further north. In the course of the Lower Cretaceous, the section between the Iberian Peninsula and Newfoundland was formed. In the Higher Lower Cretaceous and the Lower Upper Cretaceous the Bay of Biscay, whose extension extended into the Pyrenees, also spread. In the Upper Cretaceous a triple point emerged west of Ireland - one branch led into a rift system between North America and Greenland, the other widened in the Upper Cretaceous and the Cenozoic to what is now the northern North Atlantic. The first collisions occurred in the Alps (“pre-Gosau mountainous formation”).

climate

The climate in the Chalk was generally warm and balanced. It enabled some dinosaurs to advance to high latitudes in the south and north, at least in the summer months. The poles were free of ice, and accordingly the sea level was also very high, it reached its maximum value in the Unterturon. Only at the end of the Cretaceous in the Maastrichtian did a cooling and a marked regression occur.

Development of the fauna

Chalk fossils (From Meyers Konversations-Lexikon (1885–1890))
Locations of late Cretaceous marsupials (Metatheria), recorded on a paleogeographic map for the Turon

As in the preceding Jurassic period, the terrestrial macro and megafauna were dominated by dinosaurs . The Titanosauria of the Upper Cretaceous were the largest land creatures ever living. From Germany , dinosaurs from the Cretaceous period are primarily documented by numerous fossil footsteps and tracks ( trace fossils ). Famous sites are Münchehagen , Obernkirchen and Barkhausen . All three are in the Lower Cretaceous region of southern Lower Saxony. The Münchehagen site produced, among other things, the almost 30 meter long track of Elephantopoides muenchehagensis , which was caused by a large sauropod . Dinosaur body fossil (i. E. The bones and teeth) are made jungunterkreidzeitlichen fillings of Karst split in Devonian Massenkalken in the north of Sauer country known (Briloner saddle). In the locality of Nehden , it is primarily the remains of the herbivore Iguanodon . In the locality of Balve, fossil teeth prove the existence of both smaller ( dromaeosauriden ) and very large ( tyrannosaurid ) predatory dinosaurs . In Maastricht (Upper Cretaceous) on the northern edge of the Bavarian Alps, bones of a hadrosaur have been found. The most important dinosaur site in Austria is Muthmannsdorf on the eastern edge of the Alps near Wiener Neustadt . The remains of the ankylosaur Struthiosaurus and the ornithopod (and thus Iguanodon- related) Mochlodon and a theropod ( " Megalosaurus pannoniensis " ) that cannot be identified more precisely have been found there in deposits of the Campan (Middle Upper Cretaceous) .

Reptiles also lived in the chalk seas. Typical representatives of the Upper Cretaceous are the mosasaurs , close relatives of today's monitor lizards . Typical marine invertebrates of the Cretaceous period are ammonites and belemnites . In the Cretaceous period, the ammonites developed a variety of shapes that had never existed before, including housings that were spiked and / or twisted like a corkscrew (so-called heteromorphic or "aberrant" ammonites). With the species Parapuzosia seppenradensis , the largest known ammonites also lived in the Cretaceous period. A specimen from the Campan in the Westphalian Bay has a case diameter of about 1.80 meters.

The mammals of the Lower Cretaceous Jehol fauna prove that mammals had already developed a certain ecological diversity in the early Cretaceous and began to step out of the “shadow of the dinosaurs”. The Jehol fauna contains Eomaia, one of the earliest representatives of Eutheria , and this shows adaptations to a tree-climbing way of life. Particularly instructive, however, is Repenomamus giganticus , the largest known mammal of the Mesozoic era, with a skull length of almost 20 centimeters and an estimated live weight of 12 to 14 kilograms. In its rib cage, remains of young animals of the dinosaur genus Psittacosaurus were found, which is interpreted as the first unquestionable evidence that mammals stalked dinosaurs in the Cretaceous and also ate them.

Development of flora

Credneria triacuminata fossil leaf

In the Lower Cretaceous, bear moss plants ( Nathorstiana aborea ), ferns ( Weichselia , Hausmannia ), tree ferns , ginkgoales ( Baiera ), Bennettitales and conifers were the predominant plants. The coal seams of the Wealden coal in the Weser-Ems region on the edge of the Teutoburg Forest also date from this time . The first flowering shrubby plants developed during the Cretaceous . The first genus of the hardwood family was Credneria with three-pointed leaves (finds from the Harz Mountains ). In the Upper Cretaceous, many deciduous trees such as maple , oak or walnut competed with conifers such as Sequoia and Geinitzia (from the Aachen strata , Upper Santonium ). Grasses spread on the mainland and greatly changed the erosion behavior .

The chalk in Central Europe

Cretaceous rocks can be found in the Hanover area , north of the Harz Mountains, in the Teutoburg Forest at the Externsteinen , in the Westphalian Bay and in the area from Aachen to Liège . The chalk cliffs in the Jasmund National Park on Rügen are famous . Furthermore, deposits from the Cretaceous period can be found east of the Franconian Alb and on the northern edge of the Alps , in the vicinity of Dresden and Děčín ( Elbe Sandstone Mountains ), in large parts of the Czech Republic and in the Subcarpathian Mountains and between Kielce and Krakow .

Special events during the chalk

One of the significant events of the Middle Cretaceous between approx. 120 million and 80 million years ago was a huge superplume activity in the western Pacific. The 40 million year long, widespread volcanism on the Pacific Ocean floor had global effects and probably had a lasting impact on climate development.

Despite the tropical climate prevailing during the Upper Cretaceous, some studies postulate an icing phase at higher latitudes during the Turonian (93.9 to 89.7 mya). However, other specialist articles question this possibility and reject the existence of continental ice sheets or sea ​​ice cover in the polar regions of that time. On the other hand, an extensive geological investigation of South Australian regions revealed clear indications, including in the form of tillites , dropstones and diamictite , that more or less pronounced glaciers took place on the continent in the course of the Lower Cretaceous.

End of chalk

The end of the Cretaceous Period is marked by a worldwide mass extinction that affected almost all animal groups and many plant groups. There are various ideas about the causes; the best known theory is an asteroid impact on the Mexican peninsula Yucatán ( Chicxulub crater ). But also the enormous volcanism during the formation of the Dekkan-Trapps at the end of the Cretaceous could have played a decisive role. However, more recent studies consistently assume that the biological crisis at the Cretaceous-Paleogene border with a species loss of around 75 percent was caused exclusively by the Chicxulub impact.

literature

  • Harald Polenz, Christian Spaeth: Saurians - ammonites - giant ferns. Germany in the Cretaceous Period . Theiss, Stuttgart 2004, ISBN 3-8062-1887-0 .
  • Mike Reich, Peter Frenzel, Ekkehard Herrig: A sea at the end of the Cretaceous Period. The chalk . In: Biology in Our Time . tape 35 , no. 4 . Wiley-VCH, 2005, ISSN  0045-205X , p. 260-267 .

Web links

Commons : Cretaceous  - collection of images, videos and audio files

Individual evidence

  1. ↑ Oxygen content-1000mj
  2. ↑ Phanerozoic Carbon Dioxide
  3. ↑ All palaeotemps
  4. ^ Peter Bengtson, Mikheil V. Kakabadze: Ammonites and the mid-Cretaceous saga. In: Cretaceous Research. Volume 88, August 2018, pp. 90–99, doi: 10.1016 / j.cretres.2017.10.003
  5. ^ DB Norman: A Mass-Accumulation of Vertebrates from the Lower Cretaceous of Nehden (Sauerland), West Germany. In: Proceedings of the Royal Society of London. Series B, Biological Sciences. Volume 230, No. 1259, 1987, pp. 215-255, doi: 10.1098 / rspb.1987.0017 , JSTOR 36060 .
  6. ↑ Klaus-Peter Lanser, Ulrich Heimhofer: Evidence of theropod dinosaurs from a Lower Cretaceous karst filling in the northern Sauerland (Rhenish Massif, Germany). In: Paleontological Journal. Volume 89, No. 1, 2015, pp. 79-94, doi: 10.1007 / s12542-013-0215-z
  7. ^ Peter Wellnhofer: A dinosaur (Hadrosauridae) from the Upper Cretaceous (Maastricht, Helvetic Zone) of the Bavarian Alpine foothills. In: Communications from the Bavarian State Collection for Paleontology and Historical Geology. Volume 34, 1994, pp. 221-238 ( PDF (5 MB) on ZOBODAT ).
  8. ↑ Zoltan Csiki-Sava, Eric Buffetaut, Attila Ősi, Xabier Pereda-Suberbiola, Stephen L. Brusatte: Island life in the Cretaceous - faunal composition, biogeography, evolution, and extinction of land-living vertebrates on the Late Cretaceous European archipelago . In: ZooKeys . 469, January 2015, pp. 1–161. doi : 10.3897 / zookeys.469.8439 .
  9. ↑ Qiang Ji, Zhe-Xi Luo, Chong-Xi Yuan, John R. Wible, Jian-Ping Zhang, Justin A. Georgi: The earliest known eutherian mammal. Nature. Volume 416, 2002, pp. 816-822, doi: 10.1038 / 416816a (alternative full text access : ResearchGate ).
  10. ↑ Yaoming Hu, Jin Meng, Yuanqing Wang, Chuankui Li: Large Mesozoic mammals fed on young dinosaurs. Nature. Volume 433, 2005, pp. 149–152, doi: 10.1038 / nature03102 (alternative full text access: ResearchGate ).
  11. Jump up ↑ Madison East, R. Dietmar Müller, Simon Williams, Sabin Zahirovic, Christian Heine: Subduction history reveals Cretaceous slab superflux as a possible cause for the mid-Cretaceous plume pulse and superswell events . In: Gondwana Research . 79, March 2020, pp. 125-139. doi : 10.1016 / j.gr.2019.09.001 .
  12. ^ A. Bornemann, RD Norris, O. Friedrich, B. Beckmann, S. Schouten, JS Sinninghe Damsté, J. Vogel, P. Hofmann, T. Wagner: Isotopic Evidence for Glaciation During the Cretaceous Supergreenhouse . In: Science . tape 319 , no. 5860 , January 2008, p. 189-192 , doi : 10.1126 / science.1148777 .
  13. ↑ Kenneth G. MacLeod, Brian T. Huber, Álvaro Jiménez Berrocoso, Ines Wendler: A stable and hot Turonian without glacial δ 18 O excursions is indicated by exquisitely preserved Tanzanian foraminifera . (PDF) In: Geology . 41, No. 10, October 2013, pp. 1083-1086. doi : 10.1130 / G34510.1 .
  14. ^ NF Alley, SB Hore, LA Frakes: Glaciations at high-latitude Southern Australia during the Early Cretaceous . (PDF) In: Australian Journal of Earth Sciences (Geological Society of Australia) . April 2019. doi : 10.1080 / 08120099.2019.1590457 .
  15. ↑ Mark A. Richards, Walter Alvarez, Stephen Self, Leif Karlstrom, Paul R. Renne, Michael Manga, Courtney J. Sprain, Jan Smit, Loÿc Vanderkluysen, Sally A. Gibson: Triggering of the largest Deccan eruptions by the Chicxulub impact . (PDF) In: Geological Society of America Bulletin . April 2015. doi : 10.1130 / B31167.1 .
  16. ↑ Pincelli M. Hull, André Bornemann, Donald E. Penman, Michael J. Henehan, Richard D. Norris, Paul A. Wilson, Peter Blum, Laia Alegret, Sietske J. Batenburg, Paul R. Bown, Timothy J. Bralower, Cecile Cournede, Alexander Deutsch, Barbara Donner, Oliver Friedrich, Sofie Jehle, Hojung Kim, Dick Kroon, Peter C. Lippert, Dominik Loroch, Iris Moebius, Kazuyoshi Moriya, Daniel J. Peppe, Gregory E. Ravizza, Ursula Röhl, Jonathan D Schueth, Julio Sepúlveda, Philip F. Sexton, Elizabeth C. Sibert, Kasia K. Śliwińska, Roger E. Summons, Ellen Thomas, Thomas Westerhold, Jessica H. Whiteside, Tatsuhiko Yamaguchi, James C. Zachos: On impact and volcanism across the Cretaceous-Paleogene boundary . In: Science . 367, No. 6475, January 2020, pp. 266-272. doi : 10.1126 / science.aay5055 .
  17. ↑ Michael J. Henehan, Andy Ridgwell, Ellen Thomas, Shuang Zhang, Laia Alegret, Daniela N. Schmidt, James WB Rae, James D. Witts, Neil H. Landman, Sarah E. Greene, Brian T. Huber, James R. Super, Noah J. Planavsky, Pincelli M. Hull: Rapid ocean acidification and protracted Earth system recovery followed the end-Cretaceous Chicxulub impact . In: PNAS . 116, No. 43, October 2019. doi : 10.1073 / pnas.1905989116 .