Formation and Stratigraphy of the Cretaceous System
The Cretaceous is the last period of the Mesozoic Era. It started over 145 million years ago with the total duration of about 80 million years. In 1822, Jean Baptiste Julien d’Omalius d’Halloy, a Belgian geologist, distinguished the Cretaceous system in the Paris Basin. This period got its name because of its distinctive material – white chalkstone — that was commonly used in Europe. The Cretaceous Period is a complicated time in the nature formation history each stage of which played a crucial role in its determination (Mann, 2007).
The Cretaceous is divided into the Early and Late Cretaceous periods or, according to another classification, the Lower and Upper Cretaceous series. Such separation was suggested at the International Geological Congress in Berlin in 1885. However, there were some attempts to divide this period into three series where Aptian, Albian, and sometimes Cenomanian stages were a part of a Middle series (Mann, 2007).
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After the Cretaceous system establishment, Alcide Dessalines d’Orbigny, who widely used a paleontological method for this purpose, performed the further development of its stratigraphy. Instead of already existing subdivisions, he started using the term “stages”, each of which was characterized by a specific faunal complex (University of California Museum of Paleontology, n.d.).
A stratotype of Berriasian Stage was situated south-east of France near the village Berrias. In 1871, berrias chalkstones were distinguished as a separate phase, which Henri Coquand placed at the bottom of the Cretaceous system. The Valanginian Stage was singled out from Neocomian Series developed in Neuchatel, Switzerland, close to Valanginian castle. It included a set of alteration of grey, blue, and yellow chalky clay as well as solid organic limestones. There are remains of echinoids, brachiopods, sponges, moss animals, and foraminifera in the rock mass, which is 53-55 m thick. More recent researches have shown that a considerable part of layers refers to the Berriasian stage. Regarding the Valanginian stratotype itself, it has been selected unsuccessfully since its section is rich in bright layers and breaks (Birkelund et al., 1984).
In 1873, Eugene Renevier combined deposits into the Hauterivian Stage. Its name originated from the Swiss town of Hauterive. Those deposits were presented by chalky clay and oolitic limestones with the remains of ammonites, brachiopods, oysters, and echinoids. Later, these deposits were divided into two substages with two zones in each. A typical section of Barremian Stage is situated near the village Barrem (south-east of France, Durance basin) where such ammonites as Ancyloceras, Scaphites, etc. were found in limestones (Birkelund et al., 1984).
Alcide d’Orbigny first distinguished the Aptian Stage among deposits developed in the small town of Apt in the south-east of France. He classified clay with Plicatula and limestones with a big amount of ammonites as the Aptian Stage. The upper and lower boundaries of this stage are still debatable. Nowadays, it contains three substages: lower (Bedoulian), middle (Gargasian), and upper (Clansayes). These substages are named after the areas stratotype sections are developed. In 1842, Alcide d’Orbigny distinguished the Albian Stage. Its name is derived from the river Alba, the right tributary of the river Seine. Its stratotype section is situated southeastward from Paris. It is characterized by many ammonites, according to which zone division is performed. The top of the Albian stage is considered as the place of the first appearance of foram species Rotalipora globotruncanoides (Birkelund et al., 1984).
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The Cenomanian Stage was discovered in Sarthe department of France close to the city of Le Mans (old name — Сеnomanum). These deposits were initially a part of Turonian Stage. However later, being sure of significant differences in the fauna of ammonites and rudists, d’Orbigny made them a separate stage with over 800 species. Terrigenous rocks, which have been formed in the hydrodynamically unstable environment (as a result of which, there are signs of underwater breaks on several levels), represented the Cenomanian Stage. This phase is rich in ammonites, belemnites, oysters, and brachiopods (Birkelund et al., 1984).
The Turonian Stage was suggested for carbonate deposits common in the surrounding areas of the city of Tours (ancient name — Turones). There are many breaks in the section. The availability of chalk, chalky clay, and limestones characterized this stage. Its deposits are replete with the remains of ammonites, inoceramidae, oysters, rudists, gastropods, echinoids, brachiopods, as well as ostracods, foraminifer, and plant impressions. Coniacian Stage takes its name from the city of Cognac, which is located in the western part of Charente department in France. There are carbonate-terrigenous rocks here, which are full of the remains of oysters, brachiopods, echinoids, moss animals, ammonites, and rudists (Birkelund et al., 1984).
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The name of the Santonian Stage is derived from the name of the city of Sant in Charente-Maritime department in France. In 1857, Henri Coquand referred soft chalk with chalcedonies and remains of sponges, brachiopods, echinoids, and myarians to this stage. Campanian Stage takes its name from the ridge of hills of the Grande Champagne area. The position of the upper boundary in stratotype area is uncertain. It is divided into two substages and four areas, which are both full of ammonites and orbitoid (Birkelund et al., 1984).
In 1879, Andr? Hubert Dumon named the Maastrichtian Stage after the city of Maastricht in the southern part of a Dutch province Limburg. Chalk and chalky limestones with the remains of ammonites and belemnites are common to this stage. Nowadays Maastrichtian Stage is known for such deposits as Hoploscaphites constrictus. The upper boundary of this stage is distinguished by the extinction of ammonites, belemnites, many other macrofossils, and abrupt change of complexes of planktonic foraminifers and nanoplankton (Birkelund et al., 1984).
The Cretaceous Period was the time when continents kept moving. The supercontinent Panagea separated into two parts: Laurasia and Gondwana. Africa, India, and Australia also started moving in different directions, and, as a result, they formed gigantic islands south of the equator. South America and Africa were moving apart, and the Atlantic Ocean was becoming wider and wider. There were no significant catastrophes in the Cretaceous Period. Therefore, the process of evolution was going a normal way. The Earth started looking like now (Hansen, 2015).
In comparison with the Jurassic Period, a climate had changed. Due to variations in the continent position, the change of seasons became more visible. It started snowing under the poles, although there were no such ice caps as they are these days. The climate was different depending on the continent. This fact caused a difference in flora and fauna development around the globe (University of California Museum of Paleontology, n.d.).
The Cretaceous Period is the final phase of the Mesozoic Era. Therefore, its organic world has all features of the transitional phase. Despite the fact that during the Early Cretaceous Period, the dominant position belonged to mesophytic flora, the Late Cretaceous Period is highly valued for cenophytic elements — metasperms (University of California Museum of Paleontology, n.d.).
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An organic world of earth was unique. At the beginning of the Early Cretaceous Period, flora had a lot in common with the Late Jurassic one. It consisted of cycadophytes, Ginkgoaceae, and ferns. However, Bennettitales were common as well. In general, Early Cretaceous flora remained mesophytic. Nevertheless, the Barremian Stage brought the first angiosperms that started dominating in the Late Cretaceous Period. As a consequence, flora became cenophytic. Despite these were only some representatives in the Barremian Stage, the Albian is known for many species of angiosperms. Furthermore, in the Cenomanian Stage, they supersede gymnosperms. The Cenophytic aspect of the Late Cretaceous Period is defined by the development of a great number of such modern forms as oak (Quercus), beech (Fagus), willow (Salix), birch (Betula), plane, laurel, and magnolia. The Cretaceous Period was the first to see angiosperms — flowering plants. This was followed by an increase in diversity of insects that started fertilizing flowers (Wilf et al., 2004).
Flora evolution gave rise to rapid development of the animal world, including dinosaurs. They continued dominating during the Cretaceous Period. Among other vertebrates, some reptiles occupied earth, water, and air areas. There were various plant-eating and huge rapacious dinosaurs (tyrannosaurs, tarbosaurus). The appearance of snakes characterizes the Cretaceous Period. Significant evolution affected bony fish. Also, toothed birds and placental mammals started emerging (Bagley, 2013).
By the end of the Cretaceous Period, at the boundary between the Maastrichtian Stage and the Danian Age, a disappearance of coccolithophora, planktonic foraminifer, ammonites, belemnites, inoceramidae, rudists, dinosaurs, and a whole range of other groups was observed. About 50 % of radiolaria families and 75 % of brachiopod families had disappeared. The number of echinoids and crinoids had reduced, and the number of sharks had declined by 75 %. In total, over 100 families of marine invertebrates and approximately the same number of land animals and plants had become extinct. Such reduction of fauna and flora is often called “a great Mesozoic extinction”. One of the most common reasons for these extinctions is the Earth collision with an asteroid, the diameter of which could be about 10-15 km (Hansen, 2015). Signs of this accident were found in the form of “iridic abnormality” in the boundary layers of chalk and Palaeogene in the range of sections of Western Europe. Nowadays, the most probable candidate for a large crater formed on Earth as a result of an asteroid impact at the boundary between Cretaceous and Palaeogene is Chicxulub, Mexico’s Yucatan peninsula. “An asteroidal winter”, which occurred after the explosion, could cause a wide range of processes negative for organisms life — reduction of nutrition resources, disturbance of nutritive connections, temperature drop, etc. (Bagley, 2013).
In addition to the above-mentioned information, the Cretaceous Period generated the biggest flying creatures ever living on Earth. These are Archaeopteryx and Quetzalcoatlus. Nowadays, it has not decided yet which one of them was bigger. However, the Cretaceous Period gave rise to the emergence of pterosaur’s competitors — birds. Although, it has been known that the first birds appeared as far back as the Jurassic Period, their diversity had significantly grown in the Cretaceous Period. A bridging species between pterosaurs and birds called Archaeopteryx became extinct. Consequently, flying pterosaurs and birds existed as well (Hansen, 2015).
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Some Cretaceous birds are considered predecessors of modern birds. The Cretaceous Period was the time when appeared ducks, magpie geese, divers, and plovers, which are almost as similar as modern birds. Many birds of the Cretaceous Period were the final point in evolution. Therefore, they further became extinct. Nevertheless, a classification of the Cretaceous Period’s birds is quite controversial.
All in all, throughout the Cretaceous Period, life diversified and branched out. Based on everything mentioned above, one can agree that it had overcome many changes in its way, as well as many losses. However, in spite of those circumstances that happened during this period, which surely had a slightly negative effect on current nature, a significant percentage of plant and animal world managed to survive until the present days.