Chalk (period)


August 13, 2022

Cretaceous - the last period of the Mesozoic era, lasting about 80 million years (from ~ 145.0 to 66.0 million years ago). Chalk is divided into two eras: early chalk and late chalk. In the chronostratigraphic sense, it is a system that divides into two branches: lower and upper chalk. The name comes from writing chalk - a rock commonly found in the works of this period. It was introduced by the Belgian geologist d'Halloy in the first half of the 19th century.


The basic index fossils are ammonites, and in the Upper Cretaceous also inocerams, belemnites and foraminifera. The Cretaceous period / system is divided into ages / floors:


In the Cretaceous, the climate was very warm and quite humid. Even at the Pole, the average annual temperature was around 4 ° C, hardly ever below zero in winter. At the end of the Early Cretaceous, the greatest transgression in Earth's history began, making the level of the oceans more than 200 meters higher than today in the Late Cretaceous. Much of the land was inundated and numerous shallow epicontinental seas were formed. The Atlantic and Indian oceans already existed, though they were much narrower than they are today. At the end of the Cretaceous, intensive orogenic movements of the Alpine orogenesis began, leading to the folding and partial uplift of the sediments of the earlier seas. At that time, the Tatra Nappe.


Among the marine flora of the cocolith, unicellular algae were massively occurring (especially in the late Cretaceous). Their remains are an essential component of many types of limestone, including writing chalk, and are used for dating. The first angiosperms appeared in the early Cretaceous and became a staple component of the terrestrial flora in the late Cretaceous. Coniferous plants were common throughout the period, especially araucaria and slippery plants. Other gymnosperms, deciduous ginkgo, cycads and extinct in the Late Cretaceous Benetite were common in the early Cretaceous before they declined. The last seed ferns became extinct in the early Cretaceous. Spore plants appeared almost exclusively as herbaceous forms, especially ferns were common. The last woody ferns have faded away.


In the Cretaceous, there was a very lush development of protozoa, especially foraminifera, including the first planktonic species, are important rock-forming and stratigraphically. At the turn of the Jurassic and Cretaceous, other unicellular protozoa were very common - calpionellae that build some limestones and are used in rock dating. Among invertebrates living on the bottom, clams were particularly numerous, including the very important stratigraphy genus Inoceramus. Some clams with unusual horn shapes, the so-called the rudists formed reef-like structures. Brachiopods, sponges and snails were also common. Ammonites dominated in the waters of the seas. At that time, ammonites of unusual shapes appeared in large numbers - heteromorphs (e.g. the genus Scaphites), sometimes also giant forms (up to 2.5 m). The second main group of nectonic invertebrates were belemnites. In the Cretaceous period, the modern dominant group of fish, Teleostei, developed rapidly, but at the same time, sharks were at least as common. On the other hand, osteochondritis and transitional fish have deteriorated. In the early Cretaceous, Australia developed large shield-headed amphibians up to 5 m long and 0.5 tons. However, at the end of this era, the last world representatives of this group died out. Chalk is, along with yura, called the age of reptiles. Plesiosaurs lived in the seas throughout the entire period, ichthyosaurs were quite common in the early Cretaceous, but they became extinct at the beginning of the late Cretaceous, towards the end of the Cenomanian. In the Late Cretaceous, the sea was dominated by a newly formed group of sea lizards - mosasaurs, the largest of which exceeded 20 meters in length. Pos