The Chalk Formations of Europe are thick deposits of chalk, a soft porous
white limestone, deposited in a marine environment during the upper Cretaceous
Period. They appear most prominently in England. The formations are divided
into three parts: The Upper Chalk, the Middle Chalk, and the Lower Chalk.
The famous White cliffs of Dover, England are a good example of a Chalk Formation
deposit. Another good example displaying the sequence of the Chalk Formation
are the southern cliffs on the Isle of Wight, England and the quarries and
motorway cutting at Blue Bell Hill, Kent, England.
Chalk Formation consists mostly of coccolith biomicrite. A biomicrite is
a limestone composed of fossil debris ("bio") and calcium carbonate mud ("micrite").
The majority of the fossil debris comprising this chalk consists of the microscopic
plates, which are called coccoliths, of microscopic green algae known as coccolithophores.
In addition to the coccoliths, the fossil debris includes a variable, but
minor, percentage of the fragments of foraminifera, ostracods, and mollusks.
The coccolithophores lived in the upper part of the water column. When they
died, the microscopic calcium carbonate plates, which formed their shells
settled downward through the ocean water and accumulated on the ocean bottom
to form a thick layer of calcareous ooze, which eventually became the Chalk
Formation. The Chalk Formation usually shows few signs of bedding, other than
lines of flint nodules which become common in the upper part. Nodules of the
mineral pyrite also occur and are usually oxidized to brown iron oxide on
exposed surfaces. The youngest beds of the Upper Chalk formation in England
are found on the coast of Norfolk.
The Cretaceous-Tertiary extinction event defines the end of the Cretaceous,
a significant number of species (~50%) and known families (~25%) disappeared.
Plants were nearly unscathed, while marine organisms were hit the hardest.
These include a large number (~95%) of types of planktic foraminifers (excepting
the Globigerinida), an even larger number of Coccolithophores, all the ammonite
and belemnite cephalopods, and all reef-forming rudist molluscs), as well
as all marine reptiles except turtles and crocodiles. Dinosaurs are the most
famous victims of the Cretaceous extinction. Dinosaurs that were unique to
the very end of the period (such as Tyrannosaurus rex, Triceratops, and Ankylosaurus)
were wiped out. The last of the pterosaurs went extinct and the vast majority
of birds did as well, including the Enantiornithes and Hesperornithiformes.