Upper Cretaceous: Geological Epoch of Chalk Seas, Dinosaur Faunas, and the K

Upper Cretaceous: Geological Epoch of Chalk Seas, Dinosaur Faunas, and the K–Pg Extinction

Upper Cretaceous (c. 100.5–66 Ma): the final Cretaceous epoch defined by extensive chalk seas, diverse dinosaur and marine faunas, rising flowering plants, and ending with the K–Pg mass extinction

Author: Leandro Alegsa Creation date: November 13, 2022 Last updated: April 10, 2026

Overview

Upper Cretaceous refers to the final geological epoch within the Cretaceous. It spans from about 100.5 million years ago until roughly 66 million years ago.

Stratigraphy and subdivisions

The Cretaceous is commonly split into two parts, the Lower Cretaceous and the Upper Cretaceous, a division based mainly on differences in the types of rocks deposited. The Upper Cretaceous itself is divided, in ascending order, into the Cenomanian, Turonian, Coniacian, Santonian, Campanian and Maastrichtian stages.

Chalk and microscopic contributors

This interval is notable for extensive deposits of chalk, a sedimentary rock composed chiefly of tiny, calcareous plates (CaCO3) called coccoliths. Individual coccoliths are small enough to be seen with a light microscope, while their detailed structure requires an electron microscope. The coccoliths were produced by single-celled, planktonic algae known as coccolithophores and accumulated in offshore seas to form chalk beds.

Associated sediments

Interbedded with many chalk deposits are nodules of flint, a siliceous material (SiO2). The silica in flint likely derives from organisms with siliceous skeletons and indicates intervals when conditions differed from those that favored carbonate production.

Life and extinction

During the Cretaceous, dinosaurs remained the dominant large land animals. Familiar genera that lived in this time include Triceratops, Tyrannosaurus and Velociraptor. The giant marine reptile Mosasaurus was a leading marine predator. Birds (Aves) diversified further and flowering plants became increasingly important on land. The Upper Cretaceous ended with the K/T extinction event, which brought widespread loss of species across marine and terrestrial environments.

Questions and Answers

Q: What is the Upper Cretaceous?

A: The Upper Cretaceous is the last geological epoch in the Cretaceous period, which began 100.5 million years ago and ended 66 million years ago. It is traditionally divided into Lower Cretaceous (early) and Upper Cretaceous (late), because of different rocks that reflect the conditions in which they were formed.

Q: How is it subdivided?

A: The Upper Cretaceous is subdivided from lowest to highest into the Cenomanian, Turonian, Coniacian, Santonian, Campanian, and Maastrichtian ages.

Q: What type of rock can be found in chalk?

A: Chalk found in the Upper Cretaceous period is composed of countless millions of calcareous (CaCO3) plates called coccoliths, which are so small they can only just be seen with a light microscope; details require an electron microscope. The only other rock found in chalk is flint, which is siliceous (silica SiO2).

Q: What animals lived during this time?

A: During this time dinosaurs were dominant land animals such as Triceratops, Tyrannosaurus and Velociraptor while Mosasaurus was the dominant marine predator. Birds also became more diverse during this period.

Q: What plants developed during this time?

A: Flowering plants developed more and became dominant on land during this period.

Q: How did the Upper Cretaceous end?

A: The Upper Cretaceus ended with a K/T extinction event.

Author

Creation date: November 13, 2022

Last updated: April 10, 2026