Calymmian

The Calymmian is the earliest stage of the Mesoproterozoic Era of the Proterozoic Eon. It extends from about 1.6 to 1.4 billion years ago and is followed by the Ectasian. In the International Commission on Stratigraphy (ICS) framework, the Calymmian marks the initial subdivision of the Mesoproterozoic, with boundaries defined largely by radiometric ages and stratigraphic correlations rather than by a widely observable fossil horizon.

Geologically, the Calymmian records a period of crustal growth and tectonic stabilization, with widespread magmatic and metamorphic activity associated with early continental nuclei. Rock associations from Calymmian time include granitoid intrusions, felsic to intermediate volcanic rocks, and sedimentary sequences that reflect passive-margin and intracratonic basins. Globally, Calymmian rocks occur on several ancient cratons, notably in regions that are now part of Australia, North America, Fennoscandia, and central Africa. The time is important for understanding the buildup of continental crust during the Mesoproterozoic and provides insight into early crustal differentiation and the operation of plate tectonics in this era.