Aalenian

The Aalenian is a stage of the Middle Jurassic epoch, serving as the first stage of that period. It spans roughly from 174 to 170 million years ago, lying after the Toarcian and before the Bajocian. The name is derived from Aalen, a town in Baden-Württemberg, Germany, where early studies of rocks of this age helped define the interval.

In the international ICS timescale, the Aalenian is used to date rock units of Middle Jurassic age. The lower boundary is tied to global stratigraphic markers that correlate with the base of the Middle Jurassic, while the upper boundary marks the transition to the Bajocian. Correlation across regions relies on fossil assemblages, particularly ammonites and other marine organisms, as well as other biostratigraphic indicators.

Paleogeographically, the Aalenian era featured warm, generally high sea levels with extensive shallow seas in parts of Europe and Gondwanan regions. This produced abundant marine sedimentation, including limestones and shales, with more restricted terrestrial deposition in certain areas. The interval is represented by a variety of facies, from open-marine to shelf environments, depending on local tectonics and sea level.

Fossil content for the Aalenian is dominated by marine invertebrates, especially ammonites, belemnites, bivalves, and brachiopods, which provide key biostratigraphic signals. Marine reptiles occur in some basins, and plant and trace fossil records also contribute to reconstructions of the terrestrial and marginal marine environments.

The Aalenian is used by geologists to correlate Middle Jurassic rocks worldwide and to refine regional stratigraphy within the broader Jurassic sequence.