Bajocian

The Bajocian is a stage of the Middle Jurassic epoch in the international geologic timescale. It lies between the Aalenian stage to the east and the Bathonian to the west, marking a subdivision of the Middle Jurassic used in European and global stratigraphy. The Bajocian spans roughly from about 170 to 168 million years ago, though exact boundaries can vary slightly among different timescales.

The name Bajocian (from the French term Bajocien) reflects its origins in European stratigraphy, where rocks of this age were first studied and classified. In global practice, the stage boundaries are anchored by biostratigraphic markers, particularly ammonite faunas, which help correlate Bajocian rocks across different regions.

Lithologically, Bajocian rocks are commonly marine in origin, including limestones, shales, and sandstones formed in shallow seas that covered large parts of Europe and adjacent areas during this time. These deposits preserve abundant fossils, especially ammonites, belemnites, and other marine invertebrates, which are key for biostratigraphic dating and regional correlation. Terrestrial records from marginal areas indicate ecosystem changes at the edges of the sea and contribute to understanding the broader climate and sea-level fluctuations of the Middle Jurassic.

Paleogeography during the Bajocian was characterized by relatively warm conditions and high sea levels, with expansive shallow continental shelves and inland seas in many regions. The stage helps bridge the earlier Toarcian marine transgressions with later Middle and Late Jurassic developments in fauna and rock records.