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| Middle Jurassic | |
|---|---|
| Name | Middle Jurassic |
| Start | 174.1 |
| End | 163.5 |
| Timescale | Jurassic |
| Preceding | Early Jurassic |
| Following | Late Jurassic |
Middle Jurassic
The Middle Jurassic is the middle epoch of the Jurassic Period, spanning from about 174.1 to 163.5 million years ago. It follows the Early Jurassic and precedes the Late Jurassic in the Mesozoic Era; its subdivisions include the Aalenian, Bajocian, Bathonian, and Callovian stages. This interval is notable for major faunal radiations, continental reconfigurations linked to the breakup of Pangaea, and sedimentary records preserved in classic outcrops such as the Bajocian of England and the Callovian of France.
The epoch is formally defined by Global Boundary Stratotype Sections and Points established in stratotypes like the Aalenian Stage GSSP and the Callovian GSSP; biostratigraphic markers include ammonite zonations correlated with sections in Germany, Morocco, and Scotland. Radiometric ages come from isotopic studies involving U–Pb dating of volcanic ash beds in sequences near Sicily, Chile, and Wyoming. Stratigraphic frameworks integrate magnetostratigraphy from cores drilled by programs such as the International Ocean Discovery Program and lithostratigraphy tied to formations like the Bathonian White Limestone Formation and the Oxford Clay Formation.
During the epoch, the fragmentation of Pangaea advanced as the Central Atlantic widened and the Tethys Ocean expanded between the northern landmass Laurasia and southern Gondwana. Sea gateways such as the proto-North Sea and shallow epicontinental seas inundated cratons including the Siberian Craton and Bharat Peninsula margins; epeiric basins developed in regions of the Paris Basin and Neuquén Basin. Paleoclimate proxies from isotopes in foraminifera and paleosols indicate generally warm temperatures with regional humid belts reconstructed for the Kimmeridgian-adjacent latitudes, and monsoonal patterns inferred from sedimentary cyclicity in the Karoo Basin and Jiangsu Province.
Plant assemblages shifted with the rise of gymnosperm conifers such as representatives linked to the Pinaceae and Araucariaceae in deposits from Greenland to Antarctica, alongside ferns recorded from the Yorkshire and Isle of Skye floras. Marine faunas were dominated by diverse ammonites and belemnites documented in the English Channel and Moroccan Atlas, while bivalves and brachiopods persisted on shallow shelves in the Sahara. Terrestrial vertebrates saw major diversification: early neosauropods, stegosaurs, and basal theropods appear in formations like the Bajocian Stonesfield Slate and the Callovian Oxford Clay, associated with trace fossils in the Morrison Formation-proximal basins. Avian precursors and early mammals represented by morganucodontids and docodonts occur in microvertebrate sites in Wales and China; crocodylomorph and turtle lineages expanded across the Iberian Peninsula and Argentina.
Active rifting along the nascent Central Atlantic Magmatic Province margins and propagation of transform faults in the North Atlantic shaped continental fragments such as the Faeroe–Shetland Basin and South China Block. Volcanism related to mantle upwelling influenced sediment supply to basins like the Karoo and the Paraná Basin, while subsidence patterns controlled deposition of the Kimmeridge Clay-type organic-rich mudstones. Eustatic curves derived from sequence stratigraphy correlate highstands with expanded carbonate platforms in the Bahamas-equivalent realms and lowstands tied to exposure surfaces in the Caspian-adjacent shelves.
The epoch records several regional turnovers rather than a single global extinction: faunal shifts among ammonite lineages are evident across the Tethyan Realm and Boreal Realm, with provinciality increases documented in faunal provinces such as the Western Interior Seaway precursor and the Madagascar faunas. Reef ecosystems experienced reorganizations involving scleractinian corals in the Coral Triangle-analog latitudes and microbialites in restricted basins like those preserved in Svalbard. Terrestrial vertebrate assemblages underwent clade-level replacements recorded in the Gondwana successions of Argentina and South Africa.
Middle Jurassic strata host hydrocarbon source and reservoir rocks including organic-rich shales equivalent to the Kimmeridge Clay Formation and siliciclastic reservoirs in the North Sea and Gabon Basin, exploited by energy companies and regulated by agencies such as the UK Oil and Gas Authority. Ironstone, phosphate, and evaporite deposits occur in the Paris Basin and Mesozoic basins of China, with economic mining histories tied to the Industrial Revolution-era extraction in England. Exceptional fossil preservation is recorded in lagerstätten like the Bathonian Stonesfield Slate and the Daohugou Beds (noting debated ages), yielding detailed remains of insects, small mammals, and vertebrates that inform evolutionary studies conducted by institutions such as the Natural History Museum, London and the Chinese Academy of Sciences.