- Callovian Stage
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uppermost of the four divisions of the Middle Jurassic Series, representing all rocks formed worldwide during the Callovian Age, which occurred between 164.7 million and 161.2 million years ago during the Jurassic Period. (Some researchers have proposed a longer time span, from 160 to 154 million years ago, with concomitant changes in the dates of other Jurassic stages.) The Callovian Stage overlies the Bathonian (Bathonian Stage) Stage and underlies the Oxfordian (Oxfordian Stage), the lowest stage of the Upper Jurassic Series.The name for this stage is derived from the Kellaways area in Wiltshire, England, which was known as Callovium during Roman times. In England the Callovian includes strata from the Cornbrash Beds, Kellaways Beds, and Oxford Clay. The Callovian is subdivided into the Lower, Middle, and Upper Callovian, and throughout Europe each of these intervals is further subdivided into two standard Ammonite biozones (biozone). Outside of Europe the Callovian sequences are not well developed, because of gaps in marine strata, small geographic ranges among the ammonites, and the presence of long-lived species that are unsuitable for correlation. In some regions ammonite associations are present but cannot easily be correlated to European forms. However, in certain other regions and time intervals in the circum-Pacific belt (such as the Lower Callovian of Mexico), a large number of European species can be found, permitting global correlations.Carol Marie Tang
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Universalium. 2010.
