Palaeogeography of radiolarite and organic-rich deposits in Mesozoic Tethys

Siliceous and marine organic-rich deposits are sometimes associated, sometimes separate in space and time; however, both are generally accepted to be the result of high planktonic productivity. Among the siliceous marine deposits, the phtanite family facies is distinguished from the radiolarite family facies by several characteristics: They contain organic material and as a result are blackish (vs red/green for radiolarite facies), their time of deposition corresponds with strong faunal modifications and they are deposited generally in shallower environments. A palaeogeographic analysis of locations of Tethyan biosiliceous and marine organic-rich rocks, both resulting from a high planktonic palaeoproductivity, for three Mesozoic high sea-level intervals, Toarcian, Kimmeridgian and Cenomanian, show: (a) during Jurassic times these Tethyan deposits were dissociated, the siliceous deposits being closer to open ocean waters than the organic-rich ones. This is a common disposition in modern upwelling systems and suggests a common process; (b) during Cretaceous times these Tethyan deposits were often associated, i.e. both occur at the same site, and are probably the result of a different process from that in the Jurassic.