PALEOGEOGRAPHY OF RADIOLARITE AND ORGANIC-RICH DEPOSITS IN MESOZOIC TETHYS

Siliceous and marine organic-rich deposits are sometimes associated, s ometimes separate in space and time; however, both are generally accep ted to be the result of high planktonic productivity. Among the silice ous 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 radio larite facies), their time of deposition corresponds with strong fauna l modifications and they are deposited generally in shallower environm ents. 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, Toarc ian, Kimmeridgian and Cenomanian, show: (a) during Jurassic times thes e Tethyan deposits were dissociated, the siliceous deposits being clos er to open ocean waters than the organic-rich ones. This is a common d isposition in modern upwelling systems and suggests a common process; (b) during Cretaceous times these Tethyan deposits were often associat ed, i.e. both occur at the same site, and are probably the result of a different process from that in the Jurassic.