X-ray diffraction and scanning-electron microscopy have revealed that the Cenomanian and Turonian chalks of the Paris Basin contain, in their insoluble residue, a smectite-clinoptilolitecristobalite association. This association has also been described in many deep sea cores recovered by the DSDP in the North and Central Atlantic. According to many authors clinoptilolite and smectites have a volcanic origin; the origin of cristobalite and quartz in deep sea sediments is more problematical. We suggest that the smectites-clinoptilolite-"cristobalite" association in the chalk derives from volcanic material, the ultimate source of which was the active Mid-Atlantic Ridge. Paleogeographical, faunal and petrographic data clearly indicate the relationship between chalk sedimentation and opening of the North Atlantic.

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