16. SILICIFIED LIMESTONE-CHERT SEQUENCES CORED DURING LEG 8 OF THE DEEP SEA DRILLING PROJECT: A PETROLOGIC STUDY

The east central Pacific basin sediments were sampled at eight sites during DSDP Leg 8, and siliceous limestones with cherts were cored at five of the sites. Generally speaking, uppermost cherts occur as thin stringers or nodules within Oligocene calcareous sediments. The bulk of the chert, usually inter-grown with siliceous limestone, invariably occurs in middle and upper Eocene radiolarian-rich sediments. In all cases the chert is composed of both cristobalite and quartz and contains ghosts of foraminifera and/or Radiolaria. It is considered likely that the source of silica required for the chert formation was from local dissolution of siliceous micro-fossils during diagenesis. A change in oceanographic conditions is postulated for lower Oligocene times to account for the lithologic change from radiolarian-rich sediments of the Eocene to the carbonate-rich sediments of the later Tertiary. This may be the reason for the restriction of most of the cherts to Eocene sediments