PURE SILICEOUS OOZE, A DIAGENETIC ENVIRONMENT FOR EARLY CHERT FORMATION

The formation of marine opal-CT nodules or layers as early diagenetic deposits has been documented only in Antarctic deep-sea sediments. In contrast, porcellanites and cherts in land sections and Deep Sea Drill ing Project and Ocean Drilling Program drill sites are usually found i n sediment sections of Miocene age and older. During R.V. Polarstern c ruises ANT-IX/3 and 4, young porcellanites were recovered for the firs t time in contact with their host sediment in two cores from the Atlan tic sector of the southern ocean. Chemical and mineralogical studies o f these deposits and their surrounding sediments have increased knowle dge about very early chert formation. In both cores the porcellanites are embedded in sediments rich in opal-A with extremely low levels of detrital minerals, an environment that seems conducive to a rapid tran sformation of biogenic silica into porcellanites.