19. Petrology of Siliceous Rocks Recovered from Marginal Seas of the Western Pacific, Leg 31, Deep Sea Drilling Project

Siliceous sedimentary rocks of Tertiary age were recovered at three of the nine sites drilled during Leg 31 (Table 1). Only relatively small amounts of lithified material were retrieved, but these rocks are particularly interesting because they include three distinct types of siliceous rock and because each type occurs with a distinct sediment lithology. The kinds of rocks sampled are (with lithologies of the associated soft sediments in parentheses): nodular chert (nannofossil chalk ooze), radiolarian porcelanite (brown clay), and diatom porcelanite (diatom ooze) (Figures 1-8). These rocks are significant also because each different variety appears to record an early stage of diagenesis for siliceous rocks of the kinds now found in stratigraphic assemblages on land. In addition, these rocks occur entirely within the marginal ocean basin setting; their detailed description thus adds to the total characterization of sedimentary sequences formed in this setting. Marine geologists frequently apply the term "chert" to any siliceous rock recovered from the ocean basins, but its usage herein is restricted to rocks composed dominantly or entirely of quartz. The term "porcelanite" is used for less dense rocks composed mainly of cristobalite and having a subvitreous or porcelaneous luster following the recommendation of Calvert (1974). RADIOLARIAN PORCELANITE, SITE 291