Cherts from the Western Pacific, Leg 7, Deep Sea Drilling Project

Abstract

Cherts or porcellanites were encountered at DSDP 157, 158, 162, and 163 of Leg 16. Calcareous sections at DSDP 157, 158, and 163 contain discordant stringers and nodules, whereas clayey and siliceous deposits at DSDP 162 and 163 contain bedded porcellanites. The youngest cherts (late Miocene at DSDP 157) are cristobalite-rich (com- monly more than 99%). The development of these deposits in pelagic sediments only about six million years old apparently results from high heat flow and rapid sedimentation rates in the area. The oldest cherts (Campanian at DSDP 163). contain roughly equal proportions of quartz and cristobalite, suggesting that the stabler polymorph (quartz) increases with time at the expense of cristobalite. All the cherts appear to have formed from remobilized biogenous opal. There is no evi- dence of direct volcanic contributions of silica to these deposits. INTRODUCTION