Chert and Porcellanite from Deep Sea Drilling Project Site 436, Northwest Pacific

Abstract

Cretaceous chert and porcellanite recovered at Site 436, east of northern Honshu, Japan, are texturally and mineralogically similar to siliceous rocks of comparable age at Sites 303, 304, and 307 in the northwest Pacific. These rocks probably were formed by impregnation of the associated pelagic clay with locally derived silica from biogenic and perhaps some volcanic debris. Fine horizontal laminations are the only primary sedimentary structures, suggesting minimal reworking and transport. Collapse breccias and incipient chert nodules are diagenetic features related to silicification and compaction of the original sediment.Disordered opal-CT (^[101] = 4.09 Å) and microgranular quartz (crystallinity index < 1.0) are the two common silica minerals present. Some samples show quartz replacing this poorly ordered opal-CT, supporting the notion that opal-CT does not become completely ordered (i.e., d[101] = 4.04 Å) in some cases before being converted to quartz. The present temperature calculated for the depth of the shallowest chert and porcellanite at this site is 30 °C; this may represent the temperature of conversion of opal-CT to quartz. High reflection coefficients (0.29-0.65) calculated for the boundary between chert-porcellanite and clay-claystone support the common observation that chert is a strong seismic reflector in deep-sea sedimentary sections.

Biogenic chert in the Cow Head Group (Cambro‐Ordovician), western Newfoundland

“…In the modern ocean, the principal protoliths of chert are limestone, chalk, and terrigenous mud (e.g. Pisciotto, 1980;Hein &Karl, 1983).…”

Silicified Sediments and Silica Diagenesis in the Goban Spur Area of the Northeast Atlantic, Leg 80

“…These studies have yielded many new insights regarding the occurrence and the nature of various siliciffed sediments. They have also revealed very diverse features of silica diagenesis and the formation of cherts and other siliceous sediments, such as porcellanites and diatomites (Heath, 1973;Garrison et al, 1975;Keene, 1975;Kelts, 1976;Kagami, 1979;Pisciotto, 1980;Riech, 1980;Grehin et al, 1981;Hein et al, 1981; Riech, 1981).…”

“…These studies have yielded many new insights regarding the occurrence and the nature of various siliciffed sediments. They have also revealed very diverse features of silica diagenesis and the formation of cherts and other siliceous sediments, such as porcellanites and diatomites (Heath, 1973;Garrison et al, 1975;Keene, 1975;Kelts, 1976;Kagami, 1979;Pisciotto, 1980;Riech, 1980;Grehin et al, 1981;Hein et al, 1981; Riech, 1981).Previous works have identified three main diagenetic silica mineral facies in these silicified sediments: (1) X-ray-amorphous silica (opal A), (2) disordered α-cristobalite (opal CT), and (3) stable quartz (Jones and Segnit, 1971). Also, silicified sediments can be classified into two major groups: a bedded type occurring in clayey or siliceous host sediments, and a nodular type found in predominantly calcareous host sediments (Heath and Moberly, 1971;Lancelot, 1973;von Rad et al, 1978; Riech and von Rad, 1979).…”