Silica Diagenesis in Continental Margin Sediments off Northwest Africa

Abstract

Investigations using optical, scanning, and transmission electron microscopy, and X-ray diffraction of 79 samples of silicified Oxfordian to early Miocene sediments off northwest Africa yielded the following major results: 1) Only opal-CT porcellanites are present in post-middle Eocene marls and in Late Cretaceous to Eocene claystones; only quartz cherts were found in Oxfordian to Early Cretaceous limestones. 2) Opal-CT (a) precipitated from solution during intermediate diagenesis after formation of a first calcite cement and before any matrix quartz is formed; and (b) was formed by in situ replacement of radiolarian skeletons. 3) Silicification proceeds in the carbonate environment from isolated lepispheres in pores to one or more centrifugally progressing (opal-CT) silicification fronts which are later replaced by quartz from a core outwards. 4) Opal-CT and authigenic palygorskite/sepiolite in clayey sediments produce an "aggregate polarization" presumably caused by oriented growth parallel to the bedding plane. 5) The maturation of the opal-CT structure is suggested by the progressive shift of the d(4.\A) spacing with burial depth (tem-perature, ?time. 6) The ultrastructure of lepisphere characteristic interpenetra-tion of opal-CT blade is governed by tridymite twinning laws. 7) Quartz is directly precipitated only in foraminiferal tests and pores within the nodules. 8) Indicated by porcellanite relicts, quartz-replaced lepispheres, etc., all typical quartz cherts were developed by gradual maturation from porcellanites, not by direct precipitation without an opal-CT precursor. 9) The rate of opal-CT quartz transformation is much slower in the clayey sediments (730-60 m.y.) than in a carbonate environment (5-20 m.y.).