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 (temperature, ?time.6) The ultrastructure of lepisphere characteristic interpenetration 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.).

Eocene Porcellanites and Early Cretaceous Cherts from the Western North Atlantic Basin

“…If compared with the palygorskite-rich (5 to 30 per cent) porcellanites from the eastern North Atlantic (Legs 14 and 41;von Rad and Rösch, 1974;von Rad et al, 1978), the aggregate polarization in Leg 43 porcellanites is much less con¬ spicuous and often restricted to single microflasers, patches, or layers.…”

“…They can be conveniently subdi¬ vided into bedded porcellanites associated with clayey sediments, silicified turbidites with opal-CT-replaced calcilutitic matrix, and calcareous (nodular) porcel¬ lanites (Table 2). Our classification is shown in Table 1 and discussed by von Rad et al (1978).…”

“…Because of the lack of relief in the highly cemented arenites under the SEM, the opal-CT matrix was etched using H 2 SiF 6 (von Rad et al, 1978), which ex¬ posed the quartz-replaced sponge spicules without al¬ tering their surface structure (Plate 3, Figures 5-6). By decalcification with HC1, fine details of the silicification of foraminifers can be demonstrated (Plate 3, Figures 1,4).…”

“…In our brief discussion of the origin of the famous Horizon A reflector sequence, we shall only consider North Atlantic sites where Eocene porcellanites have been cored, and omit those occurrences where this re¬ flector has been identified as an erosional unconformity with a lower to middle Cretaceous/mid-Cenozoic hia- Rad et al, 1978).…”

“…The petrog¬ raphy and mineralogy were studied by semiquantitative thin-section and scanning electron microscope (SEM) analyses. X-ray diffraction analysis (XRD), kindly provided by Dr. H. Rösch, Hannover, helped to identify and estimate the contents of silica phases and other minerals (Table 2; for methods see von Rad et al, 1978). The mineralogy of isolated siliceous fossils was optically determined.…”