Major results from a petrographic study of porcellanites and cherts are as follows: 1) Acoustic Horizon A consists of isochronous but facies-independent lower to middle Eocene immature porcellanites. A biogenic silica source is inferred. The porous sediments are first cemented and later replaced by opal-CT, producing a dense or lepispheretype fabric. Porcellanites within the clayey environment include silicified (radiolarian) mudstones and turbidite sands rich in sponge spicules. Poorly consolidated Eocene chalks show gradual transi¬ tions to calcareous porcellanites, which contrast with the discrete nodules typical of more strongly lithified carbonates.2) Before chertification, early-diagenetic quartz occurs locally in porcellanites as replacement of foraminifer tests and latest-genera¬ tion cavity-filling cement.3) With increasing burial depth and age, radiolarians are trans¬ formed in situ from the original opal-A into opal-CT, and finally into quartz. The boundary between sediments containing siliceous fossils preserved as opal-A and as opal-CT coincides with the up¬ permost porcellanites of each site.4) The diagenetically mature Lower Cretaceous quartz cherts contain quartz-replaced lepispheres, suggesting a porcellanitic pre¬ cursor stage. This indicates an age-, temperature-, and facies-controlled maturation process from opal-A to quartz via opal-CT.

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.).

The Cape Bojador region of the northwestern African continental margin is apparently an unusual passive margin setting. Climatic fluctuations, local volcanism, and possibly tectonics have played important roles in the sedimentary record in this region. A transect of drilling sites connected by seismic profiles is available from the coastal Aaiun Basin (CORC 15-1), across the shelf (Spansah 51A-1), on the continental slope (DSDP Site 369), the upper continental rise (DSDP Site 397), farther out on the rise (e.g., DSDP Sites 139 and 140), and the abyssal plain (DSDP Sites 137 and 138). These data allow construction of a profile perpendicular to the margin, and reconstruction of the sedimentary evolution there.Lower Cretaceous sediment (Hauterivian) is the oldest recovered in a DSDP site along this part of the margin. Equivalent sediment was recovered from hydrocarbon exploration wells in the Aaiun Basin. Subsidence rates under the present shelf, slope, and upper rise were rapid during the Jurassic and Early Cretaceous (110 to 140 m/m.y.), and diminished since the Late Cretaceous (average 13 m/m.y.). A sequence of Lower Cretaceous Wealden-type deltaic and prodeltaic environments has been delineated from well and outcrop data: a thick sequence of sandstone and siltstone (Spansah 51A-1) grades seaward to laminated prodelta muds containing thin siderite layers (DSDP Site 397). Deltaic sedimentation gradually wanes in the Middle Cretaceous in the coastal basin. This interval is represented by a hiatus at Site 397. A Middle to Late Cretaceous transgression resulted in deposition of more pelagic black shale, nannofossil marl, chert, and limestone on the Cape Bojador slope (DSDP Site 369) and in the Aaiun Basin.

Eocene to Cretaceous siliceous sediments of four Deep Sea Drilling Project sites on the continental margin off eastern North America (Sites 603, 605-Leg 93; Sites 612, 613-Leg 95) were analyzed for their major geochemical and mineralogical composition. Under different environmental conditions (burial depth, in situ temperature, host-rock fades) of the upper slope to lower rise, conversion of biogenous opal-A to opal-CT took place in biosiliceous chalks and marls older than early middle Eocene. Dense sampling revealed the sharp character of the diagenetic silicification front (seismic reflector A c), which demonstrates a widespread, more or less synchronous (51-53 Ma), rapid change of silicoplankton fertility and preservation, which later led to the marked boundary between an opal-A and an opal-CT diagenetic facies. In clay-rich sediments on the lower rise (Site 603), the opal-A opal-CT transformation is largely retarded, whereas clinoptilolite formation is enhanced. Host-rock composition and the, consequent interstitial water chemistry, apparently controlled by palaoclimatology and paleoceanography, appear to be the most important factors influencing the rate and mode of silica diagenesis. Diagenetic alteration of the primary siliceous oozes to porcellanitic chalks and porcellanites took place only in a narrow equilibrium range of the complex geochemical sediment-pore water system. These ideal conditions were realized in our case and at most sites in the Atlantic Ocean in sediments older that 51 m.y.

Site 764 of the Ocean Drilling Program (ODP), drilled during Leg 122 in the Exmouth Plateau region, cored 200 m of Upper Triassic (Rhaetian) reef complex. This site, on the northern Wombat Plateau (northernmost Exmouth Plateau) represents the first discovery of Triassic reefal material near the Australian North West Shelf. Seismic reflection data through Site 764 show that the reef itself corresponds predominantly to a seismically poorly reflective zone. A number of regional unconformities appear to correspond, however, to traceable seismic horizons which pass with reduced amplitude through the reef, indicating stages of reef growth separated by erosion or non- deposition. Seismic facies around the edges of the reef are consistent with the deposition of wedges of prograding reef- derived detritus.Application of the seismic criteria for reef recognition established at ODP Site 764, to other seismic reflection data on the Wombat Plateau, demonstrates that a major Upper Triassic reef complex fringes the margins of the Wombat Plateau. The Wombat Plateau lies at the western end of the North West Shelf, which was part of the southern margin of a warm Tethys Ocean in the Late Triassic, at a palaeolatitude of 25- 30°S. Upper Triassic reefs are known from southeast Indonesia and Papua New Guinea, and now the Wombat Plateau, and may be common elsewhere along the outer margin of the North West Shelf. Upper Triassic reef complexes, with their associated reservoir, source and seal facies, could represent an exciting new petroleum exploration play for the entire North West Shelf. Facies analysis suggests that they are likely only on the outer shelf and slope. Shallow Triassic reef complexes are clearly identifiable using high resolution seismic reflection data. Seismic reflection data of lower resolution may well reveal the associated detrital carbonate wedges, which are more laterally extensive than the reefal core, deeper in the section.