Oceanic plateaus form by mantle processes distinct from those forming oceanic crust at divergent plate boundaries. Eleven drillsites into igneous basement of Kerguelen Plateau and Broken Ridge, including seven from the recent Ocean Drilling Program Leg 183 (1998^99) and four from Legs 119 and 120 (1987^88), show that the dominant rocks are basalts with geochemical characteristics distinct from those of mid-ocean ridge basalts. Moreover, the physical characteristics of the lava flows and the presence of wood fragments, charcoal, pollen, spores and seeds in the shallow water sediments overlying the igneous basement show that the growth rate of the plateau was sufficient to form subaerial landmasses. Most of the southern Kerguelen Plateau formed at V110 Ma, but the uppermost submarine lavas in the northern Kerguelen Plateau erupted during Cenozoic time. These results are consistent with derivation of the plateau by partial melting of the Kerguelen plume. Leg 183 provided two new major observations about the final growth stages of the Kerguelen Plateau. 1: At several locations, volcanism ended with explosive eruptions of volatilerich, felsic magmas; although the total volume of felsic volcanic rocks is poorly constrained, the explosive nature of the eruptions may have resulted in globally significant effects on climate and atmospheric chemistry during the late-stage, subaerial growth of the Kerguelen Plateau. 2: At one drillsite, clasts of garnet^biotite gneiss, a continental rock, occur in a fluvial conglomerate intercalated within basaltic flows. Previously, geochemical and geophysical evidence has been used to infer continental lithospheric components within this large igneous province. A continental geochemical signature in an oceanic setting may represent deeply recycled crust incorporated into the Kerguelen plume or continental fragments dispersed during initial formation of the Indian Ocean during breakup of Gondwana. The clasts of garnet^biotite gneiss are the first unequivocal evidence of continental crust in this oceanic plateau. We propose that during initial breakup between India and Antarctica, the spreading center jumped northwards transferring slivers of the continental Indian plate to oceanic portions of the Antarctic plate. ß

Comparison of chalk on the Ontong Java Plateau and chalk in the Central North Sea indicates that, whereas pressure dissolution is controlled by effective burial stress, pore‐filling cementation is controlled by temperature. Effective burial stress is caused by the weight of all overlying water and sediments as counteracted by the pressure in the pore fluid, so the regional overpressure in the Central North Sea is one reason why the two localities have different relationships between temperature and effective burial stress. In the chalk of the Ontong Java Plateau the onset of calcite‐silicate pressure dissolution around 490 m below sea floor (bsf) corresponds to an interval of waning porosity‐decline, and even the occurrence of proper stylolites from 830 m bsf is accompanied by only minor porosity reduction. Because opal is present, the pore‐water is relatively rich in Si which through the formation of Ca–silica complexes causes an apparent super‐saturation of Ca and retards cementation. The onset of massive pore‐filling cementation at 1100 m bsf may be controlled by the temperature‐dependent transition from opal‐CT to quartz. In the stylolite‐bearing chalk of two wells in the Gorm and Tyra fields, the nannofossil matrix shows recrystallization but only minor pore‐filling cement, whereas microfossils are cemented. Cementation in Gorm and Tyra is thus partial and has apparently not been retarded by opal‐controlled pore‐water. A possible explanation is that, due to the relatively high temperature, silica has equilibrated to quartz before the onset of pressure dissolution and thus, in this case, dissolution and precipitation of calcite have no lag. This temperature versus effective burial stress induced difference in diagenetic history is of particular relevance when exploring for hydrocarbons in normally pressured chalk, while most experience has been accumulated in the over‐pressured chalk of the central North Sea.

Burial diagenesis of chalk is a combination of mechanical compaction and chemical recrystallization as well as cementation. We have predicted the characteristic trends in specific surface resulting from these processes. The specific surface is normally measured by nitrogen adsorption but is here measured by image analysis of scanning electron micrographs. This method concentrates on the micritic matrix alone. Deep‐sea sediments are ideally suited to the study of burial diagenesis because they accumulate in a relatively conservative tectonic setting. We used material from the Ontong Java Plateau in the Pacific, where a > 1 km thick package of chalk facies sediments accumulated from the Cretaceous to the present. In the upper 200–300 m the sediment is unconsolidated carbonate ooze, throughout this depth interval compaction is the principal porosity reducing agent, but recrystallization has an equal or larger influence on the textural development. In the chalk interval below, compaction is not the only porosity reducing agent but it has a larger influence on texture than concurrent recrystallization. Below 850 m grain‐bridging cementation becomes important resulting in a lithified limestone below 1100 m. This interpretation is based on specific surface data alone, and modifies current diagenetic models.

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Density scanning by gamma transmission measurements on whole cores is here demonstrated to be valuable both as a sampling guide in the core laboratory, and for assessing the degree of induration of Palaeogene limestones in Denmark. Fracture intensity is also indicated. Moduli of elasticity calculated from acoustic velocities are systematically related to density (and thus porosity), so we have attempted to relate the degree of induration to the dynamic moduli of elasticity. The dynamic moduli of elasticity fall, as is commonly observed, above those measured from uniaxial compression tests. P-wave as well as S-wave velocities are anisotropic in the entire porosity interval (12%-44%). This is reflected in a significant anisotropy of the dynamic shear modulus at high densities, whereas the anisotropy for lower densities and for the dynamic Young's modulus and dynamic bulk modulus remain below the level of significance. The anisotropy probably reflects minor horizontal fractures formed during exhumation of the limestone and unloading of the cores. The density scanning tool thus proves a useful method for determination of geophysical and geotechnical parameters.