The crusts range in thickness from one to a few centimetres and the fragments of these crusts are irregular in shape. Their surface is always uneven and their colour ranges from white to brownish dark grey. Some slabs are impregnated along one side by ferromanganese sesquioxides, and borings occur in several samples. Serpulid tubes have been observed in one instance. The borings and serpulids suggest formation of the hardgrounds at or close to the sediment/water interface and exposure at the seafloor.
The degree of lithification is generally different on the inferred upper and lower sides of the slabs. An upward increase of lithification across the slabs is reflected by mineralogy, ultrastructure and stable isotope composition of the carbonate. X-ray diffraction analyses indicate high-magnesian calcite as the predominant carbonate with minor amounts of low-magnesian calcite and dolomite. Occasionally, large gypsum crystals are attached to the hardgrounds and sometimes smaller ones are dispersed through the carbonate matrix.
An increase in diagenesis is reflected by the passage from friable, nodular nannofossil chalk to nannofossil limestone and hard xenotopic calcite micrite. Overgrowth of coccoliths and internal cementation of the tests of planktonic foraminifera by high-Mg calcite increase from chalk to limestone. In the hard, fully cemented micrites, coccoliths can no longer be recognised in the xenotopic fabric. Pteropods occur as dissolution moulds with aragonite preserved as only tiny relics.
Carbon and oxygen isotope analyses were performed on different samples. The progressive lithification to chalk and limestone is marked by a shift in the δ18O values from +1.2‰ to +5.4‰ (PDB). This change indicates that precipitation of high-Mg calcite and possibly also recrystallisation of the original biogenic carbonate took place within cold and hypersaline brines which were enriched in 18O. The oxygen isotope data suggest that lithification and gypsum precipitation occurred under identical conditions. The carbon isotope data show progressive diagenetic change from values near +1‰ to values of +3‰. This change may reflect a contribution of methanogenetic CO2 to the hypersaline brine. 相似文献
Dolostones and dolomitic mudstones of probable Messinian age subcropping beneath the brine in the northern part of Bannock Basin and Zanclean oozes recorded on the sill separating two narrow and elongate satellite basins along the eastern part of the rim-syncline record a relief inversion during their evolution from cylindric folds to collapse structures.
In particular, the core from the eastern intrabasin sill contains two major unconformities: one separates early Zanclean oozes from late Piacenzian pelagic sediments characterized by winnowed layers, inclined bedding and incipient hardgrounds, and the second separates the latter from Middle Pleistocene pelagic sediments. The deepening is certainly post-late Piacenzian (post-M Pl 6 biozone) because sediments of that age record an actively uplifting structure.
Another argument in favour of the relief inversion, and of the youthful age of the deepening, is derived from a core raised from the northern part of the major western basins (large sill separating Maestro from Borea basins) which contains a coarse turbidite of North African origin representing an upflow turbidite similar to others recorded on the outer slopes of the Mediterranean Ridge. This unique finding in the area of Bannock Basin is preserved in the central part of the collapse basin, whereas in other settings an erosional gap is noticed at the equivalent stratigraphic position. 相似文献