Evidence of relief inversion from the rim-syncline of Bannock Basin

The last steps in the geological exploration of Bannock Basin (cruises BAN-88 and BAN-89 of R.V. Bannock) provided direct evidence of an inversion of relief that was long suspected but hitherto never recorded. 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. © 1991.