Deep Sea Carbonates: Dissolution Facies and Age-Depth Constancy

Abstract

DEEP sea drilling1 has shown the presence in many parts of the seafloor of a complex sequence of carbonate sediments showing various degrees of dissolution. On the present sea-floor, dissolution of carbonate increases with depth, leading to increasingly poorly preserved carbonate assemblages with increasing depth of deposition, until finally all carbonate is dissolved, leaving a residue of pelagic clay. This relationship between depth and dissolution has been used to interpret the carbonate profiles in drill sites in two contrasting ways. First, assuming that the present distribution of dissolution with depth was constant through time, poorly preserved carbonate skeletons and pelagic clay indicate great depth of deposition, whereas well preserved skeletons and pure chalk indicate a shallow ocean bottom2; second, neglecting tectonic movements, as well as redeposition, the distribution of age versus ocean depth of foraminifera, nannofossils and clay defines the rise and fall of solution levels3,4, such as the calcite compensation depth (CCD) and the (whole test) foraminiferal compensation depth (FCD) somewhat above the CCD.