Abstract:
Reduction of sediment porosity and increase in density under overburden pressure in the sea floor are important subjects in earth sciences. Data and samples from the Deep Sea Drilling Project allow a new look at these subjects, and are used to establish profiles of laboratory values of density and porosity versus depth in the sea floor. To construct in situ profiles, the results of consolidation tests are used to estimate the amount of elastic rebound (increase in volume) which has occurred after removal of the samples from overburden pressure in the boreholes. In situ profiles of porosity and density versus depth are constructed for some important sediment types: calcareous ooze, siliceous oozes (diatomaceous and radiolarian oozes), pelagic clay, and terrigenous sediments. There is less reduction of porosity with depth in the first 100 m in these deep-water sediments than previously supposed: 8 to 9% in pelagic clay, calcareous and terrigenous sediments, and only 4 to 5% in the siliceous sediments. From depths of 300 m the most rebound is in pelagic clay (about 7%), and the least in diatomaceous ooze (about 2%); calcareous ooze and terrigenous sediment should rebound from 300 m about 4 to 5%. Terrigenous sediment, from the surface to 1,000 m depth, probably rebounds a maximum of about 9%. Methods are described and illustrated to predict density and porosity gradients in the sea floor, and to compute the amounts of original sediments necessary to have been compressed to present thicknesses. Slightly over 2,000 m of original sediments would have been required for compression to a present-day thickness of 1,000 m of terrigenous sediments.