Abstract

Geochemical and other calculations on the total volume of sediment that should be present in deep-sea basins, based on materials released in weathering, indicate that the average thickness of sediment solids on the ocean floor should be 1–3 km. Seismic surveys in all oceans, on the contrary, report a sediment thickness on the order of 0.1–0.5 km but also reveal that, in many areas, the surface sediments are underlain by other layers whose elastic-wave-transmission velocities are intermediate between those of sediment and the “basaltic basement” rock of the oceanic crust.

Examination of the above problem in the light of soil-mechanics studies, and recent studies of the velocity of elastic compressional waves in sediments and rocks, indicates the possibility that, in many areas, the lower layers are composed of rocks formed by normal consolidation and lithification of the present types of deep-sea sediments.

Gravitational consolidation of red clay should reduce sediment pore space to values between 35 and 45 per cent at depths in the sediment between 150 and 700 m; between these porosities, pressure-chemical effects in the dewatered sediment should result in lithification to shale and marked increase in seismic velocity.

Gravitational consolidation of calcareous ooze should result in porosity reduction at a slower rate than in clay, but lithification of this material with age and pressure can result in marked seismic-velocity increases at or near the sediment surface; the discovery of deep-sea limestone in the form of lithified Globigerina oozes ranging in porosities from 5 to 68 per cent and in compressional elastic-wave velocities from 1.8 to 5.5 km/sec indicates the possible velocity variations. Various combinations of clay and calcareous material are possible and probable in many areas; high velocities under a thin cover of present-day clay deposition thus possibly represent an ancient (Cretaceous to Tertiary) layer of lithified calcareous ooze.

The present sediment thickness measured in deep-sea basins is probably the total thickness of these upper layers. If so, the thickness of all layers above the “basalt” should enter the calculation of total sediment deposited on the sea floor. A measurement of sediment thickness at any time horizon of the past or future should show about the same “anomalously thin” value for the topmost part of the slowly consolidating and lithifying sediment.

Normal, expectable deposition of sediment in many deep-sea areas, and subsequent consolidation and lithification, may explain most of the seismic discoveries in these areas and fall into line with the great geochemical balance and with the theories of soil-mechanics engineering.

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