Sediment Classification

Porosity is the void fraction of the sediments or rock.  For the uppermost layers of sediments, the voids can be up to 90% of the volume, which means the density and sound velocity will essentially be the values for sea water.  Over time compaction will remove most of the voids, and the density and sound velocity will both increase.

Permeability in the interconnectedness of the voids.  It is actually more important than porosity for oil and gas production, and fracking (or more technically, hydraulic fracturing) increases the rock's permeability.

Changes with depth, as sediment compacts and lithifies, include porosity, density, sound velocity.  Porosity decreases as the voids are eliminated, which causes density to increase because there is more sediment which typically has a specific gravity of 2.7 compared to water with 1.0, and the sound velocity increases.

Sediments reflect the latitude of deposition, water depth, and proximity to land.  All can change over time: plate motions move crust to new latitudes and often away from land, and as crust ages it cools and sinks deeper.

The following sediment types are found in the ocean:

Coccolith, wikipedia  The individual coccoliths separate when the animal dies.

Forams, wikipedia
Challenger expedition rads, wikipedia
Diatoms, wikipedia

The sedimentation rate depends on whether you measure it immediately after deposition when the porosity is high, or wait until the rock is fully compacted and the rate will be about 1/10 as large.  The rates also vary tremendously geographically: in the center of the north Pacific gyre, the rate might be 1 m/m.y., whereas in coastal areas it could exceed 1000 m/m.y.

Sediments will start with a density about 1.2 gm/cm (just slightly more than seawater) and a sound velocity of just more than 1500 m/sec (essentially the same as seawater), to go with a 90% porosity.  Once lithified, sound velocities in sandstones will range from 2.0-2.5 km/sec for Tertiary rocks to 5.5-6 km/sec for lower Paleozoic, and limestones show similar ranges.


Last revision 9/18/2016