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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 76 (1992)

Issue: 6. (June)

First Page: 792

Last Page: 804

Title: Effects of Biogenic Silica on Acoustic and Physical Properties of Clay-Rich Marine Sediments (1)



The physical properties of marine sediments are influenced by compaction and diagenesis during burial. Changes in mineralogy, chemistry, density, porosity, and microfabric all affect a sediment's acoustic and electrical properties.

Sediments from the Japan Trench illustrate the dependence of physical properties on biogenic silica content. Increased opal-A content is correlated with increased porosity and decreased grain density and compressional Previous HitvelocityNext Hit. Variations with depth in opal-A concentration are therefore reflected in highly Previous HitvariableNext Hit and, at times, inverse Previous HitvelocityNext Hit-depth gradients.

The diagenetic conversion of opal-A to opal-CT and finally to quartz was investigated at a site in the San Miguel Gap, California. Distinct changes in microfabric, particularly in the porosity distribution, accompany the diagenetic reactions and contribute to a sharp Previous HitvelocityNext Hit discontinuity at the depth of the opal-A to opal-CT conversion. Evaluation of this reaction at several sites indicates a systematic dependence on temperature and age in clay-rich and moderately siliceous sediments. In ocean margin regions, sediments are buried rapidly, and opal-A may be converted to opal-CT in less than 10 m.y. Temperatures of conversion range from 30 degrees to 50 degrees C. Much longer times (>40 m.y.) are required to complete the conversion in open ocean deposits which are exposed to temper tures less than 15 degrees C.

In the absence of silica diagenesis, Previous HitvelocityTop-depth gradients of most clay-rich and moderately siliceous sediments fall in the narrow range of 0.15 to 0.25 km/s/km which brackets the gradient (0.18 km/s/km) determined for a "type" pelagic clay section.

Relationships such as these can be useful in unraveling the history of a sediment sequence, including the evolution with time of reservoir properties and seismic signatures.

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