Reference Work Entry

Sedimentology

Part of the series Encyclopedia of Earth Science pp 1095-1104

Siliceous sediments

  • L. PaulKnauthAffiliated withDepartment of Geological Sciences, Arizona State University

Introduction

Siliceous sediments are composed of silica that has actually precipitated at or near the site of deposition or has replaced pre-existing sediments. They are distinguished from clastic or terrigeneous sediments which are made of grains derived from rocks elsewhere and physically transported to the site of deposition. In today's oceans, the dominant examples of siliceous sediments are oozes composed of microscopic silica particles precipitated biologically by diatoms and, to a lesser extent, by radiolarians, sponges, and silicoflagellates. Diatom silica is precipitated on a colossal scale in surface waters and slowly settles to the ocean floor to produce diatomaceous oozes. Although most of the opaline tests dissolve in transit, a small percentage survive and accumulate in thicknesses up to hundreds of meters. The purity of the ooze is greatest for depths below the carbonate compensation depth and in areas far removed from landmasses that can supply abundant clays ...

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