Scholle, P. A. Department of Geological Sciences, Southern Methodist University, Dallas, Texas.
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The term “chalk” is sometimes used in a broad sense for any soft, friable, or weathered fine-grained limestone; however the term is mostly restricted to pelagic (biogenic) limestones. Chalk is a uniformly fine-grained, typically light-colored marine limestone primarily composed of the remains of calcareous nannofossils and microfossils. These minute pelagic organisms (see illustration) live in surface and near-surface oceanic waters and include coccolithophores (algae) and planktic foraminifers (Protozoa). Larger fossil constituents (such as bivalves, pteropods, echinoids, or ammonites) may be present, but only in subordinate amounts. The dominant pelagic skeletal remains are composed of low-magnesium calcite and, after death, settle slowly to the ocean floor, accumulating where the sea floor lies at a depth of less than about 4 km or 13,000 ft (the carbonate is redissolved at greater depths). Typical chalk sedimentation rates are 30 m (100 ft) per million years, so chalk accumulation is also dependent on the exclusion of diluting materials such as reefal detritus or terrigenous debris (clay, silt, or sand) transported from land areas by rivers. Chalks therefore form mainly in isolated outer shelf or deeper-water settings that are far from land areas. See also: Calcite; Limestone
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