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6.19: Limey Sediments and Limestone

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    Limey Sediments and Limestone

    Lime mud is sediment composed of calcium carbonate (CaCO3) derived from the skeletal remains of shelled organisms, coral, and calcareous algae and plankton. Large amounts of lime mud is created by waves battering reefs and reef organisms (including dead corals and other calcareous skeletal material) being chewed up and excreted by reef-living organisms (Figures 6-72). With compaction and cementation (lithification) limey sediments become limestone (Figure 6.73).

    Lime mud and sand around a living coral reef limestone
    Figure 6.72. Lime mud and sand accumulating around a living coral reef. Fine limey sediments are created mostly by organisms feeding on other reef organisms. Figure 6.73. Skeletal remains of calcareous reef organisms erode and accumulate over time. "Limey" (shallow and warm) depositional environments are where lime sediments accumulate. Lime sediments turn to limestone.

    Limestone is a sedimentary rock consisting predominantly of calcium carbonate (CaCO3); the rock must have >50% calcium carbonate to be considered a limestone. Some limestones preserve large quantities of fossil material as crushed up shells or even old reef communities are sometimes preserved in nearly intact orientation of the corals and other calcareous organisms. These organic remains are made up of tiny crystals of two mineral forms of CaCO3calcite and aragonite. Aragonite is more soluble and is chemically less stable, and will usually convert to calcite with time.

    Most limestone exposed throughout the United States formed in ancient shallow marine seaways that flooded portions of the continent in the geologic past. Large regions within the United States are underlain by thick sequences of limestone rock formations representing all geologic time periods from Precambrian age to the present (Figure 6.74). In many locations the limestone beds are many thousands of feet thick. Most caverns form in limestone. Sinkholes form in limestone regions (See Sinkholes [USGS])

    Limestone is commonly used in the manufacture of lime for cement, used as building stone, and used to manufacture steel and many other products. Ancient carbonate deposits contain some of the world's largest petroleum reserves.

    Map of limestone karst regions of the United States Limestone with bryozoan fossils Brachiopodal limestone
    Figure 6.74. Map of the United States showing the location of carbonate rocks in the subsurface. Limestone rock formations occur under about 40% of the continental United States! Figure 6.75. Fossiliferous limestone. This sample from Ohio is loaded with ancient coral-like fossils called bryozoans (not corals). Figure 6.76. Fossiliferous limestone. This layer from Cincinnati, Ohio is loaded with ancient brachiopod shells that accumulated in an ancient inland seaway.