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).
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 CaCO3—calcite 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.