LIMESTONE AND DOLOMITE 



361 



Most limestone is a lithified mixture of detrital 

 grains and lime mud. The grains include whole and 

 broken fossils, pellets of mud, and oolites composed 

 of concentric layers of calcite precipitated around 

 a nucleus. A complete gradation occurs between 

 limestone composed entirely of grains and limestone 

 composed entirely of limy mud. The mud is com- 

 posed of chemically precipitated calcite and arago- 

 nite and tiny calcareous fragments from organisms. 

 The carbonate sediments respond to marine currents 

 as do land-derived noncarbonate sediments, and 

 some limestone layers are finely laminated and show 

 graded bedding or even crossbedding. 



Reef limestone is formed in place as rigid frame- 

 works build up by colonial organisms, such as corals, 

 bryozoa, and algae. Organic reefs form only under 

 ideal conditions of clear shallow marine water, warm 

 temperatures, and abundant food supply. Large reefs 

 can form only where the water depth slowly in- 

 creases. Detrital materials, including fragments 

 eroded from the reef by wave action and shells of 

 noncolonial organisms, accumulate around the reef 

 and commonly make up a much greater volume than 

 the rigid reef rock. 



Dolomite forms in some places where sea water 

 evaporates and also forms indirectly as a partial 

 replacement of calcite in limestone by magnesium 

 from calcium-depleted brines of evaporated sea 

 water percolating through limestone. Today, dolo- 

 mite forms on broad, nearly level flats just above 

 the level of high tide. These flats are intermittently 

 flooded by sea water, during either spring tides or 

 storms, and some of the water remains on the flats 

 after the flooding. The water collects in small de- 

 pressions where evaporation takes place, and vari- 

 ous minerals may precipitate as the brine becomes 

 more saline. Gypsum (CaSOi -21120), the first min- 

 eral to precipitate, depletes the brine in calcium, 

 causing a change in the ratio of magnesium to cal- 

 cium from 5:1 in normal sea water to as much as 

 50:1. Dolomite forms by reaction of the magnesium- 

 enriched brines with formerly deposited calcite and 

 aragonite. Seasonal rainfalls may bring about a con- 

 centration or purification of the dolomite by dissolv- 

 ing impurities deposited by the brine; for example, 

 gypsum, which is more soluble than dolomite, is 

 thus preferentially dissolved and returned to the 

 ocean. Dolomite also occurs in some evaporite se- 

 quences that were deposited in broad shallow seas 

 of nearly stagnant or very slowly circulating water 

 in an arid climate. In such shallow seas a salinity 

 gradient developed, whereby the salinity increased 

 as the distance from the open ocean increased. 



DISTRIBUTION AND RESOURCES 



Limestone and dolomite occur in most parts of 

 the world and have formed throughout geologic 

 time, but they are notably more abundant in rocks 

 formed after shell-secreting organisms became 

 abundant about 500 million years ago. Limestone 

 and dolomite formations are widespread in the 

 United States, and nearly every State has deposits 

 adequate for construction and other uses that do 

 not require chemically pure stone. High-calcium 

 limestone and high-purity dolomite make up only a 

 small fraction of all limestone and dolomite forma- 

 tions, and they occur in restricted areas. But even 

 so, some high-purity carbonate formations are wide- 

 spread, occurring in minable thicknesses nearly con- 

 tinuously over areas of many hundreds or even 

 thousands of square miles. Although these nearly 

 pure rocks are far less widely distributed than im- 

 pure carbonate rocks, the resources are large and 

 are sufficient to supply the growing needs of the 

 Nation for hundreds or even thousands of years at 

 anticipated rates of consumption. 



The most extensive stratigraphic units of lime- 

 stone and dolomite of the United States were de- 

 posited in the Paleozoic Era, when North America 

 was extensively covered by shallow seas. Carbonate 

 rocks make up a significant part of the thickness of 

 the Paleozoic sedimentary sequence. Exploitable 

 limestone and dolomite formations occur most ex- 

 tensively in the slightly deformed rocks of the mid- 

 continent — especially in Kentucky, Tennessee, Mis- 

 souri, and the States adjacent to the Great Lakes. 

 Formations exposed in the folded and faulted parts 

 of Appalachian and Rocky Mountains tend to be 

 linear bands parallel to regional geologic structure 

 and, thus, are more restricted in area. However, 

 some parts of these mountainous areas, as in the 

 Appalachian Plateaus and the plateau regions of 

 the Western United States, are relatively unde- 

 formed and the nearly flat-lying units are at min- 

 able depths over wide areas. 



Only a few formations will ultimately be the 

 principal sources of high-calcium limestone and 

 high-purity dolomite. The greatest quantities of 

 high-calcium limestone occur in widely distributed 

 formations of Mississippian age (table 69), whereas 

 the greatest quantities of high-purity dolomite oc- 

 cur in formations of Ordovician and Silurian ages 

 (table 70). Smaller deposits of high-quality lime- 

 stone and dolomite occur in formations of other 

 ages, and many formations that are chiefly impure 

 rock locally contain minable deposits of high-purity 

 limestone or dolomite. Some Ordovician formations 



