CARBONATE-ROCK AQUIFERS— Continued Some of the common types of karst features that develop on the land surface where limestone is exposed in the Mam- 39° ° ° moth Cave area of central Kentucky are shown in figure 28. 85 83 Recharge water enters the aquifer through sinkholes, swallow Spring Natural bridge holes, and sinking streams, some of which terminate at large Sinkholes 87° Karst valley depressions called blind valleys. These depressions, along with Sinking y stream karst valleys and sinkholes, form when all or part of a cavern K E N T U C K Y Blind valleyy roof collapses. Uncollapsed remnants of the cavern roof form 89° Mammoth Cave Area ° yy 37 natural bridges. Surface streams are scarce because most of yy yyySpring y the water is quickly routed underground through solution open- yyShale Blind valley yy y y ings. In the subsurface, most of the water moves through cav- yyyyyyy Swallow yyy y hole erns and other types of large solution openings. yy y yyyy Sinkhole y Solution cavities riddle the Mississippian limestones that yyLimestone Cavern y yy underlie the Mammoth Cave Plateau and the Pennyroyal Plain y yyyyyyyyy yyy yNOT TO SCALE y that borders the plateau to the south and southwest. Some of y these cavities form the large, extensive passages of Mammoth y y y y Cave (fig. 29), one of the Nation’s largest and best studied y yyyyy cave systems. As the cave’s network developed, surface yy yyy yy streams were diverted into the passages through sinkholes and yyShale Figure 28. The dissolution of exposed limestone flowed as underground streams through openings along bed- yyy y yyy ding planes. Sand and other sediment carried by the under- forms characteristic land-surface features, such as sinkholes, y karst valleys, and sinking streams. Solutionyy features are rare where the ground streams abraded the limestone, thus further enlarging limestone is covered with noncarbonate rocks. Few surface streams occur because yy the solution openings through which the stream flowed. Verti- most of the precipitation is quickly routed into and through solution openings in the limestone. Modified from Lloyd and Lyke, 1994 cal passages, usually developed at the intersections of joints, yyy connect the horizontal bedding plane openings. Dissolution yy yy and erosional processes are still active at Mammoth Cave. y yy Aquifers in carbonate rocks of Cretaceous to Precambrian yyyyy age yield water primarily from solution openings. Except for y a basal sandstone aquifer, the Ozark Plateaus aquifer system yyy consists of carbonate-rock aquifers whose hydrologic charac- 35° 85° Greenville 80° teristics are like those of the limestones at Mammoth Cave. ● The Silurian–Devonian aquifers, the Ordovician aquifers, the Upper Carbonate aquifer of southern Minnesota, the Arbuckle– S O U T H Simpson aquifer of Oklahoma, and the New York carbonate- ★Columbia ★Atlanta rock aquifers are all in layered limestones and dolomites of Pa- Birmingham C A R O L I N A ● leozoic age, in which solution openings are locally well devel- Augusta ● oped. The Blaine aquifer in Texas and Oklahoma likewise G E O R G I A yields water from solution openings, some of which are in ● A L A B A M A Charleston carbonate rocks and some of which are in beds of gypsum and ★Montgomery anhydrite interlayered with the carbonate rocks. n a Savannah ● e Aquifers in carbonate rocks of Tertiary and younger age c have different permeability and porosity characteristics than O c aquifers in Cretaceous and older carbonate rocks. The aquifers i t in Tertiary and younger rocks are the Castle Hayne aquifer of n a North Carolina (in Eocene limestone), the Biscayne aquifer of l t South Florida (in Pliocene and Pleistocene limestones and A ★Tallahessee interbedded sands), the North Coast limestone aquifer system F Jacksonville● Base modified from U.S. L in Puerto Rico (in limestone of Oligocene to Pliocene age), and Geological Survey digital 30° O data, 1:2,000,000, 1972 the Floridan aquifer system of the southeastern United States R (in limestone and dolomite of Paleocene to Miocene age). The I ulf of Mex D strata that comprise these aquifers were deposited in warm ma- Joe Meiman, U.S. National Park Service, G ico A rine waters on shallow shelves and are mostly platform carbon- Mammoth Cave, Ky. Figure 29. Where solution openings in limestone are large and ate deposits in which intergranular porosity is present as well ● well connected, they may form networks of cave passages such as EXPLANATION Orlando as large solution openings. The Floridan aquifer system de- this one at Mammoth Cave. The openings can be enlarged at the scribed below is the most productive and most studied example Tampa bottom as the limestone is eroded by sediment-laden streams Floridan aquifer system ● of this type of carbonate-rock aquifer. flowing through them. Figure 30. The Floridan aquifer Miami ● system underlies all of Florida, large parts SCALE 1:7,500,000 of Georgia, and smaller areas in South 0 50 100 MILES Carolina, Alabama, and Mississippi. 0 50 100 KILOMETERS Southwest South Carolina 25° Panhandle Georgia and Northeast East-Central West-Central South and Florida Northwest Southeast Florida Florida Florida Florida Florida Georgia The Floridan Modified from Figure 31. Miller, 1990 aquifer system can generally Upper confining unit be divided into an Upper Floridan aquifer and a Lower Floridan aquifer, separated in most places by a less-permeable confining unit. The Lower Floridan aquifer UPPER FLORIDAN AQUIFER locally contains cavernous zones that are extremely permeable. 35° 85° Figure 32. The thickness 80° Middle Middle Middle Middle of the Floridan aquifer system confining semiconfining semiconfining confining varies considerably. Some of unit unit unit unit the variations reflect major S O U T H warping of the Earth’s crust during deposition of the C A R O L I N A carbonate rocks that compose the system and some reflect LOWER FLORIDAN AQUIFER faulting after the rocks were Boulder A L A B Zone deposited. A M A G E O R G I A FLORIDAN AQUIFER SYSTEM 200 Fernandina n permeable A 400 a zone e 600 c 800 O 1000 Lower confining unit 00 c 200 6 i t 1200 1200 n 00 Modified from Miller, 1986 4 6 a yy1400 l 0 200 2800 0 0 1600 t 00 0 60 4 6 40 0 2000 00 800 1000 1800 A 0 4 1800 2400 600 0 00 1200 1400 600 600 80 0 6 00 1000 1 1600 F 0 1400 Base modified from U.S. 0 L 6 Geological Survey digital 30° 1 O data, 1:2,000,000, 1972 yy R 1800 1400 2400 I 1600 3000 D 1800 ulf of Me G xico A The Floridan aquifer system underlies an area of about meable confining unit (fig. 31). In parts of northeastern and 2000 100,000 square miles in southeastern Mississippi, southern northwestern Florida and southwestern Georgia, no confin- 2200 3 2400 00 Alabama, southern Georgia, southern South Carolina, and all ing unit exists and the Upper and Lower Floridan aquifers 0 2600 EXPLANATION of Florida (fig. 30). The Floridan is one of the most produc- are directly connected. Two cavernous zones, the 30002800 2800 tive aquifer systems in the world; an average of about 3.4 Area where Floridan aquifer system is thin Fernandina permeable zone and the Boulder Zone, are 3400 billion gallons per day of freshwater was withdrawn from it present in the Lower Floridan aquifer. Low-permeability yy due to intergranular gypsum 3400 3200 during 1990. Despite the huge withdrawals, water levels in 3000 confining units bound the aquifer system above and below. Thickness of Floridan aquifer system, in feet 3400 3200 the Floridan have not declined regionally; however, large de- The thickness of the Floridan aquifer system ranges 3400 clines have occurred locally at a few pumping centers. from a thin edge at its northern limit to more than 3,400 feet 600 3000 1200 3200 3000 The Floridan aquifer system is extremely complex be- in parts of southern Florida (fig. 32). The map shows the 2800 cause the rocks that compose the system were deposited in combined thicknesses of the Upper and Lower Floridan 1800 2600 2800 highly variable environments, and their texture accordingly aquifers and the middle confining unit where it is present. 2400 SCALE 1:7,500,000 0 50 100 MILES varies from coarse coquina that is extremely permeable to Some of the large-scale features on the thickness map are 3000 micrite that is almost impermeable. Diagenesis has changed 0 50 100 KILOMETERS A' related to geologic structures. For example, the thick areas 25° the original texture and mineralogy of the carbonate rocks in southeastern Georgia and in the eastern panhandle of 3200 3000 No data 3400 in many places. The principal diagenetic processes that in- Florida coincide with downwarped areas which are called the Modified from fluence the porosity and permeability of the aquifer system Southeast and Southwest Georgia Embayments, respec- 2000 Line of equal thickness of Floridan aquifer Miller, 1986 are dolomitization (which increases the volume of connected tively. In north-central peninsular Florida, the aquifer sys- system—Interval 200 feet pore space in fine-grained limestones) and calcite or dolo- tem thins over an upwarp which is called the Peninsular Fault—Vertical or nearly so. Bar and ball on mite overgrowths (which fill part or all of the connected pore Arch. Faults in southern Georgia and southwestern Alabama downthrown side space in pelletal limestone or coquina). Dissolution of the form the boundaries of trough-like grabens. Clayey sedi- AA'Line of hydrogeologic section—Section limestone has produced small to large conduits at different ments within these downdropped structural blocks have shown in figure 33 levels in the aquifer system.