Climate exerts a universal dominant influence on ecology, but processes of karstification have an equally high ecological influence in carbonate rock regions. Development of karst features depends greatly on the degree to which water containing carbon dioxide has been able to move on and through carbonate rocks and to remove some of the rock in solution. Distinctive features of many karst terranes include scarcity of soils, scarcity of surface streams, and rugged topography; less distinctive are the highly permeable and cavernous rocks, especially at the shallow depths. This high permeability gives rise to many practical problems, including (i) scarcity and poor predictability of groundwater supplies, (ii) scarcity of surface streams, (iii) instability of the ground, (iv) leakage of surface reservoirs, and (v) an unreliable waste-disposal environment. Natural karst processes in some carbonate rock regions have caused a greater restriction in the development of biota than man can ever be suspected of causing.
Unlike aquifers in nonsoluble rocks in which the permeability tends to be both inherent and fairly even in local distribution, aquifers in carbonate rocks tend to have their permeability developed through circulation of water and solution of the rock and to have an uneven distribution of permeability. The extent to which near‐surface carbonate rocks have permeability developed in them depends on the degree to which water that is high in dissolved carbon dioxide has moved through joints and other openings to places of discharge.The following well‐established generalizations indicate that the geometry of carbonate aquifers is commonly different from that of other aquifers. (1) The circulation of water and solution activity tend to be greatest in the upper part of the zone of saturation and tend to lessen with increased depth. (2) The moderately large openings in the path of bulk flow of groundwater tend to enlarge by solution action; contrastingly, the small openings not in the path of bulk flow enlarge only slightly. The texture of permeability in a few carbonate aquifers is fine and even but in most carbonate aquifers is more commonly coarse and uneven. Where the texture of permeability is fine and fairly even, as in the Biscayne aquifer of southeastern Florida, the aquifer may be riddled with a close network of solution channels or interstitial openings. Where the texture of permeability is coarse and uneven, ramifying solution channels are bounded by a large volume of relatively dense impermeable carbonate rocks. The channel network system is especially prominent in the upper part of the zone of saturation. In many cases the gross permeability seemingly decreases exponentially with increasing depth. The dynamic water table, becoming flatter and moving lower as permeability increases, is bounded above by a cavernous unsaturated zone that is a relic of the former permeable zone of saturation. With passing geologic time, the solution channel network loses its aquifer characteristics when (1) the water table moves downward to the base of the aquifer, (2) the cavernous rocks are destroyed by erosion, or (3) the aquifer becomes buried under later deposits and lies below the groundwater circulation system. The history of some carbonate aquifers includes early development under water table conditions, burial and preservation under later deposits, resurrection, and reactivation in the modern groundwater circulation system. Observable or discernible recognition can be made of (1) aquifers having fine textured permeability, (2) aquifers having coarse textured permeability, and (3) aquifers that have been resurrected and are now reactivated; each type of aquifer has distinctive characteristics that help in hydrologic evaluations of carbonate rock systems.
A worldwide overview of structural geologic settings indicates that the homoclinal flank is one of the most widespread and common of hydrogeologic systems. The geomorphic conditions on the structural basin flanks correlate with distinctive ground‐water flow patterns. Relatively undisturbed coastal plain formations and all other sedimentary rocks that are arched or warped into marginal basins are flanked at the land surface by dynamic hydrologie systems. Their aquifers, on a local scale considered to be horizontal, extend into regional tilted and beveled systems. Dips of strata that are only slightly greater than the gross slope of the land surface, but in the same direction, are prominent and produce exposures of alternating permeable and less permeable beveled formations in elongated regional bands. Differential erosion has led to cuesta landscapes in varying degrees. On these step‐like plateaus, subsequent and consequent streams have distinctive streamflow characteristics. One common type of hydrologic system is that dominated by consequent streams flowing down the structural basin, generally through reentrants in scarps. These streams capture much ground‐water runoff at the lowest exposed point in the aquifer (at the apex of the V that the stream makes with an overlying aquitard). The discharging ground water forms a natural cone of depression in the aquifer, representing a chief core of the ground‐water circulation system. An example is the large cone of depression in a Cretaceous sand aquifer in the Savannah River valley 15 to 25 kilometers south of Augusta, Georgia. Graphic modeling of hydrogeologic systems in homoclinal flanks explains the uneven distribution of ground‐water discharge. Stretches of each stream can be classified readily according to the degree of ground‐water discharge on a homoclinal flank.
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