CLASSICAL THEORIES OF CAVERN DEVELOPMENT
(This Section is taken from Reeder, 1988)
For much of this century one of the most
intensive debates in the science of geomorphology has dealt with the origin of
limestone caves. Do caves form above the water table (vadose zone), below the
water table (phreatic zone), or at the plane of the water table itself? The
various theories can be classified as follows:
1. Vadose Theories – Dwerry house (1907), Greene (1908), Matson (1909), and Malott (1937) have maintained that most cavern development takes place above the water table where groundwater velocity is greatest. Thus, fast flowing underground streams, which combine mechanical corrosion with carbonate solution, are responsible for cave development. Martel (1921) believed that the importance of flow in caves and conduits is so great that it is irrelevant to regard cavernous limestone as possessing a water table, in the sense of a single surface below which the rock is entirely saturated.
2. Deep Phreatic Theories – Cvijic (1893), Grund (1903), Davis (1930) and Bretz (1942) have argued that cave initiation and most cavern enlargement occur at random depth below the water table, often in the deep phreatic zone. The caves are enlarged by the corrosive effects of slowly moving phreatic water. A second cycle of cavern development may occur when the water table has been lowered by surface denudation, thus draining the caves of ground water and permitting vadose water and air to enter the cave. During the second cycle surface streams may enter an existing cavern system and alter cave passages somewhat by corrosion.
3. Shallow Phreatic or Water Table Theories – Swinnerton (1932), Rhoades and Sinacori (1941), and Davies (1960) have supported the idea that rapidly moving water at the water table is responsible for the solution of most caves. The elevation of the water table fluctuates with variations in the volume of groundwater flow, and there may be substantial cave development within a narrow zone above and below its mean position. However, the mean position of the water table must remain relatively constant for long periods. To explain multi-leveled cave systems a stationary water table has often been associated with periods of landscape base leveling followed by periods of rejuvenation with rapid down-cutting to the next base level.
Although most speleologists would probably agree that all three of the above theories are applicable in some instances, most would argue that the great majority of caves are formed at or near the water table. Subterranean stream invasion, normally classified as a vadose theory, has had few advocates down through the years. Malott (1937) concluded that the large caverns of southern Indiana resulted from surface streams being diverted underground and developing caverns at or near the water table. Malott (1952) cited the Lost River of Indiana as a good example of subterranean invasion by a surface flowing stream. Woodward (1961) and Howard (1963), and Crawford (1978; 1987) are prominent among others who have advocated the invasion theory of cavern development.
In discussing the hydrology of limestone terrains Smith, Atkinson and Drew (1976) suggest that the debate on the nature of limestone drainage has been between two main schools of though rather than three.
One school, taking its ideas from Grund (1903), has maintained that water circu1ation in limestones is essentially similar to that of any other fractured rock, and that caves develop as a consequence of the circulation without greatly influencing its pattern (see, for example, Davis, 1930; Swinnerton, 1932; Rhoades and Sinacori, 1941). On the other hand, the school following Martel (1910) tends to rnaintain that without caves and conduits there can be no underground circulation, or virtually none, and that the groundwater regime of limestone terrains is thus utterly different from that of other rocks (Trombe,1952). (Ford and Cullingford, 1976, p. 209).
They conclude that the two apparently opposing models of Grund and Martel are but extreme cases of a spectrum of possible drainage systems and that most limestone regions exhibit features of both schools of thought, while areas that show either to the exclusion of the other are rare.
A major problem with the classical theories of cave origin is the minor role assigned to the structural, stratigraphic, topographic, and hydrologic setting. It now appears obvious that no one theory of cavern origin and development can be applicable to all caves. Ford (Ford and Cullingford, 1976) maintains that since no two geological situations are ever exactly the same, a single generalized theory of speleogenesis is not possible. Although many factors may be common to many areas their relative importance will depend on the geological situation and therefore every area will have its own unique speleogenetic history.
However, since there are areas with very similar geological situations, it is believed that very generalized models of speleogenesis can be developed for areas that have a similar structural, stratigraphic, topographic and hydrologic setting.