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It has been generally assumed that the deep borings on Pacific atolls have confirmed Darwin’s theory of coral reef development which holds that continued subsidence results in the successive appearance of fringing reefs, barrier reefs and atolls. It is certainly true that the considerable thicknesses of shallow water carbonates encountered in these core holes necessitates subsidence; however it does not necessarily follow that this subsidence has resulted in the genetic succession of reef types advocated by Darwin. It is the purpose of the present paper to enlarge on an alternate theory first presented by MacNeil, and in so doing to demonstrate that many, if not most, of the shape attributes of modern reefs are fundamentally karst-induced rather than growth- induced.

There can be little doubt that the carbonate platforms beneath most modern reefs have suffered some degree of subaerial exposure. This general inference is warranted by the apparent thinness of Holocene shallow water carbonate deposits in conjunction with the low stand of sea level during the Wisconsin glaciation. Thus it seems logical to conclude that most modern reefs have developed on a karst substrate. The occurrence of drowned sink holes a few hundred feet deep on several modern carbonate platforms supports the same conclusion and more importantly suggests a potential for the development of considerable solution relief.

Experiments with limestone blocks indicate the feasibility of solution development of the diagnostic crosssection morphology of both barrier reefs and atolls. Tropical karst land forms are suggestive of the same conclusion. All that is apparently required is a large surface area of gently dipping beds that is bordered on one or more sides by a relatively steep slope. The dissolving action of meteoric water differentially lowers the central area relative to that immediately adjacent to the steep slopes and results in a partially or completely rimmed solution basin. Subsequent rise in sea level permits coral colonization of both the solution rim and the residual karst prominences within the basin. The resulting barrier reef or atoll, with its satellite lagoon reefs, is thus formed without recourse to a prior history of reel: development.

The attributes of the reefs themselves support this interpretation. Special pleas have been advanced for many of these, but all seem related to the development of a karst solution basin. Thus, drowned “atolls” reflect drowned karst topography; reef passes originate as drainage breaches in the solution rim; faroes are a karst product of breaching; peripheral limestone islands are exposures of the fossil drainage divide; and spurs and grooves are expressions of Iapies. These karst-induced differences in relief are perpetuated, and indeed accentuated, by reef growth, but reef growth per se has little to do with the basic configuration.

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