It has been 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 true that the considerable thicknesses of shallow-water carbonates found 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. The author enlarges on an alternate theory (first presented by MacNeil) and demonstrates that many, if not most, of the shape attributes of modern reefs are fundamentally karst induced rather than growth induced.

There is 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 recent shallow-water carbonate deposits in conjunction with the low stand of sea level during Wisconsin glaciation. Thus it seems logical to conclude that most modern reefs have developed on a karst substrate. The presence of drowned sink holes a few hundred feet deep on several modern carbonate platforms supports this 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 cross-section morphology of both barrier reefs and atolls. Tropical karst land forms are suggestive of the same conclusion. All that is required apparently is a large surface area of gently dipping beds bordered on 1 or more sides by a relatively steep slope. The dissolving action of meteoric water differentially lowers the central area relative to that adjacent to the steep slopes and results in a partly 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 h story of reef development.

The attributes of the reefs themselves support this interpretation, and 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; faros are a karst product of breaching; peripheral limestone islands are exposures of the fossil drainage divide; and

End_Page 865------------------------------

spurs and grooves are expressions of lapies. 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 resulting configuration.

It follows from this hypothesis that similar events should be recorded in the geologic record, and it is therefore interesting to note that facies relations among some supposed fossil reefs are depositionally incompatible unless an intervening period of subaerial exposure is assumed.

End_of_Article - Last_Page 866------------