Reefs are considered as largely unbedded or obscurely bedded, massive structures which are composed of solid, organically bound, in situ organisms, and which were at least potentially wave-resistant structures that rose topographically above the surrounding depositional surface. Reefs are somewhat unusual and quantitatively minor features in the geologic record, but they have received considerable attention because of their economic importance, biologic uniqueness, or distinctive facies relations.

Any model for recognition of reefs in the geologic record must allow for considerable variation in relief, size, shape, biologic composition, and facies relations. They are associated commonly with normal marine environments, but the associated complex may span from freshwater to hypersaline deposits or from euxinic to well-oxygenated conditions.

Relief and shape depend on several factors, principally the comparative rate of subsidence and growth, direction of prevailing currents, structural relation, and organic composition. Size and shape commonly are discernible, but demonstration of depositional relief is difficult in many places.

Textures of single outcrops, hand specimens, or thin sections, may be diagnostic of at least reef potential if the massive, bound relations of organisms are apparent, but commonly additional criteria are necessary. Recognition of biologic and lithologic facies relations are critical in investigation of reef and associated environments in the geologic record.

The term "reef" has been applied loosely to several structures by different workers. Locally, it has been used for merely a faunal association, even though the organisms are present as loose, discrete fragments and the rocks in which they occur are evenly bedded in moderately thin layers. The term also has been applied to carbonate lenses in noncarbonate sequences, even though these lenses are of bedded, unbound detritus, oolites, or crinoid columnals. It also has been applied to sheetlike deposits of in situ corals or algal crusts or other reef-associated organisms even though the deposit is widespread, thin, and with no demonstrable topographic expression. Massive tumbled blocks also have been considered to be reefs, particularly if the blocks are abundantly fossiliferous and occur n distinctly more thinly bedded rocks. The term "reef" also has been applied to large carbonate structures which may be truly of reef origin at their margins, but which are composed mainly of bedded, clastic debris.

End_of_Article - Last_Page 738------------