Practical Classification of Reefs and Banks, Bioherms, and Biostromes: ABSTRACT

End_Page 167------------------------------Reefs and banks form stratigraphic traps which hold more than 40 per cent of the total petroleum production in the world. They are of special importance in the Permian basin exploration. Yet there is much confusion concerning the classification and terminology of these skeletal deposits. It is the purpose of this paper to attempt to define these structures on a genetic and morphological basis and indicate their exploration potentials. Following Lowenstam (1950) and Nelson et al. (1962), the writer applies the terms "reef" and "bank" to denote the origin of the structures, whereas the terms "bioherm" and "biostrome" are used to designate the shape of the structures and their relation to the associated layered facies. A reef is a structure built by the in-place growth of organisms which have the ecological potential to act as frame-builders. It is a wave-resistant, prominent structure on the sea floor and, therefore, influences, and modifies sedimentation in its vicinity. A bank, in contrast, consists of organisms which did not have the ability to erect a rigid three-dimensional frame-work. Banks may be formed in place or by mechanical build-up following transport of the skeletal remains. Banks also are wave-resistant. They may or may not be prominent structures on the sea floor or influence the sedimentation around them. According to their formation, banks are subdivided into (1) mechanical and (2) biogenetic accumulations. Biogenetic accumulations are subdivided further into banks resulting from (a) biogenetic baffling of sediment, (b) biogenetic binding of sediment, and (c) gregarious local growth of organisms without erecting prominent structures. Thus, reefs and banks are distinctly different types of accumulations. In a reef, in situ growth of organisms is more important than sediment trapping and binding, and the organic productivity of the frame-building organisms alone is sufficient to elevate the structure above the surrounding sea floor. Frame builders in general are organisms that cement themselves to the substratum and form a rigid mass. Cementation appears to be very important in the resulting structure. Colonial hexacorals, stromatoporoids, calcareous sponges, rudistids, and crustose CaCO3-secreting red algae can act as frame builders in a reef. In contrast, in banks, the sediment-baffling and binding functions of the organisms are the main sources of sediment accumulation. Tetracorals, tabulate corals, bryozoans, crinoids, phylloid algae, pelecypods, gastropods, and brachiopods are organisms that can not erect a rigid framework. However, they can actively trap sediment and form banks. Furthermore, their local gregarious growth may lead to bank-type accumulations. A bioherm is a massive, mound-shaped structure which is in discordant relationship with the surrounding layered facies of different lithologic types. A biostrome is coarsely layered and grades concordantly into the surrounding layered sediments. According to these definitions, a reef, according to its shape and geological setting, represents a bioherm. A bank, however, may take the form of a bioherm or biostrome. Mechanically accumulated banks and biogenetic banks resulting from the sediment-baffling activity of organisms generally have the form of bioherms. Banks resulting from biogenetic binding of sediment may have the form of either bioherms or biostromes. Local gregarious growth of organisms usually leads to accumulations of the biostrome type. The fact should be stressed that the shape of a fossil structure is not necessarily the same as its shape at the time of formation. Differences in compaction capabilities of skeletal deposits versus clay or carbonate-mudrich deposits tend to exaggerate the relief of the organic bu ld-up. Examples of various types of Recent and ancient reefs and banks are illustrated and discussed. Recent coral reefs of the Florida Keys reef trend and Cretaceous rudistid reefs in Sierra El Abra, Tampico region, Mexico, are examples of true reefs. Little Molasses Reef, Florida Keys, is interpreted as a bank which was formed by mechanical accumulation of skeletal debris. Rodriguez Key, Florida Keys, and mud mounds in Florida Bay are examples of banks representing biogenetic accumulations resulting from the sediment-baffling functions of organisms. Phylloid algal mounds from the Hueco Tanks area (West Texas), Dry Canyon near Alamogordo, New Mexico, and Marble Canyon, Alamogordo, New Mexico, are ancient counterparts of these biogenetic banks. Algal stromatolites are representative of banks formed by the sediment-binding action of organisms. Cretaceous oyster beds are indicative of bank accumulations by local gregarious growth of organisms. It is very important to emphasize the fact that structures, which superficially appear to be very similar, may, in fact, have formed in very different environments and, therefore, require different genetic interpretations. End_of_Article - Last_Page 168------------