Organic reefs are defined in terms of potential to create a wave-resistant structure. The term “bioherm” may be retained for ancient reef-like organic masses of uncertain potential or doubtfully wave-resistant nature. The frames of organic reefs are built by a variety of organisms, and the reef mass includes widely varying but commonly large proportions of interstitial clastics.

Post-Paleozoic reefs built by corals and algae presumably grew at depths less than 25 fathoms and in a tropical or subtropical environment. In the case of reefs more than 150 feet thick this implies relative subsidence. For Paleozoic reefs the same general order of magnitude of depth and temperature is believed probable, especially if algae were abundant. Without regard to time, calcareous algae as abundant contributors to reef formation strongly indicate depths less than 50 fathoms and probably less than 25 fathoms. More specific limits of depth are implied by particular families of algae ranging back to Early Ordovician. The presence of calcareous green algae is presumptive evidence of a tropical or subtropical environment.

Subequidimensional or irregular organic structures known as patch reefs suggest growth in relatively quiet waters. Such structures generally grade abruptly to adjacent sediments. Table reefs are flat-topped, isolated reef masses of the open ocean, without true lagoons and with or without islands. Some resemble large patch reefs, but others were probably derived from atolls.

Linear reefs—fringing reefs, barrier reefs, and the ring-like outer reefs of atolls—are typically wave-fronting structures. The growth forms of their wave-facing elements are characteristically crustose, stubbily branching, or massive. Seaward the linear reefs grade through reef talus to enveloping sediments. The nature of the contact is related to source and sedimentary environment of offshore deposits and to slope of reef front. Fringing reefs and their fore-reef talus should characteristically show abrupt contacts with the land surface which they veneer, and both the reef proper and its talus may contain variable amounts of land-derived detritus. Barrier reefs and the outer reefs of atolls should show complex relationships with lagoonal sediments.

The Middle Devonian patch reefs of Michigan are analogous to those of existing seas, and the Middle Silurian reefs of the Great Lakes region were wave-breaking or potentially wave-resisting structures. In the Permian Capitan barrier reef the frame-building organisms presumably were sponges and calcareous algae, but physico-chemical precipitation of calcium carbonate probably contributed to the reef mass. Massive Cambrian bioherms in central Texas appear to have resulted almost entirely from the calcium-precipitating and sediment-binding activities of blue-green algae. Some Scolithus sandstones may be analogous to the annelid reefs of the North Sea. Other structures called reefs or bioherms may be depression fillings or banks of triturated organic materials that were held together, as they piled up, by concordant deposition of surrounding sediments.

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