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Abstract

A carbonate buildup is a circumscribed body of carbonate rock which displays topographic relief above equivalent sediment and differs from typically thinner equivalent deposits and surrounding and overlying rocks, Reefs are buildups which display evidence or potential for maintaining growth in the zone of waves. Need for wave resistance in reefs varies from one environment to another; wave resistance may be derived from inorganic spar cementation in the submarine or subaerial environment, organic construction of rigid or nonrigid skeletal frameworks, binding of sediment by algal mats or by rooted organisms which leave no evidence of binding in the rock, or by a combination of the above. Carbonate buildups can be classified more generally on the basis of gross constituent composition and on type and habit of skeletal constituents, where such constituents are dominant. Buildups may originate as in situ skeletal accumulations, organically induced sediment accumulations, hydrodynamic or aerodynamic sediment accumulations, or most often from interaction between organic growth and hydrodynamic forces.

Organic contributors display a varied but interrelated history in buildup formation from Precambrian to Holocene. Modern assemblages are differentiated into several major environmental regimes: (1) hermatypic Jiexacorals, red algae, green algae, foraminifers, and molluscs dominate warm, shallow marine buildups; (2) red algae dominate cold, shallow marine buildups; (3) ahermatypic hexacorals dominate deep-water marine buildups; (4) vermetid gastropods, oysters, and serpulid worms individually form buildups in various portions of nearshore, restricted marine environments; (5) blue-green algae dominate nonmarine buildups in fresh or saline water. These assemblages can be recognized to various extents back into the geologic record. The warm, shallow marine buildup assemblage has the most complete history and was dominated as well by sponges, bryozoans, pelmatozoatis, stromatoporoids, other corals, various problematical fossils, and blue-green algae at different times in the past. Bryozoans dominated some restricted marine buildups in the Tertiary.

Major types of buildups based on gross constituent composition are: (1) encrusted skeletal buildups with pervasive organic binding; (2) loose skeletal buildups dominated by whole or disarticulated shells, colonies, etc., but without preserved organic binding; (3) abraded skeletal buildups (which are mainly hydrodynamic) ; (4) lime-mud buildups; and (5) mixed buildups in which no one constituent is overwhelmingly dominant. Reefs with evidence of various degrees of wave resistance are known throughout the record of carbonate buildups. Although encrusted skeletal buildups can be considered at least potential reefs because their pervasive organic binding could provide immediate wave resistance, their presence in large reef complexes does not necessarily mean that they were the main provider of wave resistance. One must examine their position relative to evidence for turbulent water and the nature of contemporaneous wave-derived talus in order to evaluate their functional significance. Types of talus reported so far suggest that many reefs throughout the record derived wave resistance from a combination of spar cementation of debris and growth of large skeletal organisms.

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