Study of many varied carbonate complexes in ancient rocks results in recognition of three recurring patterns of shelf-margin facies and associated organic buildups. Bases for the classification are stratigraphic profile, shape and disposition of discrete carbonate bodies along the profile slope, and environmental interpretations through analysis of textural and biologic microfacies. Type I consists of linear belts of downslope bioclastic lime-mud accumulation, below the zone of active wave action and in places even below the photic zone. Organic boundstone may or may not be present. Crestal deposits on the profile are commonly barrier bars or islands of lime sand. Type II includes linear ramps or platforms composed of organic-reef knolls, commonly forming on gentle slopes at outer edges of shelf margins. Construction of such ramps may interfere effectively with wave action so that the knolls exist in moderately quiet water and can trap lime mud. Lime-sand particles accumulate in flanking beds around the knolls or in passes between them. Massive frame-building organisms may be somewhat rare within the knolls which are the growth form of the reef-builders. Interreef material is volumetrically much greater than the patches of boundstone framework. Crest of slope may be occupied by lime-sand shoals, bars, or even mudflats behind the wide, gently sloping platform. Type III is an organic reef rim of resistant framework, built like that of a modern Acropora reef into active wave base and forming the topographic crest of the profile. Growth form of reef organisms is zoned ecologically. Lagoonal environment develops behind the rim, with lime-sand bars and submarine shoals. Most shelf margins in the geologic record belong to types I and II.

These types of carbonate shelf margins and their facies sequence are observed in four common paleotectonic situations: (1) in areas of major subsidence at outer margins of great carbonate ramps built into basins or geosynclines from positive cratonic blocks; (2) within sedimentary troughs such as isolated major carbonate banks--profiles with steep slopes and narrow marginal facies belts; (3) in areas of moderate subsidence, where the same patterns of buildups fill in intracratonic basins; and (4) even surrounding second-order positive elements on shelves. Two types of small-scale organic buildups are found on shallow shelves: lime-mud mounds and patch reefs, each with its attendant facies (e.g., core, flank, crestal, capping beds, etc.).

The development of such carbonate-shelf margins is a natural consequence of the normal processes of prolific carbonate production, trapping, baffling, encrustation, organic frame construction, and inorganic and organic cementation. These processes operate in clear warm shallow water on slopes at some distance out from low-lying landmasses. Hydrography, sea-level fluctuation, organic evolution, and tectonics all operate to influence the balance of these factors and to construct particular types of shelf margins. Causal relations are not completely clear. Because the various types of shelf margins are present in all tectonic settings, rate of subsidence apparently is not so important a control as the others. Type I seems to develop with a lack of dominant framebuilders and in moderate t quiet surge. Type II would appear to develop also with moderate surge and with varied organisms as framebuilders, encrusters, or sediment trappers and in a tectonic environment of moderate subsidence. Type III develops in areas of rapid subsidence with open-sea surge or with abundant framebuilders.