Abstract This paper describes a computer program developed at the University of South Carolina to simulate the evolution of carbonate geometries and their facies responding to: (1) varying rates of accumulation; (2) eustatic sea-level variation; and (3) tectonic movement of the crust. The simulation creates two-dimensional plots of synchronous depositional sequences within sediment bodies. Rates of carbonate accumulation are modeled as a function of water depth and lateral position across the shelf. Carbonate accumulation includes in situ organic production and transport by hydrodynamic processes. Influx of clastic sediments is modeled to cause an exponential decrease in the rate of carbonate accumulation. Rates of carbonate accumulation can be further diminished with a user-defined depth-controlled wave-damping function. Eustatic variation is modeled by a fourth-order linear change in sea level over time (as described by Vail and others, 1977), with a higher order sinusoidal oscillation of sea level superimposed upon it. Reef margin and interior lagoonal facies on platforms and shelves are predicted. Modeling of these facies zones includes aggradation, progradation, backstepping, shoal development, and drowning. The Devonian Judy Creek reef complex of the western Canada Alberta basin was used both as an aid in constructing the carbonate simulation model and as an example on which to test the completed program. The Judy Creek model was constructed by the fourth co-author (J.C.W.) from the study of 100 cores and the correlation of a systematic grid of wireline log-core cross sections. Judy Creek consists of five overall shoaling-upward depositional cycles. Superimposed upon each cycle are several subcycles. Marine hardgrounds locally cap the first three cycles at the margin of the buildup. The top of the fourth cycle is a subaerial cemented surface. The top of the upper cycle is a widespread marine hardground. Subcycles consist of minor shoaling-upward sequences of lagoonal facies. The results of two versions of the carbonate model include: (1) a site-specific version of the Judy Creek area, which shows the facies location and movement within Judy Creek; and (2) a more generalized carbonate model. Both versions use similar inputs. The geometry and facies distributions of Judy Creek are simulated using a stairlike fourth-order eustatic sea-level curve (as described by Vail and others, 1977) and a low-amplitude higher order sea-level oscillation with a varying period in which sea-level fall is matched by tectonic subsidence. Maximum rates of accumulation average around 3.0 to 5.0 m/10 ka. The fourth-order sea-level variation consists of a rapid 3.5–5.0 m/10 ka rise followed by a gradual rise of 0.7–1.5 m/10 ka, followed by another rapid rise of the same magnitude, followed by another gradual rise. The higher order sea-level oscillation consists of 0.5 to 1.5-m amplitudes with periods ranging from 20 to 40 ka. Tectonic subsidence averages between 0.5 and 1.0 m/10 ka.

Abstract Abstract Observations off Bonaire, Netherlands Antilles, support the recent contention that Caribbean reefs are occasionally subjected to low-water emergence and are therefore more similar to reefs of the Indo-Pacific than previously believed. Off Bonaire, some incipient reef flats and the shallow floor between the flats and shore are sporadically awash for extended periods of time. Most of the corals (representatives of over 20% of ail known Caribbean species) that inhabit the emergent biotopes appear to be able to endure subaerial exposure without severely deleterious effects. However, destruction of Montastrea annularis either results directly from exposure or, more probably, is initiated by it. Bonaire’s shallow-water reefs are of three types. The type that occurs in turbulent water is composed almost entirely of Acropora palmata and is therefore similar to the typical, shallow-water reefs of the Caribbean. The two types that are found in calm water differ considerably in species composition from typical Caribbean reefs and differ from one another as a consequence of whether they undergo or escape emergence. Calm-water reefs that are constantly immersed consist almost exclusively of nearly continuous colonies of Montastrea annularis. Those that are intermittently emergent are the result of a succession of heterogeneous corals. Construction of the reefs of varied corals was begun by abundant large colonies of M. annularis, but most of these colonies were killed when they attained sizes at which their summits were subaerially exposed during exceptionally low stands of the sea. Following the decline of the M. annularis colonies and erosion of their coralla by boring organisms, construction of the reef framework was continued by an extraordinarily diverse coral assemblage. Framework building has been culminated upward by the establishment of exposure-tolerant corals and other cnidarians, but the diverse assemblage is maintained on the reef flanks and deeper portions of the flats. The diversity of the corals (32 of the 48 species found off Bonaire) that are involved in the formation of the sometimes emergent reefs is unusual in shallow water in the Caribbean; such diversity is usually associated only with much deeper reef zones. Because Montastrea annularis has been shown to be dominant over most of the corals of the unusually diverse, shallow-water reef assemblage in the extracoelenteric-digestion hierarchy (i.e., a “pecking order” in which representatives of subordinate coral species are consumed and thereby eliminated from competition for reef space by those that are more aggressive), the diversity of the assemblage is attributed to the initial decimation and continued control by emergence of the once prolific colonies of this coral.