Types of carbonate platforms: a genetic approach

Many types of carbonate platforms have been described, from homoclinal ramps to rimmed shelves and a full spectrum of variations in between; the distinction between these different types can be problematic. Nevertheless, classification of carbonate platforms is not just a semantic or academic issue. For example, it is clearly important for the accurate interpretation of seismic images of facies geometry and for assessing the potential of stratigraphic traps. Even though predictive efficiency of conceptual models depends on the degree of comprehension of the genetic factors controlling depositional profiles and the distribution of facies belts, current models for classification of carbonate platforms are basically descriptive and mainly based on depositional profile, size, and attachment to or detachment from a landmass. A genetic approach considers the variability of depositional profiles among carbonate platforms as a function of the type of sediment that was produced (basically grain size), the locus of sediment production, and the hydraulic energy. Three groups of carbonate-producing biota may be distinguished according to their dependence upon light: (1) euphotic (good light) in shallow, wave-agitated areas; (2) oligophotic (poor light) in deeper, commonly non-wave-agitated areas; and (3) photo-independent biota in all water-depth ranges. Several platform types in wave-dominated seas can be considered in relation to genetic factors, even when simplifying the many possible scenarios. Euphotic framework-producing biota create rimmed shelves similar to modern reef platforms. Soft-substrate-dwelling biota, which produce gravel-sized carbonate in the shallow euphotic zone, create flat-topped open shelves. Oligophotic gravel-producing biota, such as some larger foraminifera and red algae, generate distally steepened ramps. Mud-dominated carbonate production, in either euphotic or oligophotic zones, generate homoclinal ramps. Carbonate production dominated by photo-independent biota (crinoids, sponges, bryozoans, etc.) above wave base give rise to open shelves or ramps, depending upon grain size, but may produce mounds if carbonate production occurs below the base of wave/current sweeping.