Carbonate ramp depositional systems

Abstract

The classification, tectonic settings, stratigraphy and early diagenesis of carbonate ramp systems are reviewed. Carbonate ramps are common in all geological periods, but were dominant at times when reef-constructing organisms were absent or inhibited. Ramps can be subdivided into inner-, mid-, and outer-ramp environments. The mid-ramp zone extends from fair-weather wave base to normal storm wave base, although the water depths which these boundaries represent vary. An additional outer-slope environment occurs on distally steepened ramps. As with siliciclastic shelves, a range of wave-, storm-, and tide-dominated ramps can be recognised and this forms the most convenient basis for ramp classification. The carbonate productivity profile of ramps differs from that of rimmed shelves, with the inner ramp showing lower production rates than comparable shallow-water facies on rimmed shelves. The zone of greatest organic carbonate sediment production appears to have shifted from the mid-ramp to the inner ramp since the Late Jurassic. Carbonate ramps occur in most types of sedimentary basin but are best developed where subsidence is flexural and gradients are slight over large areas, as in foreland and cratonic-interior basins and along passive margins. The featureless depositional profiles of many ramps means that sequence geometries are best observed on regional seismic lines. Some show low-angle sigmoidal or shingled clinoforms, suggesting that ramps may seldom be “homoclinal”, but possess subtle slope geometries which reflect depositional environments. Because of their low-angle slopes, ramps respond differently to rimmed shelves during relative sea-level changes although results seem to be strongly dependent on the rate of relative sea-level change. During a minor fall, shallow-ramp facies belts will simply shift basinwards in a “forced regression”. In contrast, the whole surface of a steep-sloped, flat-topped rimmed shelf may be exposed so that sediment production ceases or is drastically reduced. During a major fall, shallow ramp-bounded basins may empty completely. Conversely, ramps also flood gradually, whereas rimmed shelves do so more rapidly. Homoclinal ramps develop no resedimented lowstand deposits; rimmed shelves and distally steepened ramps, in contrast, may develop lowstand talus or turbiditic wedges. Distally steepened ramps may behave more like homoclinal ramps during minor base-level falls and like rimmed shelves during major base-level falls. Many ramps consist of layered successions of several ramp sequences stacked one upon the other. Ramp “stacks” of this sort may show gross vertical accretion, but individual ramp sequences seldom appear to develop in a “keep-up” style, apart from minor organic buildups, as with many rimmed shelves. Steepening of the outer-ramp margin due to tectonism, slope inheritance, or differential sedimentation may promote the development of a distally steepened ramp or rimmed shelf. A wide variety of organisms have constructed buildups in mid- and outer-ramp environments. Isolated buildups may seed early in ramp development, accrete to wave base or sea level, and continue growth by stacking through successive ramp sequences so that depositional and diagenetic features within them are in concert with those of the shallow ramp. The location of isolated buildups on ramps is governed by tectonism, halokinesis, antecedent topography, or by the subtle slope geometry of the previous ramp sequence. Diagenesis on ramps shows some major variations compared with diagenesis on steep-sloped, flat-topped carbonate platforms. Ramp-bounded basins may form prolific petroleum sourcing and reservoiring systems and offer a range of subtle stratigraphic play types and lateral facies variations which determine reservoir quality and distribution. Isolated buildups in the mid- and outer-ramp environments represent one of the commonest petroleum reservoirs in ramp systems and tend to have their foundations in transgressive systems tracts. Grainstone and packstone reservoirs are widespread and range from shoreline carbonate sandbodies to major detached shoal complexes or shoals over offshore highs. Grainstone sandbodies, occur in both highstand systems tracts and in prograding lowstand wedges.