SHELFAL ACCOMMODATION AS A MAJOR CONTROL ON CARBONATE PLATFORMS

Changes in shelfal accommodation, sediment supply and initial depositi onal profile are the three main variables controlling the development of large-scale stratal pattern of carbonate platforms. The effects of these three variables and their distribution in time and space are dis cussed. Geometry, facies and development of third-order depositional s equences, the building blocks of the sequence stratigraphic framework, depend on their position within the second-order transgressive/regres sive cycle. The signature of depositional sequences: for a single depo sitional profile, depends on the balance between carbonate production and shelfal accommodation. High rates of sedimentation can fill all th e space created by sea-level changes and subsidence, and can maintain sub-aerial exposure conditions even during periods of long-term rise o f relative sea level. Thus the direction of movement of the shoreline, the confined or open-marine nature of facies, as well as temporal con siderations are not the driving factors to determine transgressions an d regressions. Four types of depositional sequences may develop: infil ling and forestepping during regressive phases; aggrading and backstep ping during transgressive phases. These four types of depositional seq uences do not systematically co-exist within a carbonate platform. The ir development is closely related to the amplitude and rates of the: l ong-term change of shelfal accommodation and sediment supply.