Quantifying the sequence stratigraphy and drowning mechanisms of atolls using a new 3-D forward stratigraphic modelling program (CARBONATE 3D)

This paper describes a new 3-D forward numerical model (CARBONATE 3D) that simulates the stratigraphic and sedimentological development of carbonate platforms and mixed carbonate-siliciclastic shelves by simulating the following sedimentary processes: (1) Carbonate shallow, open-marine production, dependent on water depth, restriction and sediment input; (2) Carbonate shallow, restricted-marine production, dependent on water restriction; (3) Pelagic sediment production and deposition; (4) Coarse and fine siliciclastic input; (5) Erosion, transport and redeposition of sediment, dependent on currents, slope, depth and restriction as well as sediment grain-size and composition; (6) Dissolution of subaerially exposed carbonate. In this paper the model is used to investigate the controlling mechanisms on the sequence stratigraphy of isolated carbonate platforms and atolls and to predict distinctive architectural signatures from different drowning mechanisms. Investigation of the mechanisms controlling atoll strata shows that although relative sea-level is the major control, antecedent topography, environmental setting and early diagenesis have profound influence on what stratigraphic geometries and facies develop. Hence care must be taken if sea-level curves are interpreted from real stratigraphies. Atoll drowning by fast sea-level rise, by lowered production and by repeated exposure and fast subsequent sea-level rises are investigated and different stratigraphic signatures for the respective mechanisms predicted. A fast relative sea-level rise results in a bucket-shaped morphology developed prior to drowning and a sharp transition from the platform margin facies to a pelagic cover. Drowning caused by lowered platform margin production is predicted to result in the development of a dome-shaped, shallow-water shoal over the whole platform top prior to drowning. Fourth order amplitudes of several tens of metres, typical of 'icehouse' settings, cause atoll drowning at subsidence rates where atolls subject to fourth order amplitude of only a few metres, typical of 'greenhouse' settings, can keep up with the rising sea-level. In the resultant strata, vertical facies belts are less well developed but horizontally extensive facies bands are more prominent. High fourth order amplitudes (up to 80 m) without sufficient third order scale subsidence will not lead to drowning, however, as the platform can recover in each fourth order lowstand. These results suggest that atolls might be easier to drown in 'icehouse' rather than in 'greenhouse' conditions but only in situations with suitably high rates of longer-term relative sea-level rise or sufficient lag times.