Understanding Progradation-Aggradation Ratios of Isolated Carbonate Platforms: A Forward Sediment Model Approach

We use numerical model CARB3D+ to test field-observation-derived hypotheses that describe functional relationships between carbonate system characteristics and platform geometry as defined by the progradation-aggradation ratio (P/A). The P/A describes the temporal translation of the platform margin (the break in depositional profile interpreted as the change from shallow- to Deepwater facies). Progradation tracks the relative position of the margin along the horizontal axis (seaward shifts are positive progradation; landward, negative). Aggradation tracks the relative vertical position of the margin (upward translation is positive aggradation; downward, negative). The P/A can be related to accommodation history and sediment supply; it can be calculated from outcrops, seismic profiles, etc.

Uncertainty and sensitivity analyses have shown that CARB3D+ is qualitatively consistent with modern natural systems. We employ a many-realization, ‘ensemble’ approach to glean useful information using a numerical model that represents a system for which ‘best’ parameter values are unknown and verification data are limited. Conclusions depend on the assumption that CARB3D+ adequately represents platform evolution; they are independent of choice of environmental, accommodation, and energy parameters.

CARB3D+ ensemble characteristics are consistent with the following relationships. Aggrading platforms (0≤P/A≤1) and prograding platforms (P/A>1) are the most common platform geometries. A trade-off exists between a system’s tendency to prograde and to aggrade. Initial bathymetry is more important than system characteristics in controlling whether a simulated platform will aggrade or prograde. Deep, steeply sloping basins favor aggradation over progradation; shallow, gently sloping basins favor progradation. Backstepping (A>0; P<0) is most likely in deep, steeply sloped basins with weak, shallow reef production and in systems with increasing accommodation and many high-amplitude, high-frequency sea-level oscillations. Downstepping (A<0; P>0) is rare, especially in deep basins with steep platform slopes. Direction of accommodation change and environmental conditions control a system’s capacity to downstep: strongly negative sea-level change is near-necessary for downstepping; small-amplitude, high-frequency sea-level oscillations and healthy margins enable downstepping. Facies proportions and sediment characteristics also relate to P/A; results are not presented here.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California