Controls on Depositional Geometries of Carbonate Platforms

Gregor Eberli
University of Miami, RSMAS, CSL, Miami, FL

The large-scale architecture of carbonate platforms, in particular the slope geometries, is controlled by the interplay between rates of relative sea level change and platform growth. The changing rate of this interplay causes variable slope profiles of margins that is independent of the height or age of the platform. The growth rate of the platform is dependent on the rate of carbonate sediment production and the amount of offbank transport, which varies at different locations of a platform. As a result, the slope profile varies in space and time. Variable off-bank transport can, for example, result in a backstepping of some portion of the platform while other parts of the platform still prograde. Platforms that are subjected to unidirectional trade winds expand along the sediment-rich leeward side while the windward portion of the platform, where offbank transport is minimal, aggrade nearly vertical. In other platforms, progradation is bi-directional due to seasonal switching of the wind direction. Consequently, the lateral growth potential of platforms varies widely and asymmetric platform growth is common. In a similar way, ocean currents can cause an asymmetry in platform growth. Along the Southern Marion Platform, for example, south flowing currents rework the offbank transported sediment and cause the platform to expand into a down-current direction. During the lifetime of a platform wind or current regimes and the direction of platform expansion can change, resulting in complicated platform architecture. Aggradation versus progradation is thus not only function of relative sea level change but also of sedimentation rates, which makes interpretations of rate of sea level change based on the ratio aggradation versus progradation tenuous at best.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005