AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Investigating Carbonate Platform Types: Multiple Controls and a Continuum of Geometries
(1) Dept Earth Sciences, Royal Holloway University of London, London, United Kingdom.
(2) BG, Reading, United Kingdom.
(3) Dept Earth Sciences, University of Cardiff, Cardiff, United Kingdom.
(4) Institut Francais du Petrole, Paris, France.
Current classifications of carbonate platforms use depositional gradient to separate systems into two end member types, ramps and flat-topped platforms. Facies and sequence stratigraphic predictions vary significantly between these two end-members. However, many examples exist that do not conform to this simple classification. We have used a series of 2D numerical forward model runs to investigate how sediment production, transport and other controls such as tectonic subsidence, antecedent topography, and relative sea-level oscillation interact to determine platform geometry.
Modelling results suggest that rates of offshore sediment transport relative to rates of autochthonous production are a critical factor in maintaining a ramp profile in stable cratonic settings under a constant rate of relative sea-level rise. Type of carbonate production profile, for example euphotic versus oligophotic, is not a significant control in our model cases. Both euphotic and oligophotic production profiles produce FTPs when sediment transport rates are low relative to production rates, and ramps when sediment transport rates are relatively high. These results suggest a continuum of platform types, ranging from transport-dominated, low-gradient systems, to in-situ accumulation dominated systems. A system may be transported dominated because of high-energy processes able to break down and transport even bound sediment, or because carbonate factories produce only sediment easily transportable even under low energy conditions. Breaks of slope in underlying topography and differential fault subsidence are a stronger control on platform geometry in in-situ accumulation dominated systems. Relative sea-level oscillations tend to move the locus of sediment production laterally along any slope present on the platform, distributing sediment accumulation across the whole width of the platform, suppressing progradation and steepening, and so favouring development of low-gradient systems.
Based on all these results, we suggest that simple cut-off classification into ramp and flat-topped platform types can be useful, but a more meaningful approach is to describe and predict platform strata in terms of a multiple dimension platform parameter space containing a continuum of geometries controlled by sediment production, sediment transport, antecedent topography, differential subsidence effects, relative sea-level oscillations and perhaps other as yet unappreciated controls.