Journal of Conference Abstracts

A Different Approach to Modelling Carbonate Sedimentation

Peter Ditchfield (P.ditchfield@bris.ac.uk)¹, David Waltham², Yvette Hague, Peter Smart³, Fiona Whitaker¹ & Daniel Bosence²

¹ Dept. Earth Sciences, University of Bristol, Bristol BS8 1RJ, U.K.

² Dept. Geology, R.H.B.N.C., Egham TW20 0EX, U.K.

³ Dept. Geography, University of Bristol, Bristol BS8 1SS, U.K.

Numerical simulations of carbonate platform evolution have proved valuable in investigating the complex relationships between sediment production, deposition, erosion, eustatic sea-level change and platform architecture.

However previous numerical models of carbonate platform growth have assumed a uniform distribution of sediment production and have relied on simplistic simulations of sediment transport using the diffusion. Although this approach produces realistic platform architectures it is inadequate because empirical calibration of the diffusion length is required. Furthermore, sediment transport in diffusion equation based models can only occur in one direction, thus the windward/leeward contrasts seen in the sedimentary architecture of many isolated build-ups cannot be simulated. Furthermore, existing models do not explicitly simulate the texture (grain size) of deposited sediment, although this is widely employed in the field description of carbonate facies. Simulation of texture also is needed for modelling of compaction (which may exert a fundamental control on platform architecture), and of diagenesis (textural control of permeability has a fundamental effect on the distribution of hydrological zones which in turn determine rates of many diagenetic processes).

Here we present the latest version of the Carbonate+2, a 2D finite difference model, which overcomes many of the previous limitations, by the use of more realistic hydrodynamically driven sediment erosion, transport and deposition. A user defined wind strength frequency spectrum, which includes both onshore and offshore winds, is used to model equilibrium wave climate of the model via the Phillips and Miles mechanisms for predicting wave growth. Waves are propagated across the model using Stokesian wave theory which enables the time averaged water velocity/depth profiles (i.e. currents due to wave drift), near bottom wave orbital velocities and bed shear stress to be calculated. Where critical shear stress for sediment entrainment is exceeded sediment concentration profiles are calculated for both bed and suspended load. Sediment transport is then the product of water velocity, sediment concentration and time (step length). We use two size classes with mean diameters of 500 µm and 50 µm which we take to represent allochems and mud grade material. The proportion of these two size classes in the deposited sediment enables the spatial distribution of sediment texture to be predicted for successive 2D surfaces. This in turn enables primary depositional porosity to be estimated and provides a more realistic starting point for the modelling of subsequent hydrological and diagenetic processes.

In addition the use of this hydrodynamically based approach where wave energies and water velocities are directly calculated enables the calculation of a quantitative measure of the degree of openness or restriction for any given part of the model. Such a parameter can then be used to more accurately reproduce variable rates of carbonate production with the different sub environments of the model.

Keywords: Numerical modelling carbonate Platform sedimentation

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