Self-organizing model for wavy bedform genesis

Abstract

The Maastrichtian-Danian Tuffaceous Chalk of NW Europe is a subtropical, shallow marine, bioclastic carbonate silt- or sandstone. It forms tens of metres thick sequences, characterised by a regular succession of dm-m thick laterally continuous beds. Bedding is expressed by a rhythmic vertical variation of the grainsize, structures and early-diagenetic authigenic mineral concentrations. Tuffaceous Chalk cycles are fining-upwards cycles with a coarse-grained glauconitic, wavy laminated basis and with a fine-grained, homogeneously bioturbated, lithified, bored and encrusted top (hardground). Tuffaceous Chalk cycles presumably have been deposited during 20 ka precession periods and they reflect the cyclic variation of climate, storm frequency-intensity, hydrodynamic energy and deposition rates. The relation between the periodic variation of hydrodynamics, defining sediment erosion, transport and redeposition, and the simultaneous early-diagenetic lithification and decrease of erodability is investigated with a numerical model. The model is a self-organising parallel computer that allows the simulation of the genesis of regularly spaced bedforms (ripples, dunes, waves) from an initial randomly distributed bed height. A dynamic equilibrium is reached when a train of equi-dimensional bedforms moves down-current at constant velocity. When hydrodynamic energy, water depth and deposition rates are varied properly and when erodability is dependent on simultaneous lithification, then the model simulates the genesis of wavy bedded Tuffaceous Chalk cycles as have been observed in the field.