Sedimentology of the Kerala mud banks (fluid muds?)

Abstract

Early workers have used the term “mud bank” to describe the intermittent appearance of calm patches of highly turbid water along the coastal tracts of Kerala, India during the southwest monsoon season. The term “fluid mud” is recommended as a more accurate term. A total of 32 grab and twelve core samples were collected from an area of 60 km 2 off Quilandi, Kozhikode, India to evaluate the sedimentological aspects of mud resuspension in nearshore waters. Information on sediment discharge, rainfall, salinity gradient, suspensate concentration, wave, wind and current patterns were also analysed to understand the fluid mud formation. The topmost unit in the sediment cores is represented by a soft, watery clay with a wedge-shaped geometry. The unit thickens towards the shore and the thin end lies about 9 km offshore. This expanse of fine suspended sediment in water results from interactions of suspended material concentrations, near-bottom current velocity and wave activity in the coastal zone. During the southwest monsoon, strong swells churn up the bottom clay into a thick suspension. The suspended sediment column develops a semicircular periphery in which wave energy is consistently absorbed. Lateral movement of the mud is controlled by the intensity and direction of littoral currents. SEM studies show a weakly disoriented, platy and highly porous fabric in the upper clay zone as opposed to the relatively stable, aligned, supportive framework in the lower clay layer. This suggests an easy dispersion of the upper clay zone with slight mechanical agitation in less saline coastal water. Freshwater discharge through river runoff may be an important factor in maintaining the suspension of mud due to deflocculation of clay particles. The “mud banks” of the Kerala coast differ from mud banks reported from other muddy coasts of the world in that they do not form regular relief-forming features. The transient nature of these “mud banks”, their unpredictable periodicity and the calm and turbid nature of the water column suggest that they are episodes of fluid mud formation.