Mud drapes in sand-wave deposits: a physical model with application to the Folkestone beds (early cretaceous, southeast England)

Abstract

Large-scale cross-bedded units with mud-draped bottomsets and foresets occur in several shallow-marine sand formations attributed to tidal sand waves. The deposition and preservation of mud drapes on sand waves are favoured by a large sand-wave asymmetry, a high bottom concentration of suspended mud, large time-velocity asymmetry and low strength of tidal currents, and a high eccentricity of the tidal-current ellipse. The deposits formed on a strongly asymmetrical sand wave beneath a strongly asymmetrical current during one semidiurnal or diurnal tidal cycle will be a distinctive couplet composed of (i) a compound mud drape, with an internal silt-sand parting formed by the subordinate tidal stream, overlain by (ii) a group of sandy foresets and bottomsets deposited by the dominant stream. As the tides wax from neaps towards springs, and subsequently wane toward the next neaps, the spacing of drapes between sandy foresets will at first increase and then decline, whence a bundling or clustering of mud layers, and a periodicity in the streamwise arrangement of drapes and sandy foresets, will appear within the cross-bedding set. Tidal regime and the bed-material erodibility determine the character of these spring-neap depositional cycles, or bundles. The number of sand layers, their accumulated thickness, and their range in thickness within a spring-neap depositional cycle all increase as the tidal currents grow in strength relative to the threshold speed for sand erosion. Non-tidal factors may modify the tidally dependent spring-neap pattern of drapes and foresets, among which wave action seems most important. Mud deposition is suppressed at times of heightened wave-activity, with the result that spring-neap depositional cycles become abbreviated in the number of identifiable sedimentary episodes while acquiring an exaggeratedly large range in drape spacing. Long term changes of tidal regime, such as occur between equinoxes and solstices, should be detectable as gradual changes through a long sequence of spring-neap bundles. The Folkestone Beds of the western and northeastern Weald include many thick cross-bedded units with mud drapes often visibly bundled. At three western sites, the sands are fine to medium grained, with some coarse-grade and even pebbly material. The drapes there, consisting of fine- to very-fine-grained kaolinitic silt, range in thickness mainly between about 0.002 mm and 0.02 m. The spacing between groups of sandy foresets and bottomsets changes in an orderly way along the cross-bedding sets, varying from as little as about 0.01 m to several metres. With reference to the model, and with the help of time-series and Fourier analysis, the character of the drapes themselves, and the nature of the depositional cycles to which they contribute, it seems likely that the Folkestone Beds were deposited from diurnal tidal currents of spatially changing strength assisted by a strong unidirectional current. The limitation of drapes to western and northeastern areas is consistent with the restriction of the more eccentric tidal currents to nearshore areas, even though the currents seem to have been strongest nearest to shore. The length of the depositional cycles in the Folkestone Beds-proposed to record spring-neap tidal cycles-is consistent with the slightly longer year (in terms of solar days) inferred for early Cretaceous times on various independent grounds.