The formation of convolute lamination in mud-rich turbidites

Abstract

Convolute lamination is a common fold structure within turbidite beds, attributed to the deformation of sediment during or soon after deposition of the host bed. Despite the prevalence of this feature, the formational processes are still not well understood. Possible mechanisms are investigated here through redescription and analysis of convolute lamination from the Aberystwyth Grits Group (Silurian, west Wales, UK), in which ‘convolute bedding’ was first defined. Internal bed structures have been studied in clean coastal exposures and on high-resolution optical scans of cut surfaces. Convolute lamination occurs in intervals 2 to 10 cm thick, spanning the top of the very fine sand Bouma C division through the D division of interlaminated silt and clay. Observed growth geometries confirm that the structure formed during sedimentation of the host graded bed. Folds show a down-flow asymmetry and doubly-vergent diapiric geometries (‘mushroom’-shaped structures). Grain size measurements from a modern turbidite (Icod bed, ca 165 ka, Moroccan Turbidite System) suggest that there is an optimal ‘window’ of average grain size and mud content parameter space, within which convolute lamination develops. It is proposed that this window coincides with a bed density inversion created during deposition of a graded bed as clean sand (with pore spaces infilled by water) fines upwards into mud-rich sand (with pores infilled by an increasing proportion of mud). Porosity decreases and bulk bed density correspondingly increases. The resulting unstable density gradient may lead to vertical buoyancy-driven overturn, manifest as growing folds. Subsequent amplification of the folds due to density differences and modification due to horizontal shear imposed by the depositing turbidity current may then occur. The bed density gradient stabilises with deposition of low permeability mud, suppressing further fold development. Quantitatively, both Rayleigh-Taylor instability and Kelvin-Helmholtz instability are theoretically possible in forming folds in this context.

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