Convolute Bedding

Introduction

Convolute bedding is one of the most visually striking deformation structures in the sedimentary record - laminations that were originally horizontal or gently dipping have been thrown into tight, irregular folds. But the folds are contained: the beds above and below the convoluted layer are typically undisturbed. This confinement within a single bed is one of the key features that distinguishes convolute bedding from tectonic deformation, and it points to a synsedimentary origin through rapid liquefaction of the sediment.

Description and Internal Geometry

Convolute bedding is a structure formed by complex folding or intricate crumpling of beds or laminations into irregular, generally small-scale anticlines and synclines. It is commonly - though not necessarily - confined to a single sedimentation unit or bed, and the strata above and below may show little evidence of deformation. Convolute bedding is most common in fine sands or silty sands, and the laminae can typically be traced through the folds. Faulting generally does not occur, though the convolutions may be truncated by erosional surfaces that may themselves be convoluted. ^[1]

The intensity of folding increases upward through the convoluted unit. The folds begin from undisturbed laminae at the base, grow in complexity and amplitude toward the top, and then either die out within the unit or are cut off by the upper bedding surface. The beds range in thickness from about 3 to 25 cm, though convoluted units up to several metres thick have been reported from both eolian and subaqueous deposits. ^[1]

The upward increase in fold amplitude is a diagnostic feature. It implies that deformation was concentrated near the top of the liquefied layer, where the sediment had the greatest degree of freedom to move. The undisturbed lower laminae acted as a relatively rigid substrate that the liquefied upper portion deformed against.

Origin

The origin of convolute bedding is still not thoroughly understood, but it appears to be caused by plastic deformation of partially liquefied sediment soon after deposition. The axes of some convoluted folds have a preferred orientation that commonly coincides with the paleocurrent direction, suggesting that the process occurs during or very shortly after deposition - at least in those cases. Liquefaction of sediment can be triggered by differential overloading, earthquake shocks, and breaking waves. ^[1]

The fold-axis orientation provides a useful clue in field studies. When the axes show a consistent preferred direction, and that direction aligns with other paleocurrent indicators such as flute casts or groove casts on the same bed, it supports the interpretation that convolutions formed during active sediment transport rather than after burial.

Occurrence

Convolute lamination is most common in turbidite successions, where rapid deposition and high pore-water pressures frequently trigger liquefaction. It also occurs in intertidal-flat sediments, river floodplain and point-bar sediments, and deltaic deposits. ^[1]

The common thread across all these settings is rapid deposition - sediment arrives faster than pore water can escape, leaving the framework in a metastable, partially liquefied state. Any subsequent disturbance then triggers deformation. Convolute bedding is therefore not an indicator of a specific environment so much as an indicator of a high-sedimentation-rate episode within whichever environment it is found.

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