Ball-and-Pillow Structures

Introduction

Ball-and-pillow structures record one of the most dramatic soft-sediment deformation events: a sand or carbonate bed breaking apart and sinking into the underlying mud. The resulting hemispherical masses preserve the internal lamination of the original sediment in a strikingly deformed geometry, and their presence in the rock record signals rapid burial, high pore-water pressures, and a triggering shock.

Description

Ball-and-pillow structures are found in the lower part of sandstone beds - and less commonly in limestone beds - that overlie shales. They consist of hemispherical or kidney-shaped masses of sandstone or limestone that show internal laminations. In some hemispheres the laminae are gently curved or deformed, particularly near the outer edge where they tend to conform to the shape of the hemisphere. The balls and pillows may remain connected to the overlying bed, or they may be completely isolated from the bed and enclosed in the underlying mud. ^[1]

Formation Mechanism

Ball-and-pillow structures are believed to form by foundering and breakup of semiconsolidated sand or limy sediment, owing to partial liquefaction of the underlying mud - possibly caused by shocking. Liquefaction of the mud allows the overlying sand beds or limy sediment to deform into hemispherical masses that may subsequently break apart and sink into the mud. ^[1]

The mechanism is closely analogous to the Rayleigh-Taylor instability: denser sand resting on less dense mud is gravitationally unstable. When a shock - from a seismic event, a wave, or sudden loading - temporarily fluidises the mud, the sand above it cannot be supported and deforms downward. Individual sections break away, and their internal laminae are dragged and curved into the hemispherical shape by the deformation.

Experimental Confirmation

Experimental work confirmed the mechanism: structures closely resembling natural ball-and-pillow structures were produced by applying a shock to a layer of sand deposited over thixotropic clay. ^[1]

The key finding from the experiment is that a shock is sufficient to initiate the process - the mud does not need to be pervasively liquefied in advance. Thixotropic behaviour means the mud is solid under static conditions but temporarily fluidises under shear stress. A brief impulse is enough to trigger the entire foundering sequence. This explains why ball-and-pillow structures are particularly associated with turbidite sequences, where the arrival of a turbidity current delivers exactly that kind of sudden sediment loading and impact.

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