  Kinneyia are a class of microbially mediated sedimentary fossils.
Characterised by clearly defined ripple structures, Kinneyia are generally
found in areas that were formally littoral habitats and covered by microbial
mats. To date there has been no conclusive explanation of the processes
involved in the formation of these fossils. Microbial mats behave like
viscoelastic fluids. We propose that the key mechanism involved in the
formation of Kinneyia is a Kelvin-Helmholtz type instability induced in a
viscoelastic film under flowing water. A ripple corrugation is spontaneously
induced in the film and grows in amplitude over time. Theoretical predictions
show that the ripple instability has a wavelength proportional to the thickness
of the film. Experiments carried out using viscoelastic films confirm this
prediction. The ripple pattern that forms has a wavelength roughly three times
the thickness of the film. This behaviour is independent of the viscosity of
the film and the flow conditions. Laboratory-analogue Kinneyia were formed via
the sedimentation of glass beads, which preferentially deposit in the troughs
of the ripples. Well-ordered patterns form, with both honeycomb-like and
parallel ridges being observed, depending on the flow speed. These patterns
correspond well with those found in Kinneyia, with similar morphologies,
wavelengths and amplitudes being observed.
