Rayleigh-Taylor Instability as the Origin of Certain Soft-Sediment Deformation Structures
B. Blakeney, R. Slingerland
Convolute laminations, ball-and-pillow structures, load casts, flame structures, and heavy mineral sags are thought to form from a gravitationally unstable gradient of bulk density, but little field data exist to test the hypothesis. We collected wavelengths, amplitudes, bed thicknesses, and grain sizes of deformation structures in 286 pairs of sand-mud beds, and compared the geometries to the solutions from the Rayleigh-Taylor theory for two layers with Newtonian viscosities. The data come from hummocky cross-stratified sequences in the Upper Cretaceous Parkman Sandstone Member, Elk and Big Horn basins, Wyoming, and Tertiary nearshore marine sediments from Mal Pais, Costa Rica. Stepwise multiple regression reveals that 72.3% of the variance in wavelength or spacing, ^lgr of the structures is accounted for by the thickness, h1, of the overlying (denser) bed, and 7.8% is accounted for by the thickness, h2, of the underlying bed. Although actual field geometries show denser layers of finite thickness loading less dense layers of finite thickness, wave numbers calculated as 2^pgrh2/^lgr compare favorably with those predicted from the Rayleigh-Taylor theory where an infinitely thick denser layer of viscosity, µ1, loads a lower layer of finite thickness and viscosity µ2. The mean wave numbers are not statistically different for convolute laminations, flames, and ball-and-pillow structures. However, the mean values for load casts and convolute laminations are statistically different, suggesting t at the sediment behaved hydroplastically during deformation, and that the load-cast spacings may not be adequately predicted by a model that assumes Newtonian viscosities for the involved sediments.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.