Load cast

Load casts are bulges, lumps, and lobes that can form on the bedding planes that separate the layers of sedimentary rocks.

This arrangement is gravitationally unstable, which encourages formation of a Rayleigh-Taylor instability if the sediment becomes liquefied (for instance, by an imposed earthquake shock).

The casts take on the form of slight bulges, swellings, deep or rounded sacks, knobby excrescences or highly irregular protuberances.

According to the contrast in density and viscosity of the specific layers, the wavelength produced by the instability varies considerably with values generally between a few millimeters and 10 centimeters.

It is important to note that in load casts the flame-like fingers never completely pierce the upper layer, whereas in flame structures they do.

[2] Good examples come from the Borrowdale Volcanic Series in the English Lake District and from the Carboniferous Bude Formation of southwestern England.

On the basis that liquefaction is linked to shock(s), Sims was able in 1975 to correlate the formation of load casts in modern lake deposits with historical earthquakes which had liquefied the sediments.