page_title: Load cast
section_title: Description
hierarchical_section_title: Load cast / Description
caption_attribution_description: English: Wartlike load structure in Jurassic (Hettangian) arkoses from the northern edge of the Aquitaine Basin (near Nontron, France).
context_page_description: Load casts are bulges, lumps, and lobes that can form on the bedding planes that separate the layers of sedimentary rocks. The lumps "hang down" from the upper layer into the lower layer, and typically form with fairly equal spacing. These features form during soft-sediment deformation shortly after sediment burial, before the sediments lithify. They can be created when a denser layer of sediment is deposited on top of a less-dense sediment. This arrangement is gravitationally unstable, which encourages formation of a Rayleigh-Taylor instability if the sediment becomes liquefied. Once the sediments can flow, the instability creates the "hanging" lobes and knobs of the load casts as plumes of the denser sediment descend into the less-dense layer.
  Load casts are a common kind of sole marking.
context_section_description: Load casts form on the underside of the overlying denser layer (sands, coarse sands, or gravels), which is superimposed on a less-dense hydroplastic layer (muds, silts or finer sands). The casts take on the form of slight bulges, swellings, deep or rounded sacks, knobby excrescences or highly irregular protuberances. In profile, they appear as a row of flattened, lobe-shaped masses of similar size, shape, and spacing bulging into the lower layer. Between the lobes penetrate flame-like fingers or diapir-like shapes from the underlying less-dense layer. In 3-D, the lobes reveal equant to elongated pillow shapes separated from each other by narrow clefts. In profile, the succession of lobes and fingers can be modelled as a row of semi-circular lobes touching each other at the finger tips; a characteristic wavelength L can consequently be attributed to the lobes. 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. Extreme examples have been reported with wavelengths up to 10 meters.
  Normally the lobes/pockets and the fingers/diapirs are relatively symmetrical about the vertical, but can become asymmetrical in some places. They lean then into a consistent direction, usually interpreted as the direction of the paleocurrent. Asymmetrical load casts are called squamiform or flow casts. 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.
caption: Wartlike load casts in Hettangian arkoses from the northern Aquitaine Basin
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