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Load casts are bulges, lumps, and lobes that can form on the
bedding plane In geology, a bed is a layer of sediment, sedimentary rock, or pyroclastic material "bounded above and below by more or less well-defined bedding surfaces".Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds., 2005. ''Glossary of Geology'' ...
s that separate the layers of
sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
s. 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 Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematically ...
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 (for instance, by an imposed earthquake shock). 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.


Terminology

The expression "load cast", sometimes also called a load structure, refers to a load (the denser layer) sinking into its underlying (less dense)
cast Cast may refer to: Music * Cast (band), an English alternative rock band * Cast (Mexican band), a progressive Mexican rock band * The Cast, a Scottish musical duo: Mairi Campbell and Dave Francis * ''Cast'', a 2012 album by Trespassers William ...
(mold). Related to load casts are ''
flame structure A flame structure is a type of soft-sediment deformation that forms in unconsolidated sediments. The weight of an overlying bed forces an underlying bed to push up through the overlying bed, generally when both strata are saturated with water. ...
s'', ''load waves'', and ''anticrests''. Extreme developments of load casts are ''pseudo-nodules'' and '' ball-and-pillow structures''. In these extreme cases, the hanging lobe becomes almost or completely detached from the overlying bed, resulting in apparently isolated masses of the overlying material floating in the lower bed.


History

Load casts were scientifically reported for the first time by the sedimentologist Theodor Fuchs in 1895 who called them in
German German(s) may refer to: * Germany (of or related to) **Germania (historical use) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizens of Germany, see also German nationality law **Ge ...
''Fließwülste'' (flow crests, flow warts). He also reproduced the structure experimentally. Later studies were conducted by
Henry Clifton Sorby Henry Clifton Sorby (10 May 1826 – 9 March 1908) was an English microscopist and geologist. His major contribution was the development of techniques for studying iron and steel with microscopes. This paved the way for the mass production of st ...
in 1908, Paul Kukuk in 1920, and by Robert Shrock in 1948.


Description

Load casts form on the underside of the overlying denser layer (
sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class ...
s, coarse sands, or
gravel Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone. Gravel is classifi ...
s), which is superimposed on a less-dense hydroplastic layer ( muds,
silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel ...
s 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 In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
L can consequently be attributed to the lobes. According to the contrast in density and
viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the int ...
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 A paleocurrent or paleocurrent indicator is a geological feature (typically a sedimentary structure) that helps one determine the direction of flowing water in the geologic past. This is an invaluable tool in the reconstruction of ancient deposit ...
. 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.


Occurrence

Load casts appear in very different
depositional environment In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be ...
s. They are most common in
turbidite A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing Turbidites wer ...
s, but can also occur in
fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluviog ...
and shallow-marine settings. Occasionally they appear in
lake A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...
sediments. They have even been found in layered igneous and
pyroclastic Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroc ...
successions. Good examples come from the Borrowdale Volcanic Series in the English
Lake District The Lake District, also known as the Lakes or Lakeland, is a mountainous region in North West England. A popular holiday destination, it is famous for its lakes, forests, and mountains (or '' fells''), and its associations with William Wordswor ...
and from the
Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferou ...
Bude Formation of southwestern England.


Formation

Essential for the formation of load casts is an inverted density layering, which is unstable under gravity, i.e. the
potential energy In physics, potential energy is the energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors. Common types of potential energy include the gravitational potenti ...
of the layered system is not a minimum. Load casts are an example of the instability of an interface in a gravitationally unstable arrangement of layered sediments. The instability involved is called a Rayleigh-Taylor instability, whose driving forces are due to
buoyancy Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the ...
. The instability is, however, only ''latent'' because it is dependent on
liquefaction In materials science, liquefaction is a process that generates a liquid from a solid or a gas or that generates a non-liquid phase which behaves in accordance with fluid dynamics. It occurs both naturally and artificially. As an example of th ...
to become real. The process of liquefaction implies a considerable to almost complete loss of
yield strength In materials science and engineering, the yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning of plastic behavior. Below the yield point, a material will deform elastically and wi ...
of the layer involved. This important prerequisite has been appreciated since Sorby in 1908 (and later on by Shrock in 1948), who recognized the hydroplastic condition of the lower layer. 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
earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...
s which had liquefied the sediments.Sims JD. (1975). Determining earthquake recurrence interval from deformational structures in young lacustrine sediments. ''Tectonophysics'', 29:141–152.


References


Literature

* Allen JRL (1984). ''Sedimentary structures. Their character and physical basis''. Elsevier. * Allen JRL (1985). ''Principles of Physical Sedimentology''. Chapman & Hall. * Reineck HE & Singh IB (1980). ''Depositional Sedimentary Environments''. Springer-Verlag. {{ISBN, 0-387-10189-6 Sedimentology Sedimentary structures