Load cast
Encyclopedia
Load casts are a pene-contemporaneous deformation structure and form during soft sediment deformation. These interfacial sole markings are very common and represent an expression of a Rayleigh-Taylor instability
developing in gravitationally unstable profiles with a denser
layer overlying a less-dense hydroplastic layer.
(the denser layer) sinking into its underlying (less dense) cast
(mold). Related to load casts are flame structure
s, load waves, and anticrests. Extreme developments of load casts are pseudo-nodules and ball-and-pillow structures
.
Fließwülste (flow crests, flow warts). He also reproduced the structure experimentally. Later studies were conducted by H C Sorby
in 1908, by P. Kukuk in 1920, and by R R Shrock in 1948.
s, coarse sands, or gravel
s), which is superimposed on a less-dense hydroplastic layer (mud
s, silt
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 diaper-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/diapers 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.
s, but can also occur in fluvial
and shallow-marine settings. Occasionally they appear in lake
sediments. They have even been found in layered igneous and pyroclastic
successions. Good examples come from the Borrowdale Volcanic Series in the English Lake District
and from the Carboniferous
Bude Formation of southwestern England.
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
.
The instability is, however, only latent because it is dependent on liquefaction
to become real. The process of liquefaction implies a considerable to almost complete loss of yield strength 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
s which had liquefied the sediments.
Rayleigh-Taylor instability
The Rayleigh–Taylor instability, or RT instability , is an instability of an interface between two fluids of different densities, which occurs when the lighter fluid is pushing the heavier fluid....
developing in gravitationally unstable profiles with a denser
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
layer overlying a less-dense hydroplastic layer.
Terminology
The expression "load cast", sometimes also called load structure, is self-evident, referring to a loadLoad
Load may refer to:*Structural load, forces which are applied to a structure*Cargo*The load of a mutual fund *The genetic load of a population*The parasite load of an organism...
(the denser layer) sinking into its underlying (less dense) cast
Casting
In metalworking, casting involves pouring liquid metal into a mold, which contains a hollow cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process...
(mold). Related to load casts are flame structure
Flame structure
A flame structure is a type of soft-sediment deformation that forms in unlithified 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
Ball-and-pillow structures
Ball-and-pillow structures are masses of clastic sediment that take the form of isolated pillows or protruding ball structures. These soft-sediment deformations are usually found at the base of sandstone beds that are interbedded with mudstone. It is also possible to find ball-and-pillows in...
.
History
Load casts were scientifically reported for the first time by the sedimentologist T. Fuchs in 1895 who called them in GermanGerman language
German is a West Germanic language, related to and classified alongside English and Dutch. With an estimated 90 – 98 million native speakers, German is one of the world's major languages and is the most widely-spoken first language in the European Union....
Fließwülste (flow crests, flow warts). He also reproduced the structure experimentally. Later studies were conducted by H C Sorby
Henry Clifton Sorby
Henry Clifton Sorby , was an English microscopist and geologist.-Biography:Sorby was born at Woodbourne near Sheffield in Yorkshire and attended Sheffield Collegiate School. He early developed an interest in natural science, and one of his first papers related to the excavation of valleys in...
in 1908, by P. Kukuk in 1920, and by R R Shrock in 1948.
Description
Load casts form on the underside of the overlying denser layer (sandSand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
s, coarse sands, or gravel
Gravel
Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...
s), which is superimposed on a less-dense hydroplastic layer (mud
Mud
Mud is a mixture of water and some combination of soil, silt, and clay. Ancient mud deposits harden over geological time to form sedimentary rock such as shale or mudstone . When geological deposits of mud are formed in estuaries the resultant layers are termed bay muds...
s, silt
Silt
Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...
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 diaper-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
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
L can consequently be attributed to the lobes. According to the contrast in density and viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher 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/diapers 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
Paleocurrent
A paleocurrent or paleocurrent indicator is a geological feature that helps one determine the direction of flowing water in the geologic past...
. 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 environments. They are most common in turbiditeTurbidite
Turbidite geological formations have their origins in turbidity current deposits, which are deposits from a form of underwater avalanche that are responsible for distributing vast amounts of clastic sediment into the deep ocean.-The ideal turbidite sequence:...
s, but can also occur in fluvial
Fluvial
Fluvial is used in geography and Earth science to refer to the processes associated with rivers and streams and the deposits and landforms created by them...
and shallow-marine settings. Occasionally they appear in lake
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...
sediments. They have even been found in layered igneous and pyroclastic
Pyroclastic rock
Pyroclastic rocks or pyroclastics are clastic rocks composed solely or primarily of volcanic materials. Where the volcanic material has been transported and reworked through mechanical action, such as by wind or water, these rocks are termed volcaniclastic...
successions. Good examples come from the Borrowdale Volcanic Series in the English Lake District
Lake District
The Lake District, also commonly known as The Lakes or Lakeland, is a mountainous region in North West England. A popular holiday destination, it is famous not only for its lakes and its mountains but also for its associations with the early 19th century poetry and writings of William Wordsworth...
and from the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
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 energyPotential energy
In physics, potential energy is the energy stored in a body or in a system due to its position in a force field or due to its configuration. The SI unit of measure for energy and work is the Joule...
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
Rayleigh-Taylor instability
The Rayleigh–Taylor instability, or RT instability , is an instability of an interface between two fluids of different densities, which occurs when the lighter fluid is pushing the heavier fluid....
, whose driving forces are due to buoyancy
Buoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
.
The instability is, however, only latent because it is dependent on liquefaction
Liquefaction
Liquefaction may refer to:* Liquefaction, the general process of becoming liquid* Soil liquefaction, the process by which sediments become suspended* Liquefaction of gases in physics, chemistry, and thermal engineering* Liquefactive necrosis in pathology...
to become real. The process of liquefaction implies a considerable to almost complete loss of yield strength 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
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...
s which had liquefied the sediments.
Literature
- Allen JRL (1984). Sedimentary structures. Their character and physical basis. Elsevier. ISBN 0-444-42232-3
- Allen JRL (1985). Principles of Physical Sedimentology. Chapman & Hall. ISBN 0-412-53090-2
- Reineck HE & Singh IB (1980). Depositional Sedimentary Environments. Springer-Verlag. ISBN 0-387-10189-6