Load cast

Last updated April 20, 2019

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 (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.

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of small particles and subsequent cementation of mineral or organic particles on the floor of oceans or other bodies of water at the Earth's surface. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. Before being deposited, the geological detritus was formed by weathering and erosion from the source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

Lithification is the process in which sediments compact under pressure, expel connate fluids, and gradually become solid rock. Essentially, lithification is a process of porosity destruction through compaction and cementation. Lithification includes all the processes which convert unconsolidated sediments into sedimentary rocks. Petrifaction, though often used as a synonym, is more specifically used to describe the replacement of organic material by silica in the formation of fossils.

The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume. The symbol most often used for density is ρ, although the Latin letter D can also be used. Mathematically, density is defined as mass divided by volume:

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 (mold). Related to load casts are flame structures , load waves, and anticrests.

Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process. Casting materials are usually metals or various time setting materials that cure after mixing two or more components together; examples are epoxy, concrete, plaster and clay. Casting is most often used for making complex shapes that would be otherwise difficult or uneconomical to make by other methods.

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. The resulting pattern may resemble flames. In order for the flame structure to occur, the overlying bed must be of a higher density than the underlying bed, or there must be differential pressures in the upper bed.

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.

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 limestone beds that overlie shale, but it's less common. They are normally hemispherical or kidney shaped, and range in size from a few inches to several feet.

History

Load casts were scientifically reported for the first time by the sedimentologist Theodor Fuchs in 1895 who called them in German Fließwülste (flow crests, flow warts).^[1] He also reproduced the structure experimentally. Later studies were conducted by Henry Clifton Sorby in 1908, Paul Kukuk in 1920, and by Robert Shrock in 1948.

Theodor Fuchs was an Austrian geologist and paleontologist.

German is a West Germanic language that is mainly spoken in Central Europe. It is the most widely spoken and official or co-official language in Germany, Austria, Switzerland, South Tyrol (Italy), the German-speaking Community of Belgium, and Liechtenstein. It is also one of the three official languages of Luxembourg and a co-official language in the Opole Voivodeship in Poland. The languages which are most similar to German are the other members of the West Germanic language branch: Afrikaans, Dutch, English, the Frisian languages, Low German/Low Saxon, Luxembourgish, and Yiddish. There are also strong similarities in vocabulary with Danish, Norwegian and Swedish, although those belong to the North Germanic group. German is the second most widely spoken Germanic language, after English.

Henry Clifton Sorby, 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 steel.

Description

Wartlike load casts in Hettangian arkoses from the northern Aquitaine Basin LiasLoads.jpg — Wartlike load casts in Hettangian arkoses from the northern Aquitaine Basin

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.

Sand is a granular material composed of finely divided rock and mineral particles. It is defined by size, being finer than gravel and coarser than silt. Sand can also refer to a textural class of soil or soil type; i.e., a soil containing more than 85 percent sand-sized particles by mass.

Gravel is a loose aggregation of rock fragments. Gravel is classified by particle size range and includes size classes from granule- to boulder-sized fragments. In the Udden-Wentworth scale gravel is categorized into granular gravel and pebble gravel. ISO 14688 grades gravels as fine, medium, and coarse with ranges 2 mm to 6.3 mm to 20 mm to 63 mm. One cubic metre of gravel typically weighs about 1,800 kg.

Mud is a liquid or semi-liquid mixture of water and any combination of different kinds of soil. It usually forms after rainfall or near water sources. 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.

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.

A paleocurrent or paleocurrent indicator is a geological feature that helps one determine the direction of flowing water in the geologic past. This is an invaluable tool in the reconstruction of ancient depositional environments.

Occurrence

Load casts appear in very different depositional environments. They are most common in turbidites, 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.^[2] Good examples come from the Borrowdale Volcanic Series in the English Lake District and from the Carboniferous Bude Formation of southwestern England.

Formation

Drill core with a load cast LoadCast.JPG — Drill core with a load cast

Essential for the formation of load casts is an inverted density layering, which is unstable under gravity, i.e. the potential energy 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.^[3] 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 earthquakes which had liquefied the sediments.^[4]

Related Research Articles

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A salt dome is a type of structural dome formed when a thick bed of evaporite minerals found at depth intrudes vertically into surrounding rock strata, forming a diapir. It is important in petroleum geology because salt structures are impermeable and can lead to the formation of a stratigraphic trap.

$Dike (geology) A sheet of rock that is formed in a fracture in a pre-existing rock body$

A dike or dyke, in geological usage, is a sheet of rock that is formed in a fracture in a pre-existing rock body. Dikes can be either magmatic or sedimentary in origin. Magmatic dikes form when magma flows into a crack then solidifies as a sheet intrusion, either cutting across layers of rock or through a contiguous mass of rock. Clastic dikes are formed when sediment fills a pre-existing crack.

Diapir A type of geologic intrusion in which a more mobile and ductily deformable material is forced into brittle overlying rocks

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References

↑ Fuchs, Theodor (1895). In Denkschrift Akademischer Wissenschaften, Wien, 62:369–448
↑ Thy P & Wilson JR. (1980). Primary igneous load-cast deformation structures in the Fongen-Hyllingen layered basic intrusion, Trondheim region, Norway. Geol. Mag., 117:363–371.
↑ Anketell JM, et al. (1970). On the deformational structures in systems with reversed density gradients. Roczn. Pol. Tow. Geol., 40:3–30.
↑ Sims JD. (1975). Determining earthquake recurrence interval from deformational structures in young lacustrine sediments. Tectonophysics, 29:141–152.

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

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[1] Fuchs, Theodor (1895). In Denkschrift Akademischer Wissenschaften, Wien, 62:369–448

[2] Thy P & Wilson JR. (1980). Primary igneous load-cast deformation structures in the Fongen-Hyllingen layered basic intrusion, Trondheim region, Norway. Geol. Mag., 117:363–371.

[3] Anketell JM, et al. (1970). On the deformational structures in systems with reversed density gradients. Roczn. Pol. Tow. Geol., 40:3–30.

[4] Sims JD. (1975). Determining earthquake recurrence interval from deformational structures in young lacustrine sediments. Tectonophysics, 29:141–152.