Parting lineation (also known as current lineation or primary current lineation) is a subtle

sedimentary structure Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition. Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different particl ...

in which

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 ...

grains are aligned in parallel lines or grooves on the surface of a body of sand (or lithified as a

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicat ...

). The orientation of the lineation is used as a

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 ...

indicator, although the precise flow direction (i.e. upstream vs. downstream) is often indeterminable. They are also the primary indicator of the lower part of the upper flow regime bedform.

Description

Parting lineation is a sedimentary structure commonly found on the surface of parallel-laminated sandstones. It is aligned with the current direction, the alignment holding out in places over several square meters. The lineation is formed by flat, parallel ridges which are separated by depressions or grooves. The height of the ridges rarely exceeds several grain diameters. In profile the depressions are flat-bottomed and the ridges are rounded. Ridges and depressions are arranged en echelon, so that in downstream direction the ridges pass into depressions and vice versa. The crosswise distance between individual ridges typically varies between 0.59 and 1.25 centimeters. Length as well as spacing of the ridges augments with increasing

grain size Grain size (or particle size) is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which ...

: in fine-grained sands the ridges are 3.5 to 12 centimeters long, whereas in medium-grained sands they can reach 30 centimeters in length. The long dimension of the ridges is therefore 5 to 20 times their crosswise spacing. The coarse(r) sediment fraction accumulates in the ridges, whereas dark

heavy mineral In geology, a heavy mineral is a mineral with a density that is greater than 2.9 g/cm3, most commonly referring to dense components of siliciclastic sediments. A heavy mineral suite is the relative percentages of heavy minerals in a stone. Heavy min ...

s and

mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...

trails take up an intermediate position between the ridges and the grooves.

Fabric

Statistical studies of the spatial arrangement of the sand grains (

fabric Textile is an umbrella term that includes various fiber-based materials, including fibers, yarns, filaments, threads, different fabric types, etc. At first, the word "textiles" only referred to woven fabrics. However, weaving is not ...

) show that in the horizontal plane the long axes of the grains form two symmetrical maxima oriented at an angle of 10 to 20 ° to the current direction. In the vertical plane these maxima are inclined at an angle of 8 to 10 ° into the current direction and the sand grains are imbricated.

Genesis of the structure

It is meanwhile widely accepted that parting lineation forms in the turbulent, viscous

boundary layer In physics and fluid mechanics, a boundary layer is the thin layer of fluid in the immediate vicinity of a bounding surface formed by the fluid flowing along the surface. The fluid's interaction with the wall induces a no-slip boundary cond ...

immediately above the sediment-water interface. Responsible for the shaping of the structures are streaky

vortex In fluid dynamics, a vortex ( : vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in ...

trains within the boundary layer. Downcurrent these streaks start to gradually lift off from the sediment surface until they eventually “burst”. Whenever this happens fluid rushes in from both sides. This cyclic process of lift-off, bursting and inrush of fluid exerts a shear stress on the sediment surface, which ultimately finds its expression in the spatial arrangement of the sediment grains. After all it is this lateral inrush of fluid, which “sweeps up” the grains in the grooves and redeposits them in long parallel ridges underneath the lifting turbulences. This rhythmical process is known as ''burst and sweep''.

Mode of occurrence

Parting lineation is restricted to coarse

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 as well as to fine- and medium-grained sands (i. e. to grain sizes of 16 to 500 µ). The structure very rarely occurs in coarser sediments.

Hydraulically Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counte ...

it is characteristic for the lower part of the upper plane bed regime and results from fairly high current velocities of 0.6 to 1.3 meters per second. Parting lineation forms 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. The structure is most commonly found on the

beach A beach is a landform alongside a body of water which consists of loose particles. The particles composing a beach are typically made from rock, such as sand, gravel, shingle, pebbles, etc., or biological sources, such as mollusc s ...

where it forms in flat, wet sediments due to

swash Swash, or forewash in geography, is a turbulence, turbulent layer of water that washes up on the beach after an incoming ocean surface wave, wave has broken. The swash action can move beach materials up and down the beach, which results in the ...

. Parting lineation can also be created in dewatering tidal channels. Geological formations like for instance the

Old Red Sandstone The Old Red Sandstone is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the northeastern seaboard of North America. It also exte ...

or the

Buntsandstein The Buntsandstein (German for ''coloured'' or ''colourful sandstone'') or Bunter sandstone is a lithostratigraphic and allostratigraphic unit (a sequence of rock strata) in the subsurface of large parts of west and central Europe. The Buntsandst ...

also give proof of the shallow marine character. Parting lineation has even been described from

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. Yet the structure is not only restricted to the marine environment, it can also form in

river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of ...

sediments, especially on

point bar A point bar is a depositional feature made of alluvium that accumulates on the inside bend of streams and rivers below the slip-off slope. Point bars are found in abundance in mature or meandering streams. They are crescent-shaped and located on ...

s. Parting-step lineation, which is characterized by step-like parting surfaces, has been reported by Banerjee from

varve A varve is an annual layer of sediment or sedimentary rock. The word 'varve' derives from the Swedish word ''varv'' whose meanings and connotations include 'revolution', 'in layers', and 'circle'. The term first appeared as ''Hvarfig lera'' (va ...

s deposited in

glacial lake A glacial lake is a body of water with origins from glacier activity. They are formed when a glacier erodes the land and then melts, filling the depression created by the glacier. Formation Near the end of the last glacial period, roughly 10, ...

s. Parting lineation was even artificially reproduced in hydraulic experiments. Remark: in the marine environment parting lineation doesn't have to be associated solely with the upper plane bed regime, it has been reported for instance from the erosive stoss side of

ripple mark In geology, ripple marks are sedimentary structures (i.e., bedforms of the lower flow regime) and indicate agitation by water (current or waves) or wind. Defining ripple cross-laminae and asymmetric ripples * ''Current ripple marks'', ''unidi ...

s, megaripples and

dune A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, f ...

s. This implies that the structure can form already under lower current velocities. Due to its varied and rather widespread occurrence parting lineation is not a unanimous indicator for depositional environments.

Theoretical considerations

In order to derive an expression for the crosswise spacing of parting lineation it is useful to start with the quadratic stress law: τ = 1/8*f*ρ*U_m² The

shear stress Shear stress, often denoted by ( Greek: tau), is the component of stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel to the material cross section. '' Normal stress'', on ...

τ exerted by the current within the boundary layer is proportional to the square of the current velocity U. The Darcy-Weisbach friction coefficient f and the

density 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. Mathematicall ...

of the fluid ρ are constants. Empirical studies have found a dimensionless value Z = 100 for the parallel streaks/ridges. This can be equated to: Z = 100 = ρ/η*U_t*λ where λ represents the measured spacing, U_t the shearing velocity and η the

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 fluid. We also have the following equality: U_t = (τ/ρ)^1/2 which yields after solving for τ: τ = U_t²*ρ After equating the two expressions for τ and some rearrangements one arrives at an expression for the crosswise spacing λ: λ = 100*(η/ρ)*(8/U_m²*f)^1/2 By inserting the following realistic values one finds for λ: η = 1.06*10⁻³ a*s ρ = 1000 g/m³ f = 0.01 U_m = 1 /s λ = 100*1.06*10⁻⁶*(800)^1/2 = 1.06*10⁻⁴*28.28 λ = 2.998*10⁻³ The calculated crosswise spacing λ is 3 millimeters. This value is in fairly close agreement with the experimental values measured by Allen, which are nevertheless generally 2 to 4 times higher. The discrepancy can be explained by assuming that only strongly developed streaks leave a discernible ridge.

Conclusions

Parting lineation is a very good indicator of the reigning current direction (and therefore also a good

indicator).Shelton, J. W. et al. (1974). Directional features in braided-meandering stream deposits, Cimarron River, North-Central Oklahoma. J. Sediment. Petrol., 44, page 742 – 749 An analysis of the grain fabric furthermore leads to the determination the younging direction within the sedimentary succession. The lower part of the upper plane bed hydraulic regime of relatively swift currents is characterized by parting lineation.

References

Sedimentary structures