Parting lineation

Last updated April 30, 2019

Parting lineation (also known as current lineation or primary current lineation) is a subtle sedimentary structure in which sand grains are aligned in parallel lines or grooves on the surface of a body of sand (or lithified as a sandstone). [1] The orientation of the lineation is used as a paleocurrent 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.

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.

Sandstone is a clastic sedimentary rock composed mainly of sand-sized mineral particles or rock fragments.

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.

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. [2] 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: 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 minerals and mica trails take up an intermediate position between the ridges and the grooves.

Grain 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 refers to the size of a single crystal inside a particle or grain. A single grain can be composed of several crystals. Granular material can range from very small colloidal particles, through clay, silt, sand, gravel, and cobbles, to boulders.

In geology, a heavy mineral is one 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 mineral suites are used to help determine the provenance and history of sedimentary rocks.

The mica group of sheet silicate (phyllosilicate) minerals includes several closely related materials having nearly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition. The nearly perfect cleavage, which is the most prominent characteristic of mica, is explained by the hexagonal sheet-like arrangement of its atoms.

Fabric

Statistical studies of the spatial arrangement of the sand grains (fabric) 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.

In sedimentology imbrication refers to a primary depositional fabric consisting of a preferred orientation of clasts such that they overlap one another in a consistent fashion, rather like a run of toppled dominoes. Imbrication is observed in conglomerates and some volcaniclastic deposits.

Genesis of the structure

It is meanwhile widely accepted that parting lineation forms in the turbulent, viscous boundary layer immediately above the sediment-water interface. [3] Responsible for the shaping of the structures are streaky vortex 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.

In physics and fluid mechanics, a boundary layer is an important concept and refers to the layer of fluid in the immediate vicinity of a bounding surface where the effects of viscosity are significant.

In fluid dynamics, a vortex 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 the wake of a boat, and the winds surrounding a tropical cyclone, tornado or dust devil.

The streak of a mineral is the color of the powder produced when it is dragged across an un-weathered surface. Unlike the apparent color of a mineral, which for most minerals can vary considerably, the trail of finely ground powder generally has a more consistent characteristic color, and is thus an important diagnostic tool in mineral identification. If no streak seems to be made, the mineral's streak is said to be white or colorless. Streak is particularly important as a diagnostic for opaque and colored materials. It is less useful for silicate minerals, most of which have a white streak or are too hard to powder easily.

Mode of occurrence

Parting lineation is restricted to coarse silts as well as to fine- and medium-grained sands (i. e. to grain sizes of 16 to 500 µ). [4] The structure very rarely occurs in coarser sediments. Hydraulically 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.

Silt is granular material of a size between sand and clay, whose mineral origin is quartz and feldspar. Silt may occur as a soil or as sediment mixed in suspension with water and soil in a body of water such as a river. It may also exist as soil deposited at the bottom of a water body, like mudflows from landslides. Silt has a moderate specific area with a typically non-sticky, plastic feel. Silt usually has a floury feel when dry, and a slippery feel when wet. Silt can be visually observed with a hand lens, exhibiting a sparkly appearance. It also can be felt by the tongue as granular when placed on the front teeth.

Hydraulics 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 counterpart of pneumatics, which concerns gases. Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on the applied engineering using the properties of fluids. In its fluid power applications, hydraulics is used for the generation, control, and transmission of power by the use of pressurized liquids. Hydraulic topics range through some parts of science and most of engineering modules, and cover concepts such as pipe flow, dam design, fluidics and fluid control circuitry. The principles of hydraulics are in use naturally in the human body within the vascular system and erectile tissue. Free surface hydraulics is the branch of hydraulics dealing with free surface flow, such as occurring in rivers, canals, lakes, estuaries and seas. Its sub-field open-channel flow studies the flow in open channels.

The velocity of an object is the rate of change of its position with respect to a frame of reference, and is a function of time. Velocity is equivalent to a specification of an object's speed and direction of motion. Velocity is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of bodies.

Parting lineation forms in very different depositional environments. The structure is most commonly found on the beach where it forms in flat, wet sediments due to swash. Parting lineation can also be created in dewatering tidal channels. [5] Geological formations like for instance the Old Red Sandstone or the Buntsandstein also give proof of the shallow marine character. [6] Parting lineation has even been described from turbidites. [7] Yet the structure is not only restricted to the marine environment, it can also form in river sediments, especially on point bars.

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 formed after lithification, if the sediment is preserved in the rock record. In most cases the environments associated with particular rock types or associations of rock types can be matched to existing analogues. However, the further back in geological time sediments were deposited, the more likely that direct modern analogues are not available.

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. The particles can also be biological in origin, such as mollusc shells or coralline algae.

Swash, or forewash in geography, is a turbulent layer of water that washes up on the beach after an incoming wave has broken. The swash action can move beach materials up and down the beach, which results in the cross-shore sediment exchange. The time-scale of swash motion varies from seconds to minutes depending on the type of beach. Greater swash generally occurs on flatter beaches. The swash motion plays the primary role in the formation of morphological features and their changes in the swash zone. The swash action also plays an important role as one of the instantaneous processes in wider coastal morphodynamics.

Parting-step lineation, [8] which is characterized by step-like parting surfaces, has been reported by Banerjee from varves deposited in glacial lakes. [9]

Parting lineation was even artificially reproduced in hydraulic experiments. [10] [11]

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 marks, megaripples and dunes. 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 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 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 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 [Pa*s]

ρ = 1000 [kg/m³]

f = 0.01

U_m = 1 [m/s]

λ = 100*1.06*10⁻⁶*(800)^1/2 = 1.06*10⁻⁴*28.28

λ = 2.998*10⁻³ [m]

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 paleocurrent indicator). [12] 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.

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References

↑ Boggs, S., Principles of Sedimentology and Stratigraphy, 3rd Ed., pgs. 125-126
↑ Allen, J. R. L. (1964a). Sedimentology, 3, page 89 – 108
↑ Allen, J. R. L. (1970g). Physical Processes of Sedimentation. Allen and Unwin, London
↑ Picard, M. D. & Hulen, J. B. (1969). Geol. Soc. Am. Bull., 80, page 2631–2636
↑ Wright, P. (1976). Sedimentology, 23, page 705 – 712
↑ Brynhi, I. (1978). Norsk Geol. Tidsskr., 58, page 273 – 300
↑ Stanley, D. J. (1974). Bull. Cent. Rech. Pau, 8, page 351 – 371
↑ McBride, E. F. & Yeakel, L. S. (1963). Relationship between parting lineation and rock fabric. J. Sediment. Petrol., 33, page 779 – 782
↑ Banerjee, I. (1973). Bull. Geol. Serv. Can., n° 226, page 1 – 44
↑ Karcz, I. (1974). Fluvial Geomorphology. State University of New York, Binghamton, pp. 149 – 173. Edited by M. Morisawa
↑ Mantz, P. A. (1978). Bedforms produced by fine, cohesionless, granular and flaky sediments under subcritical water flows. Sedimentology, 25, page 83 – 103
↑ 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

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[1] Boggs, S., Principles of Sedimentology and Stratigraphy, 3rd Ed., pgs. 125-126

[2] Allen, J. R. L. (1964a). Sedimentology, 3, page 89 – 108

[3] Allen, J. R. L. (1970g). Physical Processes of Sedimentation. Allen and Unwin, London

[4] Picard, M. D. & Hulen, J. B. (1969). Geol. Soc. Am. Bull., 80, page 2631–2636

[5] Wright, P. (1976). Sedimentology, 23, page 705 – 712

[6] Brynhi, I. (1978). Norsk Geol. Tidsskr., 58, page 273 – 300

[7] Stanley, D. J. (1974). Bull. Cent. Rech. Pau, 8, page 351 – 371

[8] McBride, E. F. & Yeakel, L. S. (1963). Relationship between parting lineation and rock fabric. J. Sediment. Petrol., 33, page 779 – 782

[9] Banerjee, I. (1973). Bull. Geol. Serv. Can., n° 226, page 1 – 44

[10] Karcz, I. (1974). Fluvial Geomorphology. State University of New York, Binghamton, pp. 149 – 173. Edited by M. Morisawa

[11] Mantz, P. A. (1978). Bedforms produced by fine, cohesionless, granular and flaky sediments under subcritical water flows. Sedimentology, 25, page 83 – 103

[12] 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