Parting lineation
Encyclopedia
Parting 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
). It is useful as a main indicator of the lower part of the upper flow regime bedform
, and the direction of these lineations is used as a paleocurrent
indicator.
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: 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
s and mica
trails take up an intermediate position between the ridges and the grooves.
) 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
.
immediately above the sediment-water interface. Responsible for the shaping of the structures are streaky vortex
trains within the boundary layer. Downcurrent these streak
s 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.
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. Hydraulicly
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 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. Geological formation
s like for instance the Old Red Sandstone
or the Buntsandstein
also give proof of the shallow marine character. Parting lineation has even been described from turbidite
s. Yet the structure is not only restricted to the marine environment, it can also form in river
sediments, especially on point bar
s.
Parting-step lineation, which is characterized by step-like parting surfaces, has been reported by Banerjee from varve
s deposited in glacial lake
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 marks, megaripples and dune
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.
τ = 1/8*f*ρ*Um2
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*Ut*λ
where λ represents the measured spacing, Ut the shearing velocity and η the viscosity
of the fluid.
We also have the following equality:
Ut = (τ/ρ)1/2 which yields after solving for τ:
τ = Ut2*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/Um2*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
Um = 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.
indicator). 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.
Sand
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...
grains are aligned in parallel lines or grooves on the surface of a body of sand (or lithified as a sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
). It is useful as a main indicator of the lower part of the upper flow regime bedform
Bedform
A bedform is a depositional feature on the bed of a river or other body of flowing water that is formed by the movement of the bed material due to the flow. Bedforms are characteristic to the flow parameters, and are particularly to flow depth and velocity, and therefore the Froude...
, and the direction of these lineations is used as a paleocurrent
Paleocurrent
A paleocurrent or paleocurrent indicator is a geological feature that helps one determine the direction of flowing water in the geologic past...
indicator.
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: 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
Heavy mineral
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 origins and...
s and mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
trails take up an intermediate position between the ridges and the grooves.
Fabric
Statistical studies of the spatial arrangement of the sand grains (fabricFabric
A fabric is a textile material, short for "textile fabric".Fabric may also refer to:*Fabric , the spatial and geometric configuration of elements within a rock*Fabric , a nightclub in London, England...
) 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
Imbrication
' is the arrangement of planar bodies such that they stack in a consistent fashion - rather like a toppled run of dominoes.*In roofing, imbrication is employed in the Imbrex and tegula system.*Imbrication ....
.
Genesis of the structure
It is meanwhile widely accepted that parting lineation forms in the turbulent, viscous boundary layerBoundary layer
In physics and fluid mechanics, a boundary layer is that layer of fluid in the immediate vicinity of a bounding surface where effects of viscosity of the fluid are considered in detail. In the Earth's atmosphere, the planetary boundary layer is the air layer near the ground affected by diurnal...
immediately above the sediment-water interface. Responsible for the shaping of the structures are streaky vortex
Vortex
A vortex is a spinning, often turbulent,flow of fluid. Any spiral motion with closed streamlines is vortex flow. The motion of the fluid swirling rapidly around a center is called a vortex...
trains within the boundary layer. Downcurrent these streak
Streak
Streak or streaking may refer to:*Streaking, running naked in a public place*Streaking , a method of purifying micro-organisms*Streak , the color left by a mineral dragged across a rough surface...
s 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 siltSilt
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 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. Hydraulicly
Hydraulics
Hydraulics is a topic in applied science and engineering dealing with the mechanical properties of liquids. Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on the engineering uses of fluid properties. In fluid power, hydraulics is used for the generation, control,...
it is characteristic for the lower part of the upper plane bed
Upper plane bed
In geology, upper plane bed refers to the configuration of the bed of a river that is flat and characterized by a unidirectional flow high with rates of sediment transport as both bed load and suspended load. Upper plane bed conditions can produce current lineations In geology, upper plane bed...
regime and results from fairly high current velocities
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
of 0.6 to 1.3 meters per second.
Parting lineation forms in very different depositional environments. The structure is most commonly found on the beach
Beach
A beach is a geological landform along the shoreline of an ocean, sea, lake or river. It usually consists of loose particles which are often composed of rock, such as sand, gravel, shingle, pebbles or cobblestones...
where it forms in flat, wet sediments due to swash
Swash
Swash, 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 material up and down on the beach, which results in the cross-shore sediment exchange. The time-scale of swash motion varies from seconds to minutes...
. Parting lineation can also be created in dewatering tidal channels. Geological formation
Formation
Formation may refer to:* Formation flying, aerobatics performed with several aircraft* Formation , a high-level military organization* Tactical formation, the physical deployment of military forces-Sports:...
s like for instance the Old Red Sandstone
Old Red Sandstone
The Old Red Sandstone is a British rock formation of considerable importance to early paleontology. For convenience the short version of the term, 'ORS' is often used in literature on the subject.-Sedimentology:...
or the Buntsandstein
Buntsandstein
The Buntsandstein or Bunter sandstone is a lithostratigraphic and allostratigraphic unit in the subsurface of large parts of west and central Europe...
also give proof of the shallow marine character. Parting lineation has even been described from turbidite
Turbidite
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. Yet the structure is not only restricted to the marine environment, it can also form in river
River
A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...
sediments, especially on point bar
Point bar
A point bar is a depositional feature of streams. Point bars are found in abundance in mature or meandering streams. They are crescent-shaped and located on the inside of a stream bend, being very similar to, though often smaller than towheads, or river islands.Point bars are composed of sediment...
s.
Parting-step lineation, which is characterized by step-like parting surfaces, has been reported by Banerjee from varve
Varve
A varve is an annual layer of sediment or sedimentary rock.The word 'varve' is derived from the Swedish word varv whose meanings and connotations include 'revolution', 'in layers', and 'circle'. The term first appeared as Hvarfig lera on the first map produced by the Geological Survey of Sweden in...
s deposited in glacial lake
Glacial lake
A glacial lake is a lake with origins in a melted glacier. Near the end of the last glacial period, roughly 10,000 years ago, glaciers began to retreat. A retreating glacier often left behind large deposits of ice in hollows between drumlins or hills. As the ice age ended, these melted to create...
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 marks, megaripples and dune
Dune
In physical geography, a dune is a hill of sand built by wind. Dunes occur in different forms and sizes, formed by interaction with the wind. Most kinds of dunes are longer on the windward side where the sand is pushed up the dune and have a shorter "slip face" in the lee of the wind...
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*Um2
The shear stress
Shear stress
A shear stress, denoted \tau\, , is defined as the component of stress coplanar with a material cross section. Shear stress arises from the force vector component parallel to the cross section...
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
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...
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*Ut*λ
where λ represents the measured spacing, Ut the shearing velocity and η the 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 fluid.
We also have the following equality:
Ut = (τ/ρ)1/2 which yields after solving for τ:
τ = Ut2*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/Um2*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
Um = 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 paleocurrentPaleocurrent
A paleocurrent or paleocurrent indicator is a geological feature that helps one determine the direction of flowing water in the geologic past...
indicator). 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.