Parting lineation
Srömungsstreifung is a common sedimentary structure, which occurs mainly in sands and silts. Their formation in the medium of water is attributed to turbulence in the sediment boundary layer.
History
Strömungsstreifung, Eng. parting lineation, was first described in 1859 by Henry Clifton Sorby scientifically. He was followed by German sedimentologists: 1938 Hans Cloos, 1939 W. Häntzschel, the fine, parallel striations was noticed on the surfaces of tidal sand bodies, Adolf Seilacher 1953 and 1956 A. Rabien. The term Strömungsstreifung goes back to Werner Pleßmann, who introduced him in 1961; Pleßmann different while still a coarser Strömungsriefung. The English term parting lineation was coined in 1955 by JC Crowell. For the structure but there also exist other terms such as primary current lineation ( Stokes, 1947) or streaming lineation ( Conybeare, CEB & Crooke, KAW, 1968). Lineation of the actual parting parted Mc Bride & Yeakel 1963 nor the so-called parting -step lineation from which arises secondarily in parallel along the Strömungsstreifung; is here to the step-like flaking of fine sediment layers, which preferably follows the flow direction.
Description
Strömungsstreifung is a structure of the sediment surface and is usually found in a very thin layer, horizontally stratified sandstone layers (English parallel -laminated sandstones ). The recognizable on cleavage surfaces structure retains its parallelism to the flow often over many square meters. The striation consists of flat, parallel arranged back, which are separated by grooves or furrows. The height difference is usually no more than several grain size diameter. The furrows appear in the cross section shallow, the back are rounded. Back and furrows are in their longitudinal offset (English en echelon ) to each other, ie the back go over in the direction of flow in furrows. The distance between the individual back in the transverse profile is typically 5.9 to 12.5 millimeters. Length and spacing of the back increases with the grain size of the sediment: in fine-grained sands is their length 3,5 to 12 inches, in medium -grained sands, 5 to 30 centimeters. Its longitudinal extension is thus 5 to 20 times its wavelength λ in the cross profile. In the back, the coarser sediment fraction accumulates, dark heavy minerals and mica flags take a middle position between the two extreme positions.
Structure
Statistical studies show the spatial distribution of sediment grains that their longitudinal axes in the horizontal form two symmetrical maxima that come to lie between 10 and 20 ° either side of the direction of flow. These peaks are further described in the vertical by 8 to 12 ° from the inclined flow (ie, in the flow direction of rising), the granules are thus in an imbricated manner to each other.
Formation
It is now regarded as quite certain that Strömungsstreifung immediately arises in the turbulent, viscous boundary layer above the sediment surface. The responsible for this process can be arranged in chain- like, resembling corkscrews turbulence vortex streets are moored within the boundary layer (in a different model, they are classified as " hairpins " the rolled vortex rolls - tight hairpin vortices -. Interpreted ). Downstream it is in the boundary layer to a flow of the lifting rhythmic strokes on the surface of the sediment, which in a finally burst (English bursting streaks ) passes. This in turn, however, draws a double-sided, lateral inflow of liquid by itself. The in this cyclic process ( talk burst influx ) over the sediment surface occurring shear forces then reflected in the measured distribution of grain axes reflected in the sediment. Ultimately, it is taking place at an angle of approximately 20 ° lateral inflow, which " sweeps up " the sediment grains in the furrow area and then below the heaving turbulent eddies re-deposited in the parallel back. In English, this process is logically referred to as burst and sweep.
Occurrence
Strömungsstreifung is to coarse-grained silts, and very fine-grained to medium -grained sands bound ( particle size 16-500 μ ). In coarser sediments it is very rare. Hydraulic regime is the so-called Upper horizontal stratification (English upper plane bed ) with relatively high flow velocities from 0.6 to 1.3 meters per second ( so-called " shooting " flow ).
Aktuogeologisch Strömungsstreifung occurs in various environments. The most common structure is undoubtedly present in the wet beach area, it arises when Gischtrücklauf on the flat beach sediment. They can also be found in running tidal channels. Geological formations ( such as sandstone or the Old Red Sandstone ) further confirm the occurrence of shallow marine sediments in Strömungsstreifung and even in turbidites. Strömungsstreifungen but are not limited to the marine sector, but are also found in river sediments, such as in flachschichtigen sandbanks.
Parting -step lineation, which is characterized by step-like fracture surfaces was reported by Banerjee even from Warvenllagen in glacial lakes.
In hydraulic experiments Strömungsstreifung was also produced artificially.
Note: In the marine environment Strömungsstreifungen are not associated exclusively with the upper horizontal stratification, they were already in the erosive area of ripples, mega ripples and dunes observed (ie at much lower flow rates ).
Given this more versatile spread of Strömungsstreifung whose origin is linked to turbulent, shooting streams, it is apparent that these sedimentary structure is difficult to use as a clear indicator of the depositional environment.
Theoretical considerations
Output equation for the analysis of Strömungsstreifungen is the quadratic shear stress equation:
τ = 1/ 8 * f * ρ * Um2
The force exerted by the flow in the boundary layer shear stress τ is thus the square of the mean flow velocity proportional. Constants f ( Darcy - Weisbach friction coefficient) and the fluid density ρ.
Empirical studies have provided a more or less constant dimensionless value Z of 100 for the spacing of the parallel line paths or back. The following relationship applies:
Z = 100 = ρ / η * Ut * λ
Here, λ is the respectively measured distance between the bar webs, Ut is the shear rate and the viscosity η of the liquid.
In addition:
Ut = ( τ / ρ ) 1/2 or dissolved by τ: τ = ρ * Ut2
After equating the two equations for τ and a few transformations, we finally arrived at an expression for the distance λ:
λ = 100 * ( η / ρ ) * ( 8/Um2 * F ) 1/2
When inserting the following realistic values obtained 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 distance of the line railways thus is 3 millimeters. This agrees quite well with the experimentally measured values of Allen, but that nevertheless generally are higher by a factor of 2 to 4. The discrepancy is explained by the fact that only more developed line trains leave a macroscopically recognizable back also.
Importance
Strömungsstreifung is a very good indicator of the prevailing flow direction. Moreover, the one-time storage of the sediment ( Hangendrichtung ) be reconstructed ( in thin sections ) by means of the spatial arrangement of the grain structure. The hydraulic regime of the Upper Horizontal stratification is characterized by Strömungsstreifung.