Flow streaking

Current streaking is a widespread sedimentary structure that occurs primarily in sands and silts . Their formation in the medium of water is attributed to turbulence in the boundary layer near the sediment .

history

Parting lineation ( English parting lineation ) was for the first time in 1859 by Henry Clifton Sorby scientifically described. He was followed by German sedimentologists: Hans Cloos in 1938 , W. Häntzschel in 1939 , who noticed the fine, parallel stripes on the surfaces of tidal sand bodies, Adolf Seilacher in 1953 and A. Rabien in 1956 . The term flow striation goes back to Werner Pleßmann , who introduced it in 1961; At the same time, Pleßmann distinguished between a coarser-grained flow groove . The English term parting lineation was coined in 1955 by JC Crowell . There are also other terms for the structure such as B. primary current lineation (Stokes, 1947) or streaming lineation (CEB Conybeare & KAW Crooke, 1968). In 1963, Mc Bride & Yeakel separated what is known as parting-step lineation from the actual parting lineation , which is secondary and parallel to the flow stripes; This is the step-like flaking of the finest layers of sediment, which preferably follows the direction of flow.

description

Current stripes from the Upper Carboniferous Inverness Formation, Nova Scotia, Canada

Current streaking is a structure of the sediment surface and is usually found on very thin, horizontally layered sandstone layers ( parallel- laminated sandstones ). The structure that can be seen on cleavage surfaces often maintains its parallelism to the flow over many square meters. The stripes consist of flat, parallel ridges , which are separated from each other by grooves or furrows . The height difference is usually no more than a few grain size diameters . The furrows appear shallow in the transverse profile, the backs are rounded. The back and furrows are offset in their longitudinal extension ( English en echelon ) to one another, i.e. that is, the ridges merge into furrows in the direction of flow. The distance between the individual backs in the transverse profile is typically 5.9 to 12.5 millimeters. The length and distance of the ridges increases with the grain size of the sediment: in fine-grain sands their length is 3.5 to 12 centimeters, in medium-grain sands 5 to 30 centimeters. Their longitudinal extension is therefore 5 to 20 times their wavelength λ in the transverse profile. The coarser sediment fraction accumulates in the back, dark heavy minerals and mica plumes occupy a middle position between the two extreme positions.

structure

Statistical studies of the spatial distribution of the sediment grains show that their longitudinal axes form two symmetrical maxima in the horizontal plane, which lie between 10 ° and 20 ° on either side of the direction of flow. These maxima are also inclined vertically by 8 ° to 12 ° against the flow, i.e. H. rising in the direction of flow, the grains consequently lie on top of each other like roof tiles .

Emergence

It is now considered fairly certain that flow stripes occur in the turbulent, viscous boundary layer immediately above the sediment surface. The responsible for this process can in dash-like arranged, corkscrews resembling turbulent eddies roads be made within the boundary layer (in another model are as they rolled out to "hairpins" induced drags - English hairpin vortices - interpreted). Downstream it is in the boundary layer to a rhythmic lifting of the power strokes of the sediment surface, which eventually in a burst (engl. Bursting streaks ) passes. However, this in turn entails a bilateral, lateral inflow of liquid. The shear forces occurring in this cyclical process (lifting-bursting-influx) against the sediment surface are then reflected in the measured distribution of the grain longitudinal axes in the sediment. Ultimately, it is the lateral inflow at an angle of around 20 ° that “sweeps up” the sediment grains in the furrow area and then deposits them again below the rising turbulence eddies in the parallel ridge. In English, this process is also called burst and sweep .

Occurrence

Flow stripes are linked to coarse-grained silts and very fine-grained to medium-grained sands (grain sizes from 16–500 µm). It is very rare in coarser sediments. Hydraulic regime is the so-called upper horizontal layering ( upper bed plane ) with rather high flow rates of 0.6 to 1.3 meters per second (called "shooting" flow).

Actuogeological example of flow streaking in the beach area

Actuogeologically , flow streaking occurs in different environments. The structure is undoubtedly most common in the wet beach area , it arises when the spray returns on the flat beach sediment . It is also found in running tidal channels. Geological formations (such as the Buntsandstein or the Old Red Sandstone ) also confirm the occurrence of current stripes in shallow marine sediments and even in turbidites . Current stripes are not limited to the marine area , but are also found in river sediments , for example in shallow sandbars .

Parting-step lineation , which is characterized by step -shaped fracture surfaces, was evenreportedby Banerjee from varveal layers in glacial lakes .

In hydraulic experiments, flow stripes were also created artificially.

Note: In the marine environment, current stripes are not exclusively associated with the upper horizontal stratification; they have already been observed in the erosive area of ripples , mega- ripples and dunes , i.e. H. at significantly lower flow velocities.

Due to this very diverse distribution of flow stripes, the formation of which is linked to turbulent, shooting currents, it becomes clear that this sediment structure is difficult to use as a clear indicator of the deposit area.

Theoretical considerations

The starting equation for the analysis of flow stripes is the quadratic shear stress equation :

${\displaystyle \tau =1/8\cdot f\cdot \rho \cdot U_{m}^{2}}$

The shear stress τ exerted by the flow in the boundary layer is consequently proportional to the square of the mean flow velocity. The constants are f ( Darcy-Weisbach coefficient of friction ) and the liquid density ρ.

Empirical studies have shown a more or less constant dimensionless value Z of 100 for the distances between the parallel lines or ridges. The relationship applies:

${\displaystyle Z=100={\frac {\rho }{\eta U_{t}\lambda }}}$

Here, λ is the measured distance between the lines, U _t is the shear rate and η is the viscosity of the liquid .

The following also applies:

${\displaystyle U_{t}={\sqrt {\frac {\tau }{\rho }}}}$

or resolved for τ:

${\displaystyle \tau =U_{t}^{2}\cdot \rho }$

After equating the two equations for τ and some transformations, one finally arrives at an expression for the distance λ:

${\displaystyle \lambda =100(\eta /\rho ){\sqrt {(8/U_{m}^{2}\cdot f)}}}$

Inserting the following realistic values ​​results for λ:

• η = 1.06 10 ⁻³ Pa s
• ρ = 1000 kg / m ³
• f = 0.01
• U _m = 1 m / s

${\displaystyle \lambda =100\cdot 1{,}06\cdot 10^{-6}\,\mathrm {\frac {Pa\cdot s\cdot m^{3}}{kg}} \cdot {\sqrt {800\,\mathrm {\frac {s^{2}}{m^{2}}} }}=1{,}06\cdot 10^{-4}\,\mathrm {\frac {m^{2}}{s}} \cdot 28{,}28\,\mathrm {\frac {s}{m}} }$

${\displaystyle \lambda =2{,}998\cdot 10^{-3}\,\mathrm {m} }$

The calculated distance between the lines is therefore 3 millimeters. This agrees quite well with the values ​​measured experimentally by Allen, which are nevertheless generally higher by a factor of 2 to 4. The discrepancy is explained by the fact that only more developed lines leave a macroscopically recognizable back.

meaning

Flow stripes are a very good indicator of the prevailing flow direction. In addition, the spatial arrangement of the grain structure (in thin sections ) can be used to reconstruct the former storage of the sediment ( slope direction ). The hydraulic regime of the upper horizontal stratification is characterized by flow striations.

Individual evidence

1. ^ HC Sorby: On the application of quantitative methods to the study of the structure and history of rocks . In: QJ Geol. Soc. London . tape 64 , 1908, pp. 171–233 , doi : 10.1144 / GSL.JGS.1908.064.01-04.12 .
2. ^ W. Pleßmann: Geological yearbook . tape 78 , 1961, pp. 503-566 .
3. ^ JC Crowell: Directional-current structures from the prealpine flysch, Switzerland. In: Bull. Geol. Soc. At the. tape 66 , 1955, pp. 1351-1384 , doi : 10.1130 / 0016-7606 (1955) 66 [1351: DSFTPF] 2.0.CO; 2 .
4. ^ JRL Allen: Primary Current Lineation in the Lower Old Red Sandstone (devonian), Anglo-Welsh Basin . In: Sedimentology . tape 3 , 1964, pp. 89-108 , doi : 10.1111 / j.1365-3091.1964.tb00635.x .
5. ^ E. Grumbt: Stratification types, brands, synsedimentary deformation structures in the red sandstone of southern Thuringia . In: Ber. German Ges. geol. Knowledge A . tape 11 . Berlin 1966, p. 217-234 .
6. ↑ L. Schröder: On the sedimentology of the middle red sandstone . In: Geological Yearbook . tape 82 , 1965, pp. 655-704 .
7. ^ JRL Allen: Physical Processes of Sedimentation . Allen and Unwin, London 1970.
8. ↑ MD Picard, JB Hulen: Parting lineation in siltstone . In: Geol. Soc. At the. Bull. Band 80 , 1969, p. 2631-2636 , doi : 10.1130 / 0016-7606 (1969) 80 [2631: PLIS] 2.0.CO; 2 .
9. ^ P. Wright: A cine-camera technique for process measurement on a ridge and runnel beach . In: Sedimentology . tape 23 , 1976, p. 705-712 , doi : 10.1111 / j.1365-3091.1976.tb00103.x .
10. ↑ I. Brynhi: Flood deposits in the Hornelen Basin, west Norway (Old Red Sands tone) . In: Norsk Geol. Tidsskr. tape 58 , 1978, p. 273–300 ( uio.no [PDF; accessed November 18, 2017]).
11. ^ DJ Stanley: Dish structures and sand flow in ancient submarine valleys, French Maritime Alps . In: Bull. Cent. Rech. Pau . tape 8 , 1974, p. 351-371 .
12. ↑ EF McBride, LS Yeakel: Relationship between parting lineation and rock fabric . In: J. Sediment. Petrol. tape 33 , 1963, pp. 779-782 .
13. ^ I. Banerjee: Part A: Sedimentology of Pleistocene glacial varves in Ontario, Canada, Part B: Nature of the grain-size distribution of some Pleistocene glacial varves of Ontario, Canada . In: Bull. Geol. Serv. Can. tape 226 , 1973, pp. 1-44 , doi : 10.4095 / 103421 .
14. ↑ I. Karcz: Fluvial Geomorphology. State University of New York . Ed .: M. Morisawa. Binghamton 1974, p. 149-173 .
15. ↑ PA Mantz: bedforms produced by fine, cohesionless, granular and flaky sediments under subcritical water flows . In: Sedimentology . tape 25 , 1978, pp. 83-103 .
16. ^ JW Shelton et al .: Directional features in braided-meandering stream deposits, Cimarron River, North-Central Oklahoma . In: J. Sediment. Petrol. tape 44 , 1974, pp. 742-749 .

literature

• JRL Allen: Sedimentary structures. Their character and physical basis . In: Developments in Sedimentology . tape 30 . Elsevier Science Publishers, 1984, ISBN 0-444-42232-3 .
• JRL Allen: Principles of physical sedimentology . Chapman & Hall, 1985, ISBN 0-412-53090-2 .
• M. Leeder: Sedimentology and sedimentary basins. From turbulence to tectonics . Blackwell Science, 1999, ISBN 0-632-04976-6 .