Plait Bedform

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  The picture shows a spectacular underwater bottom feature found near the east opening of the Cook Strait, lying between the North and the South Island of New Zealand. An interactive map can be found at https://marlborough.maps.arcgis.com/apps/MapSeries/index.html?appid=155a89b0beb74035bd1c4c71f6f36646 (the original figure can be found in the "Hillshade" layer of the interactive map). The feature stretches ~2300m in the southwest-northeast direction, and its peak height is 15 meters above the adjacent sea bed. The interlocking sediment braids have more distinctly defined west-east oriented crests, while the southwest-northeast ones are less distinct, and they seem to have been “grown out of” or “appended to” the east-west crests after the latter had formed.  


  The structure is likely to be composed of gravel and sand, or even cobble (see figure above where the blue ellipse indicates the location of the feature) and is located in an area where grain size transitions from fine to coarse in the west-east direction (On the interactive map, grainsize distribution can be found under the "Seafloor Classification" section). The presence of large sediment grain sizes implies the presence of strong bottom currents to put such sediment in motion. The M2 phase-lag between the eastern and western side of New Zealand is about 150-degrees (Fig.3 in Walters et al., 2010), suggesting that there can be a strong M2 throughflow due to the anti-phase water surfaces at the two ends of the strait. The depth-averaged root-mean-square tidal velocity can be above 1 m/s (Fig. 5 in Walters et al., 2010), and the principal flow direction is about north-south. Walters et al. (2010) proposes a bottom drag coefficient of about 0.02; given a current velocity of 1.25m/s, we have a friction velocity above 0.02m/s, which makes it possible for the flow to move around coarse sand but not gravels/cobbles. 


  The dimension and the (seeming) rarity of the feature may make it qualify as a “Badass Morphological Feature” (BAMF) by the definition of Phillips (2015). BAMFs are individualistic, non-conformist, and exhibits positive feedback effects. The picture provided through the interactive map shows the presence of some smaller dunes surrounding this feature. In the beginning, the energetic north-south flow might have created some west-east aligned dune-like perturbations, which are subsequently amplified by some unique, local, positive feedbacks. Once the gigantic west-east crests were formed, the cross-strait current (of which the tidal component is very weak, so wind induced transport might be crucial) and other less energetic mechanisms might bring finer sediments to be trapped between east-west crests, forming the less distinct southwest-northeast crests. Moreover, as the tidal current does not appear to be strong enough to move the dominant grainsize group in the area, could the feature be attributable to the more sporadic but also more energetic storm events? How long does it take to build such spectacular, gigantic features? Has Had this bedform shown up in earlier multi-beam surveys? What is the relation with the local geological history (before and after the last ocean transgression)? These are among the questions we and our colleagues (Stuart Pearson and Jose Alvarez) are wondering about.  


  Lastly, another interesting thing is shown in the red circle in the figure below. At the northeast tip of the feature, we seem to have a “miniature” version of the “plait bedform” which seems to be “growing” or “branching” out of a southwest-northeast crest. Can this suggest any self-similarity inherent to the system?  


  Without more field data, however, efforts for explaining such badass features may just be futile. Let’s just sit back, hold our breath, and enjoy the aesthetic feast prepared by Nature itself.  

 

Acknowledgments

Thanks to Stuart Pearson and Jose Alvarez for showing us this amazing bedform. Monster thanks to the Marlborough District Council and NIWA for collecting this amazing dataset and for making it available.

References

Walters, R.A., Gillibrand, P.A., Bell, R.G. and Lane, E.M., 2010. A study of tides and currents in Cook Strait, New Zealand. Ocean dynamics, 60(6), pp.1559-1580.

Phillips, J.D., 2015. Badass geomorphology. Earth Surface Processes and Landforms, 40(1), pp.22-33.

Shore-normal Groovy bedforms

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  Grooves, also termed gutters, furrows or runnels, are linear erosional bedforms cut into soft bedrocks (pictured left) whose formation remains a mystery. This type of bedform is usually found on steep beaches exposed to wave action. Grooves are regularly spaced, more-or-less parallel along the shore, and may be deeply incised into the seabed. The spacing ranges from decimeters to meters. Grooves are long and sinuous across the beach face. The density of grooves increases from the upper to the lower beach in the cross-shore direction. The incision depth is usually less than 1 m and reduces from the top to the bottom of the beach (Carling et al., 2018).   


  Despite the common presence of grooves on beaches, there is still no conceptual model to explain the occurrence and along-shore spacing of grooves.  Previous studies have ascribed the formation of grooves, and similar features, to wave swash (Allen, 1982), backwash (Hawkes, 1962) or the concentration of swash into the shore-normal parallel zone (Shank and Plint, 2013). The most recent simulations of wave action on the beach surface during storms use the model XBeach-G (Carling et al., 2018). Results from this model demonstrate that the formation of grooves reflects wave-induced erosional processes in the swash zone. A higher shear stress is observed during the backwash compared to the uprush. Subsequently, it is expected that groove formation is primarily the result of backwash. However, more detailed quantitative analyses are required to explain the groove formation.


  Not only is the formation of grooves unexplained, but their spacing is also a mystery. Existing experiments and numerical simulations have ascribed the along-shore groove spacing to high- and low-speed flow streakiness. Since the high- and low-velocity streakiness is reported to depend on the flow depth (H), the spacing of grooves is expected to be 1.2H-1.6H, 2H or 1.85H according to previous studies (Mohajeri et al., 2015; Kinoshita, 1967; Albayrak and Lemmin, 2011). However, it is also hypothesized that the spacing might be a reflection of flow perturbations caused by undulations in the beach surface, which could depend on both local swash and backwash, as well as large-scale storms. So far, there is no definitive explanation on groove spacing (Carling et al., 2018).  

 

Reference

Allen, J.R.L., 1982. Sedimentary Structures: Their Character and Physical Basis, vol. II Elsevier, Amsterdam 663pp.

Albayrak, I., Lemmin, U., 2011. Secondary currents and corresponding surface velocity patterns in a turbulent open-channel flow over a rough bed. J. Hydraul. Eng. 137, 1318–1334.

Carling, P., Williams, J., Leyland, J., & Esteves, L. (2018). Storm-wave development of shore-normal grooves (gutters) on a steep sandstone beach face. Estuarine, Coastal and Shelf Science, 207(March), 312–324. https://doi.org/10.1016/j.ecss.2018.04.024

Hawkes, D.D., 1962. Erosion of tidal flats near Georgetown, British Guiana. Nature 196, 128–130.

Kinoshita, R., 1967. An analysis of the movement of flood waters by aerial photography concerning characteristics of turbulence and surface flow. J. Jpn. Soc. Photogrammetry 6, 1–17 (in Japanese).

Mohajeri, S.H., Grizzi, S., Righetti, M., Romano, G.P., Nikora, V., 2015. The structure of gravel-bed flow with intermediate submergence: a laboratory study. Water Resour. Res. http://dx.doi.org/10.1002/2015WR017272.

Shank, J.A., Plint, A.G., 2013. Allostratigraphy of the Upper Cretaceous Cardium Formation in subsurface and outcrop in southern Alberta, and correlation to equivalent strata in northwestern Montana. Bull. Can. Petrol. Geol. 61, 1–40.

Pillow-hollows patterns

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Pillow-hollows (pictured left; courtesy of Ulrich Lemmin) are well-rounded, cushion-like structures commonly separated by comparatively narrow troughs. The first observations are from the bottom of Lake Geneva, Switzerland [Vernet, 1966]. Pillow-hollows have a spatial scale ranging from decimetres to meters and they serve an ecological function as shelter and foraging grounds for certain species (e.g., bottom-dwelling fish). The origin and development of the pillow-hollows are likely to depend on the interaction between near-bottom sediment properties, hydrodynamics and activities of bottom-dwelling species. Given that troughs exhibit a different stratigraphic structure compared to pillows [Brandl et al., 1990; Dominik et al., 1992], local strong near-bottom currents, e.g., Kelvin waves [Lemmin et al., 2005] and density flows acting along the slopes [Fer et al., 2002],  have been suggested as potential causes for the development of pillow-hollows [Le Dantec et al., 2013]. Since most of Lake Geneva is over 200 m deep [Stark et al., 2013], the effects of wind-induced surface currents are certainly negligible. Apart from fluid shear, biological processes may also result in the entrainment of sediment from the bottom. For example, Boyer et al.[1990] observed 30 cm deep foraging trenches made by Burbot fish in Lake Superior.  Katz et al.[2012] reported significant sediment resuspension induced by bottom-dwelling fish along the oxygenated margins of Saanich Inlet in British Columbia. These studies show that the activities of bottom-dwelling species could play some role in maintaining the depressions in pillow-hollows structures. Other processes occurring within the sediment, e.g. gas expulsion and bioturbation [Martin et al., 2005; Varas et al., 2009], could be responsible for local changes in the rheological behaviour of the sediment and consequent spatial heterogeneities in sediment erosion and deposition. Finally, slow deformation of bottom sediments under their own weight might give rise to long-term creep (a viscous-like slow deformation resulting in a net downslope transport, e.g., Mariotti et al.[2019]), possibly accounting for pillow-hollows asymmetry. Overall, this is an amazing pattern and … we really have no idea how it develops!    


Reference

Boyer, L. F., et al. (1990), Deep sediment mixing by burbot (Lota lota), Caribou Island Basin, lake superior, USA, Ichnos: An International Journal of Plant & Animal, 1(2), 91-95.

Brandl, H., et al. (1990), In situ stimulation of bacterial sulfate reduction in sulfate-limited freshwater lake sediments, FEMS MICROBIOL LETT, 74(1), 21-31.

Dominik, J., et al. (1992), Radioisotopic evidence of perturbations of recent sedimentary record in lakes: a word of caution for climate studies, CLIM DYNAM, 6(3-4), 145-152.

Fer, I., et al. (2002), Winter cascading of cold water in Lake Geneva, Journal of Geophysical Research: Oceans, 107(C6).

Katz, T., et al. (2012), Resuspension by fish facilitates the transport and redistribution of coastal sediments, LIMNOL OCEANOGR, 57(4), 945-958.

Le Dantec, N., et al. (2013), Morphology of pillow-hollow and quilted-cover bedforms in Lake Geneva, Switzerland, Flanders Marine Institute (VLIZ), Royal Belgian Institute of Natural …, 2013-01-01.

Lemmin, U., et al. (2005), Internal seiche dynamics in Lake Geneva, LIMNOL OCEANOGR, 50(1), 207-216.

Mariotti, G., et al. (2019), Soil creep in a mesotidal salt marsh channel bank: Fast, seasonal, and water table mediated, GEOMORPHOLOGY, 334, 126-137.

Martin, P., et al. (2005), A qualitative assessment of the influence of bioturbation in Lake Baikal sediments, GLOBAL PLANET CHANGE, 46(1-4), 87-99.

Stark, N., et al. (2013), Deployment of a dynamic penetrometer from manned submersibles for fine‐scale geomorphology studies, Limnology and Oceanography: methods, 11(10), 529-539.

Varas, G., et al. (2009), Dynamics of crater formations in immersed granular materials, PHYS REV E, 79(2), 21301.

Vernet, J. (1966), Prise de vues sous-lacustres dans le Leman lors de prolongees du mesoscaphe" Auguste Piccard", Corbaz.