Chevron (land form)

This article is about deposits of sediment across the earth's surface. For folds in rock layers, see Chevron (geology).

A chevron is a wedge-shaped sediment deposit observed on coastlines and continental interiors around the world. The term chevron was originally used independently by Maxwell and Haynes[1] and Hearty and others[2] for large, v-shaped, sub-linear to parabolic landforms in southwestern Egypt and on islands in the eastern, windward Bahamas.

General

The Egyptian “chevrons” are active, wind-generated dunes, but the “chevrons” in the Bahamas are inactive and have been variously interpreted.[3] The most common interpretation of large, chevron-shaped bed forms is that they are a form of parabolic dune, and that most examples are generated by wind action.

Many chevrons can be found in Australia,[4] but others are concentrated around the coastlines of the world. For instance there are chevrons in Hither Hills State Park on Long Island and in Madagascar (such as the Fenambosy Chevron), as well as in interior sites of the United States such as the Palouse region of eastern Washington State, the Great Sand Dunes National Park and Preserve, and White Sands National Monument.

Formation

According to Hansen et al. 2015, powerful storms and changes in sea level rise can explain chevrons, as the study elaborates: The lightly indurated ooid sand ridges are several kilometers long (Bahamas) and appear to have originated from the action of long-period waves from a northeasterly Atlantic source. The chevron ridges contain bands of beach fenestrae, formed by air bubbles trapped in fine ooid sand inundated by water and quickly indurated. The internal sedimentary structures including the beach fenestrae and scour structures (Tormey, 2015) show that the chevrons were rapidly emplaced by water rather than wind (Hearty et al., 1998). These landforms were deposited near the end of a sea level high stand, when sea level was just beginning to fall, otherwise they would have been reworked subsequently by stable or rising seas. Some chevrons contain multiple smaller ridges “nested” in a seaward direction (Hearty et al., 1998), providing further evidence that sea level was falling fast enough to strand and preserve older chevrons as distinct landforms.

Older ridges adjacent to the chevron ridges have wave runup deposits that reach heights nearly 40 m above present sea level, far above the reach of a quiescent 5e sea surface. Such elevated beach fenestrae are considered to result from runup of very large waves (Wanless and Dravis, 1989). These stratigraphically youngest deposits on the shore-parallel ridges are 1-5 m thick fenestrae-filled seaward-sloping tabular beds of stage 5e age that mantle older MIS 5e dune deposits (Neumann and Moore, 1975; Chen at al., 1991; Neumann and Hearty, 1996; Tormey, 2015). Runup beds reach more than a kilometer from the present coast, mantling the eastern flanks of stage 5e ridges (Hearty et al., 1998). Bain and Kindler (1994) suggested the fenestrae could be raingenerated, but the fenestrae at high elevations are widespread and exclusive to the late 5e deposits. They are not commonly found in older dune ridges (Hearty et al., 1998).

Movement of these sediments, including chevrons, run-up deposits and boulders, required a potent sustained energy source. Anticipating our interpretation in terms of powerful storms driven by an unusually warm tropical ocean and strong zonal temperature gradients in the North Atlantic, we must ask whether there should not be evidence of comparable end-Eemian storms in Bermuda. Indeed, there are seaward sloping planar beds rising to about +20 m along several kilometers of the north coast of Bermuda (Land et al., 1967; Vacher and Rowe, 1997; Hearty et al., 1998).[5]

In an alternative view, the Holocene Impact Research Group hypothesizes that the formations could be caused by tsunamis from meteorite impacts or submarine slides which lift sediment up and carry it hundreds of miles until depositing it on coastlines.[6] Part of the evidence they cite for this hypothesis is that the sediments contain tiny marine fossils; however, such fossils can be moved by the wind, just like sand. The impact idea is controversial not only because chevrons are similar to wind-blown landforms found far from the ocean, but also because it is unlikely that there have been enough large impacts and landslides to explain the observed chevrons. Moreover, some computer models and sediment-transport analysis do not support this theory. For example, the orientation of chevrons along the southern coast of Madagascar do not line up with what these models of mega-tsunamis have simulated.[7] Additional evidence against the mega-tsunami hypothesis is that the force of the water would not produce such regular bed forms.[3]

See also

Notes

  1. ↑ Maxwell, T.A. and Haynes, C.V., Jr., 1989. Large-scale, low-amplitude bedforms (chevrons) in the Selima Sand Sheet, Egypt: Science v. 243, p. 1179-1182.
  2. ↑ Hearty, Paul J.; A. Conrad Neumann; Darrell S. Kaufman (1998). "Chevron Ridges and Runup Deposits in the Bahamas from Storms Late in Oxygen-Isotope Substage 5e" (PDF). Quaternary Research. 50: 309–322. Retrieved 15 February 2013.
  3. 1 2 Bourgeois, Joanne; Robert Weiss (2009). "'Chevrons' are not mega-tsunami deposits—A sedimentologic assessment" (PDF). Geology. 37 (5): 403–406. doi:10.1130/G25246A.1. Retrieved 15 February 2013.
  4. ↑ Scheffers, Anja; Kelletat, Dieter (2003). "Chevron-shaped Accumulations Along the Coastlines of Australia As Potential Tsunami Evidences?" (PDF). Science of Tsunami Hazards. 21 (3): 174–188. Retrieved 15 February 2013.
  5. ↑ J. Hansen; M. Sato; P. Hearty; R. Ruedy; M. Kelley; V. Masson-Delmotte; G. Russell; G. Tselioudis; J. Cao; E. Rignot; I. Velicogna; E. Kandiano; K. von Schuckmann; P. Kharecha; A. N. Legrande; M. Bauer; K.-W. Lo (2015). "Ice melt, sea level rise and superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2 ◦C global warming is highly dangerous" (PDF). doi:10.5194/acpd-15-20059-2015.
  6. ↑ Gusiakov, V. Abbott, D.H., Bryant, E.A., Masse, W.B., and Breger, D., 2010. Mega tsunami of the world oceans: Chevron dune formation, micro-ejecta, and rapid climate change as the evidence of recent oceanic bolide impacts: T. Beer (ed.), Geophysical Hazards, p. 197-227; Springer Publ.
  7. ↑ "Contrary to recent hypothesis, 'chevrons' are not evidence of megatsunamis". Phys.Org. 29 April 2009. Retrieved 15 February 2013.

References

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