2017 Chapters (Layout Features)

What Are the Origins of V-Shaped (Chevron) Dunes in Madagascar? The Case for Their Deposition by a Holocene Megatsunami

Abbott, Dallas Helen; Gusiakov, Viacheslav; Rambolamanana, G.; Breger, Dee Lewis; Mazumder, R.; Galinskaya, K.

Chevrons are elongated dunes with a V-shaped pattern in map view. In some exposures, smaller Vs are nested within the larger Vs. The term chevron was first used to describe desert dunes (Maxwell and Haynes, 1989) based on their similarity to the nested chevrons used on military uniforms or in heraldry. Chevrons later were identified in coastal regions and proposed to represent megastorm deposits (Hearty et al., 1998; Kindler and Strasser, 2000). Subsequently, other workers suggested that some coastal chevron dunes were tsunami deposits (Bryant and Nott, 2001; Scheffers and Kelletat, 2003; Scheffers et al., 2008).

The proposal that some coastal chevron dune complexes represent tsunami deposits is based on three sets of observations (Scheffers and Kelletat, 2003). The first is that the long axes of many coastal chevron complexes are not oriented parallel to the direction of the prevailing wind. The second is that some chevron complexes extend several kilometers (km) inland and rise to over 100 meters (m) above sea level. Some of these chevron complexes are located on shorelines that lack beaches. In these particular cases, it is difficult to understand how either megastorms or wind could have formed the chevrons. Megastorms cannot move subaerial rock and sediment over km-scale distances with elevation gains of hundreds of meters (Cox et al., 2012; Erdmann et al., 2015). Wind cannot move sediment inland if there are no subaerial, poorly consolidated sediments on the coast.

In this chapter, we describe three chevron complexes, V-shaped, elongated dune complexes on the southern coast of Madagascar. Their origin is disputed because individual dunes are elongated along an azimuth that is close to the direction of the prevailing winds (Abbott et al., 2008; Pinter and Ishman, 2008), although their low angle of deposition generally is inconsistent with aeolian dunes. However, other characteristics preclude their derivation from modern beach deposits, although we do not discount later aeolian reworking of some chevron deposits. In particular, the Madagascar chevrons contain significant proportions of early Holocene carbonate tests resembling shells of marine foraminifera, including some that are partially dolomitized, and some that are infilled with mud. These observations suggest that marine carbonate tests in the chevrons were eroded from the continental shelf, and not from modern beaches. Furthermore, despite having lateral extents of tens of km, characteristics of the chevrons (degree of sediment sorting, carbonate content, and marine microfossil concentrations) do not change significantly along strike, as might be expected for aeolian deposits.