Sedimentology of coastal chevron deposits - tsunamigenic versus aeolian origin
Final Report Abstract
The origin of Holocene v-shaped sediment bodies, so-called ‘chevrons”, has been controversially discussed during the last years. The term ‘chevron’ has so far only been defined in terms of the morphology of the sand body, but no systematic field investigation has been conducted. The emplacement of coastal chevrons has hitherto been attributed to (mega-)tsunami inundation, most probably caused by oceanic impacts, while an aeolian origin has been excluded. The main arguments for this assumption are their dimension, height above sea level, the alignment of the central axis which is supposed to be not congruent with local wind directions, the offshore source of the material, and their grain size which should be too coarse for aeolian transport. In this project we systematically surveyed 20 field sites of proposed tsunami-related coastal chevrons in Western Australia, Central Chile, and along the US west coast. We investigated internal structures by ground penetrating radar and in trenches. Grain size, mineral composition, provenance and potential sediment sources (e.g., river, beach, beach ridge, cliff) were analyzed. Ages were obtained by radiocarbon and optically stimulated luminescence dating. If chevrons were deposited by a tsunami, all sand bodies at one site should possess the same depositional ages, the grain-size distribution should be polymodal and poorly sorted indicating various (off- and onshore) sediment sources and internal structures should be scarce or absent as the sediment was spontaneously dumped. In case of an aeolian origin, the ages of the individual chevrons may vary and internal ages will reflect the migration of the sediment body. Furthermore, foreset bedding should be present throughout the sediment body and soil horizons may represent inactive phases. Results document internal cross bedding and soil horizons. The chevrons have an unimodal grain-size distribution and are well sorted. The mean grain size of the chevron sands is 0.11 - 0.44 mm. Sediments of this grain size can be transported by aeolian forces under the local prevailing annual or seasonal wind conditions. A comparison of the chevron components with the mineral content of reference samples shows that the chevrons were indeed fed by the respective local sources. This project concludes that coastal chevrons are in no case related to (mega-)tsunami inundation. Instead, ages document their Holocene aeolian emplacement during periods more arid than today. Required wind velocities for aeolian transport of the respective grain sizes are well within the range of the prevailing local conditions and the structures are aligned in wind direction. Foreset bedding with angles of 20-27° is typical for dunes. Adjacent settings act as sources for the material. Ages decrease in landward transport direction and to the top within vertical successions, indicating landward migration and a long-term emplacement over hundreds to thousands of years. During stable, probably more humid phases, soil horizons could develop. Later on, as precipitation or wind regime changed, chevrons were reactivated. Many of the structures are currently active. The results of this study document an aeolian origin of the surveyed coastal chevrons. The previous impact tsunami hypothesis is unsustainable.
Publications
-
2012: Sedimentology of coastal chevron deposits - tsunamigenic versus aeolian origin. AGU Fall Meeting (03-07 December 2012, San Francisco, USA)
Garcia Garcia, A.-M., Spiske, M., Tsukamoto, S., Schmidt, V.
-
2012: Sedimentology of coastal chevron deposits - tsunamigenic versus aeolian origin. Of Land and Sea: Processes and Products, Geologische Vereinigung Annual Meeting (23-28 September 2012, Hamburg, Germany)
Spiske, M. and Garcia Garcia, A.-M.
-
2013: Coastal chevron deposits - sedimentology, methods and aeolian versus tsunamigenic origin. European Geosciences Union General Assembly (07-12 April 2013, Wien, Austria)
Spiske, M., Garcia Garcia, A.-M., Tsukamoto, S., Schmidt, V.
-
2014: The mystery of chevrons revealed. AGU Fall Meeting, San Francisco (USA), 15-19 December 2014
Spiske, M., Garcia Garcia, A.-M., Tsukamoto, S., Schmidt, V.
-
2015: Age Estimation for Landforms at Tolowa Dunes State Park. North Pacific Landscape Conservation Cooperative Report, 35 pages
Vaughan, P.R.
