Ripple marks in coarse tsunami deposits
Scheffers, A 2006, 'Ripple marks in coarse tsunami deposits', in A Scheffers & D Kelletat (eds), Tsunamis, hurricanes and neotectonics as driving mechanisms in coastal evolution (Proceedings of the Bonaire Field Symposium, March 2-6, 2006. A contribution to IGCP 495), Stuttgart,Germany, 2-6 March, E Schweizerbart, Stuttgart, Germany, Zeitschrift fur Geomorphologie Supplementband , vol. 146, pp. 221-233.
Ripple marks from waves or currents are usually known as minor and mostly short living features in fine sediment. Descriptions of Mega-ripples or "dunes" such as from extreme lake outbursts in Late-Glacial times in the Channeled Scablands from Lake Missoula in NW USA are rare. However, aerial pictures from the Caribbean islands Curaçao and Bonaire (Netherlands Antilles) show that well developed mega-ripple marks are present here. These ripples consist of coarse deposits which have either a bimodal character (clasts mixed with sand) or consist of uniform debris with fragments up to 1 m long. The latter show cord length of several metres with short intervals (distance between the cords), in the finer sediments patterns with dimensions of more than 30 m length and 20 m width are present. Most ripple mark patterns have been formed by flows with onshore direction, although one location shows ripple patterns which have been formed by backwash flow. In this paper we present the hypothesis that these mega-ripple marks are caused by tsunami waves. It is remarkable that these surface features in loose sediments deposited by tsunami impacts from at least 500 years ago or older are preserved over time.