First evidence of accumulation of mega boulders on the Mediterranean rocky coast of Provence (southern France)

Abstract

Abstract. An accumulation of boulders was recently discovered along the rocky coast of the Gulf of Fos located in Provence, in an area exposed to a south-westerly wave regime. The coast around this locality forms the western extremity of the calcareous Nerthe range between Marseille and the Rhône Delta. Several mega blocks are scattered to a distance of 30 m behind the coast line. The largest block (33.5 tonnes) has been transported about 39 m inland, up to about 2 m a.s.l. On the Mediterranean coast, the origin of such blocks is often attributed to tsunami-generated waves, but in the case examined here, although the origin is unclear, the differences in surface state between boulders indicates several events generated by south-westerly storms. Radiocarbon dating on several different shells collected from seven different boulders yields a wide dispersion of ages ranging from 4000 BP to the Modern Period. The differences in surface appearance, as well as the differences of fauna conservation and surface coloration, in some cases in a very fresh state, along with the dispersion of radiocarbon ages, suggest that historic storm events have affected these megablocks.

Outcomes of the Special Issue on Extreme and Rogue Waves

“…Further investigation of low probability large waves is required. Vella et al (2011) described an accumulation of boulders discovered along the rocky coast of the Gulf of Fos, Provence, France, where several mega blocks are scattered behind the coastline, with the largest block (33.5 t) transported 39 m inland. Along the Mediterranean coasts, the origin of such blocks is attributed to tsunami-generated waves.…”

Block and boulder accumulations on the southern coast of Crete (Greece): evidence for the 365 CE tsunami in the Eastern Mediterranean

“…However, the Mediterranean basin offers a good opportunity for the study of palaeo-tsunamis due to a micro-tidal regime, limited fetch reducing maximum possible storm wave heights and many active faults and other tsunamigenic sources. As a result tsunami boulder deposits have been described from across the Mediterranean region [Algeria (Maouche et al, 2009), France (Vella et al, 2011), Sicily (Scicchitano et al, 2007;Barbano et al, 2010) and the Adriatic (Mastronuzzi & Sanso, 2004), Malta (Mottershead et al, 2014;2015); Cyprus (Kelletat & Schellmann, 2002) and Greece (Scheffers & Scheffers, 2007;Scheffers et al, 2008;Vacchi et al, 2012)]. In addition, fine-grained to cobble deposits attributed to tsunamis are also common in the Mediterranean and have been described mainly from Italy, Greece and Turkey (i.e., Dominey-Howes, 1998;Vött et al, 2007Vött et al, , 2008Vött et al, , 2009Vött et al, , 2011Bruins et al, 2008;May et al, 2012;Smedile et al, 2011Smedile et al, , 2012.…”

Current progress and perspectives of the research on tsunami boulders

“…et al, 2009a, 2010a, 2011) ．そして，高波による巨 礫移動限界線を決めることができれば，それよりはるか内 陸に存在する巨礫は，その地域を襲いうる高波では説明で きず，より周期の長い津波起源だと認定できると考えられ る(後藤，2009; Goto et al, 2010a, d) ． 石垣島の東海岸などの海岸線沿いに存在する，従来から 「津波石」と呼ばれていた巨礫群は，礁縁からの距離にして 最大 1300 m も内陸にあり，上記のような台風の高波に伴 う巨礫群とは明らかに異なる分布形態を示す(第 3 図， Goto et al, 2010a, b) ．さらに，200 トンを超える巨礫も存在 し，礁池内起源のハマサンゴが多いなど，高波起源の巨礫 とはサイズ・種類とも明瞭に異なることが明らかになって きた(Goto et al, 2010a, b) ．これらの巨礫群の一部は，年代 測定結果から 1771 年明和大津波起源と考えられているが (加藤・木村，1983; 河名・中田，1994; Suzuki et al, 2008) ， 水理学的にも津波でしか説明し得ないことが明らかになっ た(Goto et al, 2010a, b) ． こうして識別された沿岸部の津波石を用いれば，津波の 履歴や規模を推定できる可能性がある．Araoka et al (2010) は，津波で打ち上げられたハマサンゴを対象に 14 C 年代測 定や U/Th 年代測定を実施し，1771 年明和大津波により打 ち上げられた津波石に加え，古文書に記録された 1625 琉球列島における， (a)台風の高波と(b)津波による巨礫運搬プロセスの違い，および(c) 台風の高波と(d)津波による巨礫分布の違い(Goto et al, 2010a を改変) et al, 2009a; Paris et al, 2011b; Terry and Etienne, 2011; Etienne, 2012) ．特に， 2011 年に出版された Marine Geology 誌の特集号 "Boulders as a signature of storms on rock coasts" は，高波で打ち上げら れた巨礫の論文を集めたものである．この特集号では，巨 礫が岩礁などから剥離するプロセス(Fichaut and Suanez, 2011; Knight and Burningham, 2011; Stephenson and Naylor, 2011)や分布様式(Chen et al, 2011; Hall, 2011; McKenna et al, 2011) ，運搬されうる巨礫の重量や移動距離に関する研 究(Goto et al, 2011(Goto et al, )などの論文が含まれる． et al, 2009Maouche et al, 2009;Scheffers et al, 2009Scheffers et al, , 2010a Barbano et al, 2010 Barbano et al, , 2012Etienne and Paris, 2010;Pignatelli et al, 2010;Switzer and Burston, 2010;Medina et al, 2011;Oliveira et al, 2011;Paris et al, 2011a;Shah-hosseini et al, 2011;Vella et al, 2011;Watt et al, 2012; Engel and May, in press)はもっ…”