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Since the last major deglaciation, about 12-10,000 years ago, paraglacial landforms widely occurred in fjord and mountain areas; mass movements such as rock-slope failure, rock avalanches, rockslides, sackungs, etc., are common (Dikau et al., 1996; Ballantyne, 2008). In north Iceland, numerous mass movements' landforms occurred, mostly in areas within Tertiary basalt formation, in a landscape characterized by steep slopes and overdeepened glacial valleys. Many of those landforms can be observed in the Vestfirðir peninsula, in central North Iceland, and in Eastern Iceland. Jónsson (1957) made some early descriptions of such landforms. He concluded that most of those landforms were formed during or shortly after the last deglaciation, in Early Holocene; at the time of these first observations, results on absolute dating were lacking. Recently, several landslides have been studied in the Skagafjörður area (Decaulne et al., 2010, Mercier et al., 2011). The aim of this study is to date several landslides in the Skagafjörður area, by combining several proxies, e.g. radiocarbon dating, teprhochronology, raised beaches. We present here the result on one of those, the Höfðahólar case-study. - The material originating from the Höfðahólar rock avalanche partly buried a succession of raised beaches. Raised beaches out in the Skagi peninsula, just west of the study area, exceeding 65 m a.s.l., have been dated older than 12,000 yr B.P. (Rundgren et al., 1997). Beaches between 43 and 50 m a.s.l. have been dated to 11,300-9,900 yr B.P. and beaches at 22-31 m a.s.l. between 9900 and 9600 yr B.P. Regression below the present sea level occurred at 9,000 yr BP. As the rock avalanche deposit does not display visible evidence of being impacted by the glacio-isostatic rebound, it is believed to be younger than 9,000 yr B.P. - Tephra layers, occurring in a peat bog on top of the rock avalanche material date the avalanche older than 4,500 yr B.P., as the H4 tephra layer is found at 140 cm depth. - 14C dating of birches (Betula sp.) trunk, branch and root pieces found 80 cm deeper than the H4 layer provide 6,070 yr B.C. calibrated age. This period, around 8,000 yr B.P. is known as the "early birch periode" in Iceland, known to have been warm and favouring tree development (Einarsson 1991, Óladóttir et al. 2001, Langdon et al., 2010). Thus, the Höfðahólar rock avalanche event occurred between 9,000 B.P. (youngest beaches) and 8,000 B.P. (14C oldest dating). Such results comfort Jónsson idea, proposing the occurrence of such landslide as the result of a rapid paraglacial crisis following the deglaciation period, due to the combined effects of glacial oversteepening, supply water throughout the lava piles during glacial melting, and glacio-isostatic uplift. |
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1Armelle Decaulne
