Chronology of Holocene rock-avalanche deposits based on Schmidt-hammer relative dating and dust stratigraphy in nearby bog deposits, Vora, inner Nordfjord, Norway

Abstract

A number of large rock avalanches from the northern slope of the 1435 m high mountain of Vora in Breim, inner Nordfjord have been mapped. The avalanche tongues dam the 4 km long Sandalsvatnet, they cover an area of 3 km 2 and the volume exceeds 100 million m3. Age estimates have been obtained by radiocarbon datings of rock dust layers in a bog and the Schmidt-hammer method. Altogether 11 large rock avalanches have occurred throughout the Holocene, of which six lobes can be recognized morphologically. Five silt laminae in a 5.5 m deep bog basin distally to the avalanche lobes are interpreted as primary sediments from dust clouds generated by rock avalanches. In addition three silt beds are interpreted as glaciolacustrine sediments related to temporary damming caused by landsliding. Average avalanche frequency during the Holocene was c. 1 per 1000 yr and during the period 7100—3600 BP it was c. 1 per 700 yr. After 3600 BP no rock avalanches have been recognized.