Denudation rates during a postglacial sequence in Northern Iceland: example of Laxárdalur valley in the Skagafjörður area
Résumé
For several decades, geomorphologists have focused on the functioning of geomorphic systems after deglaciation. The relative importance of paraglacial vs. periglacial processes has been highly debated. At present, the development of dating techniques allows to contribute to this debate. We reconstruct in this paper the geomorphic evolution of Tindastóll mountain slopes in Laxárdalur valley (Skagafjörður area, central northern Iceland), where a chronological framework can be established through tephrochronology and an assemblage of dated raised beaches. Volumetric calculations of constructed and excavated landforms were created from field data and from DEM and geographical information system techniques. Collectively, our data exhibit a first stage of paraglacial landsliding during the first half of the Holocene, followed by a stage of scree and rockglacier development (during the second half of the Holocene, but before 1100 AD). Our estimations indicate that more than 85% of the total sediment production were due to rock slope failure, and the rate of bedrock denudation due to periglacial processes was about one half of the rate of paraglacial processes. Nevertheless, paraglacial and periglacial processes cannot be seen here as antagonistic processes: they are organized in a sequence during which periglacial processes are conditioned (enhancement of bedrock denudation rates) by fracturing and consequent mass wasting. Screes and concomitant rockglaciers were indeed preconditioned by the landslide, while areas non-affected by landslides have remained mostly intact, characterized by a very low rate of accumulation due to geomorphic (periglacial) activity.