Magnetostratigraphy, K–Ar dating and erosion history of the Hafrafell volcanics, SE-Iceland

Abstract

Glacial erosion in volcanic terrain just west of the Öræfajökull stratovolcano, SE Iceland, has carved \>2-km-deep valleys. The ca. 2.8-km-thick stratigraphic sequence preserved in the mountain Hafrafell records evolution of landscape relief from relatively flat land at 4 Ma to a deeply dissected valley network today. Through geological mapping, magnetostratigraphy and K-Ar geochronology we establish that the area was first built up by lavas during the Gilbert chron, about 4 Ma. From about the same time we find the earliest evidence of glaciation. A 739-m-thick lava sequence formed, into which glaciers carved a ≥260-m-deep incision, the Hafrafell valley, during Matuyama time, \>2 Ma. The incision was subsequently filled with lava flows during upper Matuyama time, \<2 Ma. Mapping reveals 12 erosion surfaces, HR1–HR12, that formed during the last 4 Myr. The landscape evolution and erosion history of Hafrafell is divided into 6 stages with the first two occurring during the late Neogene, the Gilbert and Gauss magnetic chrons, when lava accumulation was slow and the landscape relatively flat. During stage 3, in lower Matuyama time, lava production increased by a factor of 2. During stage 4 the Hafrafell valley formed in upper Matuyama time. This stage marked clear development of more than 260-m-deep valleys. In stage 5 the Hafrafell valley was filled with subaerial and subglacial volcanic products. Finally, during Brunhes time, in stage 6, intense subglacial volcanism occurred near Hafrafell together with further valley network deepening to some 2-km-depth.