Jökull - 01.01.2014, Síða 41

Reviewed research article
Magnetostratigraphy, K-Ar dating and erosion history of the
Hafrafell volcanics, SE–Iceland
Jóhann Helgason1 and Robert A. Duncan2,3
1National Land Survey of Iceland, Stillholti 14–16, 300 Akranes, Iceland
2College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA
3Department of Geology and Geophysics, King Saud University, Riyadh, Saudi Arabia
jhelgason@internet.is, rduncan@coas.oregonstate.edu
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.
INTRODUCTION
Volcanism and glacial erosion are two opposing fac-
tors that have shaped the landscape of Iceland since
the Northern Hemisphere glaciation began. Central
to unravelling this history is detailed mapping and
dating of the stratigraphic record. Due to intense
subglacial volcanism and its northerly location, Ice-
land’s geologic record preserves information about
climate change that can be traced well into the Neo-
gene. Surrounding Öræfajökull stratovolcano in SE
Iceland are Neogene to Quaternary volcanic strata that
provide information about the onset of Earth’s most
recent sequence of glacial and inter-glacial intervals.
The present day landscape is characterized by moun-
tain "islands" located between glaciers and vast pro-
glacial outwash plains. The bedrock consists mainly
of lavas, sediments of various origins and sequences
of subglacially erupted volcanics. Numerous active
volcanoes continue to build positive topography while
outlet glaciers from the Vatnajökull ice sheet actively
erode and dissect the remaining volcanic massifs. In
the Hafrafell mountain area, we trace the erosional
evolution from a time when landscape was charac-
terized by relatively flat lavas until the present state
of deep valleys with abundant subglacial volcanism.
Our study is based on detailed geological mapping, K-
Ar age data, magnetostratigraphy and tracing of ero-
sion surfaces within the stratigraphic sequence. Stud-
JÖKULL No. 64, 2014 41