Jökull - 01.01.2014, Blaðsíða 47

Magnetostratigraphy, K-AR dating and erosion history of Hafrafell, SE–Iceland
Figure 5. Broad stratigraphic division of Hafrafell showing groups H1–H8 with erosion surfaces HR1–HR12.
– Gróf skipting jarðlaga í Hafrafelli sýnir yfirmyndanir H1–H8 ásamt rofflötum HR1–HR12.
In upper Hafrafell, marked by erosion surface
HR7 (Figures 2, 3 and 4), are two contrasting se-
quences that occur side by side and extend from about
400 to 800 m a.s.l. They differ drastically in terms of
lithology and magnetic signature. No faults are ob-
served between them. One of them is rather fresh
and not cut by dikes and has normal and reversely
magnetized lavas, whereas the other is altered and
cut by many dikes and has only reversely magnetized
lavas. The younger sequence and less altered banks
up against the other. This seems to us justifiable for
invoking a valley into which lavas accumulated. Thus,
fieldwork has established that here a valley was carved
into the bedrock and one of these sequences, forma-
tions HF20–HF31, represents a filling of an ancient
valley, henceforth referred to as the Hafrafell valley.
The timing and extent of this valley filling is an im-
portant indicator for the erosion history by Hafrafell
and probably elsewhere in Iceland.
Sampling for paleomagnetic measurements
Due to high dike density, especially within the low-
est Hafrafell strata, it is locally difficult to measure
magnetic polarity with a handheld magnetometer. To
reliably establish magnetic polarity directions for the
Hafrafell stratigraphic sequence we therefore sampled
lava flows and pillow basalts to determine their mag-
netic polarity. We drilled a total of 104 volcanic units
from 4 stratigraphic profiles in Hafrafell, namely sec-
tions HL, HP1, U and T. For each unit we drilled
4 samples with a portable two-stroke Pomeroy drill.
Samples with core tube diameter of 2.5 cm and core
length about 5 cm, were oriented in situ.
Rock demagnetization
We demagnetized cored samples at the rock magnet-
ics laboratory at the Institute of Earth Sciences, Uni-
versity of Iceland, using Molspin alternating field de-
magnetization equipment. We measured magnetic di-
JÖKULL No. 64, 2014 47