Jökull - 01.01.2014, Síða 44

J. Helgason and R. Duncan
Figure 2. Hafrafell mountain viewed from west with the peaks of Efri-Menn (center) and Neðri-Menn fur-
ther right. The Hrútfjallstindar peaks are seen behind Efri-Menn and the Öræfajökull volcano to the far right.
Numerous gullies in Illuklettar, coincide with a dike swarm. Prominent dikes are shown in red. Stratigraphic
profiles and some erosion surfaces and rock formations are also colored. Yellow lines near the base of Hafrafell,
in sections HL and HLHM, indicate southward dipping lava lobe units. A blue line in Illuklettar, at the base
of formation HF11, underlines a thick lava flow, unit HP32, that ponded at the onset of interglacial I5. Erosion
horizons HR1 to HR3 are shown with pink and cyan lines. – Hafrafell séð úr vestri, tinda Efri- og Neðri-Manna
ber við himinn. Hrútfjallstindar sjást að baki Hafrafells en lengst til hægri gnæfa tindar eldstöðvarinnar í
Öræfajökli. Fjölmörg gil í Illuklettum í Hafrafelli fara saman við gangasveim. Nokkrir áberandi berggangar
eru merktir með rauðum línum. Jarðlagasnið ásamt upplýsingum um nokkra roffleti (HR) og myndanir (HF) eru
einnig merkt á myndina. Gular línur nálægt neðri mörkum Hafrafells, í sniðum HL og HLHM, marka hrauna-
linsur með suðlægum halla. Blá lína í Illuklettum markar upphaf þykkrar hraunamyndunar HF11 á hlýskeiði
I5, þar sem neðsta lagið, eining HP32, er óvenjuþykk vegna tjarnarmyndunar. Roffletir HR1 til HR3 eru sýndir
með bleikum og ljósbláum línum.
the lavas are within the laumontite zone where deeper
crustal levels are exposed. Dikes in Hafrafell are typ-
ically of basalt composition (Prestvik, 1985).
Lava tilt and faults
Lava flows exposed at the southern part of Hafrafell
dip gently about 1◦ toward N35◦E but further north
dip increases up to 20◦ toward the NW. High local
dip values are associated with rhyolite intrusions ex-
posed on Hafrafell’s northeast side. Onyx banding in
lava vesicles reveals progressive tilting. In one case
within profile HAF (360 m a.s.l.) a tholeiite lava flow
dips 18◦/336◦ but a layered onyx amygdale dips only
7◦/358◦. Presumably, the intrusion of rhyolite caused
the lava flow to dip 7◦ in addition to a previous dip
of 11◦ (Walker, 1974). Two-stage dipping of layers,
revealed in opal and onyx fillings, is not uncommon
within lava flows on the north side of Hafrafell. On
the NE side of Hafrafell (e.g. section HS) lava flows
thicken toward the base of Hrútfjallstindar and in-
crease in dip toward 45◦. Faults are rare in Hafrafell
but a few normal faults with a throw of up to 10 m
were mapped. Faulting is not regarded a process that
has influenced relief to any extent at Hafrafell.
Lava flow direction
Several cases are found in Hafrafell where lobes of
lavas have flowed abruptly down slope. Although ex-
amples of such depositional dip are local they provide
an indicator of past relief. The earliest case of this
kind found in Hafrafell is at the base of section HL
(Figure 6). Here lobes of a basaltic lava flow of forma-
tion HF3 interfinger with an underlying conglomerate
or breccia with dip parameters 32◦/202◦ suggesting a
source slightly east of true north. Higher up in section
HL, within formation HF9, lava lenses dip between
16–30◦/165◦, suggesting a lava source slightly west
of true north. The steep depositional dips were only
noted locally.
44 JÖKULL No. 64, 2014