Jökull - 01.01.2013, Síða 46

J. Helgason and R. Duncan
GLACIAL HISTORY
Glacial–interglacial division
Lava flow units in the Öræfi district that erupted sub-
aerially clearly accumulated during interglacial inter-
vals. Subglacial eruptions typically grade upwards
from pillow lavas, block breccias to hyaloclastite
(Lescinsky and Fink, 2000; Smellie, 2000; Lough-
lin, 2002; Stevenson et al., 2006; Smellie, 2007). In
this study we have identified such volcanic sequences
as glacial. Conglomerates and tillites indicate the
action of significant thicknesses of glacial ice. In
these cases glaciers covered the area and caused ero-
sion, but without addition of volcanic rocks. Such
sedimentary units commonly alternate with hyalo-
clastites in glacial intervals. Lava flow units in the
Öræfi district that erupted subaerially clearly accumu-
lated during interglacial intervals. Subglacial erup-
tions typically grade upwards from pillow lavas, block
breccias to hyaloclastite (Lescinsky and Fink, 2000;
Smellie, 2000; Loughlin, 2002; Stevenson et al.,
2006; Smellie, 2007). In this study we have identi-
fied such volcanic sequences as glacial. Conglomer-
ates and tillites indicate the action of significant thick-
nesses of glacial ice. In these cases glaciers covered
the area and caused erosion, but without addition of
volcanic rocks. Such sedimentary units commonly al-
ternate with hyaloclastites in glacial intervals.
Erosional surfaces SR1 to SR12 in Svínafell
Erosion events in Svínafell are best represented on
surfaces that signal the end of each erosive period.
Table 3. Division of Svínafell’s rock formations into glacial and interglacial stages and their proposed correla-
tion with the geomagnetic polarity time scale. – Skipting jarðlagamyndana Svínafells í jökul- og hlýskeið og
tenging þeirra við segultímatal.
Group Form- Field Strike/ Str.gr. Mg. Mg. Mg. 40Ar–39Ar Inf. ER-surf. Glac.
ation classif. – dip of thickn.- pol. chron age Ma total fusion g-ig. stri. on
lithol. strata (m) age stage ER-surf.
S7 SV37 l, pl.ph 10◦/225◦ 153 N Brunhes <0.781 0.215(±0.012)1 I SR12
S5 SV24– var. 40◦/292◦ ca. 350 N Brunhes <0.781 I+G SR11
SV342 lithol.
S6 SV36 l, th – 12 N Brunhes <0.781 I–8 SR10
SV35 sub – 122 N Brunhes <0.781 G SR9
S4 SV23 sub, th 13◦/183◦ 160 N Brunhes <0.781 0.650(±0.092) G–8 SR8
SV22 l, pl.ph – 25 N Brunhes <0.781 I–7 –
SV21 sub, ba – 25 N Brunhes <0.781 G–7 SR7
SV20 l, ba – 25 N Brunhes <0.781 I–6 –
SV19 l, pl.ph 5◦/158◦ 25 N Brunhes <0.781 I–6 –
SV18 l, th – 40 N Brunhes <0.781 I–6 –
SV17 l, ba 20◦/156◦ 63 N Brunhes <0.781 0.724(±0.033)/ I–6 –
0.643(±0.055)
SV16 s – 18 N Brunhes <0.781 G–6 SR6 186◦
SV15 l, ba – 18 N Brunhes <0.781 I–5 –
SV14 s, sub – 160 N Brunhes <0.781 G–4 SR5
SV13 sub, ba 7◦/135◦ 190 N Brunhes <0.781 G–4 –
SV12 s, sub – 60 N Brunhes <0.781 G–4 SR4
SV11 s – 53 N? Brunhes? <0.781 G–4 –
S4 SV10 s – 80 ? ? 0.7–1.1 I–4 –
S3 SV10 s – 80 ? ? 0.7–1.1 I–4 –
S2 SV9 l, ba – 30 R U-Matuyama 1.072–1.778 I–3+G–3 SR3 213◦
SV8 l, th – 33 R U-Matuyama 1.072–1.778 I–3 –
SV7 l, th – 38 N Olduvai? 1.778–1.945 I–3 –
SV6 l,s – 3 4R L-Matuyama? 1.945–2.581 I + G–2 SR2
S1 SV5 l, th 4◦/120◦ 71 4R L-Matuyama? 1.945–2.581 I–2 –
SV4 s 14◦/75◦3 2 4R L-Matuyama? 1.945–2.581 G–1 SR1
SV3 l, th – 12 4R U-Gilbert? 3.596–4.187 I–1 –
SV2 s – 2 4R U-Gilbert? 3.596–4.187 I–1 –
SV1 l, th 7◦/300◦ 62 4R U-Gilbert? 3.596–4.187 I–1 –
Explanations: l: lava flow; s: sedimentary rock; th: tholeiite; ba: basalt andesite; pl.ph: plagioclase–porphyritic basalt; sub: rock formed under subglacial
conditions; I: interglacial stage; G: glacial stage, N: normal magnetic polarity; R: reverse magnetic polarity; SR1 to SR12: erosional stages/surfaces. 1Age
estimate based on correlation with a dated unit. 2Group S5 consists of formations SV24 to SV34. Group S7 has one formation (SV37) that is younger than
group S5. 3Dip of angular unconformity. 4R Magnetic polarity as measured with a hand–held fluxgate magnetometer in the field.
46 JÖKULL No. 63, 2013