magnetostratigraphy, k-ar dating and …...2012/11/30 · the study is based on detailed geological...

1 Helgason - Duncan: Magnetostratigraphy,, K-Ar dating and erosion history of the Hafrafell volcanics, SE-

Iceland

MAGNETOSTRATIGRAPHY, K-AR DATING AND EROSION

HISTORY OF THE HAFRAFELL VOLCANICS, SE-ICELAND

JÓHANN HELGASON1 and ROBERT A. DUNCAN

2,3

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 and 3Department of Geology and Geophysics, King Saud University, Riyadh, Saudi Arabia

Abstract – Glacial erosion in volcanic terrain by the Öræfajökull stratovolcano, SE-

Iceland, has carved over 2-km-deep valleys. There, in Hafrafell, the 2772-m-thick

stratigraphic sequence indicates evolving relief from relatively flat land to one with a valley

network. Through geological mapping, magnetostratigraphic work and age dating 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 a glaciation. A 739-m-thick lava

sequence formed during lower-Matuyama, into which glaciers carved an over 260-m-deep

depression, the “Hafrafell valley”, during Matuyama, >2 Ma. The depression was filled with

a lava sequence during upper-Matuyama, <2 Ma. Mapping reveals 12 erosion horizons,

HR1-HR12, that formed during the last 4 Ma. For horizons HR2 and HR3 the underlying

lava flow has extensive brecciation. We conclude that the lava flow brecciation associated

with erosion horizons HR2 and HR3 was caused by glaciotectonism, a process that may be a

valuable indicator of Tertiary glacial conditions. As such it may add a tool to a small

category of features that are independently used to define glaciations and may prove

valuable in tracing glacial stages of Tertiary and perhaps younger age in Iceland.

The erosion history of Hafrafell is divided into 5 stages with the first two occurring during

Tertiary, Gilbert and Gauss chrons, when lava accumulates were slow and landscape

relatively flat. During stage 3, lower Matuyama, lava production increases by a factor of 2.

The Hafrafell valley formed during stage 4 in upper-Matuyama time that marked clear

development of more than 260-m-deep valleys. In stage 5, during Brunhes, intense subglacial

volcanism continued with further deepening of the valley network to some 2-km-deepth.

INTRODUCTION

Volcanism and glacial erosion in Iceland are two opposing factors that have shaped the

country since the current ice age began. A prerequisite for tracing Iceland´s long term

landscape evolution is mapping the stratigraphic record. Due to intense subglacial volcanism

and northerly location of Iceland its stratigraphic succession preserves information on climate

change that can be traced well into the Tertiary. Surrounding Öræfajökull stratovolcano in

SE-Iceland there are Tertiary to Quaternary volcanic strata that potentially provide

information on the onset of Earth´s current ice age. In SE-Iceland two factors have for

millions of years radically influenced the stratigraphic succession, namely intensive

volcanism and glacial erosion to an extraordinary degree. The present day landscape is


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characterized by mountain “islands” located in-between glaciers and vast pro-glacial outwash

plains. The strata consist mainly of lavas, sediments of various origin and sequences of

subglacially erupted volcanics. Up to the present numerous active volcanoes provide positive

build-up of strata while simultaneously outlet glaciers from the Vatnajökull ice sheet actively

erode and dissect the remaining volcanic massifs. In the present a study of the Hafrafell

mountain, we trace erosional evolution of this area from a time when it was characterized by

relatively flat lavas until the present state of deep valleys with abundant subglacial volcanism.

The study is based on detailed geological mapping, K-Ar age data, magnetostratigraphy and

tracing of erosion horizons within the stratigraphic sequence. Studies from different sites in

Iceland have shown the first glaciations of the present ice age to extend back at least to 3-4

million years (Eiríksson and Geirsdóttir, 1996; Helgason and Duncan, 2001; Eiríksson, 2008;

Geirsdóttir et al., 2007). Presumably, most glacial events found in different parts of Iceland

were coeval. However, refined correlation across the country awaits more detailed work of

the kind presented here. Detailed work on stratigraphy, age dating and magnetic signature in

the Öræfi district (Helgason and Duncan, 2001; Helgason, 2007) may provide information

not accessible elsewhere in Iceland due to deep erosion and accumulation style caused by

volcanism in an off-axis volcanic zone.

Geology of Hafrafell

The Hafrafell research area is within the Öræfajökull Volcanic Zone (ÖVZ) that exends

between the volcanic centers of Öræfajökull in the south and Snæfell, 100 km further north

(Figure 1). In this volcanic chain, southeast of the accreting neovolcanic rift zone in NE

Iceland, crustal accretion is regarded minimal. Erosion, however, has exposed vast intrusions,

both sheet swarms and major intrusive bodies (e.g. Walker, 1974). Hafrafell, that lies some 6

km west of the Öræfajökull stratovolcano, with Skaftafellsjökull banking on its west side and

Svínafellsjökull on the east side. Immediately north of Hafrafell, is the Hrútsfjöll volcanic

massif with a highest peak of about 1875 m. Hrútfjöll volcanics formed during the Brunhes

magnetic chron (< 0.781 Ma) and are partly superimposed on the Hafrafell north end.


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Figure 1. Geological setting, Hafrafell research area in relation to Öræfajökull volcano,

neighboring volcanic centers and fissure swarms.

Through reconnaissance work Prestvik (1979) suggested a broad division of Hafrafell into 6

units. The lowest being “basaltic lava flows with fine grained layers of sediments in-between.

This unit, where alteration is most conspicuous, is frequently cut by basaltic dikes”. Above he

defined two tillite beds and three units of “hyalaoclastites and basaltic lava flows” the top one

separated by an unconformity from “hyaloclastites and basaltic lava flows” below. Prestvik

regarded the strata generally as basaltic but mentions a “silicic massif” higher up at Efri-

Menn. Part of Prestvik´s geochemical work on Öræfajökull volcano extended to Hafrafell

where he analyzed at least 5 dykes and lavas that he defined as tholeiite or dacite (Prestvik,

1985). On a geological map of SE-Iceland, scale 1:250.000, Torfason (1985) presented a

broad geological division of the Öræfi district where he regarded all of Hafrafell´s lower

strata older than 3.1 M yr, which then defined the Tertiary upper boundary.

FIELD MAPPING OF STRATIGRAPHIC UNITS


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George Walker (1959) established a field classification scheme for the Tertiary lavas

of Eastern Iceland that was used during the mapping process by Hafrafell (figures 2 and 3).

The basalt lavas are defined either as tholeitie, porphyritic or olivine basalt. It soon became

apparent, however, that the stratigraphic record kept evidence of frequent glacial-interglacial

transitions with great volumes of subglacially formed strata. Thus, the area´s geology differs

from a typical “Tertiary” lava terrain, as noted in Eastern and Western Iceland, in that

subglacially erupted rocks, hyaloclastite horizons and evidence of erosion are pronounced.

The highly diversified lithology in Hafrafell led us therefore to gather numerous stratigraphic

profiles into the cliff section. Based on correlations between profiles we divided strata into 39

lithologic formations (HF1-HF39) that are presented on figure 4. Ultimately these formations

were gathered into seven groups, H1 to H7 (figure 5). To constrain the stratigrapic evolution

suggested by the lithology we sampled a pilot section for paleomagnetic signature and

sampled key units for K-Ar age dating.

Figure 2: Hafrafell mountain viewed from west with the peak of Efri-Menn in the center and

the Neðri-Menn peaks further right. Hrútfjöll mountains are seen in the background center

with Öræfajökull volcano at the far right. Dyke swarm cuts numerous gullies through

Hafrafell at Illuklettar. Skaftafellsjökull and Svínafellsjökull extend sideways into the lower

area.


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Figure 3. Detailed geological division of Hafrafell with formations (HF1-HF39) and groups

(H1 to H8).


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Figure 4. Correlation of stratigraphic profiles in Hafrafell. Formation name, HF1 to HF39,

appears by the side of each column, and erosion surfaces, HR1 to HR12 are indicated with red

lines connecting through columns. Profile name, e.g., HL, HS or HAF, is provided at the base

of each stratigraphic column. Age determinations appear next to dated units. Lava units

drilled for paleomagnetic work are indicated with magnetic signature.

Sedimentary horizons HR1 to HR12. Intercalated between basalt lavas are up to 30-m-thick

sedimentary units, many of which suggest subglacial volcanism in the region. The

stratigraphically oldest unit of this nature is found by the quarry at Hafrafell´s south side

(figure 5), referred to as HR1.

Figure 5. On the lower insert to the right the sedimentary horizon is divided into 6 units, A to

F, whereas units A and B are shown on the left. The lower unit (A) is a breccia with

subrounded blocks embedded in hyaloclastite matrix. Upper unit (B) is a conglomerate with a

softer matrix of sand and silt. The upper insert shows unit D, a conglomerate or tillite. The

sequence begins at about 125 m a.s.l.


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On erosion surface HR1 a 12.5-m-thick sedimentary sequence is intercalated between two

tholeiite lava flows of formations HF1 and HF3 (figure 4, profile HP1). Upward the sequence

is divided into 6 units with units A and B shown on figure 5.

HR2. This horizon is stratigaphically positioned at the Matuyama/Gauss boundary.

Dykes and alteration. Numerous near vertical dykes of basalt composition cut through

Hafrafell and over a stretch of some 1600 m dykes amounted to 234 m or 14.7% at 120 m

a.s.l. Mean dyke thickness is 2.5 m (sd: 2.5 m) for 94 dykes. Dyke strike is N70°E (sd: 22°).

In addition a few near horizontal dyke sheets are observed at the south end of Hafrafell.

Dykes, sheets and rhyolite intrusions increase generally toward north where deeper crustal

levels are exposed. The south part of Hafrafell lies within the mesolite-scolecite zeolite zone

but some 2 km further north the lavas are within the laumontite zone.

Lava tilt and faults. The Hafrafell south part has only gentle dip of about 1° toward 35° but

further north dip increases up to 20° toward NW. High local dip values are associated with

rhyolite intrusions exposed on Hafrafell´s northeast side. Onyxes in lava vesicles reveal the

tilt process. In one case within profile HAF (360 m a.s.l.) a tholeiite lava flow dips 18°/336°

but a layered onyx filling within it dips only 7°/358°. Presumably, the intrusion caused the

lava to dip 7° in addition to a previous dip of 11°. Two-stage dipping of layers, revealed in

opal and onyx fillings, is not uncommon within Hafrafell north side lavas. Here the dip has

varied by a few degrees during formation of layers in the vesicle fillings. On Hafrafell´s NE

side (e.g. section HS) lava formations thicken toward the base of Hrútsfjöll with some

increase of 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 by Hafrafell.

Lava flow direction. Several cases are found in Hafrafell where lenses of lavas have flowed

abruptly down an increased slope. Although examples 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 lenses of a basalt flow of formation 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, another case is noted, within formation

HF9, where dip of lava lenses between 16-30°/165° are observed that suggest lava source


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slightly west of true north.

Figure 6. Lavas on top of the lowest sedimentary horizon have lenticular units that interfinger

with underlying conglomerate/breccias.

Table 1. Division of Hafrafell´s rock formations into glacial and interglacial stages and their

proposed correlation with the geomagnetic polarity time scale.

Group

For- mation

Field classification - lithology

Strike/dip of strata

Thick- ness (m)

Group thickness (m)

Polarity direction

Magnetic event

Magnetic chron

Magnetic age estimate, M yr

40Ar-

39Ar

total fusion age, M yr

Glacial / inter-glacial

Erosion horizon

Comment

Ref. Units.

H8 HF39 l, pl.ph 105 105 N Bruhnes C1n <0,781 0,210±0,010 I12 HR12

Dated sample: HV.

HF14-HF33

H7

HF38 s, pebc >130

995

N Bruhnes C1n <0,781 G - -

HF37 sub, band

>160 N Bruhnes C1n <0,781 G - -

HF36 sub, band

>190 N Bruhnes C1n <0,781 G - -

HF35 l, th

>120 N Bruhnes C1n <0,781 I -

Dips toward north. -

HF34 sub 180 N Bruhnes C1n <0,781 G - -

HF33 sub

>140 N Bruhnes C1n <0,781 G - -

HF32 s, till 75 - Bruhnes C1n <0,781 G HR11 Y9-Y11

H6

HF31 sub, th 55

448

R? Matuyama C1r.2r

1,072-1,778 G11 HR10 T23

HF30 l, pl.ph 5 ? Matuyama C1r.2r

1,072-1,778 I11 - HJ18

HF29 s, bcon 20 ? Matuyama C1r.2r

1,072-1,778 G10 HR9

Eroson surface below.

HJ18/HJ17


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HF28 l, pl.ph 65 R? Matuyama C1r.2r

1,072-1,778 I10 -

HJ10-HJ17

HF27 sub, pl.ph 74 R

Matuya

ma C1r.2r

1,072-

1,778

1,690±0,2

90 G9 -

Dated sample:

T22. T22

HF26 l, pl.ph 34 R Matuyama C1r.2r

1,072-1,778 I9 - T19-T21

HF25 l, olb 21 N Olduvai? C2n

1,778-1,945 I9 - T16-T18

HF24 l, th 24 R Matuyama C2r

1,945-2,581 I9 - T13-T15

HF23 l, th 25 N Matuyama C2n

1,945-2,581 I9 - T6-T12

HF22 l, th 60 R Matuyama C2r

1,945-2,581 I9 HR8 T1-T5

HF21 sub, pl.ph 30 N? Matuyama ??

1,945-2,581?? G8 - HJ3

HF20 l, band 35 N? Matuyama ??

1,945-2,581?? I8 HR7 HJ1-HJ2

H5

HF19 l, th >6

739

R? Matuyama C2r

1,945-2,581 I8 -

HS86-HS87

HF18 s, bcon 20 - Matuyama C2r

1,945-2,581 G7 HR6

HS86/HS85

HF17 l, th 90 R? Matuyama C2r

1,945-2,581 I7 -

HS64-HS85

HF16 s, bcon 24 - Matuyama C2r

1,945-2,581 G6 HR5

HS64/HS63

HF15 l, th 194 R? Matuyama C2r

1,945-2,581 I6 -

HS33-HS63

HF14 l, band 51 R? Matuyama C2r

1,945-2,581 I6 -

HS29-HS32

HF13 l, th

>100 R

Matuyama C2r

1,945-2,581 I6 -

HS1-HS28

HF12 s, bcon + l, pl.ph 8 R(T)

Matuyama C2r

1,945-2,581 G5 HR4

U60K/U60I

HF11 l, th

>246 R

Matuyama C2r

1,945-2,581 I5 - U28-U71

H4 HF10 sub, th 90 90 R Matuyama C2r

1,945-2,581

2,35±0,220 G4 HR3

Dated sample: HZ. HZ

H3 HF9 l, th+band 78 78 N Gauss C2An 2,581-3,596 I4 -

HP2:14A-29

H2

HF8 s + l, th 37

156

N Gauss C2An 2,581-3,596 3,2±0,090 G3? -

Dated sample: HL27.

HL27/HL26

HF7 l, pl.ph 88 N Gauss C2An 2,581-3,596 I3 - U5-U13

HF6 sub, pl.ph 31 N Gauss C2An 2,581-3,596 G2 HR2

HM4-HM5

H1

HF5 l, olb + pl.ph 20

161

R Gilbert C2Ar 3,596-4,187 I2 -

HL18-HL25

HF4 l, th 90 R Gilbert C2Ar 3,596-4,187 I2 -

HL6-HL17

HF3 l, th + pl.ph 35 R Gilbert C2Ar 3,596-4,187 I2 - HL2-HL5

HF2 s, bcon+br 11 - Gilbert C2Ar 3,596-4,187 G1 HR1 HF1/HL0

HF1 l, th 5 N or R(T)?

Cochiti?? C3n.1n

4,187-4,300

3,94±0,060 I1 -

Dated sample: HL1. Weak N or transitional R

HL1 or HFX

Explanations: l: lava flow; s: sedimentary rock; th: tholeiite; band: basalt andesite; pl.ph:

plagioclase– porphyritic basalt; sub: rock formed under subglacial conditions; till: tillite;

pebc: pebble conglomerate; bcon: boulder conglomerate; br: breccia; I: interglacial stage; G:

glacial stage, N: normal magnetic polarity; R: reverse magnetic polarity; HR1 to HR12:


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erosional stages/surfaces. * Age estimate based on correlation with a dated unit. ** Dip of

angular unconformity. ****R Magnetic polarity as measured with a handheld fluxgate

magnetometer in the field.

PALEOMAGNETISM

High dike density, especially within the lowest Hafrafell strata, makes it difficult to measure

magnetic polarity with a handheld magnetometer. To reliably establish magnetic polarity

directions for the Hafrafell stratigraphic sequence we therefore sampled a total of 104

volcanic units, lavas or pillow basalts. The sole purpose of collecting samples for

paleomagnetic measurements was to determine whether volcanic units are normally or

reversely magnetized. Thus, we presumed that the combined data of radiogenic age and

paleomagnetic polarity would form a basis for correlating Hafrafell´s strata with the

geomagnetic time scale.

Sampling for paleomagnetic measurements

We drilled a total of 104 volcanic units from 4 stratigraphic locations in Hafrafell, namely

sections 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. Samples were demagnetized at Dr. Leó Kristjánsson´s laboratory at the

Geophysics Division, Science Institue, University of Iceland, using. After peak field strength

of respectively 0, 10, 15 and 20 mT samples were measured using Molspin AF

demagnetization equipment. Results were corrected for lava tilt of 1°/35°.

Paleomagnetic results

The magnetic polarity signature for sections HL, HP1, U and T, together with information on

polarity direction and VGP values is presented in Appendix 1. Magnetic polarity results have

been inserted on the stratigraphic diagram for Hafrafell, figure 4. The results will be

discussed below together with results of age dating and stratigraphy under: “Stratigraphic

correlation with the geomagnetic time scale”.

AGE DETERMINATIONS


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Although the lowest exposed strata in Hafrafell are heavily cut by dykes and within the

meoslite-scolesite zeolite zone we were able to find flow units portions with low vesicle

content and surprisingly fresh interior. In the upper sequence, lower grade zeolites are still

abundant, but there also we collected relatively fresh samples from units ranging in

stratigraphic age from oldest to youngest strata.

K-Ar Analytical Methods

We prepared whole rocks for age determinations by crushing, sieving to obtain the 0.1-0.5

mm size fraction, ultrasonic washing in distilled water, drying and hand-picking under a

binocular microscope to obtain the freshest, phenocryst-free samples possible. From 5 units

with wide stratigraphic distribution, we loaded approximately 500 mg of each sample in

quartz vials, evacuated and sealed them, then irradiated these for 6 hr near the central core of

the 1 MW Oregon State University TRIGA reactor, along with neutron flux gradient monitor

FCT-3 biotite (28.03 Ma; Renne et al., 1998). Samples and monitors were analyzed in single-

step, total fusion experiments using an AEI MS-10 mass spectrometer. All gases were

extracted by radio-frequency heating of samples in Mo-crucibles under vacuum, followed by

removal of active gases via hot metal getters. We made corrections for mass discrimination

based on frequent measurements of atmospheric Ar from an on-line reservoir, and for

interfering isotopes produced during irradiation (Wijbrans et al., 1995).

Results of Age Determinations

Presented in table 2 are the results of age determinations.

Table 2. K-Ar ages for Hafrafell strata.

Sample

no.

Coordinates Strati-

graphic

height (m)

Total

fusion age

± 2s (Ma)

%K %Ar rad. Elevation

m a.s.l.

Stratigraphic

formation

HL1

N64°0114

W16°8802 5 3.93±0.06 0.411 52.1 132 Form. HF1

HL27

N64°01187

W16°8741 298 3.20±0.09 0.299 12.5 340 Form. HF8

HZ

N64°01320

W16°8688 478 2.35±0.22 0.232 1.2 435 Form. HF14

T22

N64°0213

W16°8664 1496 1.69±0.29 0.158 2.8 616 Form. HF31

HV

N64°0091

W16°8747 2720

0.215±

0.012 0.540 9.0 280

Form. HF39

Ages calculated with the following decay and abundance constants: = 0.580 x 10-10

yr-1

;

= 5.530 x 10-10

yr-1

; 40

K/K = 1.17 x 10-4

.


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STRATIGRAPHIC CORRELATION WITH THE GEOMAGNETIC TIME SCALE

Now we add results from dating and paleomagnetism to Hafrafell´s stratigraphic framework

to correlate it with the geomagnetic time scale (Gradstein et al., 2004). The suggested

correlation, shown on figure 7, is based on data from profiles HL, HP1, HP3, U and T on

figure 4 and table 2. On figure 4 the K-Ar age value of each dated unit is shown at its

position. For the lower strata three main magnetic polarity intervals are upwards from

R1 N1 R2 (figure 7).

Figure 7. Correlation of the Hafrafell magnetostratigraphy with the geomagnetic time scale

(Gradstein et al., 2004).


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Close to its base the 829-m-thick R2-sequence has been dated 2.35±0.220 Ma (unit HZ)

which places the R2-lavas age as lower-Matuyama (C2r) or in the interval 1.945-2.581 M yr.

The relatively thick N1-lava sequence below, i.e. 234-m-thick, which center was dated at

3.20±0.220 Ma, is correlated with the Gauss interval (C2An): 2.581-3.596 M yr. The lowest

strata at the south tip of Hafrafell have a date of 3.93±0.06 Ma, an age corresponding to

Gilbert near the Cochiti normal chron. The very lowest lavas sampled for paleomagnetic

signature at the south end of Hafrafell (units HP1-1 and HLX) differ from the lavas above in

that one is normal while the other is R-transitional (snown as Nt on figure 7). This suggests

that the lavas at the base of Hafrafell may be entering a normal magnetic period which is in

agreement with the age date there and would likely be the Cochiti normal (C3n.1n; 4.187-

4.300 M yr).

Higher up the section, in the Hafrafell valley filling, lavas show short magnetic events. Here

the transitions are R2 N2 R3 N3 R4. It is clear that this sequence must be older than

Brunhes but presumably not far below the Brunhes/Matuyama boundary. A unit from within

R4 was dated at 1.69±0.29 Ma suggesting that the short N2 and N3 intervals correlate with

the Olduvai chron (1,778-1,945 M yr). Within the section carapace all units are normally

magnetized (N4), e.g. groups H7 and H8, and clearly of Brunhes age or younger than 781

kyr. The age date for lavas of formation HF39 (group H8), 215± 12 ka, allows its correlation

with the third last interglacial (Mindel-Riss for the Alps; Holsteinian for N-Europe) and MIS

6 at about 190 ka.

EROSION HISTORY: NATURE AND EXTENT

Lava surface brecciation on erosion surfaces HR2 and HR3 in Hafrafell: evidence of

glaciotectonism

A total of 12 erosion surfaces are defined for the Hafrafell section: HR1 to HR12. Of

particular interest are erosion surfaces HR2 and HR3 within the lower strata, i.e. at the base

and top of the Gauss chron, respectively. These surfaces mark a transition from non-glaciated

to glaciated environment that is reflected in a lithological upward change from lava to

sedimentary rock. Here, the lava flow upper surface is brecciated. Horizons HR2 and HR3

are also of interest as they represent early glaciations in Iceland during the Gauss chron.

HR2. This horizon is well exposed near the base of profile U (figure 8a). Shown on figure 8,

in the upper right corner, is the transition from fractured lava upwards into reddish-brown


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palagonite breccia. In total the brecciation extends some 4 meters below the sedimentary

horizon where the lava flow, with a total thickness of 4.5 m, is divided into units, U3 and U4.

At the boundary a near horizontal, 15-cm-thick brown breccia, separates units U3 and U4,

shown on the lower left. The lower unit, U3, 1.5-m-thick, is considerably less fractured

whereas unit U4 above, 3-m-thick, is highly fractured or brecciated. Upwards the brecciated

lava part grades into loose fragments with brown-gray matrix between clasts.

Figure 8a. Erosion horizon HR2 in gully U. Scale is hammer near photo center (30 cm). The

dark near horizontal line in the lower left part marks the U3/U2 boundary.

HR3. The brecciation on surface HR3 (figure 8b) is variable but grading is commonly present

from the solid pristine flow interior. There, up to 1-m-long horizontal fractures, are observed,

some 50 to 80 cm below the upper flow boundary. The fractures are 5 to 20 mm wide with

light brown fillings of silt and sand sized material. Closer to the upper surface brecciation

continues and transforms the flow into blocky agglomerate as shown on figure 8b for surface

HR3. The grading continues upward still into sub-rounded sedimentary clasts and eventually

hyaloclastite breccias or pebble conglomerate.


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Figure 8b. The lower boundary of erosion horizon HR3 (profile U) with gradational change

within a lava flow from compact massive interior to brecciated upper surface. Between

blocks is fine grained hyaloclastite of sand and silt size. Upwards blocks become smaller and

clast rounding increases together with content of hyaloclastite matrix up to 50%.

The Gauss strata or the Tertiary-Quaternary transition within Hafrafell

At the end of Gauss, approximately 2.581 Ma, a thick sequence of hyaloclastite is

intercalated between lavas in Hafrafell. Erosion surfaces, HR2 and HR3, occur at the base

and top of the Gauss strata, respectively. It is clear from figure 9 that the strata from the

Gauss


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Figure 9. Stratigraphic profiles from north (left) to south (right) extending upwards from

Gilbert through Gauss to Matuyama chrons. Explanations are same as for figure 4.

normal show a distinct north-south variation. On the north side the sequence begins with

highly porphyritic lavas (section U) that grade up dip into pillow basalts some 400 m further

south (section HM). Then the next strata higher up the section are hyaloclastite sediments.

Here, tholeiite N-lavas (Gauss), inter-finger with the sediment suggesting interglacial

conditions although glaciers are probably further inland at this time. Finally, a 70-m-thick

sequence of tholeiite N-lavas blankets the sediments, being thickest at the south tip of

Hafrafell. However, there, extreme dips toward south are noted. Overall, the build-up of

strata during Gauss is in the form of lenses that are progressively added at the southern end of

Hafrafell.

Stratigraphic division: groups H1 to H8

In order to trace Hafrafell´s erosion history and landscape development we have combined

the 39 mapped rock formations into 8 groups (H1 to H8) from oldest to youngest. The groups


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are presented on figure 10 together with the 12 erosion surfaces, HR1-HR12. To better realize

the landscape evolution over nearly 4 million years we provide a 3D view of Hafrafell on

figure 11. Noteworthy is the Matuyama interval, namely groups 5 and 6. Here strata of group

6 have filled up a depression that had been carved into group 5. We refer to this, at least 260-

m-deep, depression as the “Hafrafell valley”. The walls hosting the valley consist of an at

least 739-m-thick lava sequence that formed during a relatively short interval, 1.945-2.581

Ma, or 0.64 M yr. The timing of the valley formation can be narrowed down to being older

than Olduvai, as the filling has R-lavas that predate the Olduvai subchron. An age of 2 M yr

would therefore be reasonable for the valley.

Figure 10. Broad stratigraphic division of Hafrafell showing groups H1-H8 with erosion surfaces

HR1-HR12.


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Figure 11. Relief view of Hafrafell with groups H1 to H8. The light-brown “Hafrafell valley

filling” (brown) rests unconformably on the west slope, deeply carved into a thick sequence

of lower-Matuyama age (yellow).

Main erosion stages in Hafrafell

With regard to the established stratigraphic framework above we draw together the evolution

though time of erosion in the Hafrafell area in five stages (table 3).

Table 3. Main characteristics of 5 erosion stages in Hafrafell´s landscape evolution.

Erosion

stage

Erosional

horizons

Period Estimated

age (Ma)

Formation

interval

Thickness

of volcanic

strata (m)

Volcanism Estimated

depth of local

erosion (m)

1st HR1

Tertiary

Gilbert

4.187-

3.596 HF1-HF8 161 Contniuous < 50?

2nd HR2

Tertiary

Gauss

2.581-

3.596 HF6-HF9 234 Continuous < 100?

3rd HR3-HR6

Quaternary

Lower-

Matuyama

2.581-

1.945 HF10-HF19 829

Massive

lava

production

Lava

accumulation >

erosion?

4th

HR7-

HR10

Quaternary

Upper-

Matuyama

1.945-

0.781 HF20-HF31 448

„The

Hafrafell

valley“ > 260

5th

HR11-

HR12

Quaternary

Brunhes < 0.781 HF32-HF39 1100

Major

subglacial

volcanism > 1000


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Stage 1. As strata in Hafrafell that predate the Gauss chron (> ca. 3.6 Ma) consist almost

entirely of lavas we assume that erosion was minor (< 50 m) rivers were the dominant erosive

agent.

Stage 2. Glacial remains, i.e. volcanics, date from the Gauss chron (ca. 2.6-3.6 Ma). Relief in

Hafrafell is now controlled by the sub-ice environment.

Stage 3. Volcanism during lower-Matuyama (2.581-1.945 Ma) produced an over 800-m-thick

sequence of lavas, intercalated with a few erosion horizons. Thus, locally, this extensive

volcanism by Hafrafell is likely to have evened out most landscape undulations in relatively

short time, or less than about 0.64 M yr.

Stage 4. During Upper-Matuyama or from about the Olduvai chron to almost the onset of

Brunhes (ca. 1.95-0.781 Ma) lavas were carved by glacial erosion down by more than 260 m,

thus forming the “Hafrafell valley”.

Stage 5. Volcanism from the Hrútsfjöll volcanic center during Brunhes was extensive by

Svínafell as well as erosion and valleys continued to deepen by at least 1000 m.

DISCUSSION

Landscape evolution of SE-Iceland

In considering the topographic evolution of eastern Iceland Walker (1982) states: “The

contrast between the Austfirðir and inland plateau is attributed to a departure about 5 m.y.

ago from steady state conditions, caused by a significant southward migration of the locus of

volcanism.” Thus, Walker does not mention the influence caused by glaciers on landscape

evolution that we claim is the main factor governing topography during roughly the last 3.5

M yr.

In discussing the Tertiary landscape of SE-Iceland we assume that until about 3 Ma, glaciers

had not set their mark on the region, neither in the form of a valley network nor formed the

deep depression now present below the Vatnajökull ice sheet. Keeping in mind that SE-

Iceland is regarded an off-axis volcanic regime it follows that subsidence of the volcanic

products there is much smaller than in the active accreting rift zones to the west and north.

We speculate that the major intrusive process and lack of crustal subsidence is likely to

generate local volcanoes that may be reach high elevation above the surrounding area and

actually higher than comparable volcanic centers of the active rift zones. Also, the formation

of inclined sheet swarms and major magma intrusion in SE-Iceland has added to the crustal


Iceland

build-up (Walker, 1975). There, sheet commonly amount to over 50% of the stratigraphic

sequence. Referring to the net movement caused by inclined sheets Torfason, (1979, p. 324)

states: “the sheets and the major intrusions are the major cause of the extensive uplift of

south-eastern Iceland, which is more than anywhere in Iceland.” These factors likely caused

the land to maintain relatively high elevation above sea level. Erosion, on the other hand,

counteracts the positive build-up caused by volcanism and intrusions. The time averaged

effects of these processes may have influenced elevation and relief of SE-Iceland. As the

factors governing SE-Iceland´s elevation are very different from the situation in the accreting

volcanic rift zone the question arises: “how high was the land in the accreting rift zone

compared to SE-Iceland when an ice sheet first formed in Iceland?” Specifically, this may be

difficult to estimate. Relatively, however, it may be argued that during the Tertiary, prior to

major glaciations, the SE-Iceland region was more elevated than the rest of Iceland as a result

of the off-axis volcanic process. This agrees with Geirsdóttir (2011) who argues that an ice

sheet first formed in SE-Iceland and spread from there outward to the north and west.

A flexure zone was already present in SE-Iceland in the late Tertiary (e.g. Torfason, 1978;

Klausen, 1999). This large scale feature may possibly have contributed to the regional

topography but we believe more detailed work is needed to conclusively predict its impact on

Tertiary landscape evolution.

The brecciation process associated with erosion horizons HR2 and HR3. Although

considerable time generally elapses between depositions of any two overlapping lavas, say

100 to a few thousand years, it is noteworthy that the brecciation of flow surfaces as seen by

Hafrafell is clearly a rare event. This feature is only observed on lava surfaces coinciding

with glacial erosion and the extent to which the brecciation has gone, suggests that this is not

a periglacial feature but rather a sub-ice phenomenon cause by glaciotectonism. By definition

“glaciotectonic structures include all deformations created in rock or sediment of the Earth's

crust as a consequence of glacier loading, dragging, or pushing” (James S. Aber and Andrzei

Ber, 2007). Most commonly, glaciotectonic features are described from pro-glacial terrains as

formed on the edge of flowing glaciers and such examples from Iceland include mechanism

affecting moraines (e.g. Benediktsson et al., 2010). A second scenario for glaciotectonism is

reported by Bennett et al., (2009) who describe deformation structures along the base of a

subglacially formed ridge, i.e. “large diapiric folds of diamict and gravel that appear to

represent the deformation of a pre-eruptive glaciolacustrine fan beneath a volcanic pile, either


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during, or following the eruption”. We are, however, not aware of glaciotectonic fracturing

below a thick ice sheet, affecting lava flows as in the case by Hafrafell. With reference to the

brecciation descriptions presented for horizons HR1 to HR3 we assume that these qualify

under the definition of glaciotectonic breccia. We regard the brecciation at surfaces HR1 to

HR3 as caused by the pressure of an overlaying glacier. The hyaloclastite breccias associated

with these horizons suggests subglacial volcanism nearby. This is evident for erosion horizon

HR3 that actually merges with subglacially erupted formation HF10 close by (figure 4).

Brecciation as described above has not been found for other lavas in Hafrafell and appears to

be confined to erosion horizons. It is of importance in a general sense that as the brecciation

is formed at the base of a glacier and coincides with erosion surfaces it provides an

independent criterion for determining glacial versus interglacial condition at the time of

formation and is therefore a climatic indicator equal to tillites, subglacially erupted volcanic

sequences or glacial striations.

Gauss strata in Hafrafell compared to Borgarfjörður, West-Iceland

Most magnetostratigraphic studies on Gauss age rocks (2.582-3.596 Ma) in Iceland have been

conducted on sequences that accumulated within accreting rift zones. Models of crustal

accretion, such as that of Pálmason (1980), assume symmetrical build-up across the rift zones

with major subsidence and burial at the rift axis. Away from the zone strata eventually

resurface due to erosion. Here, however, the more voluminous lavas, or those that flowed to

the rift zone edge, are preferably exposed. Consequently, the exposed sections should be

distal segments of the eruptive sequence. It is therefore interesting to compare results of the

present study, from an “off-axis” region, with results for a sequence from an accreting rift

zone. An excellent study of a rift zone sequence is from the Borgarfjörður area, west Iceland

(McDougall et al., 1977). The Borgarfjörður sequence is unique in its thorough recording of

magnetic reversals and it was in this study that the Þverá and Síðufjall subchrons were added

to the geomagnetic time scale. Both studies include the entire Gauss sequence that is about

727-m-thick in Borgarfjörður, with 115 lava flows, included 7 glacial horizons and consisted

of lavas and sediments but no pillow basalt units (McDougall et al., 1977; Geirsdóttir, 2011).

In Hafrafell, on the other hand, Gauss age strata have a total thickness of only 234 m that are

subdivided roughly into lavas: 169 m (26 flows); sediments: 34 m, and pillow basalts: 31 m,

with just two clear examples of glaciations.


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Interpretation/discussion. What has caused the great difference in Gauss-stratigraphy between

these two regions? At least three factors could contribute to the observed difference, namely:

a) Widely different accumulation rates (or volcanic production). The comparison

shows that the magnetostratigraphy in Borgarfjörður is more complete in containing a

number of subchrons that are not present in Hafrafell, namely the reverse Kaena and

Mammoth subchrons within Gauss (in addition to the normal Jaramillo and Reunion

subchrons of the Matuyama chron higher up the section). This would suggest that

even if intervals of intense erosion had occurred in Hafrafell, some remains of the

subchron strata would still be present as erosion tends to be heterogeneous in valleys

and thus leaving behind the valley walls. As this is not the case it is reasonable to

conclude that the three times thicker Gauss section in Borgarfjörður owes its thickness

to some extent to higher accumulation rates in Borgarfjörður.

b) Greater erosion in Hafrafell than Borgarfjörður. 7 glacial intervals observed for

the Borgarfjörður Gauss strata compared to only 2 for Hafrafell might indicate

considerably greater erosion in Borgarfjörður. Paradoxically, however, it can be

assumed that the SE-Iceland region was centrally located with regard to the glacial ice

sheet thus causing greater erosion there compared to the Borgarfjörður area. The

fewer glacials observed in Hafrafell could partly be caused by higher erosion there.

c) Different stratigraphic build-up as a result of processes in an “accreting rift”

versus “off-axis” environment. The strata in Hafrafell consist of small lenses that

are of much shorter dimensions, both in a vertical and horizontal sense compared with

Borgarförður (figure 9). Based on figure 9 there is a strong indication that strata in

Hafrafell were built up by progressive stacking toward south during Gauss. Clearly,

considerable relief is present in Hafrafell, while nothing comparable is observed for

the Borgarfjörður section.

Our conclusion regarding the above three factors is that accumulation rates or volcanic

production was indeed much higher during Gauss in Borgarfjörður than in Hafrafell. The

distance of Borgarfjörður from the main ice shield favoured stratigaphic “recording” there

compared to Hafrafell because subsidence and burial within the accreting rift in

Borgarfjörður was a way for the strata to “escape” erosion, an option not available for


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Hafrafell strata. Most likely, a large portion of the magmatic ascent/production in SE-Iceland

during Gauss (and other intervals as well) was in the form of intrusions whereas in

Borgarfjörður the ascent reached the surface and produced lavas.

Both Gauss sequences in Hafrafell and Borgarfjörður have glacial strata close to base or close

to 3.596 Ma. Keeping in mind that 7 glacials are found for the Borgarfjörður sequence but

only two (or possibly three) for Hafrafell it is likely that an ice sheet covered both areas as

early as or close to 3.596 Ma. This finding suggests that at this time the ice sheet was not just

confined to SE-Iceland but probably covered the Borgarjörður region as well. Two factors

may contribute to the explanation as to why so few glacials are recorded in Hafrafell during

the Gauss chron, i.e. much greater erosion in the Hafrafell area as well as slower

accumulation rates.

Erosion character of the Hafrafell stratigraphic sequence

Although erosion horizons are common in Hafrafell the stratigraphic sequence is substantial

or a total of 2772-m-thick. The 5 erosion stages derived for Hafrafell (table 3) indicate that

for the first two stages (late Tertiary) the stratigraphic sequences are “thin” whereas during

stage 3 the sequence is “thick” and clearly with massive lava production. It is tempting to

conclude that erosion during stages 1 and 2 generated relief or depression into which lavas of

stage 3 accumulated during lower-Matuyama. The accumulation during stage 4, when the

Hafrafell valley filling formed, is 448 m or greater than during both stages 1 and 2 (total of

395 m). Clearly, the Hafrafell valley filling is of local extent only.

Missing subchrons may indicate either slow accumulation rates or gaps in the stratigraphic

record. The Hafrafell sequence could only broadly be correlated with the geomagnetic time

scale in that the subchrons Jaramillo, Reunion, Kaena and Mammoth are not found in

Hafrafell.

Regional comparison of the Hafrafell stratigraphic sequence

No examples of acid tephra layer are found in the Hafrafell stratigraphic sequence indicating

that major volcanic centers with evolved magma systems did not evolve in the vicinity during

the period ca. 4 to 1 Ma. It is however known (Helgason, 2007) that the Kjós volcanic center

in Skaftafell, some 12 km to the NW, was active close to 2 Ma where sediments of acidic

composition are voluminous but not widespread.


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SUMMARY AND CONCLUSIONS

The lowest strata in Hafrafell are lavas that we correlate with the base of the Gilbert chron

(C2Ar; 3.596-4.187 M yr) at about 4 Ma. Upwards the lavas show slow accumulation rates

during the Gilbert (C2Ar; 155-m-thick) and Gauss (C2An; 234-m-thick) magnetic chrons or

roughly during the interval 4.187-2.581 Ma. Slower accumulation rates and/or intensive

erosion lasted until the Matuyama chron (2.581-0.781 Ma) when a 739-m-thick lava sequence

formed, containing 4 erosion horizons.

We attribute slower accumulation rates in the Hafrafell lower sequence in part to the off-axis

crustal build-up process. Thus, the Hafrafell lower strata accumulated through forward

stacking of lenses that were added toward south or distally, as opposed to centrally

constructed crust of the accreting rift zones.

Twelve erosion horizons, HR1 to HR12, were defined, based on tillite occurrences,

subglacially erupted pillow basalts and glaciotectonically formed breccia on lava flow

surfaces.

The erosion development of Hafrafell is divided into 5 stages. During the first two stages, of

Tertiary age, volcanism was continuous but accumulation rates were slow with depth of

erosion less than 100 m. During stage 3, i.e. lower-Matuyama, lava production increased and

accumulation exceeded erosion. In stage 4, upper-Matuyama, the Hafrafell valley formed and

landscape drastically evolved. During stage 5, or the Brunhes chron, major subglacial

volcanism took place with continuous erosion within about 2-km-deep valley network.

Glaciotectonically brecciated lavas, here described for the first time for Tertiary strata in

Iceland, potentially serve as clear indicators of glacials in the absence of other supporting

features.

Acknowledgements

JH extends thanks to Friends of Vatnajökull, a non-profit association of the Vatnajökull

national park, for financial assitance. JH received funds for an early part of this work from

the Iceland Science Fund that is gratefully acknowledged.


Iceland

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Appendix I.

Hafrafell (HL and HP1) _________________________________________________________________________________________

No N D I PLong PLat α95 J10 Po

___________________________________________________________

HP1-1 4 265.2 -59.4 237.0 -37.4 1.6 1.8 R(T)

HLX 3(2) 346.8 82.5 326.8 78.0 5.7 8.7 N

HL-1 4 203.4 -71.2 277.9 -75.8 6.5 3.6 R

HL-2 4 179.3 -67.2 345.3 -75.9 6.5 0.4 R

HL-3 4 203.1 -65.1 295.6 -69.0 10.3 0.3 R

HL-4 3(1) 188.1 -63.3 326.2 -70.3 8.5 3.4 R

HL-5 4 184.3 -64.1 333.9 -71.7 3.0 2.1 R

HL-6 4 194.1 -81.2 187.7 -79.9 3.4 3.3 R

HL-7 4 197.6 -64.9 305.6 -70.4 3.8 3.0 R

HL-8 4 205.7 -69.1 281.9 -72.6 4.1 4.1 R

HL-9 4 197.5 -65.2 305.3 -70.8 5.8 2.2 R

HL-10 4 198.3 -61.3 308.7 -66.0 5.8 3.4 R

HL-11 3(1) 185.5 -58.1 333.4 -64.6 5.3 2.6 R

HL-15 4 209.8 -37.5 304.4 -42.6 2.7 1.3 R(T)

HL-17 (4)

HL-18 4 196.7 -73.6 275.8 -80.9 3.6 2.6 R

HL-19 3(1) 236.1 -68.9 248.9 -59.4 4.1 2.2 R

HL-20 3(1) 232.5 -84.4 188.9 -69.0 9.6 2.5 R

HL-21 4 163.6 -78.6 112.1 -82.2 2.5 2.1 R

HL-22 4 197.9 -73.0 277.4 -79.9 9.4 1.8 R

HL-23 4 199.8 -70.5 287.2 -76.4 2.9 1.5 R

HL-24 4 184.9 -73.3 316.0 -84.6 5.4 3.5 R

HL-25 3(1) 216.4 -59.6 284.1 -58.4 8.2 1.0 R

HL-26 4 321.2 74.9 263.4 72.4 4.5 1.5 N

HL-27 4 317.9 76.2 272.1 71.8 7.4 0.8 N

____________________________________________________________

Hafrafell (U)

____________________________________________________________

U-1 4 280.1 -74.6 209.4 -48.6 8.8 2.4 R

U-2 3 216.0 -54.2 289.3 -53.5 9.4 0.6 R

U-7 3(1) 55.0 81.3 25.4 68.9 2.7 0.9 N

U-8 4 49.1 83.1 15.0 70.2 3.3 0.2 N

U-10 4 111.9 79.7 14.5 52.1 6.9 0.1 N

U-11 4 67.7 79.3 32.1 64.1 4.5 0.1 N

U-12 2 63.1 77.4 40.6 64.4 16.7 0.1 N

U-14 2(2) 183.0 -66.8 335.8 -75.3 2.1 1.6 R

U-17 4 234.1 -64.1 258.9 -55.4 4.7 1.2 R

U-20 4 213.6 -60.2 286.9 -60.0 3.2 3.4 R

U-21 4 219.1 -59.8 280.5 -57.5 2.2 6.2 R

U-22 4 221.6 -62.5 274.2 -59.2 2.3 3.7 R

U-23 4 207.9 -66.5 285.3 -68.9 3.2 1.7 R

U-24 2 225.0 -62.5 270.3 -57.8 4.3 4.8 R

U-25 3(1) 237.1 -67.2 251.3 -57.3 2.1 3.4 R


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U-28 4 224.1 -70.1 256.7 -65.9 2.7 1.7 R

U-29 4 237.3 -69.7 246.3 -59.7 3.5 2.9 R

U-30 3 218.4 -73.9 248.4 -71.8 12.0 0.9 R

U-31 4 239.1 -62.6 256.1 -51.8 2.3 1.6 R

U-32 5 228.0 -64.8 263.8 -58.8 2.2 3.1 R

U-33 4 165.5 -56.7 8.5 -61.8 5.4 1.3 R

U-34 4 173.5 -61.1 356.3 -67.8 4.9 2.3 R

U-35 4 168.6 -58.2 4.1 -64.0 17.0 0.9 R

U-36 4 215.4 -63.6 280.2 -62.8 6.5 1.3 R

U-37 3(1) 122.6 -66.1 74.1 -56.0 32.8 0.2 R*

U-38 4 190.9 -82.1 178.7 -78.9 10.5 0.5 R

U-39 4 160.5 -72.4 48.7 -78.6 7.5 1.8 R

U-40 3(1) 192.2 -73.1 290.1 -82.1 8.1 1.3 R

U-41 4 193.0 -81.3 185.6 -79.9 2.9 5.8 R

U-42 5 222.4 -77.5 227.7 -72.4 14.2 1.1 R

U-43 4 140.1 -82.0 127.1 -73.0 6.8 2.6 R

U-44 4 126.6 -81.3 121.8 -69.4 2.2 3.2 R

U-45 4 138.2 -79.9 113.7 -73.0 2.9 3.5 R

U-46 3 164.2 -83.1 147.5 -76.6 17.1 1.7 R

U-47 4 192.1 -70.6 304.4 -79.0 10.9 1.3 R

U-49 3(1) 154.6 -71.6 54.0 -75.4 7.3 1.2 R

U-51 4 156.4 -78.8 108.9 -79.6 4.8 3.6 R

U-52 4 146.7 -75.4 82.6 -75.1 4.2 2.3 R

U-52A 4 136.4 -65.3 59.4 -61.2 13.3 0.9 R

U-53 4 143.6 -68.2 57.0 -67.3 7.8 0.8 R

U-54 4 166.8 -70.3 24.7 -78.4 7.7 3.0 R

U-55 4 153.8 -73.0 62.3 -76.4 3.8 1.5 R

U-56 3 151.9 -70.0 51.3 -72.6 3.9 3.4 R

U-57 2(2) 147.3 -76.1 86.6 -75.7 6.1 1.4 R

U-58 4 140.2 -73.0 75.7 -70.4 4.3 2.6 R

U-59 4 137.1 -70.0 68.8 -66.3 6.5 1.4 R

U-60 3 160.0 -79.3 115.8 -80.6 7.5 6.1 R

U-60A 4 153.3 -69.7 48.4 -72.9 5.9 5.5 R

U-60B 4 159.3 -63.4 24.8 -67.8 13.3 2.8 R

U-60C 4 159.0 -64.9 27.4 -69.3 8.8 2.1 R

U-66D 3(1) 162.6 -71.6 40.0 -78.5 5.5 2.3 R

U-60F 4 184.2 -65.2 333.7 -73.2 8.6 2.1 R

U-60G 2(1) 190.8 -69.8 310.5 -78.3 9.0 1.7 R

U-60H 4 189.9 -63.3 322.6 -70.0 12.5 1.3 R

U-60I 4 186.7 -66.3 326.9 -74.3 14.6 3.2 R

U-60J 2(2) 258.0 -45.0 251.6 -28.9 13.8 0.7 R(T)

U-60K (4)

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Hafrafell (T)

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T1 4 206.2 -63.5 293.2 -66.1 3.2 5.6 R

T2 4 205.0 -63.0 295.8 -66.0 7.8 0.6 R

T3 4 208.3 -68.3 280.4 -70.7 10.1 3.4 R

T4 4 229.1 -64.9 262.6 -58.5 14.4 3.1 R


Iceland

T5 4 237.9 -72.1 240.2 -61.8 9.0 1.2 R

T6 4 32.9 81.0 24.3 75.5 3.0 0.6 N

T7 4 9.0 78.7 20.1 84.4 12.9 0.8 N

T8 2(2) 333.9 68.8 224.8 72.1 29.2 0.3 N*

T9 4 124.6 82.9 3.5 54.2 3.5 4.7 N

T10 4 353.8 43.7 172.4 51.3 18.2 3.2 N

T11 4 357.5 63.5 169.0 71.0 11.0 0.7 N

T12 4 11.6 71.9 119.9 80.9 18.9 0.9 N

T13 3(1) 228.6 -52.9 275.6 -47.6 14.7 1.2 R

T14 2(2) 236.1 -67.1 252.3 -57.6 1.5 2.1 R

T15 4 221.2 -63.9 272.7 -60.8 2.0 9.6 R

T16 4 342.3 -74.5 173.9 -35.7 17.3 1.0 R(T)

T17 3(1) 47.7 82.5 18.2 70.8 7.4 1.2 N

T18 3(1) 14.9 80.4 14.9 80.8 26.2 0.3 N*

T19 3(1) 195.9 -62.5 311.9 -67.9 2.7 3.8 R

T20 3(1) 188.4 -81.9 176.1 -79.4 6.1 4.2 R

T21 4 213.4 -85.1 180.4 -71.4 5.1 9.8 R

T22 4 199.6 -65.1 301.6 -70.0 11.6 3.1 R

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Explanations: No: Unit number. N: Number of measured samples with number of rejected

samples in brackets. D, I: Declination and inclination after tilt correction. PLong: Longitude of

virtual geomagnetic pole (VGP). PLat: Latitude of virtual geomagnetic pole (VGP). α95: Mean

field 95% confidence radius. J10: Remanence intensity (Amperes/m) after demagnetization in

10 mT AF. N: Normal polarity. R: Reverse polarity. (T) transitional.

magnetostratigraphy, k-ar dating and …...2012/11/30 · the study is based on detailed geological...

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