The Geology and Evolution of the Hjörleifshöfði Outlier: A 3D Exposure of a Surtseyan Volcano?
1
2
3
1
TIM WATTON *, D.A. JERRAM T. THORDARSON & R.J. BROWN
Volcanic
M argin
Research
C onsortium
1. Department of Earth Sciences, University of Durham, Durham, DH1 3LE (*[email protected])
2. DougalEarth Ltd., Durham DH1 4JR
3. School of GeoSciences, The University of Edinburgh, West Mains Road, Edinburgh, EH9 3JW
Tim Watton
Mý
Settlement
Highway 1
Myrdalsjokull
Vik
Map extent
10km
Associ
ations
Map
units
Stage
Interpretation
Figure
Number
18.97
A
Agglomerat
e
0
phL
V
bL
vA
Fig. 5
C&E
15
Syn-depostional
faulting
Bombs/
ejecta
lapilli
Cobbles
Boulders
Pebbles
Granules
V.Coarse
Med
Coarse
V.fine
Fine
Lithofacies
Vsl
Coset B
VStcb
D
C+E
Massive breccia , queched clasts
poorly sorted
Laminated quenched breccia
D
GHip
P2
Silicic Accretionary lapilli
L2
Upper basaltic blocky pahoehoe
lava flows
N
10
Magn. Cross Bedded
1m
Effusive Volcanic Rocks
a
P2
08
R2a with volcanic bombs
R2b
R2a
Basalt Distribution
E
B
?
20
Early Basalts
?
Debris Flow
Localized basaltic hyaloclastite and
pillow lava complex
H2
1000m
H1
20
Map (a)
PL/phL
PL/phL
bL/aLT
L1
Fault Breccia
.
Agglutinated basaltic spatter.
L2
V
40
Fig. 6 F
Fig. 6
B&D
Fig. 6 C
60
B B B BB
B BBB
B B B BB
VB
Clast supported breccia with incised bases.
HBc
Highly irregular loaded contact.
Matrix supported abundant tephra
component.
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
Clast poor interval dominant sideromelane component.
Ghip
VB
Very poorly sorted matrix supported
Clasts of :A) Pahoehoe basalt
B) Basalt Flow core
C) Sideromelane
D) Vesicular Pumice
Crude inverse grading
0
Description
Unit
Cobbles
Boulders
Pebbles
Granules
V.Coarse
750m
Trough crossbedded breccia with
some rounded clasts. Pyroclastic
lthics are common in the matrix.
Hyaloclastite material forms the
dominant clast and matrix component
Lithofacies
Clast
%
Zeolite
%
Pal. Matrix
%
60
VStcb
Fig.3
Large pebble and boulder grade
basalt clasts orientated parallel
to bedding 232/25
Erosional base to each coset
B B B BB
B BB B
B B B BB
B B B BB
B BB B
B B B BB
B B B BB
B BB B
B B B BB
B B B BB
B BB B
B B B BB
B B B BB
B BB B
B B B BB
B B B BB
B BB B
B B B BB
B B B BB
B BB B
B B B BB
B B B BB
B BB B
B B B BB
B B B BB
B BB B
µ63
µ125
µ600 µ2000
µ375
Sand
............
...........
E
GHip
Fining up to very course sideromelane fragments
Pebble and cobble size angular
clasts accumulating in troughs
Inclined Pebble lags 232/35
0
Description
Volcaniclastic tuff/ lapilli
tephra rich with pebble grade basalt fragments
Lamination of course material abundant basalt fragments
Upturned channel morphologies with abudant basalt
clasts
Lamination of course material abundant basalt fragments
Lithofacies
Clast
%
Zeolite
%
6cm
vA
Na2O+K20 wt%
6
5
4
3
B
H1- hyaloclastite clasts
& L1 lava array
1
42
46
50
54
Damage Zone
58
62
66
70
74
1
78
SiO2 (wt%)
r=
43
44
45
46
47
48
B
Debris Flow
Radius calculation
Askja
Torfajokull
50
51
52
53
Volcaniclastic distribution
and localities
GHip
E
GHip
Vent
Clast supported breccia
pahoehoe fragments
Phreatomagmatic construct
Late stage dyking of material
B
Syn-depostional
faulting
Volcaniclastic
apron
F
A
G
C
Clast supported breccia
sideromelane rich base
abundant basalt clasts
1000m
Oraefajokull 1
Katla
Eyjafjallajokull
Tindfjoll 1
Tindfjoll 2
N
A
Crack infills
1m
Vsl
30m
N
D
E
Vstcb
Rs1a
0.5m
Vsl
W
F
1m
NNE
0.7m
SE
Vsl
aLT
SiO 2 (wt%)
Snaefellsjokull
WVZ-All
Hjorleifshofdi
Kverk All
Hekla-All
Welded Exposure
Krafla All
Eyjafjallajokull 2
Askja All
Eyjafjallajokull 1
Hjor G1
Hjor G2
Hjor G3
Phreatomagmatic construct
PL
ŸTwo
phases of effusive
volcanism have occurred
during the emergence of
Hjörleifshöfði. The lower lava
sequence (L1, B, D) has been
erupted from a earlier vent.
Subsequent water interaction
has created minor
hyaloclastite and pillow lava
sequences (H2, F, Logs, 1, 2).
The upper lava (L2) is sourced
from V and fills in palaeotopographic lows. V is an
accessory vent to the larger
edifice structure.
ŸV e n t a g g l o m e r a t e s
containing xenoliths are found
at the top of the mound (a).
The vent has a composite
structure consisting of poorly
bedded pumice (which can be
oxidized) and basaltic spatter.
Bombs are also found locally
around the vent (E).
Contact R1b/P1
5m
ENE
Bomb sag I
1m
SSE
Fig.7
material
associated with lava
emplacement occurs in two
phases. P1 is characterised by
massive beds containing
numerous aligned clasts with
planar contacts to RS units. P2
P1 exposure
is highly oxidised accretionary
E lapilli interval (see lithological
descriptions).
ŸThe unwelded tbBT records
subaqueous distribution of
pyroclastic material and partial
marine reworking.
ŸWelded pyroclastic material
Rs1a/b contact
represents the final phase of
G effusive activity and is confined
to palaeo-valley systems.
ŸBoth P1 and P2 units have the
same geochemical affinity
which is separate to that of the
lava sequences (see
Geochemistry, log and fig. 4)
Bomb sag II
wcB
Grimsvotn - All
cB
Katla-All
Vestmannaeyjar All
Vsl
AR
Marine shoaling at crater rim
Fault breccia and spalling of material into crater
H1 - Hyaloclastite - L1 source
Phreatomagmatic construct
Original location of vent
A
Crater rim
Crater
H1 - Hyaloclastite - L1 source
800Kya
Phreatomagmatic construct
Relative water depth and movement estimates based upon the degree and type of reworking. The age of Hjörleifshöfði is taken from the Jarðfraeðikort
Geological Map of Icleand 1:600000 scale. This is the only recorded date for the island.
Original
Stratigraphic
Column
Stage
Depositonal
events
Relative
movement
Uplift
Subsidence
Relative water
depth at vent
Shallow
P2
L2
H3
Summary
Deep
Subsidence creating common dip direction
D
Phase 6 - Accretionary Lapilli - Late stage Katla
Phase 5 - Second phase of effusive basalt volcanism (L2)
and associated vent spatter
V
C
R2b
R2a
Phase 4 - Silicic pyroclastic deposition - external source
possible Solhiemer Ignimbrite
C ŸP y r o c l a s t i c
tbBT
Volcanic Bomb
Locations
B
D
Pebble imbricated breccia
some fragments can be rounded.
B
Localities
A
?
mltH/phL
54
0.3
0.2
L1
B
Phase 3 - Surtseyan pyroclastic activity and subsidence
created a shallow marine succession of reworked
AR
Phase 2 - Uplift, reworking (beach shoaling) early
hyaloclastite (H1) deposition
H2
R1
Extensional Faulting
H1
A
B1
Phase 1 - Pre emergent phase of surtseyan volcanism VB
Phase 1 faulting may have acted as pathways for magma intrusion in the shallow subsurface. Reactivation of faults
continued until phase 4. Phase 2 involved the continued emplacement of hyaloclastite material, reworking (due to
shoaling) and the emplacement of subaerial and subaqueous lava flows. Phase 2 lava flows thicken northwards
suggesting ponding in a large dammed crater separated from the sea. However, in the south, abundant hyaloclastite
material was still being generated. Subsidence resulted in the deposition of a shallow marine succession of reworked
volcaniclastic material (Phase 3). A distinct red fine-grained, lithic-rich (with partially quenched fragments) ignimbrite
succession fills topographic lows (Phase 4). The affinity of the ignimbrite succession is different to Hjörleifshöfthi and
may have Katla origin. Phase 5 consisted of a localized lava emplacement and marine reworking of volcaniclastic
material along the southern margin. Phase 5 basalt lavas flowed down into the crater and buried the marine
volcaniclastic sediments. Thin (1–4 m) accretionary lapilli-bearing tuff layers (Phase 6) cap the succession.
Hjor G3
Palagonite affected samples
1.0
0.8
H1- hyaloclastite clasts
&L1 lava array
FOV = 10mm
Conclusions
Silicic Zone
Highly evolved samples
Blue porosity stained
welding increasing
K2O wt%
Ÿ The evolution of Hjörleifshöfði is complex and multi-phase, however what remains is
Basalt lithics in Ignimbrite
FOV = 10mm
Ÿ The evolution of the edifice can be split into six main stages plotting the building and
0.4
0.1
Vstcb
Large apparent displacement listric faults, debris flows and subaerial
lava flows (with no quenching) at similar stratigraphic levels suggests
a dammed crater (A+B). Edifice reconstruction using the arc of
bounding listric faults indicates the diameter of this crater (assuming
lava filling, Fig. 3). The entire calculated edifice correlates well with
estimated height and width ratios (1:5) of pacific sea mounts (Smith
et al. 1988). Large damage zones of broken rafted VB lithofacies
suggests faulting post lithification. Side vent formation (map a) and
related intrusions are likely to have preserved the remaining exposed
structure.
clearly the remnants of a larger system.
0.6
Vsl
2000m
1.2
CaO/Al2O3
w=
A
r = 1116.5 m
H =175 m
W = 1200 m
Hjor G2
tcBT
0.4
?
Missing
Section
Hjor G1 Katla like basalt affinities
1.4
tcBT
Sample
1.6
0.5
Sample
Outwash
ENE
H1 - Hyaloclastite - L1 source
2
r=W H
+
8H 2
?
Scree
A
49
20m
Hekla
Pillow lavas H1
bL
42
h=
L1 lavas
E
30m
2
0
0.5m
E
Volcaniclastic and Pyroclastic Rocks
Vsl
Schematic log of Silicic section
Sample
Syn-depostional
faulting
Vent
C
Marine shoaling at crater rim
HBi
3
8
Na2O+K20 wt%
Dammed crater
limit?
Unknown pyroclastitic input - possible Katla source
P1
4
All units including separated clasts
5
7
0.5m
Ghip
W
Fig.4 ŸMajor and trace element
10
Listric fault limit
F
Pal. Matrix
%
preparation was initially conducted at
Durham University then analysed at
the XRF facility in the University of
Edinburgh by Nic Nolding.
6
Silic units
11
Late stage post erosion faulting
0.2
0.0
1.0
1.5
2.0
2.5
3.0
3.5
4.0
K2O (wt%)
4.5
5.0
5.5
6.0
0.0
0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80
0m
D
Vsl
Pebble imbricated breccia
some fragments can be rounded.
B B B BB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
5
Location: Hjörleifshöfði South C
64mm 256mm
Thinly bedded lapilli tuff abundant highlly vesicular
tephra.
............
............
......
Clast decrease higher up sequence 232/15
Geochemical Analysis
Edifice size from faulting
B
Channel fill
phL/Vsl
Bedding dip
B B B BB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
Unconformable surface (estimated 15m erosion)
2
R1 marine reworked
Interbedded lava L1
1km
64mm 256mm
B B B BB
B BB B
B B B BB
B B B BB
B BB B
B B B BB
Breccia Dmax ~1m
Fault trace
63’26,1250
18’44,0000
Location: Hjörleifshöfði South D
B B B BB
B BB B
B B B BB
B B B BB
B BB B
B B B BB
B B B BB
B BB B
B B B BB
VB
Bedding poorly defined not visible in places
Post-depostional
faulting
Fault cliff section interpretation
SSE
18
4
Bombs/
ejecta
lapilli
Coarse
ash
Fine
ash
500m
B B B BB
B BB B
B B B BB
B B B BB
B BB B
B B B BB
B B B BB
B BB B
B B B BB
Ghip
Pebble imbrication lithology as iH
containing no other lithologies
.......
......
.......
......
.......
.. .
...........
Med
Pal. Matrix
%
Zeolite
%
Coarse
Clast
%
µ600 µ2000
µ375
Sand
V.fine
Lithofacies
µ125
Fine
Description
µ63
Silt
64mm 256mm
Lithology
3
Location: Hjörleifshöfði Debris flows South B
Unit
Sand
250m
Height (m)
Bombs/
ejecta
µ600 µ2000
µ375
Cobbles
µ125
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
Fig. 6 F
Vent agglomerate with xenoliths
µ63
Boulders
Fig. 6 C
lapilli
Last stage accretionary lappilli formed
by nucleating particles in an ash
column. Possibly linked to Katla
eruptions.
Thick basal pillow lava partially
preserved. Pillows are large and
elongate as act like small tongues or
lava lobes penetrating into the water.
Spalling of rind material leads to a fine
hyaloclastite interstitial material.
Earlier dammed crater filling subaerial
flows. Joint set development from
saturated sediment contact.
Late stage flow originating from vent.
Flows down existing stratigraphy
V
10m
NNW
15
A
Granules
V
III,IV
100m
Upper lava L1
20
18
Map (a)
Map (a)
III,IV
bL/aLT
20
Vent spatter on to saturated ground.
III
D
Massive volcanic breccia
Rs1a
L1
C
24
Other Features
IV
P2
A
Lower lava L1
B1
25
Coarse
ash
bL/aLT
NNW
NNW
18
25
V
200m
Large cross bedded basaltic
hyaloclastite
32
24
Fig. 7
A&C
200m
Lower lava L1
06
Katla jökulhlaup outwash direction
5m
Outwash
N
Trough cross bedded volcaniclastic
sandstone
R1
06
L1 lavas
Late Basalts
06
N
Fault zone
Marine debris and hyaloclastite
(laminated)
Syn-depostional
faulting
C
Lower basaltic lava flows with
tephra intervals
L1
B
Post-depostional
faulting
Vent
a
14
B
Localities
A
A
L2
16
V.Coarse
cB/wcB
E
?
Fig.6
F
Laminated planar bedded lapilli tuff
Unit
IV
lhiemer Ignimbrite origin
3m
0m
H1 - Hyaloclastite - L1 source
D
15
Bombs/
ejecta
V
ó
VB
NE
200m
Marine shoaling at crater rim
Silicic Pyroclastic deposits
P1
9
Marine debris and hyaloclastite
(laminated)
VB
Debris Flow B1
A
L1 lavas fill dammed crater limiting pillow formation
Clay hosted basaltic spatter
Cobbles
wcB/cB
/vA
S
1m
Late stage accretionary lapilli follow L2 lavas
Present
Oxidised welded pumice
Fig. 7
D&G
Possible Ignimbrite flow with poor
vertical sequence exposure.
Quenching of small basalt lithics
suggests water interaction especially
at the flow base. Geochemically
separate from vent and lavas,
potential
Fault cliff section with channel infill
Damage zone
NE
Magn. Debris Flow
A
Cross Section
I
Welded pumice and spatter
10
Coarse
ash
III
C
Basaltic vent material/ spatter
a
14
P1
E
Fig.8
1km
Vesicular poorly welded pumice
06
tbLT/tb
BT
7m
Ghip
E
E
ipH
Boulders
III
3m
E
Stratigraphic Column
lapilli
V
Marine shoaling of tephra fall and
phreatomagmatically derived material,
highly palagonitised. Bombs derived
from vent.
Related to P1 tbBT possible vesicular
ejecta from vent
H1 Hyaloclastite
Slump Structure
20m
The initial phreatomagmatic
construct of the earlier edifice
has been preserved as VB
lithofacies (C,E). Reworked
hyalolclastite (A,,B,D formed
from passive quenching with
limited phreatomagmatic
involvement) forms large
slump structures with massive
cross bed sets (stage II).
Basalt clasts in H1
hyaloclastite rocks are
geochemically similar to L1
lava. L1 lavas in places pass
into pillow lavas and H1
hyaloclastite. Hyaloclastite
material is susceptible to
erosion only intrusion into the
structure seem to resist
jökulhlaups (Fig. 3).
Original location of vent
Vent agglomerate
12
NNW
N
V
V
Structure and Edifice Size
100m
Coset A
1000m
B
A+D
Hp/Lt
NNW
Coset C
Hyaloclastite
?
Trough crossbedded Volcaniclastic
Pebbles
pV,vA
II
Coset D
Geological Evolution
Stage VI
phL
Fault damage
zone
UNC
Granules
R2a/
R2b
Unconformity
E
Post-depostional
faulting
Vsl
14
Vstcb/
Vsl
Trough Cross bedded unit
A
Localities
A
?
250m
0
100m
B
100m
Interbedded volcaniclastic interval
09
Pillows, elongate; 1-3 m width by 0.5 Ghip, pL,
m. 20-30 cm quenched glass rinds.
phL
Fine to medium grained
sideromelane matrix with abundant
zeolite pore space fill.
Tabular basalt, clear core, crust,
base relationships. Show both curvicolumnar and hackley fracture.
Core well developed with columnar
joints. Flow crust breccaited into
small blocks dmax. 30cm.
Large boulders of agglutinated
basalt clasts (5-30cm). Clear dolerite
xenoliths (sharp boundaries).
Coarse
ash
Fine
ash
V
Slump Structure
Coset E
phL
Welded intervals with lithic clasts
.......
......
.......
.......
Fig.2
PL
Tephra layers
Geological Map of
Hjörleifshöfði
15
63’24,9000
18’26,5000
Distal to side vent. Basalt clasts rope
like textures each surrounded by fine
clay. Partial quenching of surfaces.
Concentric rounded small pumice
fragments thin <50cm cover.
Basalt lava with hackly fracture
V
V
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
GHip
L1 Lower lava sequence
Lower lava sequence L1
H1 Hyalo.
Silt
unit
Cobbles
Boulders
Lithology
Height (m)
Bombs/
ejecta
lapilli
Pebbles
V.Coarse
Coarse
Med
V.fine
Granules
Fine
ash
Lithology
Fine
Imbricated hyaloclastite
breccia. Well sorted laminations and large scale
crossbedded structures
VStcb
V
V
V
Hyaloclastite distribution
Vsl
B1 debris flows Upper sequence
Blocky
Lava
Hyaloclastite breccia
with jig-saw fit clasts.
Some fine sand grade
channels are observed
but discontinuous laterally.
PL
H1 Hyaloclastite
Lava
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BB B
B BB B
B B B BB
B B BB
B B
B B BB B
B B B BB
B BBB
B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B B BB
B B B BB
B BBB
B B BB
V
V
V
63’26,1250
18’44,0000
B1 Debris flows
Pillow Lava
and Pillow
lava
breccia
0
Trough cross laminated
volcaniclastic breccia.
Clasts can be rounded.
Vsl
Spalled off pillows in massive
hyaloclastite
V.Coarse
aLT
............
V
V
V
V
Coarse
Rounded
Lappilii tuff
V
V
V
..
.......
..........
.......
. ..
.........
.......
.... ..
......
. ....
V
V
phL
Large pillow lavas 1-3m
width. Brecciated pillow
rinds at base. Grading to
coherent lava flows.
V
V
V
V
Basalt lava with well developed
columnar joint sets
Med
cB
II
V
V
phL
Vsl
V
V
Fine
ash
Clay hosted
breccia
R2a
V
V
V
V
V
V
V
Fig.5
I
High angle cross bedding could
Fig. 5, A,
represent high energy Gilbert style
B, C
delta deposition from large scale flank
collapses of a hyaloclastite pile.
Supported by the presence slump
structures simplified from Watton
.
.et al
2013. Linked to L1 lavas .
Late stage lava water interaction as
Fig. 7 B
the L1 lava piles build and flows
towards the seaward side of the
mound.
Erosion unconformity, changing
Fig. 7 E
environmental conditions. Shore face
reworking accompanied by the influx
of new material leads to rounding of
the basalt clasts.
V
V
Thin tephra and basalt
lapilli
V
V
Vsl
V.fine
wcB
H2
V
V
phL
Fine
Welded
breccia with
clay hosted
material
I
V
V
V
Tephra grains and lava
flow crust in intervals
Hyaloclastite and Phreatomagmatic Rocks
Pal. Matrix
%
Zeolite
%
phL
Thin bands of hyaloclastite material
Silt
tbBT
Vstcb/
Vsl
H1
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
....
.....
63’26,1250
18’26,5000
Med
Thickly
bedded tuff
with clasts
pV
mltH,ph
L
I
Map
Coarse
Pumice
Vsl
Localized, 2-3m thick lobes. Closely
associated with late sequence L1
lava flows. Similar composition to
iBH
Vstcb unconformably overlies Ghip
.
1m thick. Clasts are composed
solely from angular to subangular
basalt. Clasts can be smoothed. The
matrix is a mix of sideromelane
glass, tephra and locally derived
pyroclastic material. Palagonite
cement
Fine to medium grained rounded
volcanic (tephra and basalt)
particles, Horizontally bedded with
bomb sag features = R2b.
Glassy silicic composition, Vesicular
upper part of P1 localized around
vent. >50% porosity.
Welded (upper), non welded (lower)
P1 interval. Fiamme streaked and
elongate glass and vesicles. Basal
unit contains clasts upto 6cm. Small
quenched basalt lithics in base of
sequence. Inverse grading. Fiamme
highly crushed in welded unit,
crushed lithics remain prominent
Basalt clasts; rope like marks, clasts
are flattened and joined.
B1
Geochemical affinity suggests relation
to side vent formation indicating
magma intrusion into crater rim.
Phreatomagmatic cone build up from
rooster tail events. Interbed zones
contain numerous incised channels
which indicate sourcing from an
effluent force.
Fine
ash
Thinly
bedded
lapilli tuff
Vstcb
Bimodal breccia unit; boulders Dmax VB/Ghip
80 cm. Clasts components; ropey
pahoehoe basalt fragments, highly
vesicular basalt blocks and tephra.
Beddding highly underlose matrix
vesicular palagonitized glass,
secondary calcite and zeolite.
-8m imbricate cross bed sets, fining Ghip/ VB
up; 50-80 cm to c.2-10 cm clasts.
Matrix well sorted medium to coarse
grained sideromelane glass with
some tephra (highly vesicular up to
1cm diameter).
II
V.fine
Cross
stratified
Lappilli tuff
mltH
Dykes
Fine
Massive
Hyaloclastite
lappilii tuff
Ghip
N/A
Silt
Granular
hyaloclastite
breccia
VB
Intrusive dolerite composition dyke
bodies cutting bedding within VB.
Lithology
Massive
matrix
supported
Breccia
D
Height (m)
Intrusive
62.38
18.33
V
V
V
V
V
Vsl
Lithology
Description
V
Clast
%
Lithofacies
Basalt lava with well developed
columnar joint sets
V
Height (m)
Code
Pebbles
Lithofacies
V
V
V
V
V
V
Description
V
V
V
V
2
Location: Hjörleifshöfði North
64mm 256mm
Stage IV/V
lss
rda
Rivers/Ocean
V
phL
Sand
Stage II/III
an
......
......
......
......
V
V
Pal. Matrix
%
Zeolite
%
µ600 µ2000
µ375
Stage II
Low elevation outcrop
r
du
Curvy columns irregular
spacing
V
V
V
Clast
%
10
V
V
V
V
Lithofacies
µ125
Stage I
High elevation outcrop
Description
µ63
R1
Legend
Location: Hjörleifshöfði North A
64mm 256mm
H1 hyaloclastite
63.58
63.39
The table below describes each of the lithofacies present in Hjörleifshöfði. Each lithofacies
relates to the map, logs and field photographs on the adjacent panel. In light of our mapping
(Fig. 2) and XRF geochemical analysis (Fig. 4) we discuss the evolution of the original and
existing volcanic edifice.
Sand
V
V
V
µ600 µ2000
µ375
Rs1a
Hjörleifshöfði is a small (~4 km ) isolated Quaternary volcanic outlier in southern Iceland that
provides an excellent exposure of a Surtseyan volcano (Fig.1). It sits in a large sandur plain
formed by glacier melt water outwash (jökulhlaup) from late Holocene subglacial activity at
Katla volcano (Lacasse et al. 1998); Aggradation of outwash sediments turned Hjörleifshöfði
from an island into part of the mainland.
V
V
µ125
H1 Hyaloclastite
2
30
µ63
Silt
Clast, Zeolite and Palagonite % estimated in the
field using grainsize analysis and area point counts
Introduction and Lithostratigraphy
1
Fig.1
Height (m)
Locality
Coarse
ash
The author
subsequent erosion of the volcanic pile.
Basal breccia
Mg#
Samples have been divided into mafic and silicic types based upon silica concentration. All major and trace element
concentrations plot within Katla fields (from Lacasse et al. 2006, Sinton et al. 2009 and the GEOROC data base). In all
plots H1 lava clasts are associated with L1 lava samples probably indicating a similar source, although there is variation
in the product. Silicic rocks share similarities with the Solhiemer ignimbrite (Tomlinson et al. 2012).
Ÿ Silicic components present in Hjörleifshöfði show different geochemistry from the rest of
the basaltic pile. They relate to an external Katla source.
Ÿ Hjörleifshöfði may be an important well exposed side vent of Katla.
References and Acknowledgements
GEOROC database - http://georoc.mpch-mainz.gwdg.de/georoc/
Lacasse, Steven Carey, Haraldur Sigurdsson, Volcanogenic sedimentation in
the Iceland Basin: influence of subaerial and subglacial eruptions, Journal of
Volcanology and Geothermal Research, Volume 83, Issues 1–2, July 1998,
Pages 47-73,
Lacasse, C.; Sigurdsson, H.; Carey, S. N.; Johannesson, H.; Thomas, L. E.
and Rogers, N. W. (2006). Bimodal volcanism at the Katla subglacial caldera,
Iceland: insight into the geochemistry and petrogenesis of rhyolitic magmas.
Bulletin of Volcanology, 69(4), pp. 373–399.
Sinton, J., K. Grönvold, and K. Sæmundsson, Postglacial eruptive history of
the Western Volcanic Zone, Iceland, Geochem. Geophys. Geosyst., 6,
Q12009, doi:10.1029/2005GC001021, 2005.
Smith, Shape analysis of Pacific seamounts, Earth and Planetary Science
Letters, Volume 90, Issue 4, 25 November 1988, Pages 457-466, ISSN 0012821X, 10.1016/0012-821X(88)90143-4.
Tomlinson, Thor Thordarson, Christine S. Lane, Victoria C. Smith, Christina J.
Manning, Wolfgang Müller, Martin A. Menzies, Petrogenesis of the Sólheimar
ignimbrite (Katla, Iceland): Implications for tephrostratigraphy, Geochimica et
Cosmochimica Acta, Volume 86, 1 June 2012, Pages 318-337,
Watton, D.A. Jerram, T. Thordarson, R.J. Davies, Three-dimensional
lithofacies variations in hyaloclastite deposits, Journal of Volcanology and
Geothermal Research, Volume 250, 15 January 2013, Pages 19-33, ISSN
0377-0273, 10.1016/j.jvolgeores.2012.10.011.
TJW would like to thank DONG E & P UK, Ltd. for the funding of this project.
Clayton Grove is thanked for field assistance and subsequent discussion.