Understanding the offshore flood basalt sequence using onshore volcanic facies analogues: An example from the Faroe-Shetland basin

Abstract

Flood basalts in associated volcanic rifled margins, such as the North Atlantic Igneous Province, have a significant component of lavas which are preserved in the present clay in an offshore setting. A close inspection of the internal facies architecture of flood basalts onshore provides a framework to interpret the offshore sequences imaged by remote techniques such as reflection seismology. A geological interpretation of the offshore lava sequences in the Faroe-Shetland Basin, using constraints from onshore analogues such as the Faroe Islands, allows for the identification of a series of lava sequences which have characteristic properties so that they can be grouped. These are tabular simple flows, compound-braided flows, and sub-aqueously deposited hyaloclastite facies. The succession of volcanic rocks calculated in this study has a maximum thickness in excess of 6800 m. Down to the top of the sub-volcanic sediments, the offshore volcanic succession has a thickness of about 2700 m where it can be clearly identified across much of the area, with a further 2700 m or more of volcanic rock estimated from the combined gravity and seismic modelling to the north and west of the region. A large palaeo-waterbody is identified oil the basis of a hyaloclastite front/apron consisting of a series of clinoforms prograding towards the eastern part of the basin. This body was > 500 m deep, must have been present at the onset of volcanism into this region, and parts of, the water body would have been present during the continued stages of volcanism as indicated by the distribution of the hyaloclastite apron.

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Jerram, Dougal A. and Single, Richard T. and Hobbs, Richard W. and Nelson, Catherine E (2009)
’Understanding the offshore flood basalt sequence using onshore volcanic facies analogues : an example from
the Faroe-Shetland basin.’, Geological magazine., 146 (3). pp. 353-367.
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Geol. Mag. 146 (3), 2009, pp. 353–367. c
2009 Cambridge University Press 353
doi:10.1017/S0016756809005974 Printed in the United Kingdom
Understanding the offshore flood basalt sequence using
onshore volcanic facies analogues: an example from the
Faroe–Shetland basin
DOUGAL A. JERRAM∗†, RICHARD T. SINGLE∗‡, RICHARD W. HOBBS∗&
CATHERINE E. NELSON∗
∗Department of Earth Sciences, The University of Durham, South Rd, Durham DH1 3LE, UK
(Received 21 April 2008; accepted 27 October 2008; First published online 26 February 2009)
Abstract – Flood basalts in associated volcanic rifted margins, such as the North Atlantic Igneous
Province, have a significant component of lavas which are preserved in the present day in an offshore
setting. A close inspection of the internal facies architecture of flood basalts onshore provides
a framework to interpret the offshore sequences imaged by remote techniques such as reflection
seismology. A geological interpretation of the offshore lava sequences in the Faroe–Shetland Basin,
using constraints from onshore analogues such as the Faroe Islands, allows for the identification of a
series of lava sequences which have characteristic properties so that they can be grouped. These are
tabular simple flows, compound-braided flows, and sub-aqueously deposited hyaloclastite facies. The
succession of volcanic rocks calculated in this study has a maximum thickness in excess of 6800 m.
Down to the top of the sub-volcanic sediments, the offshore volcanic succession has a thickness of
about 2700 m where it can be clearly identified across much of the area, with a further 2700 m or more
of volcanic rock estimated from the combined gravity and seismic modelling to the north and west of
the region. A large palaeo-waterbody is identified on the basis of a hyaloclastite front/apron consisting
of a series of clinoforms prograding towards the eastern part of the basin. This body was >500 m
deep, must have been present at the onset of volcanism into this region, and parts of the water body
would have been present during the continued stages of volcanism as indicated by the distribution of
the hyaloclastite apron.
Keywords: British Tertiary, LIP, hyaloclastite, lava sequences.
1. Introduction
Flood basalt sequences are found on volcanic rifted
margins where the positions of mantle melting an-
omalies are coincident with the rifted margin (such
as the North Atlantic Igneous Province, Paran˜
a-
Etendeka: e.g. Jerram & Widdowson, 2005). They
are significant, as the provinces contain enormous
volumes of lava; for instance, the North Atlantic
Igneous Province (NAIP), is estimated to have a volume
of 1.8 ×106km3covering an area of 1.3 ×106km2
(Eldholm & Grue, 1994), with a significant amount of
the lava sequences being found offshore. Additionally,
the flood basalts may cover pre-existing sedimentary
basins, and in many cases these basins have proven
hydrocarbon reserves in areas not covered by basalt
where conventional geophysical exploration methods
can be used (e.g. Naylor et al. 1999). Interest shown in
such basins has focused on volcanic passive margins,
particularly the Faroe Islands, Rockall and Vøring areas
and the distribution and characterization of sub-surface
volcanic successions (e.g. Fliedner & White, 2003;
White et al. 2003). An understanding of the internal
facies relationships within the lavas will provide
†Author for correspondence: D.A.Jerram@dur.ac.uk
‡Present address: Senior Geologist, Det norske oljeselskap ASA,
P.O. Box 2070 Vika, NO-0125 Oslo, Norway
valuable information on how the provinces built up
through time and may also help quantify the extent
of hyaloclastites and sedimentary sequences beneath,
which are notoriously difficult to image seismically due
to their flood basalt cover (Jerram, 2002; Spitzer et al.
2005).
In order to understand better the 3D distribution of
the significant lava sequences offshore, flood basalt
facies schemes and 3D mapping have been developed
from seismic studies (e.g. Planke, Alvestad & Skogseid,
1999; Planke et al. 2000, 2005; Thomson, 2005) and
detailed field studies of onshore successions of flood
basalts and their associated sediments (e.g. Jerram,
Mountney & Stollhofen, 1999; Mountney et al. 1999;
Jerram et al. 1999; Jerram, 2002; Single & Jerram,
2004). Recent research interest has focused on being
able to understand the 3D facies distribution of the lavas
using their geophysical properties (Planke, Alvestad
& Skogseid, 1999; Nelson et al. 2009). To improve
sub-basalt imaging, research has focused on studies
using long offset data (Fliedner & White, 2003),
low frequencies (e.g. Ziolkowski et al. 2003), the
construction of complex 2D and 3D models (e.g.
Martini et al. 2005), the application of additional
imaging techniques such as magnetotelluric inversion
(Hautot et al. 2007) and the development of multi-
method schemes such as joint inversion of gravity,

354 D. A. JERRAM AND OTHERS
magnetotelluric and seismic data (e.g. Jegen-Kulcsar
& Hobbs, 2005).
In this contribution, we have used known onshore
volcanic facies from flood basalt sequences to interpret
the offshore flood basalt sequence in a case study from
the Faroe–Shetland Basin (the GFA 99 seismic data-
set). Firstly, we briefly introduce the onshore geology
of the Faroe Islands Basalt Group with examples of the
types of volcanic facies preserved. The interpretation of
the offshore sequence is then presented concentrating
on the key volcanic sequences. These sequences are
constrained using a combination of seismic facies and
gravity modelling. The final model highlights the facies
distribution of the key volcanic horizons and sheds light
upon their evolution through time, with a significant
portion of the sequences represented by hyaloclastites
erupted into a deep water body which must have been
present during the onset of flood volcanism.
2. The geology of the Faroe Islands Basalt Group
The location of the Faroe Islands and the significant
offshore distribution of the lava cover in the Faroe–
Shetland Basin are given in Figure 1. The sub-volcanic
plays here are considered to be prospective and worthy
of exploration (e.g. Waagstein, 1988; Laier, Jørgensen
& Isaksen, 1997; Ziska & Andersen, 2005), and hence
have attracted seismic acquisition over the offshore
parts of the lava cover, and drilling in the onshore lava
sequence (e.g. Boldreel, 2006).
The igneous succession in the Faroe Islands was
erupted during Palaeogene times prior to the opening
of the NE Atlantic (e.g. Jolley, 2009 and references
therein). The lavas are all geochemically tholeiitic,
which suggests that their eruption was coincident with a
high degree of partial melting of the mantle (Waagstein,
1988). The group consists of seven formations (see
Passey & Bell, 2007), of which four formations
are composed of significant thicknesses of volcanic
rocks (Fig. 1): (1) Enni Formation, (2) Malinstindur
Formation, (3) Beinisvør›Formation and (4) Lopra
Formation.
The additional formations are related to relatively
thin sedimentary and volcaniclastic interbeds (Passey
& Bell, 2007), which unusually occur high up in the
lava sequence in the Faroe Islands (Fig. 1), whereas
elsewhere, sedimentary interlayers are often restricted
to the basal part of flood basalt sequences observed
in many onshore examples (e.g. Jerram & Stollhofen,
2002; Petry et al. 2007), where the active sedimentary
environment gets invaded and overtaken by the volcanic
system (Jerram et al. 2000).
The gross thickness of the volcanic succession in
the Faroe Islands is thought to be 6500–7000 m, of
which 3000 m are observed above sea-level (Ellis et al.
2002; Passey & Bell, 2007). The onshore distribution
of the formations is shown in Figure 1b. The Faroe
Islands Basalt Group is considered to have erupted
between c. 60.56 and 57.5 Ma (Ellis et al. 2002),
however, the dating is poorly constrained above the
Beinisvør›Formation. The Beinisvør›Formation is
overlain by the Prestfjall Formation (previously termed
the coal-bearing sequence), which consists of coal-
bearing sedimentary units and was originally dated at
around 57.5 Ma (Ellis et al. 2002). The most recent
review of the dates for the onset and cessation of the
Faroe Islands lava field, however, places the Prestfjall
Formation at about 55 Ma (see Jolley, 2009) with errors
of ±0.5 Ma, and suggests an onset of volcanism more
likely around 57 Ma based on correlations across the
NAIP.
2.a. The onshore succession
The onshore succession of the Faroe Islands Basalt
Group is exposed throughout the archipelago (Fig. 2)
and has been sampled to considerable depth through the
Lopra 1/1A borehole. Based on the detailed borehole
data through the Faroe Islands Basalt Group from
the water-borne volcaniclastics observed deep in the
Lopra-1/1A borehole to the Enni Formation observed
onshore on Streymoy and Eysturoy, the two largest
islands of the Faroe Islands chain, it is possible to
construct a schematic facies section through the whole
Faroes lava sequence (Fig. 3). This succession is
highlighted below:
The Beinisvør›Formation and Lopra Formation
represent the oldest volcanic rocks in the sequence. The
Beinisvør›Formation has a thickness of over 900 m
onshore and occurs on the islands of Mykines, Su›uroy
and V´
agar (see Fig. 1). On Su›uroy the Lopra-1/1A
borehole failed to reach the base of the lava succession
at a drilling depth of 3565 m (Hald & Waagstein,
1984). On the Faroe Islands, the top of the Beinisvør›
Formation is marked by a sedimentary sequence: the
Prestfjall Formation. This hiatus in the eruptive activity
is represented by the deposition of lacustrine sediments
and the development of a thick coal sequence which
has been mined. This zone is approximately 10 m
in thickness, but has been noted to be locally up
to 15 m (Rasmussen & Noe-Nygaard, 1970). The
formation has been geochronologically constrained by
the use of combined palynological and isotopic dating
to the age range c. 60.56–57.5 Ma by Jolley, Clarke &
Kelley (2002), and some of the deepest lavas drilled
in Lopra-1/1A have been constrained by Waagstein,
Guise & Rex (2002) at c. 58.8 ±0.5 Ma (1σ)by
Ar/Ar whole rock dating. These dates may be slightly
younger when regional correlations are used (Jolley,
2009).
The Malinstindur Formation volcanic rocks have a
thickness of about 1350 m estimated from onshore
outcrop on the Faroe Islands (Passey & Bell, 2007).
The complete succession may be seen from its base
on the island of V´
agar in the west, to Eysturoy
and other islands in the east. It starts with olivine–
phyric compound lava flows and passes upwards to
plagioclase–phyric compound flows (e.g. Figs 2b, 3).
Additionally, within the olivine flows, two different

Offshore flood basalt sequences 355
Figure 1. (a) The pre-Palaeogene structural framework of the area of the GFA-99 seismic dataset. The postulated extent of the Faroe
Islands Basalt Group in the Faroe–Shetland Basin is also shown (modified after Ellis et al. 2002). Location of the Amerada Hess
Flare-10 line is also displayed across the GFA-99 area. (b) Distribution of the Faroe Islands Basalt Group on the Faroe Islands and the
stratigraphy compiled from onshore data and the Lopra-1/1A (1981 & 1996) borehole drilled on the island of Su›uroy. The wells of
Vesmanna-1 (1980) and Glyvursnes-1 (2003) are also located (after Ellis et al. 2002). (c) Schematic log and approximate thickness of
the stratigraphy on the Faroe Islands, including the Lopra-1/1A information.
olivine–phyric suites can be identified geochemically
(low-TiO2and high-TiO2) (see Waagstein, 1988;
Rasmussen & Noe-Nygaard, 1970).
The Enni Formation consists of a mixture of simple
and compound lava flows, with a remnant thickness
of 900 m with a few hundred metres removed due to
erosion (Ellis et al. 2002; Passey & Bell, 2007) (Figs 2e,
3). The Enni Formation is considered to have erupted
during magnetic chron C24R (Waagstein, 1988), which
places this activity into a cycle of eruptive activity
which occurred prior to the opening of the NE Atlantic
Ocean.
3. Seismic interpretation of offshore
volcanic sequence
In this study, the offshore succession was interpreted
using the character and geometry of the seismic
reflections, combined with the understanding of facies
architectures of flood volcanic rocks developed from
studies of onshore exposures from key flood basalt
sequences, such as the NAIP and the Etendeka
province of Namibia (Planke, Alvestad & Skogseid,
1999; Jerram, 2002; Single & Jerram, 2004; Jerram
& Widdowson, 2005; Nelson et al. 2009). Precise
identification of boundaries is difficult on seismic
reflection data because of the complex scattering and
absorption of the seismic energy by the heterogeneous
basalts. Figure 4a highlights an example of the well-
log data available through lava sequences and the
information that we can gain in terms of rock properties
(Fig. 4b) for the different volcanic facies that occur
in flood basalts (Nelson et al. 2009). The velocity
range of different internal facies within flood basalt lava
sequences is given in Figure 4b, and this has been used
to guide the velocity models used in this contribution;
for the lava flows (tabular and compound) we use a
velocity of 4000 m s−1, and for hyaloclastites we use
3500 m s−1.
In this study we will use the following terminology
to describe the major lava sequences that we can
identify in the GFA 99 data from the Faroe–Shetland
basin: (1) Lava sequence 1 – the uppermost lavas as
identified by characteristic seismic signatures; (2) Lava
sequence 2 – the middle section of lavas as identified by
characteristic seismic signatures; (3) Lava sequence 3 –
lowermost volcanic rocks identified using seismic and
gravity, and (4) Hyaloclastite apron – representing a
large palaeo-waterbody found in the eastern side of the
offshore data identified using pro-grading foresets on
seismic data. The interpretation and modelling of the
facies of the Faroe Islands Basalt Group offshore in this
study is concentrated across the area of the commercial
GFA-99 seismic data which lies approximately 60 km

356 D. A. JERRAM AND OTHERS
Figure 2. Field examples of the Faroe Islands Basalt Group. (a) Thick tabular lavas of the Beinisvør›Formation, at Beinisvør›,
southern Su›uroy. (b) The entablature-jointed top surface of a Beinisvør›Formation tabular-type flow near Akranessker, on the north
shore of Sørv´
agsfjødur, western V´
agar. (c) Looking south from Saksun towards Malinstindur Formation compound lava flows forming
the mountain of N´
oni›in the NW of Streymoy. (d) The contrasting lava facies of compound and tabular facies in the Enni Formation
seen from Sy›radalur, west of T´
orshavn on the SW coast of Streymoy. (e) Looking south towards Stallur summit on Streymoy. (f) Soil
horizon between two tabular flow units on Su›uroy. See http://journals.cambridge.org/geo for a colour version of this figure.
SE of the Faroe Islands in the Faroe–Shetland Basin
(Fig. 1).
Geological interpretation of the GFA-99 2D seismic
dataset has covered several iterations through map-
ping observed structure then checking the candidate
interpretation with gravity anomaly data. The main
focuses of the study are the volcanic sequences in the
data. The sedimentary overburden has been grouped
into the upper sediments, equivalent to post-Oligocene
overburden, and the lower sediments, equivalent to an
Eocene–Oligocene package (e.g. Davies et al. 2004);
this is indicated on the figures and aids in the gravity
modelling. The following section studies the offshore
interpretation of the volcanic rocks through the GFA-99
dataset: how the sequence is recognized in the seismic
data, the facies interpretations and the estimated
thicknesses present within this part of the NAIP
flood basalts. To aid the description of the volcanic
rocks, three interpreted sections through lines 105, 107,
109 are presented in Figure 5.
3.a. Lava sequence 1
The top of the lava sequence 1 is recognized by a
laterally extensive reflection. This reflection starts at
about 1580 ms TWT in the west of seismic line 105
and at a similar time in each of the other W–E seismic
lines of 107 and 109. The reflection is characterized by
being the strongest amplitude reflection below that of
the sea bed, and by the rugose nature of its top surface.
The top lava sequence 1 pick is heavily affected by the
Eocene to Miocene compressional phases associated
with changes in the spreading dynamics of the NE
Atlantic (Boldreel & Andersen 1993), and the surface
is faulted in much of the GFA-99 area. A characteristic
feature of the top lava sequence 1 is the presence of
thrust faults that pierce the surface and the presence of
associated thrust-tip folds (e.g. see Fig. 5a).
The gently dipping lava sequence 1 volcanic rocks
cover over 9.4 ×103km2of the dataset area and follow
the general structural dip towards the SE into the

Offshore flood basalt sequences 357
Figure 3. Schematic diagram of the facies in the onshore
Faroe Islands succession constructed using existing studies (e.g.
Passey & Bell, 2007), as well as field observations by the authors,
and information/interpretation from the Lopra-1/1A borehole
(e.g. Waagestein, Guise & Rex, 2002).
Corona Basin where they pinch out. The Faroe Islands
Basalt Group is at its shallowest in the north of the
GFA-99 area where the lava sequence 1 is interpreted
to be close to the sea floor.
3.a.1. Thickness
The lava sequence 1 succession maintains a reasonably
constant thickness of about 1000 m (500 ms TWT)
across most of the study area, apart from where the
formation feathers out towards the south and east.
The interval is mainly composed of parallel, laterally
persistent reflections, except for in the southwest, where
a divergent reflection sequence is observed to dip down
into the region of the Corona Basin. In the north
and west of the lines, the entire thickness of the lava
sequence 1 succession consists of parallel, laterally
continuous reflections. Towards the SE, many of these
reflections pinch out and appear to downlap shallowly
as the lavas thin above dipping, divergent sequences.
This thinning provides an indication of the maximum
extent of the lava sequence 1 distal to the Faroe Islands.
The character of the sequence suggests that the lavas
pinch out close to the east end of GFA-99. This notion
is supported by the observations of Ellis et al. (2002).
Figure 4. Flood basalt rock properties. (a) Example of wireline
log information from offshore sequences of lava flows exhibiting
different facies types (data from ODP Hole 642E; Eldholm et al.
1987). (b) Ranges of velocities for different internal lava flow
facies, used to guide velocity estimates (adapted from Nelson
et al. 2009). Units: ZDEN is bulk density (g cm−3); Vp is P-wave
velocity in km s−1; GR is natural gamma ray; API is American
Petroleum Units (standard for use in wireline logs).
Mapping the various reflections from the volcanic
sequence generates the TWT thickness maps shown
in Figure 6. Using the interval velocity of 4000 m s−1
it is then possible to calculate the thickness of these
sequences and their variations across GFA-99 (Fig. 7).
The formation is at its thickest (about 1900 m) in the
north and the west, which is more proximal to the
source region for the volcanic sequences, however,
about 1400 m is calculated to be present where the
interpretation of the GFA-99 data is reliable and

358 D. A. JERRAM AND OTHERS
Figure 5. Geological interpretations of GFA-99 lines: (a) 105, (b) 107, (c) 109. Line locations are given on inset maps (see also Fig. 1).

Offshore flood basalt sequences 359
Figure 6. (a) Depth maps to volcanic horizon picks interpolated and represented as 2D surfaces in Two-Way-Time (TWT). GFA-99
grid is shown for location purposes. Note that the depth to the top of all of the interpolated reflectors increases towards the SE as
the succession dips into the Corona Basin, east of the East Faroes High. Artefacts of interpolation are apparent in the diamonding
effects observed between the 20 km spaced 2D seismic lines. (b) Two-way-time (TWT) thicknesses of the volcanic horizons of the
Faroe–Shetland succession in the GFA-99 data.
Figure 7. Isopach maps of the calculated thicknesses of the
volcanic horizons of the Faroe–Shetland succession in the GFA-
99 data in metres.
multiples are at a minimum. As stated, the preserved
onshore thickness of the Enni Formation is about
900 m, with the top of the formation missing due to
erosion. If the lava sequence 1 units were equivalent to
the onshore units it would suggest that up to about
500 m of lavas may be missing from the onshore
exposures on the Faroe Islands. This is in close
agreement with previous estimates of erosion of the
onshore sequence of a few hundred metres (e.g. Ellis
et al. 2002).
3.a.2. Facies interpretation
The seismic reflections in the NW and upper parts of the
lava sequence 1 interpretation have strong amplitudes
and are laterally persistent. Individual high amplitude
reflections may be picked over tens of kilometres
(Fig. 8a). This simple character and the lateral extent
of the reflections suggest that the volcanic rocks in
these parts of the lava sequence 1 may be of tabular-
type facies (e.g. Jerram, 2002). From onshore studies,
the Enni Formation volcanic rocks have been shown to
be composed of simple type flows of about 10 m mean
thickness (Ellis et al. 2002), intercalated with zones
of thin compound flows (Passey & Bell, 2007) (e.g.
Fig. 8b). Between many of the flows, sedimentary in-
tervals are developed similar in character and thickness
to those seen in the Skye Lava Field successions on the
Isle of Skye. The lava sequence 1 rocks are dominated
by plagioclase–phyric flows in the central Faroes (e.g.
Ellis et al. 2002 and references therein), and have
been interpreted to represent volcanism similar to the
plains volcanism (Snake River Plain) of Greeley (1982)

360 D. A. JERRAM AND OTHERS
Figure 8. An onshore analogue from the Faroe Islands for
the transition from lava sequence 1 tabular flows to the lava
sequence 2 compound-braided flows. The tabular lavas form
laterally extensive thick flows (∼10 m thick average) that may
be correlated over hundreds of metres to several kilometres.
(a) A section of GFA-99 line 207 showing the characteristics
of this architectural facies transition. (b) Cliff section looking
NE down Kollafjør›ur on the east coast of Streymoy at about
300 m thickness of Enni Formation tabular-type lava flows
(intercalated with poorly exposed compound units). Six obvious
tubular lava flow basal contacts have been indicated on this
particular mountainside section. Note the poor exposure of the
Malinstindur Formation in comparison.
(Passey & Bell, 2007). Studies on other sequences
in the NAIP such as Skye have shown that more
evolved lava types such as the hawaiites and mugearites
(basaltic–andesites) which are often found in the upper
parts of NAIP lava sequences, tend to develop more
simplistic internal and external morphologies due to
their increased erupted viscosities and inflated modal
silica contents (Single & Jerram, 2004). Given the very
clear reflectors, much of the lava sequence 1 may be
considered to be akin to these typical trap-like tabular
lava sequences. The south and eastern areas of GFA-
99 show the tabular lava sequences are linked with a
prograding sequence of reflections interpreted to be
hyaloclastites. We will discuss the detail of this facies
in the hyaloclastite apron section below.
3.b. Lava sequence 2
The interpretation in this part of the Faroe Islands
Basalt Group is more difficult than the lava sequence 1
due to a loss in seismic resolution. This is caused by the
greater depth of the lava sequence 2, the dispersive and
high acoustic impedance properties of the overlying
lava sequence 1 volcanic rocks, and also the different
internal facies architecture of the lava sequence 2,
probably due to more compound-braided flow units
such as those noted for the Malinstindur Formation in
the onshore exposures on the Faroe Islands.
3.b.1. Horizon interpretation and distribution
Whereas the top of the lava sequence 1 is a distinct,
high amplitude reflection beneath the lower sediments
(due to the high acoustic impedance contrast over the
sediment/lava interface), the intra-volcanic contrasts
are minor, unless seismically significant facies changes
occur within the succession. In much of the lava
sequence 1/lava sequence 2 interface offshore, there is
no obvious seismic boundary, and arbitrary boundaries
are inferred. The two formations are usually referred
to together in most of the literature due to the arbitrary
nature of the boundary interpreted in seismic studies.
In this interpretation, the top of the lava sequence 2
is taken as the highest amplitude TWT pick which
sits approximately 600–1000 ms beneath the top lava
sequence 1 pick, which is thought to represent the
lowest continuous simple flow reflection before a thick
sequence of predominantly compound flows of the lava
sequence 2. It should be noted that if the boundary
between lava sequence 1 and lava sequence 2 was the
same as that between the Malinstindur Formation and
the Enni Formation, onshore, then this horizon would
be separated by a thin sequence of volcanic sandstones
and breccias as in the Sneis Formation (Passey & Bell,
2007). Given the lack of a clear reflection, if such a
transition existed offshore, then the interval must be
too thin and/or not of significant velocity contrast to be
imaged.
In the SE of the GFA-99 area, the base of the
lava sequence 2 is interpreted to be the series of
high amplitude broken reflections deep in the volcanic
succession. These are interpreted to be sill complexes
at the base of the volcanic succession and form zones of
over 1000 ms TWT of strong, lozenge-like reflections.
The convergence of downlapping reflections is also
taken as a base-succession marker in this part of the
data. The lava sequence 2 is present across the entire
GFA-99 area, but thins to a minimum in the eastern
extremity of the dataset, as in the case of the lava
sequence 1, which could be due to the distance from
the eruptive source for these units.
3.b.2. Thickness
Thickness of the lava sequence 2 varies considerably
through the 2D seismic grid studied. The maximum
thickness of the formation is about 1900 m (∼950 ms)
and the lavas pinch out entirely to the southeast. Errors
associated with these values may be mainly attributed
to the difficulties of interpretation of the lava sequence

Offshore flood basalt sequences 361
2 base, and to a lesser extent the lava sequence 2 top,
as discussed above. The thickest part of the volcanic
rocks lies through the centre of the dataset where a
N–S swath of volcanic rocks has a mean thickness of
about 1300 m (corresponding TWT maps are shown in
Fig. 6b).
3.b.3. Facies interpretation
Much of the western part of the dataset contains broken,
dispersed reflections that cannot be correlated over the
large distances (kilometres) possible in the tabular-type
lavas interpreted to be present in the lava sequence 1.
This is attributed to the lavas being formed of mainly
compounded lava flow sequences similar to other
examples in the NAIP, such as onshore Faroes (e.g.
Passey & Bell, 2007) and those seen towards the base
of the succession studied in the Skye Talisker Bay study
area (Single & Jerram, 2004). The stacking patterns
are complex in the vertical section, but also laterally
as the eruptive style of these flows form compound-
braided systems in 3D (Jerram, 2002). Both the lava
sequence 2 and lava sequence 1 change laterally into
large prograding reflections of the hyaloclastite apron
discussed below.
3.c. Hyaloclastite apron (lava sequences 1 and 2)
In the south and east of the GFA-99 area, beyond N–S
line 201, the tabular-type lavas of the western parts of
the lava sequence 1 are noted to spill into a series of
basinward-dipping reflections (e.g. Fig. 5a, b). These
are interpreted to form a prograding hyaloclastite fan or
apron in the Corona Basin region and dip down towards
the ESE. Such prograding hyaloclastites that formed
from lava flows into the sea were originally described
by Moore et al. (1973) and have been previously
described in the Faroe–Shetland region (e.g. Kjorboe,
1999). Good examples are also known from onshore
Greenland in the NAIP (e.g. Pedersen et al. 1998). The
reflection sequences in this part of the lava sequence 1
are clearest on the lines 105 and 107. Although
the divergent nature of the reflections is clear, the
boundaries of any particular sequence are less clearly
defined. The hyaloclastites show complex internal
morphology in comparison to the more simple lava
types interpreted in the bulk of the lava sequence 1. The
complexity of the internal morphologies of the lavas
means that the distinction between the lava sequence 1
and the underlying lava sequence 2 is difficult to
interpret, especially through the hyaloclastite zones.
The interpretation of the presence of a lava sequence 1
hyaloclastite apron has been made by detailed picking
of the volcanic internal reflections, paying particular
attention to onlap, downlaps and pinch-out relation-
ships seen within the formations. The presence of a
hyaloclastite apron in the lava sequence 1 indicates that
the Faroe Islands Basalt Group was filling a water-filled
basin in the east of the GFA-99 area, lavas moving into
this accommodation space from their eruptive source
Figure 9. Hyaloclastite facies. (a) The east section of GFA-99
line 109 where thick hyaloclastites are developed prograding
towards the east of the section. (b) Onshore analogue hyalo-
clastite breccia deposited in the Naajat lake, west Greenland
(from Pedersen et al. 1998).
somewhere in the west. A more distinct boundary
between the lava sequence 1 and the lava sequence
2 is observed in the north and west of the data area,
where the interpreted hyaloclastites are not deemed to
be present. Figure 5a displays a basic interpretation
of the GFA-99 line 105, showing some of the more
prominent tabular-type picks in the data, and also some
of the downlapping features present in the hyaloclastite
deltaic succession.
Hyaloclastites are also interpreted to form a large
thickness of the lava sequence 2. The dipping reflec-
tions are observed to dip steeply towards the ESE
and form a body which runs NNE–SSW through the
study area. The spacing of the seismic lines is too
great (20 km) to understand whether the hyaloclastites
form individual deltas, but their widespread occurrence
in the lava sequence 2 in lines 105, 107, 109 and
201 suggest the body to be more like an apron than
individual deltas. The thickness of the hyaloclastites
in the lava sequence 2 indicates the presence of a
deep water body proximal to the subaerially erupted
lavas; the hyaloclastites prograde basinward towards
the Corona area and appear to be on a similar scale
to hyaloclastite dipping successions in west Greenland
(Fig. 9). Significantly, this accommodation space was
present and being filled by both the lava sequence 2
and lava sequence 1, and provides important additional
constraints on models used to look at the spatial and
aerial extent of uplift prior to the onset of volcanism
(e.g. Jones et al. 2002; Maclennan & Jones, 2006).
Figure 7 shows a thickness map for the interpreted
hyaloclastite volcanic rocks that are considered to

362 D. A. JERRAM AND OTHERS
be present in both the lava sequence 2 and the lava
sequence 1. This body has a maximum thickness of
about 1200 m (∼700 m s−1), with most data points
in the body clustering around the 400–600 m thick
range (Fig. 7). This represents a massive thickness of
fragmental volcaniclastic matrial that are interpreted
to have been erupting into a substantial water body.
The calculations of Ellis et al. (2002) suggest the
hyaloclastites form foresets between 150 and 500 m
in thickness. The present study confirms a similar
calculated-scale of hyaloclastite foresets.
3.d. Lava sequence 3
The depth at which the top lava sequence 3 volcanic
rocks exist in the seismic data is difficult to interpret,
because deeper structure is masked due to earth filtering
of the seismic signal by the complex overburden
succession. Its presence and structure is therefore
ratified by the use of gravity data. An interpretation is
now presented based on a combination of the seismic
reflection characteristics and gravity models. Gravity
models were built at ARK Geophysics Ltd prior to
both the collection of Faroe Islands fieldwork data and
before the seismic interpretations were finalized.
3.d.1. Horizon interpretation and distribution
It is not possible to interpret accurately the boundaries
of the succession, or if the lava sequence 3 exists at all
in more than just the three W–E lines of 105, 107 and
109. The lava sequence 3 must be also present in N–S
line 207, but its interpretation is difficult to justify to
the east of this particular line. The easterly extent of the
formation is interpreted to be coincident with the East
Faroe High. Base lava sequence 3 picks are represented
in Figure 5, by a pick based on the interpretation of sills
at the base of the succession as strong, bright seismic
reflections, and the downlap of dipping reflections.
3.d.2. Thickness
The Lopra-1/1A borehole indicates the succession to
be extremely thick beneath the Faroe Islands. The
interpretation of the seismic data predicts a thickness
of at least 3000 m (∼1630 ms) (Fig. 6). The most
reasonable estimate of lava sequence 3 thickness is
made by combining the seismic interpretation with
gravity data into a model. Along the profile of line 107
this modelling required a base-case model thickness
in excess of 2700 m (Fig. 10). This is consistent with
the observation from seismic studies and highlights the
need for a multi-disciplinary approach to help solve
basalt cover and sub-basalt imaging problems (Jegen-
Kulcsar & Hobbs, 2005; Hautot et al. 2007).
3.d.3. Gravity based interpretation
The facies interpretation is based on observations
of the geometries present within the possible lava
sequence 3 succession, and by creating gravity models
along the seismic W–E lines. An initial interpretation
of line 107 is shown in Figure 10a. This gravity
model is constructed over depth-converted seismic
data and horizon interpretations (depth converted at
ARK Geophysics), and it includes the hyaloclastite
piles of the lava sequence 2, but the lava sequence 3
is missing entirely, and there is no sub-volcanic
sedimentary succession. The densities assigned within
particular stratigraphic packages are implemented from
the ARK Field software database in combination with
a volcanic rock property database developed as part
of the SIMBA project (e.g. Fig. 4b; see Nelson et al.
2009). This provides an unsatisfactory interpretation of
the regional gravity data.
An improved gravity model of line 107 is shown
in Figure 10b. The reduction in density of the central
portion has improved the calculated gravity response
by adding sediment to the sub-volcanic part of the
succession. This will be discussed in the next section.
If we assume that the sediments do not extend to the
western edge of the model, we need some additional
mass loss to fit the observed gravity anomaly. In par-
ticular, line 107 shows dipping reflections that can be
interpreted as part of a lava sequence 3 succession. This
suggests that the lava sequence 3 may be represented
by hyaloclastites and volcaniclastics similar to those
seen more distal and basinward in the lava sequence 2
and lava sequence 1. By reducing the density of the
interpretation of the lava sequence 3 in this model of
line 107 to that of a hyaloclastite, a good fit between
the observed and calculated gravity is achieved. The
lava sequence 3 is known to form thick tabular-
type lavas in the Lopra-1/1A section, but beneath
these, the drilling was terminated in a thick pile of
subaqueously deposited volcaniclastics/hyaloclastites
(Ellis et al. 2002; Waagstein, 2006). In the area of GFA-
99, these are considered to be represented by the basinal
progradational lava delta hyaloclastites suggested by
gravity interpretation.
3.e. The sub-volcanic section
The sub-volcanic zone is the part of the dataset which
has interested the petroleum industry enough to acquire
seismic datasets such as GFA-99. The sub-volcanic
section is considered to be a potential petroleum play
(e.g. Ziska & Andersen, 2005). The top part of the sub-
volcanic rocks is marked by interpreted sill complexes.
3.e.1. Sills
Sill complexes are observed at the base of the lava
sequences of Skye, in great thicknesses on the northern
Trotternish Peninsula in particular, where over 50 m
of sills sit beneath the base of the lava succession.

Offshore flood basalt sequences 363
Figure 10. (a) Gravity model of GFA-99 line 107 (see Fig. 1 for location) built from seismic interpretation picks only. The above
model is inaccurate in several areas, particularly in the centre of the line where mass-loss is required both within and beneath the
volcanic succession. (b) Geological interpretation and model of the GFA-99 line 107 incorporating Bouguer gravity data. The observed
gravity anomaly along line 107 requires a significant volume of low-density material to be present in the central portion of the line
at a sub-volcanic level. The observed gravity profile strengthens the argument for a significant succession of LS3 hyaloclastite at the
west end of the section where the density of the LS3 geological interpretation needs to be reduced at that level in the stratigraphy. The
gravity profile interpretation is filtered to 45 km low-pass wavelength; at this wavelength, the gravity calculated from the model has a
maximum deviation of 0.7 mgal from the observed Bouguer data.

364 D. A. JERRAM AND OTHERS
Similarly, in the Etendeka flood basalts of Namibia,
the substantial Huab Sills complex again fills a large
volume of dense material at the base of the province
lava sequences (Duncan et al. 1989). In the NAIP, sill
complexes have been successfully imaged in some of
the 3D seismic data sets (e.g. Thomson, 2005) and
where limited or no significant lava cover exists (e.g.
Hansen, 2006). In the Faroe–Shetland basin, significant
sill complexes are known from seismic studies on the
feather edge of the lavas (e.g. Smallwood & Maresh,
2002). Therefore, there is significant evidence for
offshore sill complexes in this region, and in many
flood basalt examples exposed onshore there is a
common relationship of sills intruding in and around
the sediment/lava contact at the base of the lava pile.
In the GFA-99 data, high amplitude reflections fill
what is considered to be the basal zone of the lava
field. Although individual reflections are rarely over
5 km long, they are interpreted to represent a series of
sills in the Faroe–Shetland Sill Complex (Smallwood
& Maresh 2002) seated at the base of the succession
across most of the GFA data. The sills are at their most
prominent at the interpreted base of the lava sequence 2
beneath the hyaloclastite zones, and landward, beneath
the interpreted compound lava types (Fig. 5).
3.e.2. Sedimentary rocks and basement
The presence of sub-volcanic sediments and the shape
of the basement surface have been interpreted by the
use of gravity data. Figure 10 presents 2D gravity
models of the GFA line 107. These contain the greatest
amount of vertical and lateral facies variability in the
entire dataset. A simple, normally faulted basement
is interpreted from Bouguer gravity data filtered to
remove wavelengths longer than 350 km. On top of
the basement a large thickness of sediment is modelled
for a gravity data fit ratified to the 45 km high cut filter
level; this filter allows accurate gravity interpretation as
deep in the section as the top of the volcanic rocks. The
sediment maximum thickness on top of this basement
is estimated to be 6000 m. This is consistent with the
total apparent sediment thickness estimated by Kimbell
et al. (2005) in this area, predicting localized depo-
centres with up to 10 km of sedimentary succession
over basement.
4. Summary
The key volcanic facies that develop and their internal
architectures are presented in Figure 11. In the present
study we have introduced the facies variations that
occur through the observed lava sequences on the
Faroe Islands and presented a simplistic geological
interpretation of the offshore lava sequences that are
imaged in the GFA-99 dataset with the following
observations and conclusions:
(1) Common lava facies that are found in the NAIP
and particularly in the Faroe Islands succession include
packages of Tabular Simple flows (individual flows
Figure 11. (a) The development of a hyaloclastite apron pile
as the lavas prograde into a substantial water body in the distal
parts of the lava sequence 1/lava sequence 2 and potentially in
the LLS, as interpreted from seismic, Bouguer gravity modelling
and from Lopra 1/1A borehole interpretations. (b) Development
of the lava field in the Faroe Islands Basalt Group lava sequence 1
and lava sequence 2.
∼20 m thick, Compound-Braided facies of lava lobes
and flows (component flow lobes <5 m thick) (e.g.
Passey & Bell, 2007), and subaqueously deposited
Hyaloclastite facies (e.g. Fig. 11).
(2) The succession of volcanic rocks in the GFA-99
offshore data area potentially has a maximum thickness
in excess of 6800 m, which is calculated in this study.
This fits within the estimated ranges of preserved
thickness of 6500–7000 m on the Faroes (Ellis et al.
2002; Passey & Bell, 2007). Generally down to the top
of the sub-volcanic sediments, or top lava sequence 3,
where present, the succession has a thickness of about
2700 m across much of the area, again similar to
estimates of up to ∼3000 m (Fliedner & White, 2003).
A further 2700 m or more of lava sequence 3 may
exist beneath this lava sequence 1–lava sequence 2 total
thickness, as estimated from the combined gravity and
seismic modelling to the north and west of the region.
These thicknesses compare well with the estimates of
Ellis et al. (2002), who suggest the complete thickness
of the extensive volcanic units discussed to be about

Offshore flood basalt sequences 365
5550 m, combining data from the Faroe Islands and
offshore data.
(3) The extrusive activity began with the emplace-
ment of volcaniclastic material into a substantial water-
body which lay in the environs of the present-day Faroe
Islands. Their presence may be interpreted offshore
to the SE of the islands. During the lava sequence 3,
the volcanic rocks filled this former water-filled basin
and erupted into the sub-aerial environment. Thick
tabular-type lavas (flows ∼20 m thick) formed a lava
succession more than 900 m thick in the Faroes area
and this eruptive phase waned. This process is likely
to have taken place from c. 57 to 55 Ma (see Jolley,
2009).
(4) At c. 55 Ma, early in Chron24r, the lava
sequence 2 blanketed the Faroe platform with thin flows
of dominantly compound-braided facies architecture.
These are similar in facies character to those seen in the
Malinstindur Formation on the Faroes, and examples
towards the base of the Skye Lava Field (Single &
Jerram, 2004). In the east of the offshore study area,
these formed water-borne prograding hyaloclastite
fans that grew into a slope-apron of foreset-bedded
volcaniclastic material architecturally similar to the
volcanic rocks seen in West Greenland (Pedersen
et al. 1998) (Fig. 10). The convergence of the foreset-
beds marks the base of the volcanic succession in the
offshore data.
(5) The seismic response offshore suggests that the
lava sequence 1 is dominated by laterally extensive
simple flows and that the lava sequence 2 has a more
complex internal architecture akin to compound units.
(6) The Hyaloclastite Apron indicates the presence
of a significant body of water into which the lavas of the
lava sequence 2 and lava sequence 1 prograded. This
body, more than 500 m deep, must have been present at
the onset of volcanism at c. 55 Ma, and may have been
present but unfilled during the earlier volcanism.
Acknowledgements. The GFA 99 dataset was acquired by
WesternGeco and made available to this project through
the EU framework 5 project – SIMBA (ENK6-CT-2000–
00075). We would like to thank the TOTAL GRC for
funding parts of this work and for co-ordination of the
EU SIMBA project. Specific thanks go to Paul Williamson,
Claude Lafont of TOTAL, and ARK Geophysics for help
with gravity modelling. This work was completed while DAJ
was the TOTAL Lecturer at Durham University. RWH was
funded as a NERC Advanced Fellow (NER/J/S/2002/00745).
This contribution benefited greatly from reviews by John
Smallwood and Simon Passey, editorial direction from
David Pyle and Jane Holland, and additional discussion and
comments from Richard Davies.
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