Marine and Petroleum Geology 22 (2005) 109–122
www.elsevier.com/locate/marpetgeo
Mineralogical and microstructural development
of the sediments on the Mid-Norwegian margin
Carl Fredrik Forsberga,*, Jacques Locatb
a
Norwegian Geotechnical Institute, P.O. Box 3930, Ulleval Hageby, N-0806 Oslo, Norway
b
Université Laval, Québec City, Que., Canada G1K 7P4
Abstract
As a contribution to the understanding of the mechanisms and causes for Storegga Slide, the mineralogy and microstructure of samples
from a series of boreholes from the slide neighbourhood have been analysed. The results show a change from kaolinite-rich oozes through
smectite-rich fossilferous clays/clayey oozes to illite dominated hemipelagic and glacial sediments from the Brygge (Eocene to earliest
Miocene) to Kai (earliest Miocene to late Pliocene) to Naust Formations (late Pliocene to present), respectively. The deposits are interpreted
as the effect of denudation of weathered regolith and an increase in physical erosion in conjunction with the deterioration of the climate and
the initiation of glacial cycles during the late Cenozoic. During the Quaternary, climate has controlled the mixing of current transported, fine
grained sediments that are probably derived from the region around the Faeroe Islands and coarser grained glacial sediments containing a
broad spectrum of minerals eroded from the North Sea and Scandinavian mainland. Since currents have been most important during warm
periods, sediments from these periods are finer grained and contain more smectite than the glacial sediments. However, it is concluded that
the mineralogical differences between finer grained sediments from warm periods and the coarser grained glacial sediments are not great
enough to explain the ubiquitous utilisation of glacial marine/hemipelagic sediments as slip planes for the slides in the area, but that this is
probably a consequence of the differences in the grain size distribution of the sediments. Voids that may be casts left behind by gas hydrates
have been observed.
q 2004 Elsevier Ltd. All rights reserved.
Keywords: Gas hydrates; Storegga; Cenozoic
1. Background
The discovery of the Ormen Lange Gas field within the
upper part of the Storegga Slide scar (Fig. 1) resulted in
considerable interest in unravelling the Cenozoic geological history of this area. The Storegga Slide itself combined
with this interest from industry meant that the region
became one of the key areas for several multinational
programmes that included studies of slope processes (e.g.
ENAM, COSTA, STRATAGEM). As a consequence of
these studies the mid-Norwegian margin has become one
of the best studied offshore areas of the world. There is
now a well-founded model of the geological development
of the area (Bryn et al., 2005a; Berg et al., 2005; Rise et al.,
* Corresponding author.
E-mail address: [email protected] (C.F. Forsberg).
0264-8172/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.marpetgeo.2004.12.002
2005; Sejrup et al., 2000; Hjelstuen et al., 1999; King et al.,
1996, 1998).
This model shows a development from deposition of
siliceous and calcareous oozes during the early Cenozoic
(Brygge and Kai formations; Table 1) with a transition to
repeated sedimentation of stacked glacial and hemipelagic
units from the late Pliocene (Naust formation; Table 1)
through the Quaternary and until the present. The stacking
of the Quaternary slope sediments reflects the interaction
of sedimentation from the currents flowing northwards
along the continental margin and the supply of glacially
derived sediments from the hinterland (Berg et al., 2005;
Bryn et al., 2005b). The currents have had a maximum
influence during interglacial periods, and were subdued,
but still important, during glacials. During glacial
maxima, while only lasting for a few thousand years,
there was deposition of thick diamictons and glacial
debris flows on the slope from shelf edge glaciers.
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Fig. 1. The Storegga area with the positions of the investigated geoborings. The Ormen Lange gas field is outlined in white. Water depths are indicated by black
numbers and thin black lines. The Storegga Slide outline is shown by the thick black line.
Deglaciations were accompanied by short periods of local
rapid deposition from meltwater plumes (Nygård et al.,
2004). The layering indicated in Fig. 2 symbolises the
stratigraphy produced by this climatically induced oscillation of depositional mechanisms. Large landslides have
repeatedly formed in the area (Solheim et al., 2005; Rise
et al., 2005; Berg et al., 2005; Bryn, et al., 2005a; Evans
et al., 1996), and this study is one of many performed to
improve the understanding of this sliding.
Prior to these investigations only mineralogical
information from ODP sites on the Vøring Plateau and
basin had to the authors knowledge been published from
the mid-Norwegian margin (Froget et al., 1989; Krissek,
1989). The mineralogy of surface sediments of the
Norwegian Sea have, however, been studied by Eisma
and van der Gaast (1983), whereas Kuhlemann et al.
(1993), Berner and Wefer (1994), Forsberg et al. (1999),
and Butt et al. (2000) have published minalogical data
from further north in the Norwegian-Greenland Sea. We
will here present the mineralogy and microstructure from
11 sites (Fig. 1) in relation to depositional setting and
briefly discuss their contribution to the sliding in the
area.
Table 1
The ages of the Neogene formations in the Storegga region
Time period
Seismic formations/units
Late Pleistocene to recent
(0.13 Ma-present)
Mid-Pleistocene
(0.73–0.13 Ma)
Late Pliocene to (early) midPleistocene (2.6–0.73 Ma)
Earliest Miocene to LatePliocene (23–2.6 Ma)
Eocene to earliest Miocene
(55–23 Ma)
Naust units 01–03 and the Storegga
slide deposits
Naust Units 04–07, R1–R3 and S1–S3
Naust units S4–S5, u1, u2 and W
Kai formation
Brygge formation
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111
Fig. 2. Schematic diagram showing the core recovery in a stratigraphic framework for the Ormen Lange geoboring sites. The recovery is against a simplified
seismic stratigraphy. Note that the thickness of individual sub-units varies between sites, and the figure is therefore not to scale and must be viewed in a
qualitative way. No mineralogical samples were analysed from Sites OB1, OB2. Site NF (North Flank) is outside of the slide scar. See Fig. 1 for location of the
sites. Ages given in million years before present (Ma) are from Berg et al. (2005).
2. Material and methods
2.1. Sample locations and recovery
Borings with geological and geotechnical sampling
(geoborings) have been performed at 12 sites in the
Storegga area (Tjelta et al., 2002). We have investigated
10 of these, together with samples from well 6305/4-1
(Well 4-1) (Fig. 1). The sample ages span the Miocene to
the present, thereby covering pre-glacial times through the
Quaternary glaciations (Fig. 2). The sample retrieval in the
geoborings was not continuous (Fig. 2) because of
difficult ground conditions and strategic sampling imposed
by time constraints. Wireline logs were however recorded
for all holes giving continuous borehole information
(A. Solheim, pers. com., 2004). Nevertheless, the sites
have provided samples from all the Cenozoic stratigraphic
units in the area (Figs. 2 and 3) and have provided the
opportunity to investigate the mineralogy and microfabric
changes in a range of settings where the post depositional
history includes shelf areas with glacial loading as well as
deep water sites with modest rates of sedimentation.
2.2. X-ray diffraction analyses (XRD)
2.2.1. Sample preparation
All analyses were performed at the XRD laboratory at the
Geological Institute, University of Oslo.
Unoriented bulk samples were prepared by filling
crushed, dried sediment into a standard holder for powder
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Fig. 3. Idealized cross-sections through the geoborings demonstrating the sampling strategy and the units recovered. Two cross-sections are represented. The
upper one through the northern part of the Storegga Slide (Sites 22, NF and 28—see Fig. 1) where only a shallow layers have moved, and the lower one through
the deep slide scar (Sites 20, 19_2, 31,99,27, Well 4-1, 17 and 18_2). The paler layers are the marine/hemipelagic layers, the darker ones, layers from glacial
deposits. The Storegga slide sediments have a variable shading.
XRD analyses. Oriented clay fraction samples were
prepared by filtering the less than 2 mm size fraction
obtained from a settling tube and inverting them onto
ceramic holders. To differentiate swelling minerals such as
smectite from chlorite and illite, samples were treated with
ethylene glycol (glycolation) vapour overnight in an
exsiccator. Samples were also heated to 550 8C for about
2 h to remove the kaolinite peak so that a positive
identification of chlorite could be achieved (Fig. 4).
Minerals were classified manually according to standard
procedures (e.g. Carroll, 1974). Quantification was performed digitally using a Unix-based script previously used
by Forsberg et al. (1999) and Butt et al. (2000, 2001). The
script subtracts the background level and finds the number
of X-ray counts under selected peaks between the half peak
heights on each flank. Peaks can be chosen from any
combination of diffractograms. The X-ray counts obtained
can then be processed in a spreadsheet or similar software
where weighting factors etc. can be applied. For the present
work processing was accomplished using Unix-based
scripts with weighting factors from Biscaye (1965) for
clay and Ramm (1991) for the bulk samples (Fig. 4). The
kaolinite/chlorite ratio was determined from detailed scans
of the peak between 248 2q and 268 2q (Biscaye, 1964).
Peaks common to different diffractograms (e.g. the quartz
peak) were used to normalise the different scan amplitudes
to those of the glycolated runs. No internal standards were
used, so the quantification assumes that the sum of all the
quantified minerals is 100%. The same minerals were
quantified in all samples. Silica was the major constituent in
four samples from the Brygge formation (Well 4-1).
Because of the amorphous nature of silica, the quantification
of the silica content was not performed. The results for these
samples therefore only reflect the relative mineral distributions, not the silica content.
2.3. Scanning electron microscopy (SEM)
2.3.1. Sample preparation
Samples used for SEM work were prepared by first
freezing them in liquid nitrogen. The surfaces to be
observed were obtained by cutting while the sample was
still frozen and were perpendicular to the bedding plane.
Cutting while frozen normally ensures that the surface is as
planar as possible and that the porosity is well exposed to
the viewer. For samples which are coarse, usually at a clay
content of less than 30%, it may, however, be difficult to
obtain planar surfaces. The top of the layering is up on all
SEM images.
Magnifications used for SEM analyses varied from 20 to
a maximum of 20,000. During inspection micrographs were
taken at increasing magnifications. Characteristics of the
microstructure (orientation, alignments, grain sizes,
shapes), types of particles present and features of special
interest were noted. During inspection X-ray dispersive
spectrometer (XREDS) analyses were performed on single
particles to provide the relative abundance of various
C.F. Forsberg, J. Locat / Marine and Petroleum Geology 22 (2005) 109–122
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Fig. 4. Examples of diffractograms recorded: the left hand diagram from a smectite rich sample, the right hand one from one rich in illite. The numbers next to
the curves refer to scan descriptions below the plots. All runs were performed on a Philips X’Pert X-ray diffractometer with a q-q goniometer using an
automatic divergence slit and Cu Ka radiation. Peaks used for quantification are indicated on the plots. Weighting factors used for quantification of clay
fraction: smectite-1; illite-4; kaolinite-2; quartz-2, feldspars-1; calcite-1. Weighting factors, bulk samples: total clay minerals-20; quartz-7.5; potassium
feldspar-3.7; plagioclase-4.2; calcite-1.6; dolomite-1.6; pyrite-3.
elements in the grain or the aggregate as an aid to
identification.
low feldspar contents in the Brygge and Kai formations
(Well 4-1 and Site 28). Through the Naust formation,
however, there is considerable variation in the sediment
composition.
3. Results
3.1. Mineralogy
Most samples contain a similar suite of minerals (Figs. 5
and 6). The exceptions are the samples from Well 4-1 and
Site 28 that were from Eocene to Early Miocene siliceous
oozes (Brygge formation) and the Miocene to Pliocene Kai
formation, respectively. Where sub-unit boundaries occur
between samples they have been picked from wireline logs
and correlation with high resolution seismic data (Solheim,
pers. com., 2004). Site 31 demonstrates the variability of
results within a single sub-unit (R2). In the following, only
general trends will be presented whereas details can be
examined in Figs. 5 and 6.
3.1.1. Mineralogy of bulk samples
The minerals quantified were quartz, potassium feldspars, plagioclase, calcite, dolomite, pyrite and the total clay
mineral content. The mineral concentrations reflect the large
scale stratigraphy (Fig. 5) showing high clay mineral and
3.1.2. Mineralogy of clay fraction
The clay minerals smectite, chlorite, illite and kaolinite
as well as quartz, potassium feldspars, plagioclase, calcite
and pyrite were quantified in the clay fraction. The clay
mineral composition (Fig. 6) reflects the stratigraphy
showing differences in composition between the Brygge,
Kai and Naust formations. The mineral fraction of the
Brygge formation has high kaolinite contents whereas
chlorite is absent. The Naust W and Naust U units are only
sampled at Site 28 but can be seen to be rich in smectite
compared to all but a few of the samples from the remaining
units that are otherwise dominated by illite. It should be
noted that the chlorite content increases from the Kai
formation into the Naust formation.
3.2. SEM analyses
3.2.1. Microfabric
From the many SEM pictures taken for the various layers
investigated as part of the Ormen Lange project, only a few
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Fig. 5. The results of the mineralogical analyses of the bulk sediment samples. The red lines between columns represent selected horizons. Horizon names appear in the columns.
C.F. Forsberg, J. Locat / Marine and Petroleum Geology 22 (2005) 109–122
Fig. 6. The results of the mineralogical analyses of the clay sized fraction of the samples. The red lines between columns represent selected horizons. Horizon names appear in the columns.
115
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were selected to illustrate the different signatures related to
the main pre-Storegga slide stratigraphic units encountered
in this area. The intervals chosen are the Kai formation
(clayey ooze), the Naust W (hemipelagic) unit, the Naust S3
(debris flow or diamicton)and the Naust R2 (hemipelagic)
sub-units. Their respective typical SEM pictures have been
assembled in Fig. 7.
The microfabric, i.e. the layout of the sediment skeleton,
is very much influenced by the nature of the sediments,
its sedimentary environment, mode of deposition and
post-depositional evolution, and burial stress (Mitchell,
1993). So, the end product, observed today, can be the result
of many processes which have been interacting over time.
The orientation of particles has been evaluated qualitatively.
In many samples, the orientation of clay particles around
coarser (silt or sand) particles is quite intense. In fact, it
seems that if clay particles lie close to any flat or larger
continuous surface, they will get moulded onto it (Fig. 8).
The Kai formation consists of clayey ooze deposits rich
in both siliceous and calcareous microfossils with secondary
Fig. 7. SEM examples of the main facies found in the Storegga area. (a–b): R2 layer at borehole 31, sample 7 at a depth of 97 mbsf; (c–d): S3 layer at borehole
27, sample 5 at a depth of 248 mbsf; (e–f): Naust W layer at borehole 28, sample 5 at a depth of 61 mbsf; (g–h): Kai formation at borehole 28, sample 10 at a
depth of 110 mbsf. In each case, two microphotographs are provided with scale bar at 100 and 10 mm respectively. The small square on the lower magnification
microphotographs show the location of the larger magnification one. The orientation of the sample is normal. Symbols are as follows: DiZdiatom; FoZ Fossil;
MiZmica; OZorganic matter; PZpyrite; SiZsilica; SmZsmectite.
C.F. Forsberg, J. Locat / Marine and Petroleum Geology 22 (2005) 109–122
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Fig. 8. Micrograph of an amalgamated crust that has formed where it was in contact with the larger particle seen in the upper left hand part of the picture. The
crust can be seen to be very thin and does not appear to influence the structure in the clay below (Site 31, sample 18C, 104.2 mbsf) b. Intricate hole with a thin
crust that is interpreted to have formed around a nodule of gas hydrates that decomposed during climatic amelioration (Site 19_2, sample 28-I, 225.8 mbsf).
porosity related to the presence of the various microfossils
(Fig. 7g). As is often the case with siliceous deposits
(Tanaka and Locat, 1999), diatoms’ intra-skeletal porosity
is filled with framboı̈dal pyrite (Fig. 7h). This formation is
particularly rich in smectite which can be seen in Fig. 7h
forming a fine texture of flocculated particles less than 2 mm
in size. Another characteristic of this formation is its higher
content of organic matter, exceeding 2% by weight. Still in
Fig. 7h, a piece of organic matter can be seen. Although its
composition has been confirmed with XREDS, it also
exhibits a typical shrinkage upon freeze-drying leaving a
gap which is about 5 mm in width.
The base of the Naust W unit represents sedimentation in
a hemipelagic environment. This can also be seen at the
microstructure level with a homogeneous texture (Fig. 7e)
and a microfabric that still contains a few flocs (smectite ?)
that may have been protected from greater compaction by
larger silt particles like the mica grain identified in Fig. 7f.
The various debris flow/diamicton sequences reflecting
the glacial or stadial phases on the margin during the
Pleistocene have produced a series of thick layers composed
mostly of glacial sediments with some reworked marine
material. At small magnifications they usually showed
numerous silt particles floating in a well oriented clayey
matrix (Fig. 7c and d). The development of flow features and
the post-depositional compaction may be responsible for the
high degree of orientation of the platy particles.
The Storegga slide deposits exhibit a typical flow like
structure as shown in Fig. 9. Here this sample does not
contain large amounts of fine sand but most of the coarse
particles are more or less aligned with respect to the
horizontal plane. Since some marine sediments may be
reworked during the slide event, it does contain some
microfossils, often as debris (Fig. 9b). The example shown
in Fig. 9 is from a sample taken at a depth of only 9.6 m so
that the degree of compaction is not very high and which
therefore suggests that the orientation observed in Fig. 9b is
only due to the flow phenomena.
4. Discussion
4.1. Influence of environmental changes
The sampling at site 28 provides a good opportunity to
compare two different microstructures resulting from two
Fig. 9. Microphotographs of the Storegga slide deposit at site 18, sample 1 taken at a depth of 9.6 mbsf. The fossil (Fo) seen in (b) consist of calcite and may be
a foraminifera fragment. The two microphotographs are provided with scale bar at 100 and 10 mm, respectively. The small square on the lower magnification
microphotographs show the location of the larger magnification one. The orientation of the sample is normal.
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Fig. 10. Micrographs from Site 28 showing how crusts from contact with larger particles are better developed in the Naust formation (a and b) than the
underlying Kai formation (c and d). Annotations indicate various minerals identified through XREDS analyses.
distinct sedimentary environments. A series of four SEM
pictures of both the Naust W and Kai B units have been
assembled in Fig. 10 to illustrate the control that the
presence of a microfossil skeleton has on the fabric of the
matrix. Surfaces in contact with coarser particles, even at a
fairly shallow depth, are much more amalgamated than
those in deeper samples. In Kai B (Fig. 10d), the matrix
which surrounds the grain is still quite open and very porous
whereas the surface in Naust W (Fig. 10b) is almost
polished. This is an illustration of the sort of ‘potential
strain’ which can be released once the yield strength of the
ooze is exceeded and the skeletal support collapses. A
significant difference is also to be expected in terms of
permeability and compressibility for the units, the ooze
being more compressible and permeable than the Naust
sediments. In the Kai formation there are little or no
aggregates, but rather flocs of fine clay particles, which
allow the observation of the floc porosity (Fig. 10).
The presence of flocs and the lack of signs indicating in
situ mineral transformations in the SEM study is evidence
pointing to the detrital origin of the minerals detected.
In general, the weathering of rocks produces clay minerals
in quite well defined trends (e.g. Jackson et al., 1948). The
clay minerals produced depend on the intensity and duration
of the weathering. In warm humid climates, kaolinite is the
end product with smectite and illite as precursors. The
stratigraphic sequence of clay minerals in our samples
follows this trend. The depositional period from the
Miocene to the late Pliocene is the time interval during
which Northern Hemisphere glaciations were initiated. The
oozes sampled in the Brygge formation (Well 4-1) only
have a very small terrigenous component with a significant
kaolinite fraction, whereas the sediments in the upper part of
the Kai formation sampled at Site 28 have a significant
contribution from both biogenic and terrigenous sources
(Fig. 7). The terrigenous fraction is smectite-rich. The
greater terrigeneous contribution may reflect the increased
denudation rate on land and the smectite the deeper erosion
of weathered rocks (i.e. rocks that are not completely
weathered). However, oceanic currents flowing northwards
have probably given the sediments their contouritic seismic
facies (Bryn et al., 2005b) and may also have transported
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sediments from the continental margin to the south and
therefore from the region around the volcanogenic Faeroe
Islands. Smectite is a common product of weathering of
basaltic igneous rocks (Blatt et al., 1980). In concordance
with this, Eisma and van der Gaast (1983) have mapped
high smectite contents in the sediments around the Faeroes.
Irrespective of whether the smectite is from erosion of the
hinterland or the Faeroes region, the increased detrital input
to the sediments is a signal of greater erosional activity and
is interpreted to reflect that the pre-glacial land areas were
probably eroded by fluvial processes and covered by
weathered rocks and soils in which the content of kaolinite
was high. We therefore suggest that the progressive
denudation by increased glacial activity gradually exposed
less weathered and more crystalline rocks resulting in the
mineralogical changes observed. The lower part of the Kai
formation has not been sampled. Any transitional changes
closer to the Brygge formation are therefore unknown.
Krissek (1989) presents a similar stratigraphic development
and interpretation to ours from ODP Leg 104 on the Vøring
Plateau.
The base of the Quaternary, Naust W, is dominated by
hemipelagic deposits from before the first continental shelf
glaciation (1.1 Ma, Haflidason et al., 1991). However, there
is a significantly lower biogenic component than in the Kai
formation (Fig. 7). The increase in the relative importance
of physical erosion is probably signalled by the appearance
of chlorite, a mineral that is associated with low grade
metamorphic rocks and an early stage of weathering of
igneous and intrusive rocks. Glacial ‘flour’, fine grained
non-clay minerals in the clay fraction, is found throughout
the Kai and Naust formations and also demonstrates the
effect of physical erosion. Support for this notion comes
from seismic stratigraphy (Berg et al., 2005) that shows the
Naust U2 sub-unit to be the first glacial debris flow deposit,
signalling glacial expansion to the shelf edge. Debris flow
deposition did not, however, reach Site 28 where Naust U is
glacial marine/hemipelagic (Fig. 10). The distal position of
Site 28 in relation to most of the other sites (Fig. 1), may
also explain the high smectite contents because current
transport from the Faeroes to the south is probably relatively
more important in this setting as demonstrated by the results
of Eisma and van der Gaast (1983) who mapped a high but
decreasing smectite content in the Norwegian Sea from the
Faeroe Islands and northwards along the Norwegian margin.
The Naust formation was deposited under the influence
of the Quaternary climatic fluctuations. In order to examine
the changes more closely, the results have been compared to
grain size analyses. However, grain size analyses were not
performed on the same samples as the mineralogical
samples, but within each sub-unit variations in the total
clay mineral content calculated from the bulk XRD scans
can be seen to reflect similar changes in the clay content
from grain size analyses (Fig. 11). Such a correspondence is
as one should expect because the clay minerals have the
highest relative concentrations in the clay fraction and
119
means that the total clay mineral content can be used as a
rough proxy for the grain size of the sediments on which the
mineralogical analyses were performed.
There is a higher smectite (Fig. 12) content in samples
with higher clay mineral concentrations, i.e. the more fine
grained samples. Furthermore, the relative concentration of
feldspars is lower and carbonates higher in bulk samples
with high clay mineral contents (Fig. 13). The more fine
grained interglacial or interstadial units as demonstrated by
samples from Sub-unit R2 (Fig. 7) exhibit a homogeneous
microfabric and can be distinguished from the debris flow or
diamicton deposits by its much larger content of microfossils (e.g. foraminifera and diatoms), a smaller content of
dispersed silt particles and a lesser degree of orientation of
the platy particles in the clayey matrix. Because feldspars
are depleted and smectite may be enriched in weathered
soils and sediments, the results are interpreted to reflect the
relative contribution of erosional products of crystalline
rocks on one hand and from sedimentary rocks and
weathering combined with ocean current transport on the
other. The availability of most of the clay minerals from
physical erosion is demonstrated by their widespread
occurrence in the North Sea area (Irion and Zollmer,
1999). However erosion of crystalline rocks is interpreted to
have been associated with glaciations whereas a greater
relative contribution from sedimentary rocks, weathering
and current transport are thought to signal warmer periods.
Currents were also active during glacials and there has been
reworking of interglacial sediments during glacial periods.
The result is that that there is a gradational change in
sediment composition both laterally and stratigraphically
that reflects changing climatic and oceanographic
conditions.
The marine /glacial marine sediments are more sensitive
(loose strength upon deformation; Berg et al., 2005;
Kvalstad et al., 2005) than the diamictons and debris flow
deposits, and is a property that has been interpreted as the
main cause of their ubiquitous utilization as slip planes for
the slides (Berg et al., 2005; Bryn, et al., 2005a; Kvalstad
et al., 2005). While these layers contain more smectite than
the glacial sediments from the same site, we do not think
that the differences are great enough to explain this
phenomenon. We are of the opinion that the grain size
distribution and the consequences this has for the physical
properties (e.g. higher plasticity index and water content;
Mitchell, 1993) is of greater importance.
4.2. Indications of gas hydrates
Almost polished surfaces observed surrounding coarser
particles embedded in the clay matrix is seen on surfaces of
silt or sand particles in contact with the clayey matrix
(Figs. 8 and 10b). In marine clays the particle arrangement
is usually flocculated, with edge to flank contacts. The crust
formation observed here, where the distinction between
individual particles disappears, may perhaps be termed
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Fig. 11. The clay content of the samples (CC) compared to the total content of clay minerals (TCM) in the bulk samples. The right hand columns represent the clay content.
C.F. Forsberg, J. Locat / Marine and Petroleum Geology 22 (2005) 109–122
121
embedded particle. The cast therefore appears to have been
produced by a solid that has dissolved without disturbing the
matrix. A tentative interpretation is that is was produced
during an earlier presence of gas hydrate, that disappeared
during a climatic amelioration. Gas hydrates are not stable
at the site at present, but were so during glacial periods.
5. Conclusions
Fig. 12. Ternary diagram showing the relative concentrations of Illite,
smectite and chloriteCkaolinite for the clay fractions of Naust formation.
The shading indicates the total content of clay minerals found in the bulk
sediment.
‘cold bounding’ (Mitchell, 1993; Locat et al., 2003) and
involves the transformation from edge to flank to flank to
flank contacts. The kinetics and mechanisms involved in the
process are not known, but our observations here show that
the effect is limited to a very thin zone (Fig. 8) close to the
larger particles which suggests it is a surface phenomenon.
Fig. 8b shows an intricately shaped void with a coating
(one of several in this sample) surrounded by an undisturbed
clay matrix. The latter seems to exclude the extraction of an
Both the mineralogy and microfabric reflect the changes
in depositional processes that have occurred in the study
area.
There is a compositional transition from the Miocene
Brygge formation through the late Pliocene Kai to the
Quaternary Naust formation that reflects a change in oceanic
circulation and climate in conjunction with growing glacial
activity that involves
1. An increase in terrigenous and a reduction in biogenic
sedimentation.
2. Mineralogical changes from kaolinite-rich Miocene
sediments through smectite-rich late Pliocene to illitedominated Quaternary deposits that is interpreted to
reflect gradual denudation of a weathered regolith and
sedimentary rocks and an increased erosion of crystalline
rocks.
Fluctuations in the Quaternary mineralogy is mostly
related to the inter-fingering contribution from glacial and
glacial marine and hemipelagic processes that can be seen in
both the microfabric and the fluctuations in the mineral
contents. Higher smectite and calcite concentrations reflect
contouritic and biogenic deposition, respectively, and are
associated with the more fine grained sediments from
interstadial and interglacial periods. Coarser, more glacially
influenced sediments have higher feldspar concentrations
reflecting a higher contribution from physical erosion.
The more fine grained nature of hemipelagic/glacial
marine deposits is thought to be more important than their
somewhat higher smectite content in determining their
ubiquitous use as slip planes for slides in the Storegga
region.
Voids interpreted to have been left behind after
dissolution of gas hydrates may have been detected.
Acknowledgements
Fig. 13. Ternary diagram showing the relative concentrations of feldspars,
quartz and carbonates in the bulk sediments of the Naust formation. The
shading indicates the total content of clay minerals found in the bulk
sediment.
Norsk Hydro ASA and partners in Ormen Lange license
are gratefully acknowledged for their support of this work
and the release of the both these and associated data sets.
Referees are acknowledged for their suggestions and
comments that were very helpful in shaping the final
manuscript. The work has also in part been financed by
122
C.F. Forsberg, J. Locat / Marine and Petroleum Geology 22 (2005) 109–122
the EUROMARGINS programme. This is International
Centre for Geohazards (ICG) contribution number 66.
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