Merging the concepts of pseudokarst and paleoseismicity in Sweden: A unified theory on the formation of fractures, fracture caves, and angular block heape

Abstract

Sweden has thousands of caves cut into the bedrock, two thirds of which occur in the
crystalline bedrock, and hence represent pseudokarst phenomena. The formation of these
caves can only be understood in terms of paleoseismics. In this paper, we review the parallel
evolution of the concept of pseudokarst caves and of the concept of paleoseismic activity
in Sweden, and combine both concepts into a unified theory on the formation of fractures,
fracture caves and angular block heaps.

Full text

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International Journal of Speleology
Off icial Journal of Union Internationale de Spéléologie
*morner@pog.nu
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Abstract: Sweden has thousands of caves cut into the bedrock, two thirds of which occur in the
crystalline bedrock, and hence represent pseudokarst phenomena. The formation of these
caves can only be understood in terms of paleoseismics. In this paper, we review the parallel
evolution of the concept of pseudokarst caves and of the concept of paleoseismic activity
in Sweden, and combine both concepts into a unified theory on the formation of fractures,
fracture caves and angular block heaps.
pseudokarst, fracture caves, block caves, paleoseismics, methane venting tectonics
Received 21 August 2018; Revised 5 October 2017; Accepted 7 October 2018
Mörner N.-A. and Sjöberg R., 2018. Merging the concepts of pseudokarst and paleoseismicity
in Sweden: A a unified theory on the formation of fractures, fracture caves, and angular block
heaps. International Journal of Speleology, 47 (3), xx-xx. Tampa, FL (USA) ISSN 0392-6672
https://doi.org/10.5038/1827-806X.47.3.2225
Merging the concepts of pseudokarst and paleoseismicity
in Sweden: A unified theory on the formation of fractures,
fracture caves, and angular block heaps
Nils-Axel Mörner1 and Rabbe Sjöberg2
1Paleogeofysics & Geodynamics, Stockholm, Sweden
2Obbola, Umeå, Sweden
International Journal of Speleology 47 (3) xx-xx Tampa, FL (USA) September 2018
The author’s rights are protected under a Creative Commons Attribution-
NonCommercial 4.0 International (CC BY-NC 4.0) license.
INTRODUCTION
The Swedish Speleological Society has a database
including 2871 classified caves (SSF, 2018). Out
of those, 410 (14.3%) refer to true karst caves, 309
(10.8%) refer to shore caves (including “tunnel caves”),
and the vast majority of sites (1919 or 66.8%) refer
to block and fracture caves. This is an effect of the
fact that Swedish bedrock predominantly consists of
crystalline bedrock, especially gneisses and granites.
The word “pseudokarst” has a somewhat unclear
definition. It implies, however, that it is not a question
of dissolutional weathering, but coastal erosion (shore
caves), bedrock fracturing (the cave formation of
which has been termed quite differently; viz.: granite
caves, bedrock caves, boulder caves, block caves,
block heaps, etc) and scree deposits.
In this paper we review the evolution of observing,
documenting and understanding such pseudokarst
caves and scree accumulations in Sweden. Previous
standard references were by Sjöberg (1994a, b),
Mörner (2003) and Mörner & Sjöberg (2011).
The Swedish speleological society (Sveriges Speleolog-
Förbund, SSF) was formed in 1966 by Leander Tell. It
issues a journal named “Grottan” (The Cave), has a
publication series named “Svenska Grottor” (Swedish
Caves) and a database “Grottdatabasen” (SSF, 2018).
In 2007, SSF hosted the Baltic Speleological Congress
on the Island of Gotland (Gustafsson, 2007), and in
2011, it hosted the Second International Conference
on Granite Caves (Mörner & Sjöberg, 2011).
Figure 1 shows the distribution of caves registered
in the SSF database (SSF, 2018). Table 1 gives the
distribution of numbers and lengths of pseudokarst
caves in Sweden: the majority are less than 25 m long
and the longest 2,633 m long. The lengths of the 10
longest pseudokarst caves are given in Table 2.
Fracture caves and block caves are formed by
tectonic processes and typically consist of fracture
patterns and angular blocks with sharp edges, as
illustrated in Fig. 2. Tunnel caves (Sjöberg, 1986a,
1990) are formed by littoral erosion as illustrated in
Fig. 3.
Leander Tell is regarded as the “Father of Swedish
Speleology” with a benchmark book (Tell, 1955),
followed by other books (Tell, 1962, 1969) and a number
of cave catalogues (Tell, 1963, 1966, 1970, 1974).
Caves have, of course, been observed far back in
history. In the early 17th century, King Karl XI was
carried up to a shore cave hanging in the mountainside
at 120 m a.s.l. in Ångermanland (Fig. 4), where the
rate of uplift has been at its maximum. During his
travel to Lapland, Carl von Linné visited this cave
and several other caves along the Bothnian coast
(Linnaeus, 1732). Caves were often hiding places
for robbers and malicious people. In pre-historic
times, however, they seem to have been avoided for
mythological reasons, rather than used for habitation
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International Journal of Speleology, 47 (3), xx-xx. Tampa, FL (USA) September 2018
Fig. 1. Distribution of 2871 caves (red dots) registered in the Swedish
cave database (SSF, 2018). The caves named refer to the six caves
in the “gallery” examples plus two MVT sites discussed in the text.
Length in m Number
>2500 1
1000-2500 1
500-1000 4
250-500 4
100-250 25
50-100 38
25-50 144
10-25 463
<10 1323
Table 1. Distribution of pseudokarst caves (in total 2054) with respect
to their length.
Name of cave County Length
(m)
Type of
cave
Bodagrottorna Hälsingland 2,633 fracture
cave
Hölickgrottan Hälsingland 1,340 fracture
cave
Almekärrgrytet Småland 610 fracture
cave
Örnnäset Hälsingland 520 talus cave
Strångbergsgrottan Jämtland 510 fracture/
block cave
Gillberga gryt Uppland 500 fracture/
block cave
Ö. Klövbergsgrottan Södermanland 362 block cave
Frugrottan Södermanland 320 fracture/
block cave
Töllsjögrottan Västergötland 300 talus cave
Stora
Trångbergsgrottan Västmanland 250 talus cave
Table 2. List of the 10 longest pseudokarst caves in Sweden.
or other cultural activities. Only occasionally do we
find ancient tools in Swedish caves.
FRACTURE CAVES AND BLOCK CAVES
There are numerous caves in the Precambrian
crystalline bedrock of Sweden associated with
fractures and block tectonics. There are also heaps
of huge blocks, within which there are caves. These
structures have nothing to do with weathering
and dissolution, but refer to broken up fragments
from the bedrock surface. They are all examples of
“pseudokarst”, although they usually have been
termed “fracture caves”, “block caves”, “boulder
caves”, “granite caves” and “talus caves” (it should
be noted that the word “boulder” has been used in
Sweden for big blocks without respect to presence or
absence of rounding).
Because Swedish geological mapping is divided into
a solid rock branch and a Quaternary branch, caves
and loose block heaps happened to fall in between
those units, and therefore were largely ignored in the
geological mapping. Instead, the study of Swedish
block caves came to be driven by the spirit and
curiosity of individual persons like Bergsten (1943),
Agrell (1981, 1982), Isacsson (1982, 1990), Sjöberg
(1987a, 1994a), and Mörner (2003).
Deformational till and bouldery moraines
In a very few cases boulder heaps can be explained
as “deformation till” in the sense of Dreimanis (1969)
and Mörner (1973). This concept has been generally
ignored in Swedish Quaternary geology, however.
Fig. 2. A typical seismotectonic cave consisting of huge angular
blocks torn out of the bedrock due to a major paleoseismic
event in postglacial time. Only a seismic event is capable to
move these huge blocks anti-gravitationally upwards (which is
the case here, as it is in many other cave sites).
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Some De Geer moraines (terminal end moraines)
consist of big boulders and have become known as
“bouldery moraines”. The majority of “pseudokarst
caves” in Sweden have nothing to do with ice-
marginal till accumulations, however. Neither have
they anything to do with “glacial tectonics”.
In a few cases, however, bouldary moraines of a
proposed seismic origin have been described by De
Geer (1938, 1940) and Mörner (1990), suggesting that
the bedrock surface was broken up into loose blocks
by earthquakes when still in subglacial position
(Mörner, 1978, 2017b, c).
Fig. 3. The Räckebergskyrkan tunnel cave located at an elevation of
180 m a.s.l. It is the largest and highest elevated tunnel cave in Sweden.
It can be regarded as an example of a tunnel cave.
Bedrock fracturing, seismotectonics
and paleoseismology
Open bedrock fractures, granite caves, boulder
caves, block caves and block heaps imply fracturing
of the former surface of the crystalline bedrock. In
association with glaciation and deglaciation, the
Scandinavian bedrock became smoothly polished
and striated (Mörner et al., 2008, Fig. 9). Therefore,
fractures and faults in this glaciated bedrock
surface provide evidence of postglacial fracturing.
This fracturing of previously solid bedrock must be
understood in terms of seismotectonics (Mörner,
1978, 2003; Sjöberg, 1987a, 1994a). Today, Sweden
is a country of low to moderately-low seismic activity.
At the time of deglaciation, however, Sweden has, in
the last decades, turned out to be an area of high-
seismicity (Mörner, 1991, 2003). Therefore, there is no
longer any geodynamic problem in referring Swedish
“pseudokarst” phenomena to seismotectonics (Mörner
& Sjöberg, 2011). This applies not only to the deglacial
phase (e.g., Mörner, 1991, 2003) but also to the Late
Holocene (Mörner, 2007, 2009, 2017b).
Fig. 4. A shore cave (“Kungsgrottan”) is hanging in the steep slope
of Mt. Skuleberget at a level of +120 m, which is the level of the first
Littorina sea level peak at 7000 C14-yrs BP. This implies a minor
retardation in the rapid shorelevel displacement (relative uplift),
providing time enough for a littoral cave to be eroded.
From parallel concepts to unified theory
The earliest observation of postglacial fracturing
and interpretation in terms of earthquake origin come
from De Geer in 1879 (De Geer, 1940, p. 124). De
Geer observed that big angular blocks were sometimes
concentrated in the end-moraines. He termed such
moraines “seismic moras”.
In 1929, von Post published interesting data from
Säffle in SW Sweden. Old drawings (dated 1915) from
the clay pit of Älvängen record extensive sedimentary
deformations in the form of folds and faults (von Post,
1929). The site was later revisited by Mörner (Mörner,
2003, p. 256).
Bergsten (1943) was the first to demonstrate that
the block cave of Torekulla was a product of tectonics.
A similar view was expressed for the Trollegator
(Trollgatere) cave site (Mårtensson and Nilsson, 1971;
Bergsten, 1976). Other observations and suggestions
of a seismotectonic origin were given by Agrell (1981,
1982), Sjöberg (1987a), and Isacsson (1990).
In 1994, Sjöberg took his PhD at the institute of
Paleogeophysics & Geodynamics at Stockholm
University on a thesis entitled “Bedrock caves and
fractured rock surfaces in Sweden: occurrence and
origin”. He identified six possible causes to the
cave formation: (1) glacial tectonics, (2) freeze and
thaw processes, (3) methane venting, (4) postglacial
unloading (and stress release), (5) hydrofracturing,
and (6) seismotectonics. The external examiner (Dr.
Douglas Grant from Canada) said: “I think you have
identified all possible processes, and I cannot think of
any others”. In his thesis, Sjöberg (1994a) evaluated
the possible processes against the characteristics of
53 pseudokarst caves distributed over almost the
whole of Sweden. The result is given in Table 3. It
shows that seismotectonics is the most probable
origin for 52 sites (and possibly for the remaining one
site, too). Therefore, seismotectonics emerged as the
most probable origin of the Swedish fracture caves,
block caves or boulder caves, whatever name we may
use for these pseudokarst caves in the crystalline
bedrock, or within heaps of loose angular blocks.
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International Journal of Speleology, 47 (3), xx-xx. Tampa, FL (USA) September 2018
This was a major step forward in the understanding
of Swedish non-karstic cave formation. It marked a
paradigm shift in Swedish speleology.
Fig. 5. Uplift of the Fennoscandian shield in the last 13000 years with
contours in hundreds of meters of total uplift (from Mörner, 2003),
giving rise to extension in the vertical, radial and tangential directions
(Mörner, 1991).
is characterized by low to moderately low seismicity.
He proposed (Mörner, 1991) that the rate of glacial
isostatic uplift with rapid extensional forces in the
vertical, radial and tangential dimensions initiated
the high seismicity during deglaciation when the rate
of uplift peaked and amounted to tens of cm per year
(0.5 to 1.0 mm per day). This is illustrated in Fig. 5.
Process Impossible Possible Probable
Glacial tectonics 37 15 1
Frost deformation 30 20 3
Methane venting 40 11 2
Stress release 10 29 14
Hydro-fracturing 10 30 13
Seismotectonics 0 1 52
Table 3. Sjöberg’s semi-quantitative analyses of possible mechanisms
behind the formation of 53 Swedish fracture and block caves (Sjöberg,
1994a).
A paradigm shift in Swedish seismology
In the 1950s and 1960s, the Fennoscandian Shield
was generally assumed to be of exceptional stability.
Shoreline diagrams by Nilsson (1968) in Sweden and
Hyyppä (1966) in Finland were claimed as evidence
of this stability. However, Mörner (1979, 1980) and
Donner (1980) demonstrated that those diagrams are
no longer tenable.
In the 1960s, two major steps forward were made:
(1) the repeated levelling identified irregularities
(Asplund, 1968; Mörner, 1977a), and (2) C14-dated
shorelines identified the presence of tectonic fault-
lines where the uplift isobases were bent (Mörner,
1969, 1977a).
In the 1970s, there were several important events:
(1) the project on “postglacial earth movements”
was initiated in 1973 as a part of the International
Geodynamics Project (GDP), (2) because of the
achievements within the project (Mörner, 1975), the
international GDP group asked Mörner to organize
an international conference in Sweden, (3) this
conference was held in Stockholm in 1977 together
with a field-excursion through southern Sweden
(Mörner, 1977b, 1980), (4) Mörner (1977c) reviewed
uplift irregularities and traces of possible paleoseismic
events in Sweden (it seems that this was the first time
the word “paleoseismic” was used), (5) Lundqvist and
Lagerbäck (1976) published the first account of the
Pärve Fault in northern Sweden, (6) Mörner (1978)
published the first account on paleoseismics in the
Stockholm area, (7) Mörner became the leader of
the INQUA Neotectonics Commission and started
issuing the Neotectonics Bulletin, which became
an international source for the evolution of the
understanding of neotectonics and paleoseismics.
The Bulletin came out in 19 annual issues from 1978
to 1996. Mörner (1985) summarized available solid
rock evidence, geomorphic evidence, sedimentological
evidence and geophysical evidence indicating
paleoseismic events in postglacial time. Additional
observational data followed from sedimentary data
(e.g., Mörner & Tröften, 1993; Tröften, 1997), block
cave generation (Sjöberg, 1994a) and bedrock faulting
(Lagerbäck, 1990; Mörner, 1990).
In 1991, Mörner presented a mechanism for
the interpretation of the observation that the
deglacial period was characterized by frequent high-
magnitude seismic events, whilst the present period
In 1999, we felt ready for an international
demonstration and discussion of our field data on
seismotectonics and liquefaction (Mörner et al., 2000).
An international excursion was organized (Mörner,
1999). It gave a thorough profile across the whole
of Sweden from Umeå in the north to Båstad in the
south. The excursion was attended by 40 specialists
from all over the globe. The discussions were very
fruitful, and lay the ground for extended analyses
(Mörner, 2003).
Our working methodology was novel in the
combination of fault data, sedimentary data,
liquefaction structures, turbidite levels, varve
chronology and tsunamite data (Mörner et al., 2000;
Mörner, 2003, 2011). Magnetic grain rotation was
also documented (Mörner & Sun, 2008).
A unified picture takes form
As an outcome of the description of the Boda cave by
Sjöberg (1994a), we were able to force the nuclear waste
handling organization to finance a major research
project on the Boda cave and related phenomena in
the surrounding areas (Mörner et al., 2003).
A group of international experts (Professor Franck
Audemard from Venezuela on liquefaction, Doctor
Sue Dawson from Scotland on tsunami deposits and
Professor Andrej Nikonov and Doctor Dmitri Zykov
from Russia on block motions and seismicity) joined
our research group at the Institute of Paleogeophysics
& Geodynamics at Stockholm University in 2000-2001.
The Boda cave was investigated and documented in
detail. Thanks to the varve chronology, the creating
earthquake could be dated at varve year 9663 BP
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(Mörner, 2003, 2013a, 2016a). It was linked to a major
liquefaction event deforming sediments over an area of
at least 80x40 km. It also set up a tsunami wave of at
least 12.5 m height that was recorded over an area of
125x30 km. A turbidite was spread over the seafloor
over a distance of 210x40 km right in the annual
varve of varve-year 9663 BP. Simultaneously, fracture
caves and block caves were created and recorded over
an area of 50x50 km. The primary fault has a scarp
height in the order of 10-20 m. All this speaks clearly
of a causation earthquake of M > 8 (Mörner, 2003),
later fixed at M 8.5-8.6 (Mörner, 2017a).
The Boda cave investigation (Mörner et al., 2003)
probably represents one of the most well-documented
paleoseismic events in the world. It is thus the “core
stone” for the formation of a unified picture of the
mechanisms, characteristics and distribution of
paleoseismic events in Sweden (Mörner, 2003; 2013b;
2016b, 2017a).
International anchoring
The above-mentioned international excursion in
1999 marked an important step in our formulation
of the concept of a high-seismic activity in Sweden in
former times (Mörner, 2003).
As a part of the 33rd International Geological
Congress in Oslo in 2008, we organized an excursion
on Paleoseismicity and Uplift of Sweden in two parts:
A, from Umeå to Stockholm, and B, from Stockholm to
Båstad (Mörner et al., 2008; Sjöberg, 2008). It marks
another key point in our studies.
In 2011, Mörner & Sjöberg (2011) organized and
hosted the Second International Conference on
Granite Caves in 2011. It offered another occasion
to direct the limelight on Swedish fracture caves
and block caves. The guidebook (op. cit.) provides a
representative illustration of various forms of “granite
caves” in Sweden.
SCREE DEPOSITS
In his study of pseudokarst caves in Sweden,
Sjöberg (1994a) included “caves in collapsed rock
walls” as his “type 2” caves, and identified 21-23 of
them. In the present SSF database, there are 33 scree
accumulations registered. The distinction between
talus deposits successively accumulated over time
and the sudden breaking-off and falling down of huge
rock masses is sometimes a fine one. In the first case,
it is a matter of time and gravity. In the second case,
there must be a release force, which seems logical to
ascribe to seismic ground shaking (at least, nowadays,
when this process has become well documented all
over Sweden).
The sites Pyhävaara, Slåtterdalsgrottan and Ulorna
(Sjöberg, 1994a, Figs. 26, 27, and 29), indeed, seem
to represent momentous breakings-off of huge rock
volumes, which are likely to have been triggered by
paleoseismic events. The movement of the Pyhävaara
scree can be directly connected with the Lainio Fault
movement (Lagerbäck & Witschard, 1983), which
strongly supports the conclusion of Sjöberg (1994a).
In association with the 9663 BP paleoseismic event
in the Hudiksvall area, huge scree masses were
detached at Blackåsberget and Storberget (Mörner et
al., 2003, p. 109-111), hosting caves. A characteristic
feature observed was that the scree accumulations at
the foot of the scarps were not located at the logical
(gravitationally straight-down) position, but lay tens of
meters outside, as though moved by an extra ground
shaking motion (i.e., paleoseismics). The Blackåsberget
rock scarp and scree are shown in Fig. 6.
Fig. 6. The Mt. Blacksåsberget bedrock scarp is traversed by slope-parallel fractures with a big-boulder scree
accumulation beneath, hosting caves (modified from Mörner et al., 2003; Sjöberg, 1994a). The scarp is high
and steep and strongly fractured (left). The scree has not just fallen down gravitationally, but also been moved
tens of meters laterally (blue arrows to the right) due to earthquake forces.
The Eastern Klövberget caves occur in a huge scree
deposit (Fig. 7). Here we are able to demonstrate
that the entire mass moved in one big scree unit,
the movement of which is most likely to have been
initiated by a strong earthquake. The simultaneous
mass movement is indicated by the preservation of
the original fracture patterns in the scree masses
(Fig. 7). According to Mörner & Sjöberg, (2011; Stop 6
of the Local Excursion) this may imply a novel model
of proving that a scree moved as one huge unit in
response to an earthquake.
It should be stressed that numerous scree
accumulations, some of which may contain block
caves, represent successive gravitational talus
movements of rock fragments, and are of an aseismic
origin.
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Fig. 7. The Eastern Klövberget caves occur in a huge scree accumulation (from Mörner & Sjöberg, 2011). Because the fracture pattern can
still be identified in the scree masses, the entire scree must have moved in one huge mass. This is indicative of a paleoseismic origin.
METHANE VENTING TECTONICS (MVT)
Björklund (1990) was the first to suggest that
methane venting might have something to do with
the fractured bedrock observed both in Finland and
Sweden. It was followed by general discussion of
this concept (Mörner, 1993), and a comprehensive
study of granitic caves in Sweden (Sjöberg, 1994a).
Both authors appreciated the possibility of this
mechanism, but neither was then able to present
any clear observational facts. Definite evidence came
with the study of the Boda Cave (Mörner, 2003) and
the comprehensive presentation on Methane Venting
Tectonics (Mörner, 2017b, c).
Mörner recognized at least 13 sites in Sweden, and
1-2 sites in Finland of methane venting tectonics
(Mörner, 2017b, 2017c). Out of the 13 Swedish sites,
10 date from the late glacial period (with 4 from the
9663 BP event) and 3 from the Late Holocene.
Figure 8 illustrates our interpretation of methane
venting tectonics triggered by the 9663 BP earthquake,
and the deformational structures recorded at the site
of deformation at the Boda Cave (Mörner at al., 2003;
Mörner, 2017b).
North of Hudiksvall, lies the Skålboberget site
(Mörner et al., 2003, p. 105-109; Sjöberg, 2009;
Mörner, 2017a-c; ). It refers to a 25 m high cone
(with a base of about 110x170 m at an elevation of
+32 m) consisting of angular blocks, and with gigantic
blocks at the top (with volumes in the order of 8,000 m3).
A minor cave at the base includes rounded beach
boulders (Fig. 9), indicating that the cone formation
occurred after the uplifting beach was at sea level at
3200 BP (Mörner, 2003, p. 105-109), because there
Fig. 8. Map of the Boda Cave surface (a) with 13 minor centres of
deformation (red dots), the mode of initial expansion followed by
contraction (b) and model (c) of interaction between seismic ground
shaking and methane venting (from Mörner et al., 2003; Mörner, 2017b).
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Fig. 9. The Skålboberget site represents the deformation at an
explosive methane venting (from Mörner et al., 2003; Mörner, 2017b).
The cone of blocks is 25 m high, and has some gigantic blocks at
its top (a). The block cone contains several caves; one at the base
contains rounded littoral boulders (b), indicating a deformation in
Late Holocene time when sea level had fallen to +26 m at 2900 BP.
are no traces of marine abrasion of the cone itself.
A 12 m high tsunami wave was generated in direct
association with the bedrock deformation (Mörner &
Dawson, 2011; Mörner, 2017c). The tsunamite layer
was dated at older than 2,411 ± 50 cal. yrs BP. The
venting of methane affected the lake environment,
and most C14-dates were strongly affected by dead
carbon (Mörner, 2017c), with the largest deviation
being 18412 ± 378 cal. yrs BP instead of 2900 BP
(the time of the event). This contamination provides a
good confirmation of dead carbon from the methane
venting. The methane venting tectonic took place
when sea level was at +26 m, which occurred at 2900
BP (Mörner, 2017c, p. 9).
400,000 m2) at the level of +15 m (Mörner, 2017c).
This level (with a lot of fine structures documented
in 2018) corresponds in age to about 2800-3000 BP
(Mörner, unpublished).
Fig. 10. Kvarnberget, a 25 m high cone of huge blocks hosting caves
(from Mörner & Sjöberg, 2011; Mörner, 2017a, b). At the top of the block
cone there are some gigantic blocks (a). The cone is surrounded by
depressions (b), which, via an 800 m long and 10-15 m deep canyon
cut into clay deposits, end in a huge delta at +15 m. This implies that
the methane venting episode took place at about 2900 BP.
Kvarnberget refers to a site south of Stockholm,
which can only be understood in terms of violent
methane venting (Mörner, 2017b, c). It was first
observed by Sjöberg (2009), structurally investigated
in detail by Mörner (Mörner & Sjöberg, 2011) and
fully presented in Mörner (2017b, c). Later field
studies in 2018 (Mörner, unpublished) have further
pinpointed the age determination at about 2900 ±
100 BP.
Kvarnberget is a 25 m high cone of blocks broken
loose from the solid bedrock beneath. The block cone
is full of minor caves. At the top there are two gigantic
blocks (Fig. 10). The cone is surrounded by erosional
troughs continuing in a 10-15 m deep river canyon cut
in clay deposits, which ends in a huge delta (of about
SHORE CAVES
Shore caves are another type of pseudokarst. They
are formed by littoral wave actions, and have little or
no relation to seismotectonics. Hence our discussion
is brief.
De Geer (1902) and Lindström (1902) were both
pioneers in the study of shore caves. Whilst De
Geer correctly understood that shore caves were
predominantly formed by stones and blocks moved
by the littoral wave action, Lindström advocated
streaming water. Munthe (1920) gave a general
account on the occurrence of “shore caves” (Swedish:
strandgrottor) in Sweden.
The “tunnel-cave” (Sjöberg, 1986a) is a special type of
shore cave abraded along a single sub-vertical fracture
(Fig. 3). The caves have a typical well-rounded cross-
section. In a number of papers, Sjöberg addressed
the occurrence and formation of these caves (Sjöberg,
1981, 1982, 1983a, b, 1984, 1985, 1986b, c, 1987a,
1991, 1994b). He found that the formation of these
caves needed fractures that were directed toward
the sea (with a long relative fetch) and the prevailing
winds. Furthermore, the volume of abraded rock
was, at the time of formation, depending on sufficient
access of abrading material in the form of washed till.
In northern Sweden, the rate of uplift is still very
fast: up to 10 mm/yr with an exponentially increasing
rate back in time (Mörner, 1979). This means that
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International Journal of Speleology, 47 (3), xx-xx. Tampa, FL (USA) September 2018
littoral erosion can only have had a short duration
of activity at a single elevation when passing through
the shore zone. Nevertheless, Sjöberg (1982, 1983b,
1984, 1987a, 1991, 1994b) reported numerous
tunnel caves hanging in the hillsides inland, far above
present sea level (Fig. 4). They seem to date from the
Littorina Sea period of the Baltic and to represent
minor retardations in the relative sea level rise, due to
the effect of eustatic oscillations (Mörner et al., 2008).
At Lidberget, south of Umeå, there are at least five
good examples of littoral caves (Sjöberg, 1982, 1991).
They are concentrated at 90-110 m above sea level,
and are associated with an extensive fracture system
(Mörner et al., 2008, Fig. 26), formerly filled with
dykes of dolerite (Sjöberg, 1983b).
Some caves and abraded depressions may have
got their primary erosion by water-flow under high
Fig. 11. The Boda cave as seen from the air: a hilltop transformed into
a large field of huge, angular blocks from the underlying crystalline
bedrock (from Mörner at al., 2003).
Fig. 12. Map of the sub-surface caves and passages of a total length of
2633 m (from Mörner et al., 2003). The caves have been followed 25 m
down, but the roots of the cave seem to go down 100-150 m.
Fig. 13. Mode of deformation as recorded in two cross-profiles (from Mörner
et al., 2003). It shows the interaction of an extension force deforming the old
bedrock surface into blocks moved upwards and outwards, followed by a
contraction force with blocks falling back
hydrostatic pressure along fractures and bedrock
irregularities in subglacial position (Mörner,
2003, p. 238).
CAVE EXAMPLES
After the above theoretical review of Swedish
pseudokarst caves, we now turn to a representative
“gallery” of some of the cave structures actually
recorded.
The Boda Cave
The Boda Cave at Iggesund represents a former
bedrock hill deformed into a seemingly chaotic
field of angular blocks (Fig. 11). The sub-surface
contains a system of cave passages of a total
length of 2,633 m. The cave system was skilfully
mapped by Alf Sidén. His map (Fig. 12) agrees
perfectly well with our air photography (Fig. 11) as
further discussed in Mörner et al. (2003). Though the
surface may look chaotic (Fig. 11), a closer analysis
reveals that much of the surface remains more or less
in place (as seen in glacial striae and rock veins). This
indicates that the deformation took place in two steps;
first a nearly “explosive” extensional deformation, and
second a falling-back contraction as illustrated in
Fig. 13. This mechanism (Mörner et al., 2003) also
seems to apply to most of the other bedrock block
caves in Sweden.
The cave system goes down 25 m below the surface.
The root fractures of the cave system seem to reach
at least 100-150 m below the surface, however,
judging from our deep drillings. The surface gives an
impressive view of huge angular blocks where one
side is a smoothly polished bedrock surface from
the pre-deformational glacial surface (Fig. 14). This,
too, is a characteristic criterion of Swedish bedrock
block caves. Figure 15 shows a bedrock disc trapped
in an open fracture, vividly illustrating the mode
of deformation: extensional fracture opening and
throwing out of the disc, followed by a subsequent
contraction trapping the disc when it fell back.
From the violence of deformation recorded at the
Boda Cave, one might suspect that it was formed close
to the epicentre of a high-magnitude earthquake. This
is not the case, however. The epicentre lies 12.5 km to
the NE. Therefore, methane venting tectonics is likely
to have been an additional or primary process (Fig. 8),
as further discussed in Mörner et al. (2003) and
Mörner (2017b).
Alf’s Gryt
Alf’s Gryt is another bedrock cave formed by the
9963 BP paleoseismic event (Mörner et al., 2003). The
sub-surface cave includes deltaic deposits of sandy
silt with grits, sucked into the cave when extensional
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International Journal of Speleology, 47 (3), xx-xx. Tampa, FL (USA) September 2018
Fig. 14. The surface of the Boda cave area (from Mörner et al., 2003;
Mörner, 2017c); partly a chaotic block field (below) and partly huge
blocks fractured loose from the bedrock and “vibrated” more or less in
position (above).
Fig. 15. A bedrock disk stuck right in its falling-back motion by a
contraction fracture (from Mörner et al., 2003). This provides firm
evidence of an extensional opening followed by contraction. To throw
a piece of rock up into the air can only be achieved by a violent
seismic event.
Fig. 17. Mehedeby (from Mörner, 2003; Mörner & Sjöberg, 2011). The
bedrock is cut up into large blocks with the fracture pattern still visible.
Caves occur underneath and between the blocks.
Fig. 18. Gillberga Gryt (Sjöberg, 1994a; Mörner, 2003) deformed by
an earthquake at about 10160 BP. The deformation produced a heap
of big blocks with quite large caves underneath.
forces lifted and fractured the roof (Fig. 16). Violent
methane venting tectonics may have been the driving
force (Mörner, 1017c).
Mehedeby
Site Mehedeby (Sjöberg, 1994a) refers to a bedrock
hill totally fractured into huge angular blocks (Fig. 17).
This can only have been achieved at a major
earthquake. An age of about 10,000 varve years BP
was assigned (Mörner, 2003, 2017c, d).
Gillberga Gryt
Gillberga Gryt refers to a fracture and block cave
about 100 m long. It is described in detail by Sjöberg
Fig. 16. Alf’s Gryt; a cave formed at the 9663 BP paleoseismic event (from
Mörner et al., 2003; Mörner, 2017b). A bedrock area of about 60 × 70 m was
fractured into blocks, which were moved anti-gravitationally upwards (top profile
and lower right). This forced sediments to become sucked into the cave and
deposited in a deltaic manner (lower left).
(1994a), with some additional notes in Mörner (2003,
2017c, d). It was formed at a paleoseismic event at
10160 varve years BP. Fig. 18 gives a view of the
block deformation. Explosive methane venting seems
– totally or partly – to have been involved (Mörner,
2017c).
Trollberget
This site was investigated quite recently (Mörner
2017b, c). It consists of huge angular blocks broken
loose from a previous smooth bedrock surface, well
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International Journal of Speleology, 47 (3), xx-xx. Tampa, FL (USA) September 2018
striated and polished by glacial motions (Fig. 19). No
doubt, it represents a severe deformation in postglacial
time, maybe at around 10200 varve years BP. It seems
to be a quite clear case of explosive methane venting
tectonics (Mörner, 2017c).
Fig. 19. Trollberget (from Mörner, 2017c); a site of quite violent bedrock deformation hosting several sub-
surface caves. The old glacially polished bedrock surface is well visible on the loose blocks, providing solid
evidence that the deformation must have taken place in postglacial time.
Fig. 20. Pukeberg (from Mörner, 2017e); a glacially formed solid bedrock hill is
now strongly faulted and fractured with individual blocks moved laterally.
Fig. 21. The mode of deformation at Pukeberg (Sjöberg, 1994a; Mörner,
2017e); extension opened the fractures so that a 60 cm round erratic
Pukeberg
This is quite an interesting site (Sjöberg, 1994a;
Mörner & Sjöberg, 2011). The cave and its surrounding
area were recently investigated in detail (Mörner,
2017e). It seems obvious that it was formed by a
combination of earthquake forces and methane venting
tectonics (Mörner, 2017c). The age of deformation is
estimated at about 10300 varve years BP.
The Pukeberg cave lies inside a hill that was solid
and glacially well polished at deglaciation, but is now
totally deformed into huge blocks and open fractures
(Fig. 20).
The mode of deformation is well recorded in Fig. 21.
An initial opening of fractures allowed a well-rounded
erratic block of 60 cm diameter to fall into the cave
below. A subsequent partial contraction of the fault
fractures now no longer allows a block of that size
to pass.
Other sites
The bedrock block caves illustrated above just
represent a selection of available sites. Several other
sites are discussed in Sjöberg (1994a). Torekulla
Kyrka and Trollegater, for example, are block caves of
very similar type to those of Boda (Fig. 11), Gillberga
(Fig. 18) and Trollberget (Fig. 19).
COMPARISONS
Block and fracture caves of a seismotectonic
origin have been described in Finland
(Mörner, 2010). In recent years there
have been several reports on paleoseismic
fracturing of the crystalline bedrock in
northwest Russia (e.g., Rodkin et al., 2012;
Shvarev & Rodkin, 2017; Gorbatov et al.,
2017; Nikolaeva et al., 2018). Cooper &
Mylroie (2015a, b) described “large fractures,
and upon failure, large talus blocks, assisting
pseudokarst cave development“ from the
Alternatively, Mehedeby, Gillberga Gryt, Trollberget
and a few others may have been formed all at once at
major paleoseismic event occurring at about 10000
varve years BP and amounting to M 8 to M >8 (Mörner,
2017d).
block could fall down into the cave below, followed by
contraction so that the fractures above the cave are now far
narrower than the size of the erratic block.
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International Journal of Speleology, 47 (3), xx-xx. Tampa, FL (USA) September 2018
northeast of USA. An international comparison of
potential records of methane venting tectonics (MVT)
is given in Mörner (2017b, c).
CONCLUSIONS
After the preceding reviews of Swedish cave history
and the evolution of paleoseismic understanding, we
can now sum up some of the main conclusions with
respect to the origin of fracture caves and block caves
in Sweden:
• Sweden has an excellent database of speleological
objects (SSF, 2018). The vast majority of sites
(1919 or 66.8%) refer to fracture caves and block
caves.
• The fracture caves and block caves (also known
as boulder caves, bedrock caves or granite caves)
have taken some time to be understood. With the
thesis by Sjöberg (1994a), a seismotectonic origin
became the most logical explanation. With the
Boda Cave Project it became indisputable (Mörner
et al., 2003).
• Methane venting tectonics has recently been
proposed as the causational process of some heaps
and cones of angular boulders (Mörner, 2003;
2017b, c). The most surprising thing with some
of those block caves and block concentrations is
that they took place in Late Holocene time, with
four events occurring at about 3000 BP.
• We trust we have now successfully joined the
concept of fracture caves and block caves in
Sweden with the concept of a high paleoseismic
activity in deglacial time as well as in Holocene
time into a unified theory, where we claim that
the majority of those caves were formed by
seismotectonic processes and sometimes by
methane venting tectonics.
ACKNOWLEDGEMENTS
R.S. developed the understanding of Swedish caves.
N.A.M. developed the understanding of neotectonics
and paleoseismicity in Sweden. Both ideas merged
at the Institute of Paleogeophysics & Geodynamics
at Stockholm University with the thesis by R.S. in
1994, where most fracture caves and block caves
were explained in terms of paleoseismics. Most of
the papers by Sjöberg can be found on ResearchGate
and Linkedin, and those by Mörner on ResearchGate.
Mörner’s paleoseismic monograph of 2003 can be
obtained directly from author (via mail order). The
2011 conference guidebook and Sjöberg’s monograph
on tunnel caves can be ordered from Sveriges
Speleolog-Förbund. We acknowledge very fine and
useful comments by Dr. Trevor Faulkner and Dr. Raúl
Pérez López, which undoubtedly improved the paper.
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