Facies (2010) 56:157–172
DOI 10.1007/s10347-009-0192-6
O R I G I N A L A R T I CL E
Carbonate mud mounds, conglomerates, and sea-level history
in the Katian (Upper Ordovician) of central Sweden
Mikael Calner · Oliver Lehnert · Michael Joachimski
Received: 30 October 2008 / Accepted: 21 April 2009 / Published online: 15 May 2009
© Springer-Verlag 2009
Abstract The Katian (Upper Ordovician) facies succession of the Siljan district, central Sweden, records some
of the most prominent environmental changes in the
Ordovician of Baltoscandia. These changes include two
separate phases of major sea-level drawdown that were
of basinwide and presumably global importance. The
Wrst regression and lowstand terminated an entire
generation of carbonate mud mounds (the Kullsberg
Limestone) and resulted in the formation of polymict
carbonate conglomerates (Skålberg Limestone) belonging to the Amorphognathus superbus Zone. New stable
isotope data from the Amtjärn quarry shows that this is
immediately after the peak of the Guttenberg Carbon
Isotope Excursion (GICE), which reaches a 13C peak
value at 3.3‰ in the uppermost Amorphognathus tvaerensis Conodont Zone. A second major regression and
sea-level lowstand is manifested by palaeokarst morphologies in the Slandrom Limestone, which formed
close in time to the comparably minor Waynesville positive carbon excursion in the basal Amorphognathus
ordovicicus Conodont Zone. The widespread exposure
associated with this latter lowstand terminated carbonate
production in much of the basin, and, during the subsequent Xooding, organic-rich, graptolitic shale formed
across most of Baltoscandia. The two corresponding
M. Calner (&)
GeoBiosphere Science Centre, Lund University,
Sölvegatan 12, 223 62 Lund, Sweden
e-mail: [email protected]
O. Lehnert · M. Joachimski
GeoCenter Northern Bavaria, University of Erlangen-Nürnberg,
Schloßgarten 5, 91054 Erlangen, Germany
e-mail: [email protected]
sequence boundaries are amalgamated at the top of truncated carbonate mud mounds in the Siljan district,
resulting in a pronounced Middle Katian hiatus in the
immediate mound areas.
Keywords GICE · Conglomerates · Black shale ·
Glaciation · Katian · Siljan district · Sweden
Introduction
The Katian and Hirnantian stages (Upper Ordovician) are
today recognized as time intervals with repeated changes in
the global carbon cycle, inferably reXecting climatic
changes on a global scale (Bergström et al. 2007). Recent
studies by Kaljo et al. (2004, 2007), Bergström (2007), and
Bergström et al. (2007, 2009a, 2009b) on 13C chemostratigraphy have summarized up to six positive 13C excursions through this interval (Bergström et al. 2009a, Wg. 2).
The oldest of these excursions, the ChatWeldian (Early
Katian) Guttenberg Isotope Carbon Excursion (GICE) was
originally identiWed by Hatch et al. (1987) and has subsequently been discussed from Laurentia also by Patzkowsky
et al. (1997) and Ludvigson et al. (2004). The array of associated changes in fauna and facies during this event were
Wrst summarized from Baltoscandia by Jaanusson (1976). It
has more recently been detailed by Ainsaar et al. (1999,
2004), almost exclusively based on sections in the East Baltic
area, and collectively referred to as the ‘Middle Caradoc
Facies and Faunal Turnover’. The causes for the carbon
cycle anomaly and the biotic event are not yet fully understood, although several authors have proposed climatic
cooling and/or a glaciation in this stratigraphic interval
(e.g., Ainsaar et al. 2004; Tobin et al. 2005; Saltzman and
Young 2005). In Baltoscandia, the GICE is followed by
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three, or possibly four, positive 13C excursions in the
Katian before the prominent and well-known Hirnantian
Isotope Carbon Excursion (HICE; Marshall et al. 1997;
Kaljo et al. 2001, 2004; Ainsaar et al. 2004). These smaller
excursions have subsequently been identiWed in Upper
Ordovician type strata of North America, and partly correlated with the East Baltic record, and were named the Kope,
Fairview, Waynesville and Whitewater 13C excursions by
Bergström et al. (2007, Fig. 1; 2009a, Wg. 2).
The recurrent Upper Ordovician 13C excursion has challenged the hypothesis of an isolated brief glacial episode in
the latest Ordovician (Brenchley et al. 1994, 2003) and the
timing for the onset of Icehouse conditions following the long
Lower Palaeozoic Greenhouse period (e.g., Kaljo et al. 2003a,
2003b; Ainsaar et al. 2004; Saltzman and Young 2005). However, the shift from Icehouse to Greenhouse in this time interval is a matter of debate and oxygen-isotope studies on
conodont apatite by Buggisch et al. (2008) show no correlation between positive shifts in 13C and cooling, of which the
latter is expressed by increasing values in 18O. Instead, the
GICE is a period of warming according to 18O data, but has
been interpreted as a cooling event based on the carbon isotope shift (e.g., Young et al. 2005). Detailed studies of the
environmental evolution during the corresponding time interval are therefore important in gauging the relative eVects of
diVerent stable isotope excursions. Investigations on the environmental evolution during the Katian in Sweden are few and
the aim with this paper is to present new sedimentary and stable isotope data from sections in the classical Siljan district of
central Sweden, representing the Central Baltoscandian Confacies Belt of Jaanusson (1976; Scandinavian basin of Ainsaar
et al. 2004) and the cratonic interior of Baltoscandia (Fig. 1).
We concentrate on a stratigraphical interval encompassing the Kullsberg Limestone, Skålberg Limestone, Slandrom Limestone, Fjäcka Shale, and the Jonstorp Formation,
and our data set mainly derives from the classical and now
abandoned quarry at Amtjärn. Comparison is made to
nearby localities as well as with the Borenshult-1 drillcore
from Östergötland (Calner and Lehnert 2008; Fig. 1). The
recent recognition of the beginning of the GICE in the lowermost Kullsberg Limestone (Bergström et al. 2009b; see
also data in Tobin et al. 2005) has provided initial information about the time relationship between mound growth and
this global positive carbon excursion. Additional, complementary stable-isotope data from the upper Kullsberg
Limestone are presented herein to help constrain the precise
timing of mound formation and termination.
Geological setting and local stratigraphy
The Lower Palaeozoic succession in the Siljan district is
preserved in an impressive Middle Devonian astrobleme
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with a diameter calculated at ca. 65–75 km [see review by
Henkel and Aaro (2005) and the recent International Geological Congress 2008 Field excursion guide no. 10 by
Lindström et al. (2008)]. Radiometric dating methods have
more recently conWrmed the close coincidence in time
between this impact event at 376.8 § 1.7 Ma and the Frasnian–Famennian mass extinction event (Reimold et al.
2005). Strata in the remaining ring structure are generally
steeply inclined or slightly overturned. Fault planes with a
few meters of vertical displacement or even overthrusting
of entire formations are evident in several of the abandoned
or still-active quarries in the area.
The Upper Ordovician part of the succession (Fig. 2) is
typiWed by the development of two separate carbonate mud
mound generations separated by a thin veneer of shale and
argillaceous carbonates formed in an open-marine, relatively shallow intracratonic sea. The carbonate mud
mounds are conWned to the Upper Sandbian through Lower
Katian Kullsberg Limestone and the Upper Katian to
Hirnantian Boda Limestone, respectively (see Jaanusson
1982; Ebbestad et al. 2007; Fig. 2). The mounds range in
thickness from several tens to more than 100 m and have
diameters of several hundreds to more than 1,000 m, the
Boda mounds being markedly larger than the Kullsberg
mounds (Jaanusson 1982, p. 27; see also Bergström et al.
2004a). In spite of the impressive size of the mounds, both
generations and the intervening bedded limestone can be
studied in individual quarries in the Siljan district.
The overall stratigraphical relationship between the Kullsberg carbonate mounds and the adjacent strata was outlined
in brief by Jaanusson (1982). He noted that the oldest beds
that rest on the carbonate mound Xank facies was a ‘nodular,
argillaceous calcarenite’, which he referred to as the Skålberg
Limestone (see Jaanusson 1973, 1982, p. 27). Towards the
top of the mounds, these strata were successively replaced by
the Moldå Limestone, the Slandrom Limestone, and the
Fjäcka Shale, the latter draping the mound (Jaanusson 1982).
Accordingly, the stratigraphical framework associated with
the mounds varies between diVerent localities depending on
what part of the mound is exposed. This stratigraphical relationship, with successively younger units overlying the
mounds towards their tops, was explained by Jaanusson
(1982) as a postmortem burial of the mound without the
inXuence of any relative sea-level changes: ‘when the growth
of a mound had ceased, deposition on the top of the mound
did not resume until the level of surrounding sediments
began to approach the level of the mound top’ (Jaanusson
1982, p. 26–27). Hence, the higher parts of the mounds
should be associated with a hiatus––due to non-deposition––
that increase in magnitude towards the top. These younger
units have traditionally been considered as inter-mound
facies (e.g., Jaanusson 1982, p. 29). As shown herein, this is
an unfortunate term because most of these strata formed after
Facies (2010) 56:157–172
159
Fig. 1 Location of the Siljan
district in central Sweden and
Ordovician palaeogeography of
Baltoscandia. a Map of the
Siljan astrobleme with the
ring-shaped distribution of
Palaeozoic rocks. b Distribution
of the Kullsberg and Boda
carbonate mud mounds and the
location of the quarries
investigated for this study
(a, b are modiWed from Ebbestad
et al. 2007). c General palaeogeographical reconstruction of
the Baltoscandian basin showing
the position of the Siljan district
and the Borenshult-1 core in
Östergötland (modiWed from
Nielsen 2004; Stouge 2004).
Note that all investigated
locations belong to a deeper part
of the basin––the Central
Baltoscandian Confacies Belt of
Jaanusson (1976, 1995)
the termination of the Kullsberg mounds and there is no evidence for the existence of productive mounds during their
formation. Accordingly, since these sediments were not produced by the mounds this is an ‘inter-mound facies’ only in a
geographical sense.
An updated review on the Ordovician geology and biostratigraphy of the Siljan district was presented by Ebbestad
et al. (2007). A detailed summary of Ordovician conodont
biostratigraphy and its relation to published carbon isotope
stratigraphic work in the Siljan area is given by Bergström
(2007).
Methods
The south entrance section at the Amtjärn quarry was
documented. As far as the relationships between the
mound and enclosing facies are concerned, this is at present the best exposure with the most complete stratigraphy. About 50 representative rock samples from the
diVerent stratigraphical units were collected for preparation of polished slabs. Another 43 small rock samples
(sample series AMT1–AMT44) were collected for wholerock analysis of stable carbon isotopes. The isotope samples
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Fig. 2 Stratigraphic framework
of the Late Sandbian through
Hirnantian of the Siljan district
and correlation of this international stratigraphic standard with
the regional zonation (grey
shading denotes hiatuses). See
Bergström (2007) for a recent
discussion on the conodont
biostratigraphy of this interval
and Ebbestad et al. (2007,
Fig. 3) for full references to the
non-conodont zonation.
ModiWed from Ebbestad et al.
(2007)
were drilled from these samples using a micro-drill.
Stable-isotope analyses were performed using a Kiel III
device connected to a ThermoFinnigan 252 mass spectrometer at the Geocenter of Northern Bavaria, in Erlangen, Germany. 13C and 18O values are reported in per
mil relative to V-PDB (Vienna Peedee belemnite). Accuracy and precision of the carbon-isotope measurements
were checked by replicate analysis of standards NBS19
and laboratory standards. Reproducibility was better than
§0.05‰ (1).
For comparison, coeval strata were studied and
sampled in various detail at the nearby Kullsberg and
Skålberget quarries as well as at the locality Fjäcka, the
latter representing facies formed at a greater distance
from any mound structure (this locality was treated by
Ainsaar et al. 2004). Comparison is also made to the
Borenshult-1 core in Östergötland, which has been
recovered to function as a reference section for studies
on the Upper Ordovician of Sweden (Calner and Lehnert
2008).
A series of papers from Baltica (e.g., Ainsaar et al.
1999, 2004; Bergström et al. 2009b; Kaljo et al. 2003a,
2003b, 2004, 2007; Meidla et al. 2004; Schmitz and
Bergström 2007) have clearly illustrated the value of
carbon-isotope stratigraphy for basinwide correlations
across the diVerent facies boundaries. It has similarly
been demonstrated that some of the events may well be
correlated to Laurentian basins (Saltzman et al. 2003;
Kaljo et al. 2004; Young et al. 2005; Bergström et al.
2007). For this reason, carbon-isotope stratigraphy is
herein used for correlations to other sections in Sweden
and the East Baltic.
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General facies relationships at Amtjärn quarry
The bulk of the Kullsberg Limestone in the Siljan district is
formed by high-relief, stromatactis-bearing carbonate mud
mounds that are composed of a massive and pure micritic
core with a halo of fossiliferous, stratiWed pelmatozoan
wackestone, packstone and grainstone Xank beds (Hadding
1941; Jaanusson 1982; Riding 2002; Tobin et al. 2005). The
facies relationship between one of the carbonate mud
mounds and the adjacent strata is excellently illustrated in the
Amtjärn quarry. The Kullsberg carbonate mud mound core
facies is now excavated but originally attained a length of ca.
260 m and a thickness of 34–35 m (Thorslund 1935). Mapping of the southern part of this quarry for the present study
conWrms the general stratigraphical observations of Jaanusson
(1982) although we hesitate to apply the term Moldå
Limestone to any of the strata (because of lack of strata similar to the Moldå Limestone in its type section at Fjäcka). The
currently exposed succession includes, from below, the
Kullsberg Limestone (Xank facies), Skålberg Limestone,
Slandrom Limestone, Fjäcka Shale and the lower portions of
the Jonstorp Formation (Figs. 3, 4). The uppermost ca. 6 m
of the underlying Dalby Limestone have previously been
exposed (Thorslund and Jaanusson 1960, their Ludibundus
beds in Wg. 12; Bergström 2007, p. 37) on top of the quarry
and just west of the measured section, but this exposure is not
readily accessible. The thick Kinnekulle K-bentonite, which
deWnes the upper boundary of the Dalby Limestone in many
places in Sweden, has not been documented from the
Amtjärn quarry (S. M. Bergström, pers. comm. August 2008)
where the contact between the Dalby Limestone and the
Kullsberg Limestone is gradational (Bergström 2007).
Facies (2010) 56:157–172
161
Fig. 3 Field-based sketch
showing the narrow gully that
forms the southern entrance
section at the Amtjärn quarry
and the identiWed rock units.
Strata are tilted by about 90°
with successively younger strata
exposed to the east. Black dots in
the map illustrate sampling levels for carbon-isotope analyses
(sample series AMT1–AMT44
in Fig. 5; samples 1–12 were
taken in the southern part of the
main quarry whereas samples
13–44 were taken in the southern
wall of the southern entrance
section, with some overlap).
Encircled letters with arrow
indicate the position of the camera for photographs a–i in Fig. 4.
Note also that this Wgure is used
as a base for interpretation of the
larger-scale stratigraphical relationships in Fig. 9. The proWle to
the right was measured along the
southern wall and thus illustrates
only the upper part of the
section, starting near the reference clay between units A and B
(see Fig. 5 for a full section)
Facies and chemostratigraphy at the south entrance
section
Overview photographs of the studied sections are shown in
Fig. 4, whereas the measured proWle and 13C data are
presented in Fig. 5.
Kullsberg Limestone
The Kullsberg Limestone is well exposed in the western
wall of the main quarry (Fig. 4a), in the south-western most
part of the main quarry (Fig. 4b), and in the western parts,
including the high-wall, of the south entrance section. The
strata are thin to thick bedded and the dominating lithofacies is a medium to coarse-grained pelmatozoan wackestone to packstone with a dark greenish matrix (Fig. 6a, b).
Bedding planes often yield abundant larger fossil fragments
and disarticulated macro-fossils, notably brachiopods and
bryozoans showing little or no abrasion. This main part of
the Kullsberg Limestone has 13C values concentrated
around 2‰ with a slight increase in the uppermost two
meters of unit A (Fig. 5).
The uppermost 1.70 m of the Kullsberg Limestone show
notably thicker bedding than underlying strata and are mainly
composed of a reddish-colored skeletal grainstone (unit B in
Fig. 3; Fig. 6d). The lowermost part of this grainstone unit
yields small-scale, irregular stromatactis with two clearly vis-
ible cement zonations (Fig. 6c; see Tobin et al. 2005 for a
description of stromatactis in the Kullsberg Limestone). This
uppermost meter of the Kullsberg Limestone yields the acme
of the GICE with a maximum 13C value of 3.3‰ (Fig. 3).
Recent work in North America shows conclusively that the
entire GICE is within the Amorphognathus tvaerensis conodont Zone (e.g., Young et al. 2005) and the same seems to be
the case also in Baltoscandia where the index conodonts are
very scarce in the critical interval (S. M. Bergström written
comm. 2009; see also Bergström 2007). Accordingly, based
on carbon isotope stratigraphy also the youngest parts of the
Kullsberg Limestone at the Amtjärn section belong to the
upper Amorphognathus tvaerensis conodont Zone and not in
the Amorphognathus superbus Zone as shown in the stratigraphic charts of the volume by Ebbestad et al. (2007). The
boundary to the overlying Skålberg Limestone is disconformable.
Skålberg Limestone
The Skålberg Limestone belongs to the Amorphognathus
superbus Zone, has a patchy distribution in the Siljan district,
and has previously been reported as a tectonic breccia or a
slump breccia (Bergström 2007). The unit is only a few meters
thick and may thin out over short distances within a quarry
(Jaanusson 1982). It is currently poorly exposed in the type
section at the Skålberget quarry, but well exposed at Amtjärn.
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Facies (2010) 56:157–172
䉳 Fig. 4 Photo-plate showing the main lithological units at the Amtjärn
quarry. a View from southeast of the western quarry wall, exposing
steeply dipping crinoidal packstone facies of the lower Kullsberg
Limestone. b Lowermost portion of the studied section (younger strata
towards the left). The northern opening to the south entrance section
is in the left-most part of the photo. White dots illustrate the positions
of stable isotope samples 1–12 (the sample to the far right being sample no. 1, AMT1). c Lower part of the studied section. The complex
bedding in the upper part of C2 and enclosing C1 and C3 units suggests
minor tectonic overprint in this part of the succession. d Middle part
of the studied section, mainly showing the well-bedded Slandrom
Limestone (D1–D3). Note the successive, upward increase in bed as
well as parasequence thicknesses. Hammer for scale. e Detail of C3
conglomerate. The large clast below hammer is 0.64 m across. f Detail
of the two lowermost parasequences in the Slandrom Limestone. g Detail of the uppermost part of unit C2 represented by skeletal packstone
with abundant solution vugs and rust. h Detail of C1 conglomerate.
Note matrix-supported fabric and rounded lithoclasts. Several of the
clasts in the photograph are also shown in Fig. 6. i Detail of the graptolitiferous Fjäcka Shale
Based on the overwhelming majority of rounded clasts
that are matrix-supported, we re-interpret this deposit as a
conglomerate, although with local inclusions of breccia. At
Amtjärn, the Skålberg Limestone can be subdivided into
three subunits, represented by a lower, 1.20-m-thick polymict carbonate para-conglomerate (C1), a middle, well-bedded and 1.75-m-thick pelmatozoan packstone-grainstone
unit (C2), and an upper, 2.20-m-thick polymict carbonate
para-conglomerate (C3). Clast size in the two conglomerate
units is generally 5–15 cm, but rarely ranges up to ca. 1 m
in the C3 unit (cf. Fig. 4e). Clast shape is variable but generally rounded to subrounded, sometimes with oblate forms.
A small portion of the clasts is highly angular and still
retains the form of a broken stratum. In the C1 unit, the clast
composition is generally a medium to coarse-grained
pelmatozoan wackestone or packstone that sometimes
show weathering-induced alteration of the matrix. In addition to these lithologies, the C3 conglomerate unit includes
clasts of glauconitic, skeletal grainstone. The form and
lithology of a set of lithoclasts are shown in Fig. 6g–l (see
also Fig. 4e, h).
The middle pelmatozoan packstone-grainstone unit (C2)
can be further diVerentiated in three subunits, herein informally named a, b, and c (Fig. 5). The lowermost unit (a) is a
pelmatozoan packstone that closely resembles the pelmatozoan-rich Xank facies of the Kullsberg mounds (cf. Fig. 6a,
b, e). This unit is overlain by a pelmatozoan grainstone unit
(b) with abundant, rust-Wlled solution vugs in the upper
part. The b-unit is also topped by a rusty, pitted surface
(Fig. 4g) that may be a disconformity surface. The c-unit is
a packstone rich in pelmatozoan and bryozoan skeletal
fragments (Fig. 7a).
The 13C values decline in the C1 and C3 units and show
a small peak in the interjacent C2 unit.
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Slandrom Limestone
The Slandrom Limestone is 3.02 m thick in the south
entrance section and strikingly well bedded (Fig. 4d, f).
The lithology is generally a massive, pale olive green,
grey or brown, very Wne grained mudstone to packstone
with a Wne bioclastic hash originating from pelmatozoans and trilobites (Fig. 7a–e). The limestone alternates
with thin beds of calcareous mudstone and greenish, calcareous shale. Other beds are nearly devoid of skeletal
grains and are very similar to the so-called ‘aphanitic
limestone’ (a very dense, evenly textured and extremely
Wne grained limestone) reported from the Slandrom
Limestone elsewhere in Baltoscandia. These plain-looking lithologies also include thin, horizontal, and calciteWlled vugs. Based on the variation in bed thickness and
argillaceous content, this unit can be separated into three
parasequences (D1–D 3; Fig. 4d, f). The 13C values
range from ca. 0.3–1.2‰, showing an increase in values
upwards with the highest 13C values observed in the
uppermost parasequence (D3; Fig. 5).
At Fjäcka, in the north-eastern part of the Siljan district, the thickness of the Slandrom Limestone is 7.20 m.
The primary bedding is partly disrupted and especially
the upper ca. 2 m of the unit has a splintery appearance
with sharp, angular facets and easily falls apart. From
this upper part, spar-Wlled solution pipes are evident
from cut and polished surfaces. Similarly, solution pipes
and meteoric diagenesis is observed also in the Borenshult-1 core in which the Slandrom Limestone is only ca.
1 m thick.
Fjäcka Shale
The Slandrom Limestone is sharply overlain by the Fjäcka Shale, which is 5.8 m thick at Amtjärn and consists
of dark brown to black, graptolitiferous shale (Fig. 4i).
The lowermost part is slightly calcareous. In the northern part of the nearby Kullsberg quarry the Fjäcka Shale
overlies a Kullsberg mound with a conspicuous unconformity (Fig. 8).
Jonstorp Formation (lower part)
Only the lowermost ca. 4 m of the Jonstorp Formation
were studied and sampled. The lowermost 0.5 m (f in
Fig. 3) is a Wne-grained skeletal wackestone (Fig. 7e)
that is nearly identical to the lithology of the Slandrom
Limestone. Above this thin unit, the Jonstorp Formation
is represented by a greyish-green, nodular pelmatozoan
wackestone (Fig. 7f), very similar to the lower Jonstorp
Formation in e.g., the Borenshult-1 core in Östergötland.
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Fig. 5 Sedimentary proWle and
13C record from the southern
entrance section of the Amtjärn
quarry showing close relationship between mound termination
and the peak of the GICE. Black
data points denote samples taken in the southern part of the
main quarry (samples 1–12)
whereas red data points denote
samples taken in the southern
wall of the southern entrance
section (samples 13–44). Note
that the Skålberg conglomerates
occur within the falling limb of
the GICE. Note also strongly
negative 13C values in the topmost part of the Slandrom Limestone, inferably due to meteoric
diagenesis at this level. A local,
provisional sequence stratigraphic framework is included:
HST highstand systems tract,
FSST falling stage systems tract,
LST lowstand systems tract, TST
transgressive systems tract. Correlation to other areas, where
palaeokarst occurs at the top of
the Slandrom Limestone suggest
that the transgressive surface at
the top of that formation at Amtjärn also is a sequence boundary
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Facies (2010) 56:157–172
Fig. 6 Photo-plate showing carbonate facies types at the Amtjärn quarry
(units A–C3). a Argillaceous, greenish pelmatozoan wackestone (dense)
typical for the Xank facies of the Kullsberg Limestone (unit A). b Greenish
pelmatozoan wackestone (Kullsberg Limestone, unit A). c Reddish, skeletal packstone-grainstone with well-developed stromatactis in the youngest
Kullsberg Limestone Xank facies (lowermost unit B). d Pelmatozoan
grainstone of the Kullsberg Limestone (unit B). e Pelmatozoan wackestone
of the Skålberg Limestone (lower unit C2). f Pelmatozoan grainstone of the
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Skålberg Limestone (lower unit C2). g Skeletal wackestone lithoclast of the
C1 conglomerate. h Weathered pelmatozoan wackestone–packstone lithoclast of the C1 conglomerate. i Pelmatozoan wackestone lithoclast of the C1
conglomerate. j Pelmatozoan packstone lithoclast with joints (C1 conglomerate). k Glauconitic pelmatozoan grainstone lithoclast of the C2 conglomerate. l Pelmatozoan wackestone lithoclast of the C2 conglomerate. The
color variegation in samples d and f is due to later weathering. Scale bars
1 cm
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Fig. 7 Photo-plate showing carbonate facies types at the Amtjärn
quarry (units D–G). a Pelmatozoan-bryozoan packstone from uppermost part of C2-unit. b Bioturbated, skeletal packstone from the Slandrom Limestone (D2-unit). c Skeletal wackestone from the Slandrom
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Limestone (D2-unit). d Aphanitic limestone from the Slandrom Limestone (top of D3-unit). e Skeletal wackestone from the lowermost Jonstorp Formation (F-unit). f Skeletal wackestone with dark-greenish
argillaceous partings from the lowermost Jonstorp Formation (G-unit)
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167
Fig. 8 A clear-cut truncation of the Kullsberg carbonate mud mounds
is locally evident. The photograph shows a conspicuous unconformity
between one of the mounds and the overlying Fjäcka Shale (overlain
by Quaternary gravel). Two sequence boundaries previously reported
from open-shelf facies in Baltoscandia are amalgamated here (see
text). Note that it is primarily the Xank facies of the mound that is exposed here. The cut-out strata here and elsewhere in mound facies of
central Sweden inferably formed the Skålberg conglomerates. Northern wall in the Kullsberg quarry. The part of the section seen on the
photo is about 6 m high
Interpretation of the facies succession
cally lower on the peripheral mound Xank facies (e.g., at
Amtjärn and Skålberget). Importantly, the micro-facies
variety of the lithoclasts (e.g., bioturbated skeletal wackestone, glauconitic grainstone, and pelmatozoan packstone)
in the conglomerates implies that a series of diVerent depositional environments were subaerially exposed. This provides further evidence for a major sea-level fall. The
roundness of the cobbles similarly implies that they were
reworked in extremely shallow water for a prolonged time
(perhaps on the shoreline created by exposure of the
mound), although encrustations, which could possibly be
expected, have not been identiWed. The clasts that are conspicuously angular or still retain a stratum form were obviously not heavy reworked and are interpreted to reXect a
short transport of collapsed strata, possibly due to karstiWcation. The large size (ca. 0.5–1.0 m) of some of these
clasts suggests that the depositional slope along the margins
of the partly exposed mound was relatively steep.
Thorslund (1932) brieXy investigated the unconformity
at Kullsberg quarry and reported several vertical Wssures
that penetrated the mounds. These Wssures were interpreted
to be of tectonic origin (Thorslund 1932, p. 154; Jaanusson
The local depositional conditions and the position of the
studied section relative the (now excavated) mound core can
be understood simply by reconstructing the map of the southern entrance section. Such reconstruction clearly shows how
the Skålberg, Slandrom, and Fjäcka Formations can be traced
laterally into or over the previous mound structure (Fig. 9).
Accordingly, the measured succession in the south entrance
section represents the proximal mound Xank facies as well as
the strata that formed during (the conglomerates) and after
(the Slandrom Limestone) exposure and termination of the
mound. This discussion instead focuses on the depositional
history in terms of relative sea level.
The angular unconformity at the Kullsberg quarry
(Fig. 8), and a disconformity at the same level at the Skålberget quarry (Jaanusson 1982, p. 29), provides evidence for
substantial weathering and erosion of the Kullsberg
mounds before deposition of the Fjäcka Shale. Accordingly, the most signiWcant strata in the region from a depositional point of view are the Skålberg conglomerates
because they rest disconformably and (palaeo) topographi-
123
168
Facies (2010) 56:157–172
Fig. 9 Inferred stratigraphical
relationships based on the
Amtjärn quarry. The southern
entrance section is in the rightmost part of the Wgure. The
Wgure gives an idealized
interpretation of the stratigraphy
developed in the nearby
Kullsberg and Skålberget
quarries
1982, p. 27). However, the authors described the Wssures as
a few centimeters to a little more than one decimeter thick
and Wlled with Fjäcka Shale that displayed a micro-brecciated texture when studied with a hand lens. The Wssures
displayed widenings and the contacts to the shale were
sharp (Thorslund 1932, pp. 148–149). One Wssure that
Thorslund never got the chance to study in the Weld
(reported by Isberg 1917, p. 217 and seen by quarry workers) was even wider and penetrated the entire Kullsberg
mound as well as associated bedded limestone. Also, this
Wssure was Wlled with Fjäcka Shale showing primary bedding planes and well-preserved fossils as well as a limestone breccia (Thorslund 1932, p. 150). We have not been
able to re-study these Wssures but, based on Thorslund’s
description and our own observations on the facies succession, we put forward that these Wssures may represent karst
structures, although more detailed studies are needed to
really test this hypothesis. The partly disrupted bedding
associated with the Skålberg conglomerates at Amtjärn and
Wssures within lithoclasts (Fig. 6j) are herein interpreted as
a later tectonic overprint of the succession.
The overlying Slandrom Limestone yields Wrm evidence
for a second regression and subaerial exposure. We have
identiWed abundant palaeokarst morphologies in the Slandrom Limestone at Fjäcka as well as in the Borenshult-1
core. Fresh-water diagenesis at this stratigraphical level is
further supported by strongly negative carbon isotope values in the Borenshult-1 core, at Amtjärn (herein; Fig. 5), as
well as at Fjäcka and in the Jurmala-R1 core in Latvia
(Ainsaar et al. 2004, Fig. 6).
Interpretation of the Katian sea-level history
We propose two phases of regression and sea-level lowstand during the studied time interval. The Wrst regression
is expressed by the prominent facies change at the contact
123
between the Kullsberg Limestone and the Skålberg Limestone and occurred immediately after the peak of the GICE
in the upper Amorphognathus tvaerensis Conodont Zone
(based on biostratigraphy of Bergström 2007). The relationship between the Kullsberg mounds and sea level has previously been discussed by several authors and their common
conclusion is that the mounds formed in shallow water or as
a response to sea-level lowering:
1. Dronov and Holmer (1999) made an early attempt to
subdivide the Ordovician of Baltoscandia in depositional sequences. They included the Kullsberg Limestone in their Kegel sequence but did not provide clear
data whether it was a transgressive or regressive unit
although they mentioned that the coeval strata in Estonia belonged to the shallowest part of the sequence.
2. Nielsen (2004) states that the argillaceous upper part of
the Skagen Limestone and local Kullsberg Limestone
signals the Frognerkilen Lowstand Event in the uppermost part of the Diplograptus foliaceus graptolite Zone
(Nielsen 2004, p. 88). We argue that these strata rather
formed during the preceding Keila Drowning Event and
that the conspicuous Frognerkilen Lowstand Event is
reXected by the Skålberg Limestone only. See Owen
et al. (1990, pp. 22–23) for deWnition of the Frognerkilen
Formation in the Oslo-Asker type district of Norway.
3. Tobin et al. (2005) investigated the oxygen isotopic
composition of well-preserved marine cements in the
Kullsberg Limestone and suggested an up to 15°C
decrease in sea water temperature coinciding with
mound formation. If this extraordinarily high value is
correct, these data can only be explained by a glaciation and thus lowering of the sea level.
4. Cherns and Wheeley (2007, p. 457) interpreted the
Kullsberg mounds as formed during cooling of climate
and regression. This interpretation was based on general facies-similarities with the younger Boda Event
and with regard to the regional geology.
Facies (2010) 56:157–172
In contrast to these published studies, we suggest that the
Kullsberg mounds formed during a transgression and sealevel highstand, but that they were abruptly terminated by a
regression that started shortly after the peak of the GICE
(Fig. 5).
Arguments for mound growth during transgression
and sea-level highstand
There is now a good understanding of how the mounds
relate to the development of the GICE. The 13C data of
Bergström et al. (2009b) indicate that the beginning of the
GICE is in the lowermost Kullsberg Limestone, whereas
our data show that the peak of the GICE is in the uppermost
meter of the mound-Xank facies. It can therefore be concluded that the mounds grew and attained their large sizes
during the build-up or rising limb of GICE. 13C correlation
of the strata at Amtjärn quarry to areas where sedimentation
was not aVected by mound growth shows that this part of
GICE and, thus, the Kullsberg mounds correlate perfectly
with sedimentary facies reXecting a deepening of the depositional environment. This is the case at the nearby Fjäcka
locality, but also in the more remotely located Borenshult-1
core in Östergötland in which the common grainstone
facies of the Dalby Limestone is succeeded by more argillaceous strata in the uppermost ca. 3.5 m of the unit. This
argillaceous facies continues above the Kinnekulle K-Bentonite where it is referred to as the Skagen Limestone. A
small conglomerate of limestone clasts at the top of the
Skagen in the Borenshult-1 core marks a sequence boundary without any evidence of preceding shallowing. In addition, 18O data based on conodont apatite from coeval strata
in Kentucky and Minnesota including the GICE point to a
warming event after a short-term cooling connected with
the mega-eruption producing the Deicke K-bentonite
(Buggisch et al. 2008, and unpublished data of Buggisch),
which represents one of the major volcanic ashfalls in
Phanerozoic history (Bergström et al. 2004b).
In addition to these data, there are other aspects that are
of primary importance when sea-level changes in carbonate-dominated successions are discussed. First, due to the
interaction of so disparate controlling factors as ecology of
the carbonate-producing biota, available accommodation
space, and diagenesis, carbonate systems are most productive during transgressions and sea-level highstands (e.g.,
Schlager 1991). This is particularly true for tropical Xattopped carbonate platforms, but is likely to also be important
in open-shelf settings because the environmental criteria
(e.g., light, nutrients) are principally the same. Secondly,
true regressions result in generally thin lithosomes independent of the depositional system (although lowstand strata
may be thick in clastic depositional systems). This became
obvious when Weld-based sequence stratigraphy became
169
important in the 1990s. It was realized that many regressive
units had been overlooked because of the low temporal
resolution of seismic stratigraphy and a new systems tract
was therefore introduced (falling stage systems tract). Thin
regressive lithosomes are particularly documented for carbonate-dominated successions because carbonates lithify
early and are easily dissolved under subaerial conditions.
Carbonate production is also suppressed by the associated
inXux of terrigenous materials and shallowing of the photic
zone during regressions. For these reasons, carbonate systems export very little sediment to the basin when exposed.
Therefore it seems logical that larger mounds or bioherms
and aggrading successions of limestone rarely represent a
sea-level lowering but a sea-level rise.
Arguments for mound termination during a regression
and age of the sequence boundary
The evidence for a regression and exposure in the latest
stage of mound growth is based primarily on the deposition
of the conglomerates of the Skålberg Limestone. The existence of some hiatus between the erosive top of the mound
and the basal conglomerate of the Skålberg Limestone
seems therefore reasonable. Our new 13C data from
Amtjärn together with unpublished 13C data from the
Borenshult-1 core show that the ‘Skålberg lowstand’
correlates with a conglomerate at the top of the Skagen
Limestone in Östergötland and with the sequence boundary
reported at the Keila-Oandu stage boundary in the East
Baltic area by Ainsaar et al. (2004, pp. 122–123). This
boundary is associated with substantial erosion and a hiatus
in the northern Estonian shelf sections (Fig. 1), whereas the
correlative conformity is interpreted as being the transgressive surface at the base of the Variku and Mossen Formations
in the Livonian basin (Ainsaar et al. 2004, p. 123). Being a
time-line, this sequence boundary can therefore be dated as
of early Middle Katian age.
Palaeokarst in the Slandrom Limestone
The palaeokarst identiWed in the Slandrom Limestone at
Fjäcka needs to be discussed in brief here because the associated unconformity is exposed a few meters above the
Skålberg unconformity in several places, e.g., in the
Amtjärn and Skålberget quarries. The palaeokarst denotes a
second substantial environmental change in the Katian,
which is also manifested by the carbon isotope record (the
2nd Late Caradoc positive carbon excursion of Kaljo et al.
2007 and the Waynesville positive carbon excursion of
Bergström et al. 2007). The palaeokarst is immediately
overlain by the conspicuously brownish-black, organic-rich
Fjäcka Shale (formed during the early Amorphognathus
ordovicicus chron) across wide areas of Baltoscandia and
123
170
this abrupt shift in facies certainly reXects one of the most
impressive environmental changes in the Ordovician succession of the continent (Calner and Lehnert 2008). In most
locations, the Skålberg unconformity and the karst-related
unconformity at the top of the Slandrom Limestone is separated by at least a few meters of strata, e.g., at Amtjärn, in
the Borenshult-1 core, and in the East Baltic area Ainsaar
et al. (2004). They are, however, amalgamated at the top of
truncated carbonate mud mounds in the Siljan district,
resulting in a pronounced Middle Katian hiatus in the
immediate mound areas. For this reason, it is not yet
possible to judge whether the vertical Wssures in the
Kullsberg mounds were formed during the Wrst or the
second lowstand or if they formed during the Wrst and were
further reworked/widened during the second lowstand.
Since the inWll is reported to be Fjäcka Shale, however,
available data suggest formation during the second
lowstand when the Slandrom Limestone was subjected to
karst weathering.
Conclusions
The studied interval represents an extraordinary succession
of strata and yields some of the most pronounced facies
shifts in the Ordovician of Baltoscandia. Our study gives
evidence for at least two separate phases of regression in
the Katian of the Siljan district, and suggests that erosion
locally removed substantial parts of the stratigraphic
record. The study provides a basis for reconstruction of
Katian sea-level history in Central Baltoscandia:
• The Wrst regression and sea-level lowstand is implied by
the conglomerates of the Skålberg Limestone. They rest
disconformably on the peripheral mound-Xank facies of
the Kullsberg mounds and topographically well below
the truncated top of the mounds. The variation in lithoclast composition suggests exposure of several subenvironments in proximity of the mounds. The
conglomerates belong to the Amorphognathus superbus
Zone and formed immediately after the peak of the
GICE, reaching a 13C peak value of 3.3‰ in the uppermost Amorphognathus tvaerensis Conodont Zone.
• The second regression and sea-level lowstand is evidenced by palaeokarst morphologies in the Slandrom
Limestone (at Fjäcka) that formed close in time to the
Waynesville positive carbon excursion in the basal
Amorphognathus ordovicicus Conodont Zone. This palaeokarst has been identiWed also in the Borenshult-1 core
in Östergötland, but not at Amtjärn where coeval strata
might be cut out.
• Based on carbon-isotope records the two sequence
boundaries in the Siljan district are likely to correlate
123
Facies (2010) 56:157–172
with two sequence boundaries previously reported from
the East Baltic area by Ainsaar et al. (2004, Fig. 7).
Hence, they are of basin regional importance.
• The reported conglomerates and palaeokarst morphologies are of particular interest because they formed in a
relatively deep part of the basin, the Central Baltoscandian Confacies Belt, and thereby imply signiWcant sealevel lowstands.
• These sea-level lowstands may reXect a series of Upper
Ordovician glaciations and put additional support to an
early start of the Late Ordovician through Early Silurian
Icehouse period.
Acknowledgments We are grateful to Stig M. Bergström, Jan-Ove
Ebbestad and Åsa Frisk for many interesting discussions on the Upper
Ordovician stratigraphy of Sweden. Valuable comments from the
reviewers Stig M. Bergström and Patrick McLaughlin further improved the manuscript. MC acknowledges the Swedish Research
Council (VR) for support over many years and Crafoord for a grant to
recover the Borenshult-1 drillcore. OL and MJ are grateful to the support by the Deutsche Forschungsgemeinschaft (DFG grant to W. Buggisch, Bu 312/59).
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