UNIVERSITY OF GOTHENBURG
Department of Earth Sciences
Geovetarcentrum/Earth Science Centre
Sedimentology of a coarse-grained,
outsized-ripple bed from
the Lower Jurassic, Sose Bugt,
Bornholm, Denmark
Anders Eurenius
Lorenz Lindroth
ISSN 1400-3821
Mailing address
Geovetarcentrum
S 405 30 Göteborg
Address
Geovetarcentrum
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Bachelor of Science thesis
Göteborg 2015
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Geovetarcentrum
Göteborg University
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SWEDEN
Sedimentology of a coarse-grained, outsized-ripple
bed from the Lower Jurassic, Sose Bugt, Bornholm,
Denmark
Anders Eurenius & Lorenz Lindroth, 2015. University of Gothenburg, Institution of Earth
Science; Box 460, 40530 Göteborg, Sweden.
Abstract: The purpose of this paper is to find sedimentological support to discuss a possible
hypothesis for the genesis of a bed displaying outsized ripple forms (OSR), which are unique
within the stratigraphic context of the Sose Bugt Member of the Rønne Formation. Possible
hypotheses for origin of the OSR material include earthquake generated tsunamis, storm
waves and normal or extreme tides. Sose Bugt, on the south-western coast of the Danish
island of Bornholm, is the location of the type section of the Sose Bugt Member, which has
been preserved due to faulting and subsidence generated along the Thörnquist zone. Methods
include grain-size analysis, grain-shape analysis, grain mineralogy, stratigraphic logging and
paleocurrent analysis. The results show that the OSR consists of coarse-grained quartz sand
with predominantly sub-rounded, spherical, polished and frosted grains. The OSR shows
ripple lengths between 16 and 70 cm and ripple heights between 3 and 8 cm. They also show
large variation in spacing between ripple crests and a lack of internal stratification.
Conclusions include that the OSR may have been deposited during normal tidal- and wave
activity and the grain size may have had a role in producing the larger ripple forms. Transport
by wind is likely to have occurred before deposition (which accounts for frosting).
Earthquakes along the Thörnquist zone may have generated waves, or indirectly caused a
change in basin geometry that have allowed for storm waves or extreme tides to erode coastal
sediment not previously accessible. This may have been responsible for transporting coarse
material to the intertidal zone.
Keywords: Sose Bugt, outsized ripples, ripple morphology, grain-size analysis, tidal deposit
1
Sedimentology of a coarse-grained, outsized-ripple
bed from the Lower Jurassic, Sose Bugt, Bornholm,
Denmark
Anders Eurenius & Lorenz Lindroth, 2015. University of Gothenburg, Institution of Earth
Science; Box 460, 40530 Göteborg, Sweden.
Sammanfattning: Syftet med denna artikel är att hitta sedimentologiskt stöd för att diskutera
en möjlig hypotes för genes av ett lager av anomalt stora vågformer (ASV) vilka är unika i
stratigrafisk kontext i Sose Bugt Ledet av Rønne Formationen. Möjliga hypoteser för genes av
ASV inkluderar jordbävningsinducerade tsunamis, storm vågor och normala eller extrema
tidvatten. Sose Bugt, på den sydvästra kusten av den danska ön Bornholm, är platsen för en
sektion av Sose Bugt Member, som has bevarats genom förkastningar och subsidens längs
Thörnquist zonen. Metoder inkluderar analys avkornstorleksfördelning, analys av kornform,
korn mineralogi, stratigrafisk loggning och analys av paleo-strömriktning. Resultatet visar att
ASV består av grovkornig kvartssand med övervägande halvrundade, sfäriska, polerade och
frostade korn. Vågformerna i ASV visar längder mellan 16 och 70 cm och höjder mellan 3
och 8 cm. De visar också en stor variation i avstånd mellan vågtoppar avsaknad av intern
struktur. Slutsatserna är att ASV möjligtvis avsatts under normala tidvatten- och våg aktivitet
och att kornstorlek kan ha haft en viktig roll i att producera vågformerna. Vindtransport har
troligen skett före deposition (vilket förklaras av frosting). Jordbävningar längs Thörnquist
zonen kan ha genererat vågor eller indirekt orsakat en förändring i bassänggeometri som har
gjort det möjligt för stormvågor eller extrema tidvatten att erodera kustsediment som tidigare
inte varit tillgängligt. Detta kan ha bidragit till att transportera grovt material till
tidvattenzonen.
Nyckelord: Sose Bugt, anomalt stora vågformer, vågforms morfologi, kornstorleksanalys,
tidvattenavsättning
2
Table of Contents
1 Introduction ............................................................................................................................. 4
2 Study area ................................................................................................................................ 5
2.1 Location ............................................................................................................................ 5
2.2 Geological background ..................................................................................................... 5
3 Methods ................................................................................................................................... 8
3.1 Sampling ........................................................................................................................... 8
3.2 Field measurements .......................................................................................................... 8
3.3 Lab analysis ...................................................................................................................... 9
4 Results ................................................................................................................................... 10
4.1 Stratigraphic log ............................................................................................................. 10
4.2 Paleocurrent .................................................................................................................... 10
4.3 Grain-size distribution .................................................................................................... 12
4.4 Grain textures & mineralogy .......................................................................................... 14
4.5 Ripple morphology & internal character ........................................................................ 15
5 Discussion ............................................................................................................................. 17
5.1 Interpretation of grain- mineralogy and texture of the OSR........................................... 17
5.2 Genesis of the OSR......................................................................................................... 18
5.3 Conclusions .................................................................................................................... 20
6 Acknowledgements ............................................................................................................... 21
7 References ............................................................................................................................. 22
3
1 Introduction
The purpose of this paper is to interpret the genesis of an outsized ripple (OSR) bed within a
tidal sequence located in Sose Bugt, Bornholm, Denmark. The coarse-grained sand ripples
display a grain size greater and morphology distinctly different than other ripples in the
sequence. The purpose is further to describe this unique layer and discuss hypotheses about its
genesis. This is done by measuring the distribution, shape, roundness and mineralogy of
grains within the ripple material, as well as length, height and paleocurrent of the ripples
themselves. Measurements are also made on the sequences above and below the ripple bed to
provide a comparison and a context for deposition.
The relationship between current- and oscillation flow-velocities of fluids, and the effect on
ripple morphology of different grain size, composition, sediment supply and textures, has
been substantially studied both in field and in lab experiments (Barton & Lin, 1955; Costello,
1974; Guy et al., 1966; Hill et al., 1969; Pratt & Smith, 1972; Pratt, 1973; Southard & Harms,
1972; Stein, 1965; Williams, 1967, 1970; Willis et al., 1972). However, there are only a few
studies on ripples similar to those of the OSR. Textbook examples commonly discuss isolated
ripples, which are current- and wave ripples of insufficient sediment supply, usually related to
lenticular bedding and tidal environments (Reineck & Singh, 1975; Nichols, 2001). There are
also several studies of barchan ripples, which are crescent-shaped isolated ripples that are
predominantly formed by wind, or unidirectional currents and oscillatory flows in subaqueous environments, such as tidal flows and river channels (as cited in Endo et al., 2004).
However, most of these studies (Finkel, 1959; Long & Sharp, 1964; Hastenrath, 1967; Khalaf
& Al-Ajimi, 1993) focus on the wind generated morphologies. The development of
subaqueous barchan ripples have been studied in laboratory experiments using oscillatory
flows (Endo et al., 2004) and unidirectional currents (Endo et al., 2005). These studies
however, were made on very fine sands with an average grain size of 0,1 mm. Literature
dealing with the development of coarse-grained isolated and barchan ripples in relation to
flow velocities is substantially lacking.
The stratigraphy of Bornholm and Sose Bugt has also previously been studied. Sellwood
(1972) studied the influence of tidal-flat deposition during the Lower Jurassic. Surlyk (1995)
made a detailed sequence stratigraphic analysis of the units at Sose Bugt. Graversen (2004;
2009) has by studying the basin tectonics of Bornholm using seismic sequence stratigraphy,
provided good context of its transgressive-regressive history.
4
2 Study area
2.1 Location
The island of Bornholm is situated in the Baltic Sea, between Sweden, Denmark and
Germany. The type section of Sose Bugt Member of the Rønne Formation is located directly
east of Sose Bugt, on the southern coast of Bornholm, roughly 9 km east of Rønne (Fig. 1A).
2.2 Geological background
The geology of Bornholm owes much of its diversity to the tectonics that have driven
extensive faulting and subsidence along the Thörnquist zone. This has served to preserve
much of the sediment from the Mesozoic strata (as cited in Graversen, 2004).
The central and northernmost part of the island is largely dominated by crystalline basement
rock of magmatic and metamorphic origin. The southern and western parts consist of
sediment and sedimentary rock from the Mesozoic and Paleozoic (Fig. 1B).
B
A
Sose Bugt
Figure 1. (A) Geographical location of Bornholm. (B) Geological map, the arrow points to the study
area. Modified from Sellwood (1972).
Tectonic setting
Bornholm lies within the Thörnquist zone, which is an intraplate fault zone that strikes
northwest-southeast. It forms a border between the stable cratons of the Baltic Shield & East
European Platform to the east and the Northwest European Craton to the west (Fig. 2A). The
5
Thörnquist zone consists of a northern branch and a southern branch (Fig. 2B), which
converge and form a graben/horst structure (Fig 2C). The Bornholm high is situated
distinctively higher than the surrounding subsided blocks (Surlyk et al., 1995).
C
B
A
Figure 2. Tectonic setting of Bornholm. (A) The Thörnquist zone in relation to surrounding cratons.
(B) Location of the northern branch (KSS & BSS) and the southern branch (TTZ). (C) Bornholm and
surrounding blocks, study area indicated by arrow. Modified from Graversen (2009).
Bornholm during the Jurassic
Figure 3 shows a generalized description of
the paleogeography of northern Europe
during the Jurassic period. Bornholm was
situated on the margin of a shallow epereic
sea, which was in turn connected to the
Tethys Sea which covered what today is
Mediterranean Europe (Sellwood, 1972).
Graversen (2004) describes the development
of facies in the Jurassic sediments exposed at
the
southwestern
coast
as
being
predominantly marine and increasingly
terrestrial upwards. They are divided into
three genetic units (Fig. 4). The first and
lowermost genetic unit comprises the
Figure 3. Paleogeographic map of northern
interfingering of marine and continental
Europe,
location of Bornholm indicated by arrow.
sediments of the Rønne Formation and the
From Surlyk et al. (1995).
marine sandstone of the Hasle Formation.
Deposition of these formations was contemporary to subsidence and faulting attributed to the
6
Thörnquist zone. The second and middle
genetic unit is the Sorthat Formation, which
constitutes coastal- and delta plain deposits
which gradually develop into shore face and
lagoon deposits. The third and uppermost
genetic unit is the Bagå Formation, which
consists of coarse-grained, non-marine
alluvial fan deposits as well as deposits of
lacustrine clay, sheet wash and debris flows.
Tidal Environments
The Sose Bugt Member is interpreted to
contain mainly tidal sediments which may
correspond to the intertidal zone (Surlyk et al.,
1995). Tidal flats are commonly found in
broad and shallow coastal regions and develop
in various environments, such as barrier
islands, deltas, estuaries, lagoons and Chenier
plains (Daidu et al., 2013). Herringbone crossstratification, flaser to lenticular bedding and
organic-rich salt-marsh muds are typical of
tidal sequences. The alternating type of
bedding, where high energy deposits sand and
low energy deposits clay, is explained by
daily fluctuating tidal energy levels. It is the
result of reversing current direction and
velocity, imposed by the advancing and
retreating of tides (Prothero & Schwab, 1996).
The type section
The Munkerup Member constitutes the lower, and the Sose Bugt Member constitutes the
upper part of the Rønne Formation
Figure 4. Shows the ages and related tectonic
(Graversen, 2004). The lower section of the
events of the units found at Bornholm. Sose Bugt
Sose Bugt Member consists of a lacustrine Member indicated by dashed line. Modified from
to estuarine succession (Surlyk et al., 1995).
Graversen (2004).
It starts with a well sorted, fine-grained, amalgamated sand bed displaying wave-ripples and
hummocky to swaley cross-stratification (Surlyk et al., 1995). This is overlain by wavy
bedding consisting of sand and clay, typical for tidal flat deposits (Reineck & Wunderlich,
1968). Occurring near the top of the wavy bedding is the OSR bed, which is the target of this
study. The age of these sediments are of Hettangian-Sinemurian age (Fig. 4), and has been
determined by correlation of palynomorph assemblages from the Sose Bugt Member with
other deposits of the Lower Jurassic in Denmark (Koppelhus, 1991). Sose Bugt Member
makes up the top of the Rønne Formation, which is overlain by the Hasle Formation.
7
3 Methods
3.1 Sampling
The bulk size of samples needed to be collected were estimated using standards described in
Evans & Benn (2004), although availability of material required smaller samples.
Outsized ripples
To obtain the amount required for grain size analysis, 10 of the largest ripples in the OSR bed
were selected for sampling. Approximately 0,5 kg of material were collected, which was
gently scraped into plastic containers, carefully avoiding the often pyritized upper part of the
ripple forms.
Sand from the wavy bedding
Approximately 0,05 to 0,1 kg per sample was taken from the sand layers within the wavy
bedding, 10 samples above and 10 samples below the OSR.
3.2 Field measurements
Stratigraphic log
A stratigraphic log was produced by selecting sections (Fig. 5) that would represent the
outcrop as a whole. Two vertical sections spread roughly 2 meters apart were chosen and
logged with a resolution of 2 cm. Sand vs clay ratio was made by using photographic
documentation to estimate the average amount of clay and sand within 10 cm vertical
sections. This was done for the entire logged section.
Figure 5. Outcrop of the section described in this report. The vertical rectangles show where logging
occurred. The OSR occurs near the top of the log. Shovel for scale.
8
Ripple length, ripple height & paleocurrent
Paleo-current direction of sand ripples within the wavy bedding was interpreted by digging
out the ripple forms, and measuring the orientation of the ripple crest. This was done on
several of the sampled beds, 7 measurements above, and 14 below the OSR. On the OSR bed,
10 measurements of ripple-crest orientation were made. For each of the ripple forms, the
ripple length was measured as the distance from the trough on one side to the trough on the
other side of the same ripple, and ripple height was measured as the distance from the ripple
bottom to the ripple crest.
3.3 Lab analysis
Grain size analysis
Using a sample splitter, 100g of OSR material was dispersed in 0,05M Na-Pyrophosphate for
approximately 30 hours. The reason for this was that the OSR sand contained some finer
material, which we assumed infiltrated the ripple bed after deposition. Separation of fine and
coarse material was done using wet sieving. Fine fractions (< 4 ɸ) were weighed separately,
and the grain size distribution of the coarse material was measured using sieves with half ɸ
intervals and a mesh size ranging from -3 to 4 ɸ. Grain size distribution for the sand of the
wavy bedding was measured using sieves with half ɸ intervals and a mesh size ranging from 2 to 4 ɸ.
Mineralogy, Shape, Roundness & Frosting
By further splitting of the OSR samples, a subsample of 3-4g was obtained. For each of the
ten samples, 100 grains was analyzed with standard binocular microscopy. Observations and
interpretations of mineralogy and textures were made visually on each grain. Classification of
shape and roundness were made after Powers (1953).
Statistical tools
Grain size statistics were produced using Gradistat (Blott & Pye, 2001), ripple crest trend was
plotted using GEOrient and ternary diagrams were produced using an Excel add-in.
9
4 Results
4.1 Stratigraphic log
The stratigraphic log from Sose Bugt (Fig. 6 and 6A) show how sand and clay dominates
through the section, though the relative amounts of sand vs. clay varies widely throughout the
section.
Lower sequence
The first 0 to 0,6 m consist exclusively of very fine sand and displays wave ripples,
hummocky cross stratification and erosional surfaces. In between layers of clay, is layers of
medium sand that appear frequently from 0,6 to 1 m, but disappears entirely from 1 to 2 m.
Here the amount of clay increases rapidly from 40% to 80%.
At 2 m, the layers now consist of very fine sand and silt which appear less frequent than
previously. Here, the amount of clay reaches its maximum of 90%.
From 3 to 4 m, the sand layers increase in thickness, although they do not occur more
frequent. This is accompanied by a gradual decline in clay, from 90% to 50%.
Outsized ripple bed
The OSR are found between 4 and 4,1 m. The amount of clay is 50%, which is unchanged
since upper part of the lower sequence.
Upper sequence
The section ranging from 4,1 to 4,8 m starts with an abrupt increase in clay from 50% to 70%.
A thin layer of poorly sorted silt and sand is found at 4,2 m. At 4,3 m lies another layer with a
grain size similar to that of the OSR. Very fine sand and medium sand is found within a layer
at 4,6 m. Above this layer the amount of clay reaches its minimum of 40%.
Two layers of medium sand accompanied by an increase of clay to 70%, are found at 4,8 m. It
is followed by a decrease in clay which reaches 40% at 5 m. From 5 to 5,4 m, the occurrence
of very fine and medium sand layers increase rapidly. This is accompanied by an increased
amount of clay, ranging from 70% to 90%.
4.2 Paleocurrent
The ripple crest orientation is shown in relation to the stratigraphic log (Fig. 6). Figure 6B
displays a NNW/SSE (163/343˚N) trend of in three of the bottommost sand layers, located
between 0,5 to 1 m. The trend changes at 4 m, where the OSR displays a NW/SE (130/310˚N)
trend (Fig. 6C). The trend changes slightly to the north (144/324˚N) in the uppermost sand
layers, which are located between 4,1 and 5,4 m (Fig. 6D).
10
D
Mean:
144/324
Data Points:
7
C
Mean:
130/310
Data Points:
10
B
Mean:
163/343
Data Points:
14
A
Figure 6. Shows the stratigraphic log in which A1-A10, B1-B10 and MR1-MR10 are sample names.
Sample locations are marked with dashed lines. Indicated on the x-axis are the dominant grain sizes.
(A) Clay/sand ratio in percent. (B, C & D) Ripple crest orientation, mean and number of data points.
Paleocurrent indicated with arrows. Measured layers indicated by dashed lines.
11
4.3 Grain-size distribution
Lower sequence
The sand layers in the bottom of the lower sequence (Fig. 7A) consist mostly of very fine
sand and silt, B10 corresponds to the amalgamated fine sand bed described earlier. Samples
B9 – B7, in which the largest fraction is medium to coarse sand, belongs to the cross stratified
lenticular bedding of the lower sequence. Samples B6 – B1 consist of very fine sand, and
represent the upper part of the lower sequence (Fig. 6 & 7).
Outsized ripple bed
Grain size distribution and statistics (Fig. 7B & Table 1), show that the OSR material is
moderately to moderately well sorted and range from coarse sand to granule with a mean of
very coarse sand. The silt and clay fraction constitutes less than 2% of the total in all samples.
All samples have a unimodal distribution and classifies as gravelly sand. Kurtosis is either
leptokurtic or very leptokurtic, and skewness is either symmetrical or finely skewed.
Upper sequence
The upper sequence (Fig. 7C) comprises sand layers ranging more widely in grain size
distribution than in the lower sequence. Common for many of the samples are great variations
within samples, a good example of this is samples A1 and A2. They have a peak grain size of
fine sand and coarse sand respectively, but both show a bulk distribution between coarse sand
and clay. The sample A2 has a mean grain size similar to that from the OSR. As a group, the
sand in the upper sequence is finer than the OSR but coarser than the lower sequence.
Table 1. Summary of grain size statistics and statistical descriptions after Folk & Ward (1957), sorted
according to mean grain size (fine to coarse).
12
C
B
A
Figure 7. Grain size distribution, phi- and Wentworth scale. The red line indicates the average of all
samples within the particular grain size. The green line is a cumulative curve based on average
values. The smoothed histograms on the right hand side of the graph display the grain-size
distribution for each sample, the shade of gray correlate to the bars of the adjacent histogram. (A)
The bottom plot displays a combined histogram of all the samples from the lower sequence (B1-B10).
(B) The middle plot displays a combined histogram of all the samples of the OSR bed (MR1-MR10),
samples ordered from fine to coarse. (C) The upper plot displays a combined histogram of all the
samples from the upper sequence (A1-A10).
13
4.4 Grain textures & mineralogy
Mineralogy, Shape, Roundness & Frosting
As seen in figure 8 and 9A, the OSR consists
almost entirely of quartz with nearly no
feldspar and lithic fragments. This varies
only slightly among the samples.
A
B
Figure 9B show that it is predominately
spheres that constitute the largest part of
most samples. Sorting in combination with Figure 8. Shows the OSR material. (A) In the field.
(B) In the lab, after separation from clay.
roundness (Table 1 & 2), show that this
sediment classifies as super mature sand (Folk, 1951).
Table 2 shows that the OSR are largely comprised of sub-rounded grains (78,4%), and minor
parts of sub-angular (11,7%) and rounded (7,9%) grains. Frosted surfaces of grains vary
between 50% and 80% and have no apparent correlation to roundness.
Figure 9. (A) Mineralogy plotted as QFL, quartz is dominating in each sample. (B) Shape diagram
showing distribution of spheres, discs and rods.
Table 2. Percentage of frosting and roundness. Sub-rounded grains are the dominating population
within each sample. Percentage of frosted grains ranges between 52% and 78%, with an average of
64%.
14
4.5 Ripple morphology & internal character
The individual ripples of the OSR are shown in figure 10, where samples 1 through 10 are
located in lateral succession from right to left. Ripple length (the distance from the trough on
one side to the trough on the other side of the same ripple) ranges from 16 cm to 70 cm and
ripple height (distance from the ripple bottom to the ripple crest) ranges from 3 cm to 8 cm,
ripple indices (ripple length/ripple height) range between 3 and 12 (Table 3). Except for MR7
which has an elongate right side, the ripples appear to be symmetrical or asymmetrical. There
is no apparent connection between shape and position in the succession. Transverse
continuation (along the ripple crest, which in this case is inward in the wavy bedding) is
limited and there in no visible internal stratification. The bottom of the samples MR3, MR6,
MR7 and MR9 show trough shaped lower contacts.
OSR size and shape parameters
(cm)
Ripple length (L)
Ripple height (H)
Ripple Index (L/H)
MR1 MR2
26
3
9
20
5
4
MR3
MR4
MR5
MR6
17
3
6
17
5
3
40
8
5
21
3
7
MR7 MR8
70
6
12
16
3
5
MR9 MR10 Avg.
20
4
5
41
6
7
29
5
6
Table 3. Summary of ripple length- and height measurements. Ripple Index is length divided by height.
15
Figure 10. Shows the OSR. Ripple length is measured from one side of the trough to the trough on the
other side of the same ripple. Ripple height is measured from the bottom of the ripple to the top of the
crest. Ripple length = L and ripple height = H, measurements in cm. Orange dashed line indicate the
top of the ripple form, black dashed line indicate the bottom (where visible).
16
5 Discussion
The lower sequence and the upper sequence seem to have been deposited in a similar
environment. It is possible that this may have been part of an upper intertidal flat, as implied
by Surlyk et al. (1995). The grain size (Fig. 7) and morphology (Fig. 10) of the OSR, shows
that it is unique in comparison to its stratigraphic context (Fig. 6 and 6A). Although the A2
bed is somewhat similar, it is interpreted as composed of sediment eroded from the OSR bed
and re-deposited. This bed shows a similar grain size distribution but not the anomalous shape
and size of the OSR. Further discussion on stratigraphic context can be found in Lindroth &
Eurenius (2015).
5.1 Interpretation of grain- mineralogy and texture of the OSR
According to Folk (1951) the super mature quartz could possibly indicate an origin of fluvial
channels, however it strongly suggest an origin of aeolian dunes or beach- and offshore bars.
It is likely that the material have been reworked many times in each of these environments.
This indicate that the sorting of the OSR material may be attributed to reworking in previous
environments, which further suggests that the clay may have been derived from the overlying
layer and infiltrated the sediment post deposition.
The OSR consist of between 52% and 74% frosted grains. However, by further comparing
frosted and polished grains with Schneider (1970); the remaining 26% to 48% is likely
displaying a polished texture. Because determining frosting is a very subjective procedure,
there is a possibility that the grain count results are in error; many of the grains identified as
frosted might in reality be polished.
The presence of both frosted and polished grains would indicate reworking in both marine and
aeolian environments (Cailleux, 1943; Zimdars, 1958; Nichols, 2009). It is possible that
different types of reworking have occurred in the same local area. Beach bars and ridges are
typical environments in proximity to tidal flats (Daidu, 2013) that would have been exposed
to both aeolian and marine processes. It is possible that such an environment could contain
both frosted and polished grains. However, as indicated by the work of Surlyk (1995), there is
no evidence of beach bars or ridges in the Sose Bugt Member. Another likely explanation is
mixing of sand grains, where frosting and polish can be inherent features from the previous
environment (Reineck & Singh, 1975).
It is difficult to make any substantial interpretation of provenance region based on sediment
characteristics and textures, which is likely to be attributed to reworking in close proximity to
the area of deposition. However, classification and petrography commonly used to distinguish
the provenance of principal sandstone types can be applied for a modest interpretation based
on mineralogy and composition. The sediment of the OSR is almost exclusively quartz (98%)
and contains virtually no matrix. If this material was lithified sandstone, it would classify
according to Pettijohn et al. (1987) as a quartz arenite. These are in many cases the product of
extensive sediment reworking and weathering, in which the climate of the source area has a
major influence (Tucker, 2001). Because Bornholm during the Jurassic was warm, humid and
17
with a seasonal variety (Mehlqvist et al., 2009), the climate may indeed have had a role in
producing the pure quartz sediment of the OSR.
Besides the fact it must have
contained quartz, it is impossible
making any inference of source
rock composition. It is to be noted
however, because quartz is
sometimes second cycle sediment,
further studies of overgrowths
could possibly distinguish between
a magmatic or sedimentary source
(Tucker, 2001).
Figure 11. Comparison between the results of OSR
mineralogy (left) and provenance terranes (right). From
Dickinson et al. (1985).
Comparing mineralogy (Fig. 11) plotted in QFL with related provenance terranes (Dickinson,
1985), indicates that the sediment is likely from a craton interior or recycled orogen.
However, this serves only to confirm the paleogeographic setting of Bornholm during the
Jurassic, in which it was situated near the border of the Laurentian plate (Sellwood, 1972).
5.2 Genesis of the OSR
The OSR are made by waves
The OSR contain no crossbedding or internal stratification indicative of depositional
mechanism, which is often the case with coarse grained ripples (Leckie, 1988). This make
distinguishing current- and wave generated ripples difficult. However, genetic classifications
from Reineck et al. (1971) can be used to classify the OSR based on shape and size
parameters. Ripple indices (Table 3) suggest that they could be either current- or wave
generated ripples, except for sample MR4 which is clearly classified as a wave ripple. Further,
the OSR are underlain and overlain by tidal sediments, which confirm a marine deposition.
Normally seen in current- and wave generated ripples is a transverse continuation and regular
spacing between crests and troughs, along with superimposed crossbeds due to migration
(Reineck & Singh, 1975). These features are not present within OSR. They are clearly single
ripples with no physical connection to each other, or to any kind of layering. This suggests
that they could be what Reineck & Singh (1975) call ‘isolated’ ripples which are also
sometimes, referred to as ‘starved’-, ‘incomplete’- or ‘barchan’ ripples. They are typically
produced by currents or waves; the main difference is an insufficient sediment supply. This is
an important factor that causes the ripples to form as disconnected, incomplete ripple crests in
the troughs of the substrate. In clay dominated tidal flats this type of ripples is normally what
constitutes the lenses in lenticular bedding (Reineck & Singh, 1975).
It is evident that several of the OSR show trough shaped lower contacts rather than flat
surfaces and more resemble lenses than ripples, which is visible in samples MR3, MR7, MR9
and MR10 (Fig. 10). It is possible that the OSR were deposited in the depressions of the
clayey substrate or that depressions were made by the loading of the overlying sand.
18
With normal current- and wave ripples it is possible to interpret the water depth, and current
or shear stress velocities needed to produce a specific ripple type (Harms et al., 1975).
However, accurately determining depth or flow velocity with this method is not a
straightforward procedure in this case. It requires assumptions about either flow velocity or
water depth to make any inference about the other. Further, it would be necessary to specify if
the OSR were deposited by currents or waves to make any assumption regarding current- and
oscillatory flows, which is difficult because the OSR seem to show influence of both.
Although, flume experiments by Endo et al. (2004; 2005) have previously shown that finegrained (0,1 mm) isolated ripples in oscillatory- and unidirectional flows, can change shape
from straight-crested isolated ripples to 'barchan'-shaped ripples in less than 30 minutes.
Presuming that similar processes occur in coarse-grained isolated ripples; the shape seen in
MR7 for example, could possibly be explained by the ‘barchan’ shape documented by Endo et
al. (2004). However, if these are barchan ripples, the ripple crests of the 'horns' (the curved
outer edges) of the ripples would have different direction, which should also likely appear as a
larger variation in the measured ripple-crest orientations. The OSR show little variation in
ripple-crest orientation, which suggest that this is not the case.
The OSR are unique in the sequence
The OSR show a unique grain size, much coarser than any sand found elsewhere in the lower
and upper sequence. Coarse materials, like the OSR is not likely to be transported or
deposited in a tidal zone that predominantly only transport and deposit clay and fine to
medium sand. Because the OSR material cannot be found anywhere else, it is suggested that
although the material may have been deposited by processes in the tidal flat, it is possible that
another process is responsible for transporting it there.
Hypothesis I: Normal wave activity
One explanation for the OSR is that they were deposited under normal tidal conditions, but
some event caused transport of very coarse sand to the area. Reineck & Singh (1975) imply
the possibility of coarser sands to form larger ripples than finer sands under the same shear
stress conditions. It suggests that normal tidal- and wave activity may have been responsible
for producing the ripple forms, but does not explain the occurrence of the unique OSR
material.
The direction of the paleocurrent of the upper sequence does not appear to be significantly
different from the OSR bed, which support the hypothesis of deposition during normal tidaland wave activity. However, the small difference could also be explained by a change in
current- or wave direction which may be related to altered basin geometry. This possibility is
further discussed in the section below.
Hypothesis II: Storms or extreme tides
One possibility is wash-over events from waves and currents under the influence of storms or
extreme tides, which would be consistent with Surlyk et al. (1995). They suggest that
deposition took place under a significant influence of waves and storms and describe the
sequence as a transgressive, wave dominated, restricted, marine environment, likely in the
outer zone of a wide estuary. However, because of the cyclic nature of storms and tides, it is
19
likely that this would be indicated by the appearance of OSR material elsewhere in the upper
and lower sequence.
Hypothesis III: Tectonic events
Another explanation is earthquake generated waves or currents. This does not necessarily
implicate a change in the basin geometry of Sose Bugt, but could have occurred anywhere
along the Thörnquist zone. Such an event is likely to erode and transport surrounding coastal
sediment in a way similar to that of wash over-events from storms and extreme tides, but can
at the same time be explained by deposition from a single event. A possible origin for the
coarse material seen in the OSR is tidal channels. They normally have much higher energy
and therefor usually coarser material than the rest of the tidal zone (Boggs, 2001). An
earthquake-generated wave would explain the energy needed to transport material from the
tidal channel to the intertidal flat.
Another possibility is a change in basin geometry, which is not unlikely based on the active
subsidence and faulting taking place in the Thörnquist zone during the Jurassic (Graversen,
2004). A change in basin geometry may have allowed for erosion and transport of sediment
from places that was not previously accessible. Transport could have occurred by storms- or
tidal currents and waves, as previously suggested. Again, this would mean that the OSR
material is likely to appear in the upper sequence, which it does not.
5.3 Conclusions
The anomalous shape and size of the OSR could be related to grain size, while occurrence
within the stratigraphic sequence suggest that deposition must have occurred in a marine
environment predominantly characterized by normal tidal- and wave activity.
Grain textures of the OSR indicate that both wind and water may have been dominating
agents of transport. These features may have been obtained in a coastal environment possibly
containing beaches or beach barriers.
The stratigraphic context, unique grain size and textures of the OSR suggest that, of the
hypotheses discussed, the most likely is a single event that may have been related to
earthquake-induced waves or currents of the Thörnquist zone. However, the genesis of the
OSR could also be explained by waves or currents from extreme tides and storms. Either of
these could be responsible for eroding beach, beach barrier or even tidal-channel sediment,
and transporting it to the intertidal zone.
Based on mineralogy and composition of the OSR, it is possible to confirm the
paleogeographic setting of Bornholm during the Jurassic, and its location near the border of
the Laurentian plate.
20
6 Acknowledgements
This paper is part of a Bachelor of Science course at the University of Gothenburg where field
and lab work is funded by the Institution of Natural Science. For providing this project, we
wish thank our supervisor Mark D. Johnson, as well as examiner Rodney Stevens, who both
contributed with their knowledge, advise and support as well as reviewing of this paper. We
also wish thank Erik Jansson, Sebastian Pokorny, Sara Lidén and Maria Granberg for
providing useful peer reviews.
21
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