UNIVERSITY OF GOTHENBURG Department of Earth Sciences Geovetarcentrum/Earth Science Centre

ISSN 1400-3821 B877 Bachelor of Science thesis Göteborg 2015

Mailing address Address Telephone Telefax Geovetarcentrum Geovetarcentrum Geovetarcentrum 031-786 19 56 031-786 19 86 Göteborg University S 405 30 Göteborg Guldhedsgatan 5A S-405 30 Göteborg SWEDEN

Sedimentology of a coarse-grained, outsized-ripple bed from

the Lower Jurassic, Sose Bugt, Bornholm, Denmark

Anders EureniusLorenz Lindroth

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.

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

2

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

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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

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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 sub-

aqueous 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.

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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).

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

Sose Bugt

A

B

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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).

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

interfingering of marine and continental

sediments of the Rønne Formation and the

marine sandstone of the Hasle Formation.

Deposition of these formations was contemporary to subsidence and faulting attributed to the

Figure 3. Paleogeographic map of northern

Europe, location of Bornholm indicated by arrow.

From Surlyk et al. (1995).

A

B C

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Figure 4. Shows the ages and related tectonic

events of the units found at Bornholm. Sose Bugt

Member indicated by dashed line. Modified from

Graversen (2004).

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 cross-

stratification, 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

(Graversen, 2004). The lower section of the

Sose Bugt Member consists of a lacustrine

to estuarine succession (Surlyk et al., 1995).

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.

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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.

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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.

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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).

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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.

B

C

D

A

Mean:

144/324

Data Points:

7

Mean:

130/310

Data Points:

10

Mean:

163/343

Data Points:

14

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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).

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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).

B

C

A

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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.

Figure 9B show that it is predominately

spheres that constitute the largest part of

most samples. Sorting in combination with

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%.

Figure 8. Shows the OSR material. (A) In the field.

(B) In the lab, after separation from clay.

A B

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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.

Table 3. Summary of ripple length- and height measurements. Ripple Index is length divided by height.

OSR size and shape parameters

(cm) MR1 MR2 MR3 MR4 MR5 MR6 MR7 MR8 MR9 MR10 Avg.

Ripple length (L) 26 20 17 17 40 21 70 16 20 41 29

Ripple height (H) 3 5 3 5 8 3 6 3 4 6 5

Ripple Index (L/H) 9 4 6 3 5 7 12 5 5 7 6

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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).

17

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

18

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).

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.

Figure 11. Comparison between the results of OSR

mineralogy (left) and provenance terranes (right). From

Dickinson et al. (1985).

19

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 fine-

grained (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 tidal-

and 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

20

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.

21

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.

22

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