4.4 Genesis of the Skånings-Åsaka Push Moraine

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

Skånings-Åsaka push moraine

- a formation process,

paleo glacial tectonics

and sedimentology study

Hampus Johansson Brian Karlsson Tufuga

ISSN 1400-3821 B1016 Bachelor of Science thesis Göteborg 2018

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

During the Younger Dryas, the ice sheet in Scandinavia re-advanced and several end-moraines were formed in Middle Swedish end-moraine zone (MSEMZ). One of these end-moraines is the Skånings-Åsaka push moraine. Near Bränningsholm in Skara municipality, an exposed outcrop of the moraine is present where our investigation was conducted. With the help of sketching and structural measurements, the provenance and morphology of Skånings-Åsaka moraine was investigated. The moraine was found to consist of interbedded sand, predominantly fine sand. Multiple clay and silt layers were also found in the exposure. In the sand and clay layers faults and folded structures where found. We suggest that the Skånings- Åsaka moraine was formed in three main steps. During the first step, the ice sheet in MSEMZ advanced and pushed up the southern part of Skånings-Åsaka moraine. In step two, the ice retreated and outwash fans in the east supplied sand which was deposited in a near-shore marine environment. Simultaneously clay was deposited further out in the ocean. The Skånings-Åsaka moraine was now situated above sea level in the area. During the last step, the ice sheet re- advanced and deformed the sediment that builds up the Skånings-Åsaka push moraine. The Skånings-Åsaka push moraine is an excellent example of a subaquatic push moraine, formed of sediment deposited under or near the sea level but now exposed subaerial.

Keywords: Push moraine, Younger Dryas, MSEMZ

Sammanfattning

Under Yngre Dryas så ryckte istäcket i Skandinavien tillfälligt fram igen och flera ändmoräner bildades i södra Mellansverige. En av dessa ändmoräner är Skånings-Åsaka, en push morän belägen i MSEMZ (Middle Swedish end-moraine zone). Nära Bränningsholm i Skara kommun finns en blottad skärning av moränen där denna studie genomfördes. Med hjälp av skissning och strukturmätningar har Skånings-Åsaka moränens ursprung och morfologi undersökts. Moränen fanns till största delen bestå av sand, huvudsakligen finsand. Ett flertal ler- och siltlager fanns också representerade. Sanden visade upp både veckade och förkastade strukturer. Vi föreslår att Skånings-Åsaka moränen har bildats i tre steg. Under det första steget avancerade isen i området fram och tryckte upp södra delen av Skånings-Åsaka moränen. I steg två drog sig isen tillbaka och i öster förde outwash fans med sig sand som avsattes nära strandlinjen i en marin miljö. Samtidigt avsattes lera längre ut i havet. Skånings-Åsaka moränen befanns sig nu över havsnivån i området. Under det sista steget drog isen återigen fram och deformerade sedimenten som utgjorde Skånings-Åsaka moränen. Skånings-Åsaka push morän är ett utmärkt exempel på en subaquatic morän som bildats under havsytan men som nu är exponerad ovan land.

Nyckelord: Ändmorän, Yngre Dryas, MSEMZ

Table of contents 1. Introduction ...... 1 1.1 Purpose and aim ...... 2 1.2 Location of the study area ...... 3 1.3 Previous studies ...... 5 2. Methods ...... 7 2.1 Field work ...... 7 2.2 Laboratory work ...... 8 2.2.1 Grain size analysis ...... 8 2.2.2 Grain composition ...... 9 3. Results ...... 10 3.1 Sketch of the outcrop ...... 10 3.2 Structural geology ...... 14 3.2.1 Reverse faults ...... 14 3.2.2 Normal faults ...... 16 3.2.3 Folds ...... 17 3.3 Laboratory results ...... 18 3.3.1 Grain composition analysis for sand fractions ...... 18 3.3.2 Dry sieving of sands and pipet analysis results ...... 19 4. Discussion ...... 20 4.1 Depositional setting of moraine sediment ...... 20 4.2 Provenance of moraine sediment ...... 21 4.3 Deformation process ...... 22 4.3.1 Folds and faults...... 22 4.4 Genesis of the Skånings-Åsaka push moraine...... 24 4.5 Error sources ...... 26 4.5.1 Field work: ...... 26 4.5.2 Laboratory: ...... 26 5. Conclusions ...... 27 6. Acknowledgments ...... 28 7. References...... 29 8. Appendix ...... 31 8.1 Dry sieving ...... 31 8.2 Pipet analysis ...... 33 1. Introduction

During the Late Weichselian deglaciation, a sudden climate change took place and the rapidly warming global climate got colder again. The abrupt climate change occurred approximately 12900 years ago and is called the Younger Dryas event (Carlson, 2010). This stadial lasted for circa 1300 years and caused a significant cooling. The Scandinavian ice sheet pushed forward again in certain parts, like in south central Sweden, contemporaneously the ice sheet in Finland and Norway created end moraines due to the halting of the ice sheet and the re-advancement of it caused by the Younger Dryas (Johnson, Benediktsson & Björklund, 2013). The climate effect was global, and glaciers grew in many other parts of the world as well (Benn & Evans, 2010).

Prior to the Younger Dryas was the Allerød interstadial, which was characterized by warm and humid climate, where the Scandinavian ice sheet was melting and shrinking in size, the climate was almost comparable to today's (Mangerud, 1987). Possible causes for the Younger Dryas event are credited to the reduction of the Atlantic Meridional Overturning Circulation (AMOC) (Leydet et al., 2018). Other causes such as meteorite impacts have been suggested by recent studies, where worldwide meteorite impacts would have aided in the rapid cooling of the climate (Wolbach et al., 2018).

During this swift cold chock caused the ice sheet in southern central Sweden to move south across the land and formed the Middle Swedish end-moraine zone (MSEMZ), which consist of several end-moraines formed during the Younger Dryas (Johnson et al., 2013). The ice margin moved south in this area during the Younger Dryas stadial, however many oscillations of the ice margin occurred with minor back and forth movements that caused the ridges created in the MSEMZ (Johnson et al., 2013). These end moraines were formed when the ice re-advanced, at the front of the ice margin.

This paper will focus on the Skåning-Åsaka push moraine, one of seven push moraines in the MSEMZ (Johnson et al., 2013), located in Skara municipality, specifically an outcrop near Bränningsholm.

When the ice advanced, sediments in front of the ice margin were pushed together and deformed, these processes formed ridges now called push moraines. A push moraine is a glaciotectonic ice-marginal moraine which forms when sediment is deformed and pushed together by the ice margin when it advances. Push moraines are interesting because they can say something about the speed of the advancing ice and the paleoclimate (Benn & Evans, 2010).

1.1 Purpose and aim

The aim of this study is to describe the composition and structure of the Skånings-Åsaka push moraine and to come up with a reasonable interpretation for the formation process. The structural geology of the outcrop is to be investigated, which will result in the production of a sketch of a section in the outcrop. The provenance of the moraine sediments will be investigated since they will reflect the depositional setting of the moraine. Few other outcrops at Skånings- Åsaka are present, more knowledge of these outcrops is needed to understand the area. We seek to widen the knowledge of the Skånings-Åsaka push moraine and the activity of the Scandinavian ice sheet in general by conducting this study.

1.2 Location of the study area

The Skånings-Åsaka push moraine is located in southern Sweden, west of Mt. Billingen (Fig. 1), a DEM(Digital elevation model) of the study area can also been seen in Fig. 2. The MSEMZ consist of seven ridges west of Mt. Billingen where Skånings-Åsaka is one of these ridges (Johnson et al. 2013). The studied outcrop is situated near Bränningsholm and can be reached via a small dirt road. The outcrop lies in NW-SE direction with the coordinates: 58°26'34.7"N 13°34'33.0"E. The outcrop is 22 meters long, with a height of around 8 meters at the crest of the ridge (Fig. 3). The outcrop section of the moraine is perpendicular to the backside of the moraine which strikes N20°E-S20°W.

Fig. 1. Map of the MSEMZ west at Billingen. The outcrop is located in the eastern part of Skånings-Åsaka ridge (marked with a red star in the map). A) shows a cross section through the seven moraine ridges B) Map of Sweden showing were the study area is located (the grey rectangle).Modified from Johnson et al. (2013).

Fig. 2. DEM model of study area. Red star marks the outcrop.

Fig. 3. Photo of the exposure of the Skånings-Åsaka moraine near Bränningsholm. Ice flow from right to left.

1.3 Previous studies

The moraines and the inter-moraine flats in MSEMZ were interpreted to be composed of primarily clay by Johnson and Ståhl (2010), deposited in front of the ice in a low-energy marine environment. Foraminifera and ostracods found in the clay indicated an artic marine environment. Age dating showed that the clay was deposited during the Younger Dryas.

Large amounts of sedimentation occurred spurred by the sediment-rich meltwater from the melting ice sheet and settled on the relatively shallow seafloor in thick layers. These inter- moraine flats have clay layers of 10-15 meters thickness and make up the majority of the sediments in between the seven ridges of the MSEMZ (Johnson & Ståhl, 2010). The clay building up the ridges in the MSEMZ is deformed which indicate a deposition prior to the ice re-advanced through the area were the clay later was deformed by the pushing of the ice margin.

Johnson et al. (2013) wrote about the Ledsjö moraine situated north of the Skånings-Åsaka moraine. Their results show that the Ledsjö moraine is a push moraine and is dominated by clay with several sand lenses which are deformed. The formation of the push moraine occurred during two small re-advances of the ice. In the first advance, marine clay was deformed and created a ramp of debris-flow sediment. Subglacial meltwater resulted in interbedded sand and clay layers which later were deformed by the advancing ice. Before the second advance, sand was deposited on the ridge. The re-advance caused faulting in the sand and have created a complex network of various shear zones.

The MSEMZ push moraines are similar to Late Weichselian moraines in western Iceland formed in front of marine-terminating glaciers investigated by (Sigfúsdóttir, Benediktsson & Philips, 2018). The glaciers which were active there in the Late Weichselian deformed glaciomarine sediments that later got exposed in the coastal cliff region Belgsholt and melabakkar-Ásbakkar, Melasveit, Iceland. A detailed study of the sedimentology showed several subaquatic push moraines that were formed by active glacier advance. Their study also showed that the moraines were built up by proglacial thrusting and folding of marine sediments and additionally deformation of ice-marginal subaquatic fans. When the glaciers retreated from the area, glaciomarine sedimentation continued. The sequence of depositional and deformation events in Iceland is strikingly similar to the MSEMZ.

According to Bennett (2001), push moraines are important because they can help us understand the palaeoenvironment in the Pleistocene. Their occurrence depends on special glaciological conditions which make them useful in palaeoglaciological investigations. Push moraines are also valuable to investigated because they are potential equivalent to thin-skin tectonics. Fig. 4 shows an example of a push moraine from Iceland.

Fig. 4. A push moraine from HöfEabrekkujökull, Iceland (Bennett, 2001).

All of these studies are relevant to our study because they deal with push moraines formed in similar ways, mainly subaquatically under the Younger Dryas event.

2. Methods

2.1 Field work

The field work for this study was carried out during five days in April 2018.

A portion of the exposed moraine was selected and cleared by spade and knife. The surface was divided into a grid with two rows, each consisting of four squares. The areas of the squares are 1×1 m and were divided by cord and nails into a grid, separating the cells so that each cell could be sketched separately (Fig. 5). A detailed sketch of the exposed surface was made in the field on graph paper. Various beds in the exposure were sketched according to grain sizes. The grain- size determined units that were sketched were clay, silt, fine sand, medium sand and coarse sand. Folds and faults in the beds were also added to the sketch. The scale of the sketch in the field was 1:5. The sketch from the field work was scanned and transferred to PowerPoint software where it was redrafted to make the final complete sketch in the results part (Fig. 6).

Fig. 5. The sketched exposure of the moraine, showing how it was divided.

Strike-and-dip measurements of folds and faults were measured with a Brunton compass. The right-hand rule was used. The measurements were plotted on stereonets with Stereonet 10 software for stereographic projection (Allmendinger, Cardozo, & Fisher, 2011). Folds and faults measured in the section were marked on the sketch (Fig. 6).

A total of 16 samples were taken from the outcrop for further analysis in the laboratory. Three samples each of medium sand, fine sand, silt and clay were collected. Four coarse sands were also collected.

2.2 Laboratory work

Laboratory work was conducted at GVC (Gothenburg University, Department of Earth Sciences). The samples used can be seen in Table 1 below.

Table 1. Samples collected at the outcrop. 3 silt B1S, B2S, A3S* 3 clay A1C, B1C, A4C 3 fine sand A4FS, A3FS, A2FS* 3 medium sand A2MS, A3MS, A4MS 4 coarse sand A1GS, A2GS, B1GS, B4GS Pebbles Samples marked with * were discarded due to errors in the lab work (See Error sources in the Discussion). The laboratory work included three different parts: wet sieving, dry sieving and pipet analysis.

2.2.1 Grain size analysis

Wet sieving was done for the three silt and three clay samples in order to filter out coarser material than 63 µm. The samples were sieved into a 1000 ml cylinder. The coarse material left in the sieves were put into beakers and inserted into an oven in order to dry out the water from the samples for dry sieving.

For the dry sieving, the material left from the wet sieving bigger than 63 µm was put into a stack of sieves ranging from -1 to 4 Φ and then inserted into a shaker machine for 15 minutes to sort out the different grain sizes. The different sieve fractions were then weighed and plotted on histograms and cumulative frequency diagrams along with the pipet analysis results. The grain sizes -1, 0 and 1 Φ were saved for petrologic and mineralogical analyze by microscopy which can be seen in the results part (Table 2).

Additionally 10 samples from beds of sand ranging from fine to coarse fractions with pebbles in were dry sieved to further investigate the grain size distribution. These samples were used to create plots of grain size distribution and cumulative frequency diagrams for the fractions -1 to 4 Φ.

Pipet analysis was done for the three clay and three silt samples that earlier were put in six different 1000 ml cylinders. An agitating rod was used to agitate the sediments before withdrawal with a pipet. For each of the six cylinders, seven beakers were prepared for each withdrawal. The withdrawal times were calculated by a formula based on Stoke’s Law (Equation 1), a total of seven time intervals were given for withdrawals for each cylinder. 20 ml was sucked up by the pipet each withdrawal to represent 1/50 of the entire sample.

The last step was done by weighing the dry samples and calculating the aliquot to determine cumulative frequency for clay and silt fractions ranging from 5-10 Φ which was combined with the -1 to 4 Φ fractions from the dry sieving of the silt and clay and plotted on the same diagram in the results part, the cumulative frequency for the sand samples was also plotted on the same diagram (Fig. 16).

Equation 1. Formula used to calculate withdrawal times. 2 Tmin= (Depthcm)/(1500*A*dmm )

A=0.0907 T°C + 1.764

The skewness, sorting and mean values for the nine sand samples was calculated according to the Folk and Ward (1957) grain size parameters standard.

2.2.2 Grain composition

Grain composition was identified for sand grains from five randomly selected samples and where further investigated. The samples were initially examined to create a useful list of mineral and rock types. A sample splitter was used to get samples containing around 200-500 grains. The samples were analyzed by microscope. A chart of mineral and petrological classification was created and each sample was counted and classified. Roundness and mineralogy/petrology was studied under the microscope.

3. Results 3.1 Sketch of the outcrop

The sketch created gives a representation of a portion of the moraine and covers an area of eight square meters. The results from the sketching are shown in Fig. 6. The majority of the outcrop consists of sand, primarily fine sand. Layers in the outcrop are deformed and seem to be orientated in a similar direction, dipping approximately 30 ° towards the backside of the moraine. Five faults and three folds where found and measured and are marked in the sketch. One of the folds, B2, has been reverse faulted by fault b (Fig. 6).Medium sand and fine sand layers have been displaced. Two minor faults were found in fold A2 (Fig. 7). These were not measured due to their small size.

A possible shear zone is marked with a dashed line in the sketch, mainly separating the clay layer and the fine sand layer. In the fine sand layer, an s-shaped fold was found (Fig. 8) within the shear zone. Two types of different clay, red and more grayish clay can be seen in the section. There is a red continuous clay layer (Fig. 9) with a thickness varying from 4 to 15 cm spanning the entire sketched area. There is another red clay layer in the bottom left part of the section which crosscuts fault c.

Some fine sand lenses were also found in the outcrop mainly imbedded within the clay layers. There is a massive coarse sand layer present in the upper parts of the section, the coarse sand in the bottom right has plebes and lenses of both fine and medium sand. Silt is mainly present at two places in the middle right of the sketch, some of the silt appears to be partly folded in fold B2. One of the silt layers have small blebs of fine sand present.

Pebbles were found in three parts of the section. One 3 cm pebble was found in the medium sand at middle left in the sketch. In the clay layer present in the lower middle of the sketch, several pebbles were found. The biggest with a size of 6 cm. The medium sand in the lower left contains many small pebbles and also some shale pieces.

On the surface of the proximal side of the moraine, boulders were found (Fig. 10).

Fig. 6. Sketch of the outcrop. The folds are labeled in capital letters and the faults in small. See legend for further explanation of sediment type.

Fig. 7. Small faults were found in fold A2 marked in red.

Fig. 8. S-shaped fold in the possible shear zone in fine sand. Arrows shows sense of shear.

Fig. 9. A distinct red clay layer can be seen in the upper part of the cell.

Fig. 10. Boulders were present in the upper part of the sand at the proximal side of the outcrop.

3.2 Structural geology

Three reverse faults, two normal faults and three distinct folds were identified. Additionally, we identify a clay bed that we suggest to be a shear surface for a low-angle thrust fault. The orientation of these structures are presented in Figures 11-15. The position of the structures can be found marked in the sketch (Fig. 6).

3.2.1 Reverse faults

Fold B2 and reverse fault b can be seen in Fig. 11 with a combined stereoplot, displaying the fold limbs and poles as well as trend and plunge of the fold axis. The layers folded are fine sand and medium sand. A silt bed has also been partly folded. The

Fig. 11. Reverse fault b: Displaced 6-7 cm. Strike and dip of fault: 356° & 87°. Fold B2: Trend and plunge of fold axis: 189, 6° & 18, 5°. See sketch in Fig. 4 for position exact position. Red star marks position of fold axis.

Reverse faults a, b & c are shown on a stereoplot (Fig. 12) and can be seen marked on the sketch (Fig. 6). The strike and dip of fault a: 360° & 70°, b: 356° & 8°, c: 330° & 71°.

Fig. 12. Strike and dip for reverse faults a, b and c.

3.2.2 Normal faults

Normal faults d & e are shown in Fig. 13. Normal faults d & e are displaced 4-5 cm each. The faults d & e can be seen in the sketch (Fig. 6).

Fig. 13. Strike and dip values for fault d: 192° & 50° and e: 189° & 69 °.

3.2.3 Folds

Fold A2 is displayed in Fig. 14 combined with stereoplot for the fold limbs and poles as well trend for the fold axis. The layers folded are fine sand and medium sand. See Fig. 6 for position in the sketch.

Fig. 14. Fold A2 with belonging stereoplot in the upper right corner. Trend & plunge of fold axis: 6,6° & 9,2°. Red star marks position of fold axis.

Fold A3 was plotted on stereonet (Fig. 15), see Fig. 6 for position in the sketch. The folded layers are fine sand and coarse sand.

Fig. 15. Strike and dip for beddings in fold. Red star marks position of fold axis.

3.3 Laboratory results

3.3.1 Grain composition analysis for sand fractions

The composition of coarsest materials from the dry sieving of the silt and clay samples ranging from -1 to 1 Φ were analyzed. The grain composition study shows quartz and quartz with feldspars to be the dominating composition in the sediments. Some quartz included pyrite inclusion. Sub-angular grains are the most common. The results from the microscopy analysis are presented in Table 2.

Table 2. Microscopy analyzed sand grains from clay and silt samples. Sample Grain size Φ Composition of sand grains A1C -1, 0, 1 Quartz with Pyrite inclusions B1C -1, 0, 1 Subangular quartz grains with pyrite, feldspar and biotite. Some black material, possibly shale. A4C -1, 0, 1 Subangular quartz and sandstone fragments. B2S -1, 0, 1 Mostly subrounded quartz with some rock fragments, shale, biotite and sandstone. A3S -1, 0, 1 Angular quartz with pyrite and feldspar. Some black material.

Grain composition of sand grain fractions revealed a clear dominance of quartz grains. A stable amount of fine grained mafic material was also found in every sample. However, shale, likely Alum Formation shale, was present in small amounts in every sample as well as mica. Quartzs with feldspars were also a common occurrence. The results from the grain composition analysis of five samples are presented in Table 3.

Table 3. Mineral composition and grain count of sand grains. Mineral: Quartz Well Fine Quartz Mica Shale Other n= Sample % rounded grained with % % % quartz black feldspar % rock % % A3S 38.4 1.0 25.2 31.9 1.7 1.7 0.0 580 A1GS 20.7 0.4 13.7 61.4 2.9 0.4 0.4 241 A2MS 62.0 1.7 18.3 8.0 2.7 2.0 6.3 300 B1GS 71.8 1.2 18.4 7.1 0.6 0.3 0.6 337 B4GS 58.8 0.0 24.1 11.8 4.1 0.6 0.6 170

3.3.2 Dry sieving of sands and pipet analysis results

The results from dry sieving and pipet analysis were put together in a cumulative frequency diagram (Fig. 16). Sand= blue, silt=grey, clay=green. The sand fractions is situated in the span from -1 to 4 Φ. Silt and clay have more of the finer grains than sand. See Appendix for histograms and cumulative frequency for each sample.

Fig. 16. Cumulative frequency for samples.

The skewness gave values from-0.86-0.59, the mean value was from 1.18-3.13 and the sorting gave values from 0.85-1.85. Results for the sand samples are presented in Table 4. The nine sand samples in Table 4 can be seen in Fig. 16.

Table 4. Results of sand grain-size statistics and parameters. Sand sample Sorting Mean Skewness A4FS 1.85 3.13 -0.20 A2MS 1.45 1.80 0.14 A1GS 1.29 1.30 -0.36 A2GS 1.15 1.20 0.10 B1GS 1.23 1.47 0.45 A3MS 1.50 1.77 0.30 A3FS 1.72 2.80 0.12 A4MS 1.58 2.27 -0.86 B4GS Pebbles 0.85 1.18 0.59

4. Discussion 4.1 Depositional setting of moraine sediment

The interbedded sand and clay in the exposure we interpret as evidence for a near-shore marine environment. We think that the sand probably was deposited in shallow marine waters (less than 10 m).

The distribution of grain sizes in dry sieving showed an expected result. The grain sizes in sand followed a trend where they increased in amount when the Φ value was high. The silt showed to have finer grains than the sand and the clay had even more weight percentage of fine grains, suggesting a low energy depositional environment (Fig. 16). The clay was deposited in front of the ice margin in a marine environment further out than the silt and sand, where the particles could settle. The coarse fractions such as sand could be explained by a depositional environment closer to the shoreline, where the energy is higher and waves could have deposited these. Silt would have been deposited somewhere in-between the further out marine clay and the near shore sand. The coarse sand layers may indicate a position closer to the shoreline. The sorting results from the sand samples gave values from 0.85-1.85 (Table. 4) which according to Fairbridge and Bourgeois (1966) would classify the sands to be moderately sorted to poorly sorted. The depositional environment for sand with this sorting are glaciofluvial sands, shelf sands below the wave base, continental shelf sands, river sands and lagoonal sands (Fairbridge & Bourgeois, 1966) which strengthens a marine depositional interpretation. The skewness of the sand can say something about the depositional setting, for example beach sands tend to have negative skewness. However, the results from our sands ranged from -0.86- 0.89 (Table. 4) and are too divergent to say anything about the depositional setting of the sands (Tucker, 2001). A lot of sand grains were also found in the silt and clay samples, which could indicate that the sand layers have undergone folding and been mixed up with the clay. In the sketch (Fig. 6) lenses of sand can be seen.

The dominance by quartz grains shown by the grain composition results (Table 3.) is expected due to quartz high resistance properties to erosion and its high value on the Mohs scale giving the mineral it’s hard to erode property. Fine grains like silt and clay were also found in sand samples.

The subaerial outwash sediments to the east within a few kilometers are much coarser than the sediments found at the outcrop.

4.2 Provenance of moraine sediment

The sedimentology indicates that the sediment was derived in a nearshore setting and likely from the east. The provenance of the sand grains confirms this. Provenance of sediments reveals some grains of shale which we expect is derived from mount Billingen and which characterizes the outwash sediments found in Valle Härad (Kallin, 2017). When the ice moved forward it eroded and picked up shale from the Billingen plateau, which was deposited at the ice margin when the Skånings-Åsaka push moraine was formed. In contrast to Johansson (1937) who claimed the shale to be derived from the drainage of the Baltic Ice Lake, which he thought, would explain the provenance of the shale in the MSEMZ.

These subaerial outwash plains were formed simultaneously with the push moraine formation and occur 1.2 km east of the Bränningsholm site in Valle Härad. These supplied sand to the ocean in front of the retreating ice margin which then was deposited near shore and built up the Skånings-Åsaka push moraine when the re-advance of the ice sheet occurred. The outwash sediments originate from meltwater streams coming from the ice margin east of Bränningsholm outcrop (Kallin, 2017).

The clay was deposited in front of the ice margin and was later mobilized by the re-advance of the ice sheet during the Younger Dryas which now builds up large parts of the Skånings- Åsaka push moraine including the studied outcrop near Bränningsholm.

The boulders present at the proximal side of the moraine (Fig. 10) are too big to have been transported by anything but the ice. They indicate where the ice margin was during the formation of the ridge since they were found only on the proximal side of the moraine where the ice margin was.

4.3 Deformation process

Since the clay has been folded, it must have been deposited before the re-advance of the ice sheet and been folded when the ice advanced. When the re-advancement of the ice occurred it should have created shear zones as seen in the sketch (Fig. 6). We interpret the possible shear zone found at Bränningsholm outcrop to have similar features to the imbricate thrust fans in a push moraine in Iceland shown in Fig. 17. This structure would have been created by compressional forces such as when the ice margin re-advanced and pushed sediments together. In the shear zone an s-shaped fold was found (Fig. 8). This is a non-cylindrical fold, formed during high shear strain in the shear zone.

Fig. 17. An example of imbricate thrust fan in a push moraine. From Bennett, (2001).

4.3.1 Folds and faults

The fold axis should in theory be perpendicular to the biggest stress σ1. This would mean that in order for this to be true the fold axis should be orientated perpendicular to the movement of the ice, which is from north to south as the ice re-advanced.

This would mean that the majority of the fold axes would have a trend of 90° assuming the right hand rule. However, the fold axes measured in this study gave trends of 6,6°, 191° and 189,6° which speaks against the ice flow direction being from north to south. Perhaps this could be explained by a small local different flow direction of the ice sheet, where the main ice sheet moved from north to south but at the specific locality in Bränningsholm the ice would have flowed from West-northwest. In fact, the axial planes of the fold axes are orientated nearly parallel to the strike of the moraine ridge at the outcrop (Fig. 18).

Reverse faults in the moraine indicate thrusting and compression which match the structures created at the front of advancing surging glaciers (Benn & Evans, 2010). Advance of the ice which occurred during the Younger Dryas in south central Sweden (Johnson et al. 2013) would create similar structures which we have found in the investigated section. We interpret these structures to be caused by compressive stress formed at the margin of the ice as it re-advanced.

Normal faults in the moraine indicate a decompressional setting. When extension occurred, stress was decreased which causes the hanging wall to slide down. This was caused by gravitational forces. Gravity processes started to affect the ridge when the ice left and the pressure released at the end of the Younger Dryas and normal faulting took place.

The fold axes, normal faults and reveres faults strikes and trends was plotted on a map of the study area (Fig. 18) where the orientation of the push moraine ridge at the outcrop was marked with a yellow line, displaying the relation of the orientation of the structural measurements and the push moraine ridge. The strike of the ridge and the strike of the structural measurements seem to be related and are roughly orientated parallel to each other.

Esker

Fan of stream channels

Fig. 18. The strike of the Skånings-Åsaka push moraine for the studied outcrop near Bränningsholm is marked with a yellow line and the blue star shows the position of the outcrop, the red arrow is the proposed ice flow direction. The black line is the profile of an esker situated west of the outcrop. The elevation of the esker can be seen in bottom right corner.

4.4 Genesis of the Skånings-Åsaka push moraine.

In the beginning of the Younger Dryas glaciomarine clay was deposited in thick layers due to the huge sediment supplied from the melting ice sheet. When the climate got cold enough the ice sheet started to advance, the ice margin pushed sediments together and clay in front of the ice margin was mobilized into a ridge (Fig. 19) the sediment that got thrusted and deformed by the ice was subaquatic. After the ice had formed the ridge it briefly retreated and the sea covered the whole area again, except for the newly created ridge which had been pushed up and was subaerial at this point (Fig. 20). Simultaneously outwash fans in the east supplied sand to the area near Bränningsholm and the outcrop. The sand was deposited in a nearshore marine environment on both sides of the ridge (Fig. 20).

We estimate the sea level at the time of the end-moraine formation to be 125-126 MASL. The evidence for this is a fan-and-esker combination 1 km west of the outcrop at Bränningsholm (Fig. 18). The combination of the esker and fan clearly indicate that it formed when ice was at the Skånings-Åsaka push moraine. The fan is composed of rounded gravel up to cobble size. We interpret the channels to indicate that the fan was subaerial, and also suggest that the distal end of the channels indicate the former sea level (Fig. 18). This level is between 125 and 126 MASL. The sea level behind the moraine that existed prior to the re-advance that deformed the sediment in our outcrop likely was about the same level considering the time interval and distance. This would define the area in between the sea and ice margin as subaerial (Fig. 20).

Finally the ice re-advanced and deformed the sand and clay into a complex shear zone, folding of the layers and reverse faulting occurred due to the increased compression by the re-advancing ice margin. The direction of the ice re-advance was with a component of West-northwesterly at the outcrop (Fig. 21). When the ice once again retreated and the pre-boreal times began the Skånings-Åsaka push moraine had been formed, normal faulting occurred due to the decreased tension from the ice margin caused by gravitational processes.

Fig. 19. First step in the model of Skånings-Åsaka push moraine formation. The blue star marks the position of the outcrop.

Fig. 20. Second step in the formation of the Skånings-Åsaka push moraine. The blue star marks the position of the outcrop. The boundary of the sea and outwash fans was probably somewhere near the outcrop.

Fig. 21. Third step in the formation of Skåning-Åsaka push moraine. The blue star marks the position of the outcrop. The red arrows marks the local flow direction.

4.5 Error sources

4.5.1 Field work:

Only a small piece of the moraine was examined and sketched due to prevailing weather conditions. The field work was conducted in the beginning of April when the ground was still partly frozen which made the digging and excavation of the section bothersome. We would have liked to expand our section area which could help to reveal more structural features, however, we think the chosen section will represent the entire geology of the moraine at the outcrop near Bränningsholm well enough for this study.

Sampling: When samples were classified in the field they could have been wrongly classified since silt and clay are very similar in the field and sediments were frozen.

4.5.2 Laboratory:

Dry sieving: 60-80 grams of sample was supposed to be used for each dry sieving of the sands according to the guidelines for dry sieving. For a few of the samples however, we did not have enough sample mass left to fulfill the standard requirement so they were run anyways with a lesser mass, as low as 40 grams was used since we did not have time and means to go back and retrieve more samples. This could affect the grain size distribution results. One fine sand sample (A2FS) which was supposed to be used was failed due to a problem with the weighing.

Pipet analysis: One of the pipet analyses failed (A3S). We suspect this to be caused by dispersals of the sediments during the analysis. This gave decreasing values for cumulative frequency which is not possible so the results from this analysis were not presented in the paper.

5. Conclusions

 The depositional settings of the moraine sediments indicate a near-shore marine environment. Finer material such as clay was deposited further out than the silt and sand where the energy was lower.  Provenance shows that clay was deposited in front of the ice margin when glaciomarine sedimentation was high. Shale was eroded from mount Billingen and deposited at the ice margin. Sand originates from the outwash plains in the east and was later redeposited by the sea in a near-shore marine environment. Coarser fragments could have been deposited by waves, when energy was higher.  Deformation processes show that the strike and dip of the faults as well as the orientation of the fold axial planes are parallel to the former ice front. We suggest this to support a local paleo flow direction of the ice at the outcrop to be West-northwesterly.  The formation process consists of three steps. The first step was when the ice advanced and pushed up sediments to form the ridge. The second step included the retreat of the ice and the sea covering the area, deposition of sand from outwash plains in the east occurred. The newly formed ridge was subaerial during this step. Finally, the last step was when the ice re-advanced and deformed the sediment layers of the moraine.

6. Acknowledgments

We would like to thank Dr. Mark D Johnson. He helped us with field work and laboratory advising as well as being a helpful supervisor throughout the whole project. He introduced us to the MSEMZ, and helped us understand the local geology. He also provided us with some pictures that were used in this report.

7. References

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8. Appendix 8.1 Dry sieving

8.2 Pipet analysis