A B

UNIVERSITY OF GOTHENBURG
Department of Earth Sciences
Geovetarcentrum/Earth Science Centre
Evolution of the
Hällesåker Deltas in
Lindome Valley, South
East of Gothenburg
From a sedimentological aspect
Johanna Bäckström
ISSN 1400-3821
Mailing address
Geovetarcentrum
S 405 30 Göteborg
Address
Geovetarcentrum
Guldhedsgatan 5A
B700
Bachelor of Science thesis
Göteborg 2012
Telephone
031-786 19 56
Telefax
031-786 19 86
Geovetarcentrum
Göteborg University
S-405 30 Göteborg
SWEDEN
Sammanfattning
Målet med detta arbete är att utreda Hällesåkersdeltats evolution i Lindomeåns dalgång, sydost om Göteborg. Evolutionshistorien har utretts i samarbete med Andersson (2012) och Wahlström (2012) vilka har använt sig av Georadarundersökningar samt LiDAR‐data. Där detta arbete kommer att fokusera på sedimentologi där loggar över åtta utskärningar har gjorts. Dessa utskärningar har valts utifrån intressanta platser identifierade utifrån en LiDAR‐karta, framtagen av Wahlström (2012). I kartan har områdets morfologi beskrivits. Loggarna har bearbetats med hjälp av programvaran SedLog, 2.1.4. Sedimentets olika sammansättning och strukturer har beskrivits och utifrån detta har depositionsmiljö tolkats. Resultat och tolkning av sedimentloggarna har senare korrelerats till undersökningar utförda av Andersson, (2012) och Wahlström (2012). Detta har resulterat i en depositionsmodell för Hällesåkerdeltat. Depositionsmodellen föreslår att deltat har byggts upp i huvudsakligen två steg. Det första steget skulle vara att den isavsatta Ålgårdsbackamoränen har dämt upp dalen åt öster och sediment har deponerats upp till ryggens hösgsta punkt. Det andra steget är då sedimentet har spillt över ryggen och deltat har börjat progradera västerut i dalgången och byggt upp deltat allt eftersom havsnivån succesivt har sänkts. Lindomeån har sedan eroderat sedimentet som resulterat i dagens synliga terasser, plattåer och branter. Nyckleord; Hällesåker, Lindome, deltautveckling, sedimentologi, sedimentloggar, Gilbert‐type delta, foreset beds, topset beds. Abstract
The aim of this thesis is to investigate the evolution of the Hällesåker delta, located in the Lindome valley southeast of Gothenburg. This investigation has been done in co‐operation with Andersson (2012) and Wahlström (2012) where they have used GPR and LiDAR in their research. This paper will focus on the sedimentology, where sediment logs over eight outcrops have been constructed. Those outcrops have been chosen from a geomorphologic map over the area, constructed from LiDAR‐data compiled by Wahlström (2012). Sediment logs have been processed in the program SedLog 2.1.4. Different structures and sediment composition have been carefully studied and described in the logs and from this, an interpretation of the depositional environments has been done. Result and interpretations of the sediment logs have later been correlated with research done by Andersson (2012) and Wahlström (2012). This has resulted in a depositional model for the Hällesåker delta which suggests that the progradation took place in mainly two stages. The first stage is sedimentation on the eastern side of the Ålgårdsbacka moraine ridge and after sediment spilled over the crest of the ridge; the second stage with progradation of the delta towards west in the valley began during successive sea level fall. When the sedimentation of the Hällesåker delta was finished, the Lindome River later eroded the landscape and as a result, terraces, plateaus and scarps are seen today.
Keywords; Hällesåker, Lindome, delta evolution, sedimentology, sediment logs, Gilbert‐type delta, foreset beds, topset beds. i Content
Sammanfattning ....................................................................................................................................... i Abstract .................................................................................................................................................... i 1 Introduction .......................................................................................................................................... 3 1.1 Aim................................................................................................................................................. 3 1.2 Study area ...................................................................................................................................... 3 1.3 Background .................................................................................................................................... 3 1.1.1 Quaternary history in the region ............................................................................................ 3 1.1.2 Deglaciation in the study area ................................................................................................ 5 1.1.3 Deltaic environments ............................................................................................................. 6 2 Method ................................................................................................................................................. 7 3 Results and interpretations .................................................................................................................. 9 3.1 Legend ........................................................................................................................................... 9 3.2 Log descriptions and interpretations ............................................................................................ 9 4 Discussion ........................................................................................................................................... 23 4.1 Sediment logs .......................................................................................................................... 23 4.2 Delta evolution ........................................................................................................................ 25 6.3 Recent delta research .................................................................................................................. 26 4.4 Compared to earlier research ..................................................................................................... 27 5 Conclusion .......................................................................................................................................... 27 Acknowledgements ............................................................................................................................... 28 6 Cited works ......................................................................................................................................... 29 ii 1Introduction
from an old delta can be seen. The delta is suggested to be built up from both glaciaofluvial and fluvial deposits (Hedendahl, 1997; Wedel, 1971). 1.1Aim
The aim of this paper is to determine the evolution of the delta in the Lindome valley with help from recent studies done by my colleagues Andersson (2012) and Wahlström (2012), together with my own work in sedimentology. Our recent work includes studies with ground penetrating radar (i.e. GPR) and geomorphology with LiDAR: Light Detection And Ranging, i.e. new national height data‐base, (NNH). With this collected information, together with my own research in sedimentology, I will be able to suggest an interpretation of the depositional environments and the delta‐evolution since the last glaciation. Studies of outcrops in connection to different plains and plateaus in the area have been carried out and have been chosen after being identified with LiDAR by Wahlström (2012). The outcrops contain information about sedimentary structures, thickness, orientation and differences in the grain‐size distribution between the layers deposited during the delta evolution. Finally, sediment logs over each outcrop and a composite log will be constructed and later compared with the results from my colleagues Andersson (2012) and Wahlström (2012). In co‐operation with them, an interpretation of the delta evolution in the area since the last glaciation will be accomplished. 1.2Studyarea
The study area in Hällesåker is located 20 km southeast of Gothenburg in southern Sweden (Figure 1 & 2). Field work has been carried out in an east‐west oriented valley were remnants 1.3Background
1.3.1Quaternaryhistoryintheregion
Since the beginning of the Quaternary period, 2,5 Ma, there have been several extensive glaciations. The Quaternary period is subdivided in to two epochs; Pleistocene and Holocene, whereof Holocene is the epoch we are living in today, starting around 12 000 BP. Figure 1 & 2. Showing the geographic position of the study area; Lindome valley in Sweden (stadskartan.se).
3 The ice first left the southern parts of Sweden around 14 000 BP, and retreated in three different directions; to the NW in the eastern parts, N in the central parts and to the NE in the westernmost parts of Sweden, see Figure 3 (Curt Fredén, 1994). therefore there is a scarcity of deposits left from older glacial periods (Fredén, 1983). The delta in the study area has been deposited during and after the deglaciation, therefore a summary of this time period will follow. As Fredén (1983) has described, the latest glaciation started about 115 000 BP, and had its LGM (last glacial maximum), 20 000‐18 00 BP. Although there are some contradictions concerning timing of the LGM, recent studies The latest glacial was the Weichsel‐stadial, and also the coldest one during the Quaternary time. The Weichsel‐stadial eroded remnant sediment from earlier glaciations and Figure 3. A map showing the ice‐retreat in southern Sweden with deposited marginal moraines and their ages (Lundqvist 2001). 4 show that there was a greater time‐span for this cold period than earlier suggested. These investigations have come to the insight that the ice‐melting was very complex in southern Sweden due to recession of the ice in three directions (Lundqvist, 2011). evolved to that of today. Traces from this successive land uplift can be seen as paleo coastlines. In the investigated area, the marine limit is estimated to 91 m.a.s.l. based on interpolations between local marine‐limit determinations from marine limits in Ålgårdsbacka, Kungsbacka SE, Sandsjöbacka and Göteborg SE. This research by Påsse is According to Fredén (1983), several bigger marginal moraines were deposited during the ice retreat; one is the Gothenburg marginal moraine with a NW‐SE direction, deposited during Older Dryas 12 600‐12 800 BP. Determination of the age is done by C‐14 dating. Recent dating suggests an older age of 14 500 BP in calibrated ages (Jan Lundqvist, 2011), whereas the ice‐retreat continued, additional marginal moraines were deposited, Figure 3. As described by Fredén (1983), deglaciation in the study area was affected both by climatological fluctuations and the morphology of underlying bedrock; therefore, there was a successive ice retreat which can be seen through several smaller end moraines situated on the southern valley‐side in Hällesåker. Figure 4. Shore‐line displacement curve, where curve a is for Sandsjöbacka and b is for Ålgårdsbacka (Påsse, 1987). Due to the thick ice cover, the underlying crust had been depressed, and since the ice left the area, successive land uplift has taken place. In the beginning of the uplift the land rose about 30 mm/year, which today have decreased to about 2 mm/year (Fredén, 1983). compiled in to a shore line displacement curve for the investigated area, Figure 4 (Påsse, 1987). 1.3.2Deglaciationinthestudyarea
During the Older Dryas, the ice started to melt away due to rapid climate changes (Lundqvist, 2004). The underlying valley with E‐W orientation was uncovered and revealed at this time, stretching from Ålgårdsbacka in the east to Hällesåker in the west. This valley is dominated by steep slopes on the north side, while the southern side is more gentle. It is suggested by Fredén (1983) that the pattern of the ice melting was influenced by climate fluctuations and topography of the bedrock in the area. To begin with, the land rose faster than the sea level until about 9000 BP which lead to forced regression during this time. After 9000 BP, the sea level rise exceeded the land uplift which in turn lead to drowning of low laying terrain, also recognized as the PGT (i.e. postglacial transgression). This event can be connected to areas covered by postglacial clay in areas lower than 20‐25 m.a.s.l. in the study area. From about 7000 BP the land started to rise faster again and the coastline has since 5 The shape of the Lindome valley, with a narrower part in the east which widens to the west, Figure 2, is a result of the erosive effects of the retreating ice (Wedel, 1971). Wedel (1971) stated that the possible direction of the ice‐margin retreat were eastward, and during a re‐advance of the ice, an end moraine located in Ålgårdsbacka was created. The retreat continued, and during a still‐stand at Ingsjön in Inseros, sediment was accumulated up to the crest of the end moraine in Ålgårdsbacka. Further accumulation of sediment took place on the western side of the moraine ridge in Ålgårdsbacka after the sediment spilled over the crest. At this point, the delta started to build up and prograde westward in the valley (Wedel, 1971). Gilbert‐type deltas are often formed quickly (Cofaigh, 2007) and are often found under the marine limit. The delta composition varies depending on the amount of material and type of material transported (Jan Lundqvist, 2011). The characteristics of a Gilbert‐type delta is the three tripartite structure (Fig. 5) composed of topset, foreset and bottomset beds (Kostic 2005). There are several models for Gilbert‐type deltas with steeper, intermediate and lower gradients, in all cases gravitational and turbidity deposits are the dominating depositional processes (Benn & Evans, 1998). Bottomset beds are built up by finer sediments i.e. sand and clay deposited by distal turbidity currents and falling out from suspension. On top of these bottomsets, foreset beds can be found, which are dipping layers constituting of sand and gravel. The dip is caused by slumping of the coarser foreset, out over the earlier deposited bottomset. As 1.3.3Deltaicenvironments
The deltas at Hällesåker are produced from sediment deposited in the outflow of melt‐
water where sediment bearing water reaches calmer, standing water. A decrease in in the water velocity decreases the carrying capacity Figure 5. Showing a schematic picture of a Gilbert‐type delta with the typical three tripartite structures (Suffolk, 2011). of the water and coarser material settle first, while finer sediment often settle from suspension in more distal parts (Hedendahl, 1997). the delta builds up it will begin as a subaqueous fan and it will eventually reach the water surface and become a delta. At this point, the delta top will develop a braided river system where coarser material is deposited with resulting horizontal layering. This upper part of the delta is called the topset bed (Kostic 2005).
The studied delta has been suggested by Wedel (1971) and Hedendahl (1997) to be of Gilbert‐type. 6 To understand delta evolution, four important factors have to be taken into consideration; the accommodation space, water discharge, sediment supply and relative sea level fall (Hedendahl, 1997) (Giosan, 2007). was used to create the sediment logs. After studying the LiDAR‐map over the area, delta deposits and other geomorphic features were identified and mapped (Wahlström, 2012). After reconnaissance in field, positions for sediment logging were found and plotted on a map over the study area. By excavating outcrops at plotted locations, Figure 6, sediment logs have been constructed and described. The size of the outcrops varies depending on the ease of excavation. 2Method
Instruments and equipment used in field was; shovel, LiDAR‐map, compass, knitting needles, soil map; Kungsbacka NE from SGU, plotting‐
paper (in millimeter), measuring tape, trowel and a camera. At every location, a log over the sediment sequence has been compiled on mm‐paper and photographs were taken. SedLog, 2.1.4 is a program for modeling and Possible position for Ålgårdsbacka moraine ridge. Figure 6. Map showing geomorphic features and locations of sediments logs in the study area. Modified from Wahlström (2012). 7 To be able to interpret and get a better understanding of the depositional environments and distance to the ice during the delta formation, the structures and grain‐
size distribution in the sediment have been studied carefully. By using compass and knitting needles, the strike and dip of different layers have been determined. Finally, the sediment logs have been compiled into a composite log over the area. Together with result from studies based on GPR made by Andersson (2012) and a geomorphologic map Figure 7. Interpolated well data‐map showing approximate depth to bedrock (Andersson, 2012). Figure 8. Bedrock profile over the Lindome valley, based on well data (SGU, 2012) and NNH´s elevation data. Described in m.a.s.l (Andersson, 2012). 8 The lowest 5 m of the log with silt and sand shows dips of 17‐20 ᵒ towards the west, and a repetitive change in grain size from sand to silt can be observed in this part of the log (Fig. 12B). Ironstaining is present in this sequence. Coarse sand also occurs (Fig. 11,12C). over the area compiled by Wahlström (2012) a history of the delta evolution in the valley will be presented. Profiles showing depth to bedrock, Figure 7 and bedrock altitude, Figure 8 has also been used to interpret the depositional history in the Lindome valley. 3Resultsandinterpretations
3.1Legend
Sediment logs of the investigated outcrops will follow and a legend over represented sediment and structures can be seen in Figure 9. Sediment Symbols
Water‐ escape structures Clasts Figure 10. Stratigraphic sequence showing log 1 with a coarsening upward sequence containing coarse topsets and dipping foreset beds with finer material dipping between 17‐21ᵒ W. Figure 9. A legend which describes the sediment and sedimentary structures in following logs; 1‐8.
The upper meter contains coarsest material with subangular to subrounded pebbles represented. Cross bedding, laminations and scour and fill structures are observed (Fig. 11, 12A). 3.2Logdescriptionsand
interpretations
Description LOG 1 Log 1 (Fig. 10) is located in Hällesåker in a steep slope facing NW, 25 m.a.s.l. and stretches from the soil surface and 6 m downwards. The sediment exposed consist of a sequence of bedded sand with silt with a overlying bed of pebbly sand (Fig. 11 and 12 A). 9 1m
Figure 11. Photo from log 1 showing the topmost 2 m in the entire outcrop. that the pebbly sand appears to lie directly on the finer‐grained forest beds. Interpretation LOG 1 The upper 1 m is interpreted to represent topset beds over westward dipping foreset beds found in the lower 5 m. These deposits form a typical Gilbert‐type delta sequence. The ironstaining is interpreted to be a result of oxidating states while groundwater has been running, therefore it is indicating on former groundwater table (Prothero & Schwab, 2004). This sediment in the upper 1 m is typical of braided river environments as shown by the grain size, channel structures and cross bedding (Benn & Evans, 1998). Topset beds are typicaly fond on top of foresets along an erosional surface which is representing the bottom of the stream (Prothero & Schwab, 2004), however, the contact between pebbly sand beds at the top and the finer dipping layers is confusing. Figure 11 and 12A show Possible depositional mechanism for the forest beds is turbidity flows or grain flows (Plink & Ronnert, 1999). Variation in grain size can be result of varying energy of the flows, leading to varying carrying capacity of the water. Finer grain sizes indicates on low energy flows, likely from suspended material in turbidity currents. 10 Figure 12. Picture showing log 1 with sediment description and interpretation of depositional processes. Photo A represents the top 1 m, photo B the middle section of the log, down to 5 m and photo C the bottom 1 m. 12 Description LOG 2 soil surface and 150 cm downwards. The upper 75 cm of the log is coarse layer with sandy matrix and high amount of pebbles, up to 15 cm in diameter, subrounded in shape. After 75 cm, sandier material is observed with some big cobbles. The bottom, 100‐130 cm contains sand and silts with laminations. Only the uppermost 30 cm exposed. This shows on layers dipping 7 to 10ᵒ towards the west. Log 2 is found behind the People’s Park in Hällesåker, in a steep slope facing to the south, Figure 13. The log stretches from the Interpretation LOG 2 The coarser material in log 2 is interpreted to be topset beds deposited by a braided river system. There is no clear imbrication but there are some tabular clasts that are oriented parallel to the bed. This could be a result of internal sliding of the clasts (Hedendahl, 1997). The bigger cobbles found in the finer matrix is hard to interpret. They can be drostones, but no clear evidence for this is found in the sedimentary structures surrounding the cobbles. Also the subrounded shape indicates on transportation rather than instant deposition, Figure 14. Figure 13. Stratigraphic sequence from log 2, showing a coarsening upward sequence with gently dipping layers between 7‐10ᵒ W. Figure 14. Picture showing log 2 with sediment description and interpretation of depositional processes. 13 Description LOG 3 Interpretation LOG 3 Log 3 is found close to the football filed in Hälllesåker, in a slope facing SE, 27 m.a.s.l, and stretches from 1,5 m below the delta top and continues 2,5 m downwards. The coarser material is slope material, not the delta top, Figure 15. The sequence is sandy, from very fine to coarse sand with graded beds observed. The topmost layer contains coarse sand and gravel with pebbles around 1‐3 cm in diameter. The size of the pebbles decreases gradually downwards. The topmost 30 cm in Figure 16 could be braided river deposits. But it is more likely coarse material as a result from slumping down the steep slope and therefore it is not suggested to be the delta top. This is hard to distinguish due to strong soil processes. The dipping layers of silts and sands are interpreted to be foresets deposited by turbidity currents. This interpretation is strengthened by Andersson (2012) where he describes distinct foreset near log 3 seen I the GPR models. At 70 cm down in the log, finer sediments begin to dominate with layers dipping 12 to 14ᵒ towards west. Laminations are represented at 60‐140 cm. At the bottom of the log, at 150 cm this layer grades in to a massive layer with homogenous, coarse sand. Only the top 100 cm are exposed. Graded beds can be result from rapid sedimentation from high density turbidity currents, which have deposited a carpet of coarser material first, and on top of this, finer sediments. More massive beds are probably deposited by continuously flowing turbidities or of high density type, also grain flows are possible. Figure 15. Showing a stratigraphic sequence over log 3 with a coarser layer on top and finer sediment further down with dipping layers, 12‐14ᵒ W. 14 Interpretation LOG 3 Figure 16. Picture with sediment description and interpretation of depositional processes in log 3. Photo A represents the top 50 cm in the log, photo B the middle section between 50‐150 cm and photo C the bottom section, down to 2,5 m. 15 Description LOG 4 Interpretation LOG 4 Log 4 is located in an outcrop facing south, 68 m.a.s.l, Figure 17. This sequence is dominated by sand, fine to coarse where the topmost 25 cm contains pebbles, 1‐5 cm in diameter. At 25‐150 cm, Figure 18 A, a thick sand layer, fine to coarse in size are found with laminations observed. The lowermost part, 210‐300 cm is silty‐clay, Figure 18 B. Only the top 90 cm of the clay is exposed. Hard to distinguish any dip of the clay and silt. Both reversed and normal grading are observed in the sequence. The thin topmost layer in Figure 18 A, with coarser material is interpreted as braided stream deposits and the bottom with silty clay are probably bottomset beds, Figure 18 B. It should be taken in to consideration that the coarser top also could be slope deposits due to the steep slope, as suggested in log 3. Graded beds observed, are probably deposited from high density turbidity currents, where traction carpets is interpreted to be the transportational process (Hedendahl, 1997). Variation in thickness and grain size in the beds are results from shifting turbidity currents, from high to low density and more or less energy of the flows. The coarser fractions need higher concentrations of material and more energy of the flows to be in suspension. Silts and clay is likely deposited from suspended material in calm and deep environments. Since no distinct dip of the silty‐clay is observed, it can be interpreted as bottomset deposits that accumulated before sandier forest beds. Figure 17. Showing stratigraphic sequence over log 4 with a coarser top followed by finer layers which are dipping in the interval of 20‐22ᵒ W, interpreted as foreset beds 16 A B Figure 18. Picture with sediment description and interpretation of depositional processes in log 4. Photo A represents the top 120 cm and photo B the bottom section between 210‐290 cm. 17 Description LOG 5 Interpretation LOG 5 Log 5 is found in Ålgårdsbacka, just under the estimated HK, 91,3 m.a.s.l, in a steep slope where the outcrop is facing to the S‐SW, Figure 19. The log stretches from the soil surface and 4 m downwards. The top 50 cm in the log contains sand with high amounts of pebbles, up to 15 cm in diameter, subrounded in shape. In the interval 50‐260 cm no observation has been done. At 260 cm sand is dominating with high amount of pebbles. Laminations and dipping layers, 18‐20ᵒ are observed at 350‐400 cm. The top of the log with coarser material, Figure 20 A, agree with topset composition. Depositional mechanisms are interpreted to be braided river due to the shape of the pebbles. The finer material of silts and sands are likely deposited by suspended material from turbidity currents. Lamination indicates on shifts in the flow velocity on the water and variation in amount of material in transport where the thicker beds are interpreted to be deposited by steady turbidity currents due to their thickness and composition (Kostic 2005). 18‐20ᵒ Figure 19. Showing a stratigraphic sequence over log 5. Much coarse material and rich vegetation on top and further down sand beds with dipping between 18‐ 30ᵒ W. 18 A B C Figure 20. Picture with sediment description and interpretation of depositional processes in log 5. Photo A represents the top 50 cm in the log, photo B at 350‐370 cm and photo C the bottom 40 cm in the log. 19 Description LOG 6 Interpretation LOG 6 Log 6 is located in Ålgårdsbacka, in a ridge facing north, 78 m.a.s.l. The log is stretching from the soil surface and 120 cm downwards. This log is completely dominated by sand. Grains sizes are fine to medium in size, Figure 21. Structures like laminations and soft sediment deformation are observed, (Fig. 22 A). At the lower part in the log, 80‐120 cm, homogenous, very fine sand are found with a big boulder (Fig 22 B). Dipping layers, 6‐20ᵒ E are observed in the interval 40‐80 cm. The topmost 20 cm in the log is interpreted to be result from rapid sedimentation, due to the plane bedding. This is probably caused by turbidity currents of surge type where material have settled from suspension in several stages, Figure 22 A. The deformed sediment is interpreted as convolute bedding and are likely a result from water escape due to rapid sediment load. Slumping and sliding from upper slope during episodic discharge regimes are also likely causes (Kostic, 2005). The big boulder, Figure 22 B, are interpreted as IRD or a dropstone, deposited directly from the ice which is strengthened by structures in surrounding sediment where laminations and drape‐like structures are observed (Kostic, 2005). Figure 21. Showing stratigraphic sequence in log 6. Mostly fine sand with varying structures, some layers dipping between 6‐20ᵒ E. Big boulders are found and interpreted as dropstones. 20 A B Figure 22. Picture with sediment description and interpretation of depositional processes in log 6. Photo A represents the entire sequence in log 6 and photo B is showing IRD/dropstones found. Description LOG 7 Log 7 is located in a steep slope facing the Lindome River to the south, 25 m.a.s.l, Figure 23. The log stretches from the soil surface and 3 m downwards. The topmost 20 cm in the log is sandy with moderate amount of pebbles. This layer grades in to silty‐clay where only the topmost 3 m of this bed is exposed. Shell‐
fragments have been observed. 21 Figure 23. Showing stratigraphic sequence in
log 7, containing mostly homogenous clay with some shell‐fragments. Interpretation LOG 7 Interpretation LOG 8 The coarser top 20 cm are interpreted as braided river deposits overlying a thick clay bed. This silty clay is most likely accumulated during marine deep water environments due to the shell‐fragments represented, Figure 23. Observed structures like soft sediment deformation are interpreted as convolute bedding; a result from water escape. Both the convolute bedding and the plane bedding indicates on rapid sedimentation (Prothero & Schwab, 2004). Description LOG 8 The lack of dipping layers in addition to the grain size composition makes it hard to interpret this log. Due to the fine grain size composition it can be interpreted as bottomset beds. But this is hard to tell, Figure 25. Log 8 is located in a small canyon in N‐S direction near Ålgårdsbacka, 36 m.a.s.l, Figure 24. Log 8 is stretching from 4 m below the soil surface and 160 cm downwards. Sands, silts and clay are represented. Thickness of the sand beds varies throughout the whole sequence. Structures like laminations, plane bedding and soft sediment structures are observed below 100 cm. At the bottom part, 140‐160 cm homogenous sand is dominating. No dipping layers are observed in this log. Figure 24. Showing stratigraphic sequence over log 8 with mostly sand with different sizes and structures, clay and silt are represented further down. 22 Figure 25. Picture with sediment description and interpretation of depositional processes in log 8. bed 4, which often exceed into coarser sand bed, 5. Clay and silt are found at the bottom as bottom set beds, 6. After comparison between the sediment logs, a trend where the coarsest materials containing boulders up to 50 cm in diameter were found most eastward in the valley, Figure 22, photo B, and coarsest material westward is seen in Figure 14 with cobble‐
sizes of < 11 cm in diameter. This agrees with the general progradation model for a Gilbert‐
type delta where the finest material should be found in the most distal parts in the delta. A diagram over clast size variations of the coarsest clasts found in each log has been done, where variations in clast size from west to east in the Lindome valley are shown, Figure 26. The coarsest material are found in log 5 and 6, most eastward in the valley while the finest material is found in log 7 and 8 where both are interpreted as bottomset beds. Finally a composite log has been compiled to give a generalized and simplified picture of sediment and sedimentary structures represented in the study area, Figure 27. Most of the logs are showing some coarser top layer, 1 which is followed by a finer layer of sand; most likely dipping foreset beds in bed 2. Bed 3 contains sand in varying sizes, and different sedimentary structures are often represented i.e. cross‐beds, scour and fill, laminations and convolute bedding. This is followed by even finer sand with laminations/beds with varying thickness in 4Discussion
4.1Sedimentlogs
Altogether, the sediment in the logs are showing on structures typical for a Gilbert‐
type delta with pebbly and sandy topset beds overlying sandy, westward dipping forest beds. However, not all of the logs show this very clearly and there is some exceptions. 23 Figure 16. Clast‐size variations in the sediment logs, from west to east in the Lindome valley. Distance between log 1 and log 5 is 2,5 km. Considering the angle of the dipping foreset beds, an average dip in the interval of 16‐22ᵒ W can be seen. Thus a trend where all beds are dipping westward has been noticed and it is clearly an indication that delta progradation was from east to west (Wedel, 1971). Log 7 and 8, Figure 23 & 24 differ remarkably in contrast to the other logs. Both log 7 and 8 lack topset and foreset structures and instead marine clay can be seen in log 7 (Fig. 23) and sand with structures like convolute bedding and laminations in log 8 (Fig. 25). This area is therefore hard to tell whether it is part of the delta or not due to lack of foreset structures, it could however be that these deposits are parts of the bottomset beds. This variation is also noticed by Andersson (2012). His results with GPR shows complex structures with poor penetration depth and undulating reflectors, interpreted as a mixture with both till material and clay. No foreset beds are represented with GPR or in the sediment logs. It is suggested that this complex area in Ålgårdsbacka (close to log 7 and 8), with till‐
like deposits covered by clay has forced the river to flow towards the north in the valley due to higher resistance against erosion. Another important factor is that the entire Figure 27. A generalized and simplified composite log over sediment and sediment structures in the study area. Layers 1; often contains coarse material, 2; dipping sand layers, 12‐22ᵒ W. 3; sand layer with varying composition and different structures like cross‐beds, laminations, scour and fill and convolute bedding, 4; fine sand with bedding and laminations, 5; homogenous sand, often coarser, 6; silt and clay deposits. area is dipping from south to north due to underlying bedrock topography. According to Wahlström (2012), those above described factors are probably the reason for scarps and terraces seen throughout the valley. Several terraces with lower relief are identified on the northern flank of the valley, while the southern side shows fewer and higher terraces with steeper scarps. This could be a result of more resistant material. Log 1 (Fig. 12, photo A) is the only log showing obvious topset beds with typical structures like scour and fill and cross‐beds. These structures are typically results from braided river deposits. The structures can indicate a change in sediment transportation, from more deep water accumulation to 24 braided river deposits. Also log 2,3 & 5, Figure 13, 15 & 19 are showing forest beds, but the coarse material on top is harder to interpret here. lead to deep‐water environment with extensive clay accumulations covering the valley floor and the till‐like deposits west of Ålgårdsbacka. This has been noticed in GPR‐
profiles by Andersson (2012) and correlated to log 8 and log 7, where log 7 is made up from marine clay, Figure 23. During continuous recession, sand was deposited throughout the whole valley on the eastern side of the Ålgårdsbacka moraine ridge and filled up to the crest, and backset structures were deposited. Remnants from this moraine ridge with backsets can be seen in Figure 24 in Andersson (2012). It is suggested that the sediment later spilled over the crest of ridge and created forsets into the west, and the deltas in Hällesåker started to form. Those deltas on the western side of the ridge are suggested to have developed in three stages. A correlation to the shore displacement curve by Påsse (1987) has been done, Figure 28, and an interpretation of this correlation will follow: The sedimentation could have taken place in three stages where surface A+B is suggested to be in relation to each other, and were deposited after the moraine ridge, about 12 500 BP. Those surfaces have a height of 87‐
89,9 m.a.s.l, and this is likely an indication of the lowest possible level of HK. So HK in the study area can be in the interval of 87‐89, 9 m.a.s.l rather than 91 m.a.s.l which has been suggested in earlier investigations by Påsse (1987) and in Hedendahl (1997). Next stage probably includes surface C+D+F which can be assumed to lie in the same height interval between 38‐46 m.a.s.l and can therefore be correlated to be deposited in an age interval of 10 500 ‐10 200 BP. Sediment were filled up to at least the level of surface D, 4.2Eventhistory
The result from this work, together with studies by Andersson (2012) and Wahlström (2012) have resulted in following interpretation of the depositional model over the Hällseåker delta. Wedel (1971) and Fredén (1983) have suggested that the ice retreat and depositional pattern of the sediment in the Lindome valley were strongly affected by the topography in the area. To get a better understanding of this, a profile over depth to bedrock in the valley has been compiled by Andersson (2012); from this profile conclusions about the depositional pattern during the delta evolution can be drawn. When the ice retreated from the westcoast, the Gothenburg moraine was formed and during continuous retreat several smaller end moraines and recessional landforms were deposited in the Lindome valley. Remnants from smaller end moraines are seen on the southern valley flank in Hällesåker today, (Fig. 7). A few hundred years after deposition of the Gothenburg moraine, the ice had receded to the easternmost parts of the valley and deposited till‐like sediment in the upslope of a depression in the underlying bedrock, between 40‐49 m.a.s.l in Ålgårdsbacka. This depression is described in a bedrock profile, Figure 8 from Andersson (2012). Later, the Ålgårdsbacka moraine was deposited on the height just east of Ålgårdsbacka in the narrower and easternmost part of the valley. It is suggested that the entire valley was water filled during and after this deposition which 25 Figure 28. Correlation between height profile in the study area , based on LiDAR‐data and a shore displacement curve from Påsse (1987), (Walström, 2012). 44‐46 m.a.s.l, and therefore it is suggested that surface C had at least the same height as D and is later eroded, down to today’s level of 41‐47 m.a.s.l, therefore it is suggested to be an erosional surface. Mainly due to its lack of forest structures which were not observed in neither GPR‐profiles, nor sediment logs and also due to the undulating soil surface. Further west in the valley, G+H+E is suggested to be in relation to each other in an height interval of 22‐30 m.a.s.l, and can be correlated to the shore displacement curve to an age interval of 9 900‐9 600 BP. This correlation of the surfaces to the shore displacement curve is very rough and lack in ability to explain all observed heights and angles. It is hard to interpret any individual surface and determine a specific time period for each surface since the erosion in the area probably was very high. Therefore just approximate age intervals can be interpreted for these surfaces, and the appearance of those can be a result of several processes: (1) Delta deposition at a paleo‐sea level without any significant post‐depositional erosion, or; (2) deposition followed by natural erosion by the Lindome River, or; (3) anthropogenic redistribution of sediment in the valley, or; (4) a combination of the above mentioned scenarios. The final processes have been incision by the Lindome River which has created scraps and terraces both on the northern and the southern valley flank throughout the valley. Where the material have allowed the river cutting down, the delta deposits have been more preserved than in areas with more resistant material where a more meandering flow has developed and eroded the sediment, as surface C. 4.3Recentdeltaresearch
According to Andersson (2012), easternmost and more elevated areas has shown on GPR‐
measurements with lower penetration depth and stronger reflectors, while distinct foreset beds and scour and fill structures have been identified only west of Ålgårdsbacka. Dips of the foreset beds are indicating delta progradation from east to west, observed in both sediment logs, Figure 11 and GPR‐
profiles in Andersson (2012). The LiDAR‐map constructed by Wahlström (2012) show areas with generally lower gradients more to the west compared to east in the study area. Features like scarps are identified on both the north and south side of 26 5Conclusion
the valley where the southern side show less terraces, they are however bigger and have steeper scarps compared to the northern side. This is suggested to be a result from more resistant material on the southern side which has forced the river meander more towards the northern valley flank. 
The identified terraces are suggested to be a result of the meandering river which has cut down in the valley floor. Terraces found in field are identified with LiDAR‐data and exact elevation was defined for each. 

4.4Comparedtoearlierresearch
As in earlier work by Wedel (1971), it is suggested that the Ålgårdsbacka moraine ridge easternmost in the valley has hindered the sedimentation westward. Before sedimentation on the western side of the ridge took place, back set beds have been deposited on the eastern side. 


Due to lack of forest beds in surface C, it is suggested that this is a erosional surface and the complexity is a result of high amount of erosion and more investigation is needed to explain it in more detail. It is also suggested that the lowest plateau should be deposited during PGT (Hedendahl, 1997). This could be true, but in this work it is suggested that all clay on the valley floor is accumulated before PGT since the valley lack in delta deposits below 22 m.a.s.l. It could be that clay deposited during PGT has covered earlier delta deposits, but further investigations need to be done to solve that. 

27 There have been two main stages in the depositional model; first, sediment accumulation east of the moraine ridge in Ålgårdsbacka took place and the second stage when sediment spilled over the crest in to the west of the ridge. Dipping direction of the forest beds in both sediment logs and GPR‐profiles indicates on east‐west progradation of the delta. Main deposition of delta sediment was subaqueous. The composition of the delta is typical for a Gilbert‐type delta. Main depositional processes were gravitational processes like turbidity currents and later from fluvial transportation. There are more and lower terraces and slopes observed on the northern valley side than the southern side where steeper slopes are observed. Age determination of the plateaus and surfaces with help from the shore displacement curve is hard to do due to high amount of erosion. Surface C is suggested to be an erosional surface due to its complexity and lack of forests. Acknowledgements
I would like to thank Per Wedel for discussing his earlier research in Hällesåker in more detail with us, and also thanks to my supervisor Mark D. Johnsson for help during the project and Gothenburg University for founding this project. I would also like to thank Fredrik Andersson, Vera Bouvier, Erik Bäckman, Johanna Ljungdahl and Stina Ranjer for critically reading my paper. You were a great help! Greatest acknowledge to my colleagues Fredrik Andersson and Carl‐Anton Wahlström, for a great and fun collaboration. Thanks for all valuable help in field and unfailing support throughout the entire writing process! 28 6Citedworks
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