Bertil Ringberg

Micromorphology and petrography of Late Weichselian glaciolacustrine varves in southeastern Sweden

Abstract The purpose of this investigation is to describe and interpret the sedimentology and petrography of Late Weichselian varves in southeastern Sweden in order to determine their nature and origin. It is focused on the microscopic evidence for glaciolacustrine varve sedimentation in the Baltic Ice Lake and the possibilities of making a detailed facies classification of the sediments in an area with an established varve chronology. The material examined was cores taken from five representative localities in an area below the highest shoreline of the Baltic Ice Lake in the provinces of Skane and Blekinge, i.e., below an altitude of 55–65 m. The investigations included chemical analyses, XRD, microscopy (polarized) and SEM-EDX investigations of the summer and winter layers of the varves. The results of the chemical, clay mineralogy (XRD) and microscopic examinations of the bedrock and mineral fragments indicate that the material in the different facies of the varved clay is mainly produced by moderate alteration and erosion from the local bedrock of predominantly granitoid rocks and of residual kaoline deposits. It is possible to distinguish microscopic evidence of four varve types differing in texture depending on the water depth and how close to the ice the different facies were deposited, i.e., if they were deposited proximally, distally or extramarginally in relation to the ice margin. It is also possible to relate these differences to changes in the palaeoclimate during the deglaciation of the area in the Bolling and Older Dryas chronozones.

Upper late Weichselian lithostratigraphy in western Skåne, southernmost Sweden

Abstract Different methods have been used to produce a new informal upper Late Weichselian lithostratigraphy for western Skane. Special attention is payed to the genesis of the uppermost diamicton, the Malmo till, which was formed during the Low Baltic readvance. The different opinions about the latter unit and the existence of the Low Baltic ice stream are discussed. The lithostratigraphy related to the shoreline displacement during the Late Weichselian and to the outbursts of the Glacial Lake Vomb during the final deglaciation is shown. The 100 years old finds of the arctic codfish (Boreogadus saida) in the Lomma clay is 14C dated and the most reliable age is 13,700 B.P. The expected age was 12,800–13,000 B.P. The latter age is calculated from the shore displacement curve and corresponds to the final deglaciation at the beginning of the Bolling chronozone.

Readvance and retreat of the Late Weichselian Low Baltic ice stream in southernmost Sweden – a review

Abstract The aim of this study was to review old and new evidence for the readvance and retreat of the Low Baltic ice stream. The review is based partly on new field observations from southernmost Sweden and new dates relating to the marginals of the ice stream. The diamicton deposited by the Low Baltic ice stream is mainly a subglacial till (Malmö till) rich in clay and chalk. In addition, there are clasts and microfossils in the till derived from the bedrock in the Baltic area. This suggests that they were transported up to 700 km. The likelihood is that the ice moved upon a subglacial bed of mainly fine-grained, deformable sediments. It is also possible to think that the ice shut-down of the deforming bed-driven ice stream, was the result of sediment exhaustion. The sharp limits and low level of the Malmö till, and the sharp limit between an area of low relief hummocky moraine and an area of high relief hummocky landscape, are indicative of deposition from a lowland ice stream. The dating of the deglaciation of the Low Baltic ice stream shows that the ice stream existed approximately 18 000 to 14600 calendar years BP. At the end of this period the first stage of the Baltic Ice Lake opened.

Image analysis of rhythmites in proximal varves in Blekinge, southeastern Sweden

Abstract Image analysis is evaluated as a method to study the nature of the rhythmites in thick proximal varves in the bottom bed of the Bredakra glaciolacustrine delta, southeastern Sweden. The method used was greyscale and colour profiling on digital images. The greyscale curve reflects the relative grain size of the sediment and the results show that it is possible to graphically reproduce both the thickness variations of the rhythmites and the gradation between the different sublayers. Data confirm earlier field observations from the area that the rhyhmites in the proximal varves reflect the diurnal transportation in the meltwater streams. The mean value of six measured varves shows that the summer layers consist of c. 50 diurnal couplets. If the analysed laminae are formed of unmistakable diurnal couplets the duration of the intense melting period was c. 50 days during the Late Weichselian deglaciation of the Bolling Chronozone (G1-1e) in southeastern Sweden. Future work will show if the image analys...

Sedimentology of a glaciofluvial deposit at Ekeby, East Central Sweden

Abstract The Ekeby site, south of Stockholm, has been chosen to elucidate glaciofluvial sedimentation processes soon after the end of the Younger Dryas. Detailed lithofacies and grain-size analyses of gravel, sand and fine sediment sequences in a gravel pit reflect changes in the depositional environment. Four main facies assemblages have been distinguished. The lowermost part of the sequence was probably deposited in a subglacial conduit environment. The middle part of the sequence reflects a transition to a proglacial subaqueous environment. The upper part of the latter sequence probably occurred during increased melting of the ice. The sequence was then covered by beach gravel formed during the regression after the drainage of the Baltic Ice Lake.

Sedimentological evidence of a meandering Younger Dryas subglacial conduit: Horn, SE central Sweden

Abstract The lateral displacement of a subglacial conduit, consisting of an area of subglacial conduit-mouth deposits covered by various subglacial and flow tills, is recognized in an abandoned gravel pit at Horn (near Nykoping). The displacement took place during a cold period (Younger Dryas) when the subglacial discharge of meltwater towards a proglacial basin was low. The sedimentary succession contains glaciofluvial deposits overlain by lodgement and flow tills. The superposition of tills may reflect oscillations of the ice in the frontal zone of the Scandinavian ice sheet during the Younger Dryas stadial. The tills are suggested to have been deposited in the conduit-mouth environment. The larger extent of the glaciofluvial unit, as compared to the limited range of the outcropping flow tills, is interpreted as evidence of lateral movement of the subglacial conduit in the frontal zone of the ice sheet. The study confirms a limited subglacial meltwater drainage in the study area during the Younger Dryas.

Local clay-varve chronology in the Karlskrona-Hultsfred region, southeast Sweden

Abstract A local varve chronology from southeast Sweden between the town of Karlskrona in the province of Blekinge and Hultsfred in the province of Småland is presented. The chronology covers approximately 800 varve-years. The glaciolacustrine varves were deposited in the Baltic Ice Lake during the Late Weichselian deglaciation. The study includes 60 connected and 56 unconnected varve series and shows that the ice recession rates vary between 75–125 m/year and 250–340 m/year in the southern and northern parts of the area, respectively. An abrupt change from thin to thick clay varves was found in the northern part of the area. The change has been correlated with a similar change of silty varves in the bottom bed of one of the delta plains close to the highest shoreline. This change in the meltwater deposition has been hypothetically correlated with the transition between the Older Dryas (GI-1d) and Alleröd (GI-1c) chronozones, or around 13,750 GRIP years.