Sub-till Sediments Research Articles

Genesis of hummocks found in tunnel valleys: an example from Hörda, southern Sweden

In the south-central sector of the former Fennoscandian Ice Sheet, imprints of the sub-glacial hydrological system are present as “glaciofluvial corridors,” formed by glacial meltwater at the ice–bed interface during the Bølling–Allerød warm period. Many of these are interpreted as tunnel valleys and are commonly characterized by hummocks on their valley floors. Contemporary ice sheets produce increased amounts of meltwater as a consequence of global warming, and occasionally it is observed that meltwater is suddenly released from supra- and subglacial lakes, suggesting a highly dynamic subglacial hydraulic system. Studies of the imprints and deposits from such systems on formerly glaciated terrain can expand our knowledge of ice-sheet response to increased meltwater production. Here, we study sediments exposed in two hummocks within the tunnel valley at Hörda, south Sweden. One of the investigated hummocks is superposed by a small esker. This hummock consists of a diamict interpreted as a subglacial traction till, observed to be overlain by esker sediment. A second hummock displays deformed sediment at its base, which is glaciotectonically intercalated with above-lying diamict, a sub-glacial traction till. The sub-till sediments, interpreted as proglacial outwash, were deformed by overriding ice. The sediment was dated using optically stimulated luminescence (OSL), inferring a late MIS 4 or early MIS 3 age, congruent with other observations of sub-till sediments in south Sweden. The investigated hummocks on the floor of the Hörda tunnel valley are interpreted to have been formed by sub-glacial fluvial erosion simultaneous with tunnel valley formation, most probably during the latest deglaciation of the area.

Stratigraphical signatures of glacier activity, marine processes and a possible tsunami in the Leirfjorden fjord‐valley system, north Norway

An integrated interpretation of on‐ and offshore stratigraphical records at Leirfjorden, north Norway, reveals new aspects of the area's palaeoenvironmental history. The study is based on marine sparker data and well‐exposed sections on land that were analysed for their sediment facies, mineralogy and fossil assemblages. Existing research and new radiocarbon dates provide a chronological framework for the interpretation. The late Younger Dryas Nordli substage type locality in the Leirfjorden catchment is revised and found to reflect local glacial activity, maybe a collapse of stagnant ice rather than glacier advance, while late Younger Dryas to Preboreal glacier re‐advances south of Leirfjorden and near Ranfjorden are here named the Bardal substage. The stratigraphical record includes pre‐Younger Dryas, valley‐crossing, glacial striae and old till with provenance of resistant bedrock typical of more elevated mountain areas. It differs from younger till units representing topographically controlled glacier movement. Part of the Leirfjorden fjord‐valley system is located between the main glacial and fluvial drainage paths affecting the sediment supply. As a result, highstand deposits are indistinct and fluvial sediments form only a minor part of the forced‐regressive systems tract. Instead, the valley fill overlying till and subtill sediments is dominated by the deglacial transgressive tract and a forced‐regressive systems tract with composite marine deposits and various marine erosion surfaces. A special event bed is interpreted as a possible tsunami deposit caused by seismicity and/or mass‐wasting in the fjord following glacier retreat. The study highlights the stratigraphical complexity of interconnected fjord and sound systems in a low accretion setting.

Subglacial conditions and Scandinavian Ice Sheet dynamics at the coarse-grained substratum of the fore-mountain area of southern Poland

Abstract The fore-mountain areas of southern Poland are locally composed of the coarse-grained sediments of alluvial fans, which created unusual conditions under the advancing Scandinavian Ice Sheet during the Elsterian glaciation. This highly permeable substratum potentially enabled rapid outflow of meltwater from the ice sheet base, thereby reducing the water pressure and strongly influencing the ice sheet dynamics. The subglacial conditions and the relationship between the ice sheet behaviour and its coarse-grained substratum were studied at the foreland of the western Carpathian Mountains. The sedimentological and structural analysis of the till and related sediments that were deposited above the alluvial gravel of the fore-mountain fans are presented. The study indicates that despite the high permeability of the coarse-grained substratum, it did not slow the ice sheet movement. Conversely, the ice sheet moved mainly due to basal slip and locally shallow deformations. This was a consequence of very high basal water pressure, which resulted largely from the presence of permafrost that restricted subglacial groundwater outflow. In addition, the ice sheet substratum was inclined opposite to the direction of its movement, increasing the pressure of the subglacial water. Numerous subhorizontal sandy laminae within the till indicate that the meltwater from the ice sheet base was drained by a water film along the ice/bed interface. The water escape structures within the till and subtill sediments indicate the occasional instability of the ice sheet hydrological system and suggest that the meltwater was periodically stored in the ice sheet base. Temporal changes occurring in the ice sheet hydrological system might indicate variations in the ice sheet behaviour; i.e. phases of relatively fast ice flow and phases of ice stagnation. The latter were probably correlated with the freezing of the ice margin to its base. The study shows how the coarse-grained substratum could potentially influence ice sheet behaviour and provides a broader look on the factors controlling the dynamics of the Pleistocene ice sheets, which soft-sediment substrata in many areas were texturally varied.

Interplenivistulian (MIS 3) environmental changes recorded in sub-till lake deposits at Wildno, Dobrzyń Lakeland (Polish Lowland)

A complex of clays and sandy-silt deposits with mollusc remains covered by a till correlated with the Main Stadial of the Vistulian (Weichselian) Glaciation was investigated at Wildno in the Dobrzyń Lakeland (Polish Lowland). A few taxa of snails: Bithynia tentaculata, Valvata piscinalis, Theodoxus cf. fluviatilis, Valvata sp., Viviparus sp. as well as bivalves Pisidium cf. amnicum and Sphaerium sp. were identified amongst poorly-preserved shell fragments. No pollen grains were present in mollusc-bearing deposits, whereas the pollen spectrum of clays lying below the latter points their accumulation in oligotrophic water body in a cool climate (Pediastrum kawraiskii) and woodless area. Radiocarbon age of shell detritus (4 samples) ranges from >43.7 cal ka to c. 32.1 ± 0.8 cal ka. Both the geological situation and the results of dating suggest that sub-till sediments with molluscs were deposited in the upper part of Interplenivistulian (MIS 3) = the Grudziądz Interstadial, while accumulation of clays may be correlated with its earlier and cooler part.

Sub-till glaciofluvial sediments at Hultsfred, South Swedish Upland

In a gravel pit near Hultsfred, at the eastern margin of the South Swedish Upland, gravelly, sandy and silty-sandy beds covered by a thick, stratified gravelly diamicton are exposed. Large folds and shear-planes indicate post-depositional deformation of the sorted sediments, which according to OSL dating have a maximum Middle Weichselian (∼60 ka) age. The sand and gravel are interpreted as glaciofluvial sediments that were deposited laterally during a Middle Weichselian deglaciation, or possibly subglacially during the Late Weichselian. The large-scale deformation was due to glaciotectonics, caused by an overriding wet-based ice sheet during the last deglaciation. The subglacial hydrostatical pressure was high and led to the formation of downward-injected clastic dykes. A basal till, which forms the cover moraine in the area, caps the succession. The fairly frequent occurrence of sub-till sediments on the South Swedish Upland demonstrates the limited effect of glacial erosion in parts of this area. During the Last Glacial Maximum, the South Swedish Upland was an ‘island’ of slow-moving, polythermal and at least partly non-erosive ice, surrounded by faster ice-streams in the Baltic and in the Skagerrak. The ice sheet may have left its main impact on the landscape during short periods during advance and deglaciation only.

ESR/OSL ages of long‐debated subtill fossil‐bearing marine deposits from the southern Kola Peninsula: stratigraphic implications

The occurrence of sandy clay deposited in a warm marine environment just below the till of the last glaciation has created controversy about its age and stratigraphic position in the sedimentary basin of the Kola Peninsula. Data on marine microfauna, diatoms, malacofauna and pollen composition indicate that during the period when the sandy clay was deposited the climate was similar or even warmer than at present. According to 14C dates, sedimentation of the sandy clay occurred around 40 ka BP. Based on these data, some researchers have attributed these marine deposits to the third Late Pleistocene Belomorian (sensu Lavrova 1960) interglacial transgression. At the same time there are geological indications suggesting re‐deposition of these subtill sediments. To solve this problem we have reinvestigated the subtill interglacial marine deposits from the Varzuga section (˜66.4° N and 36.6° E). Four different marine shell species and enclosing sandy clay sediments taken from the subtill marine unit of the section were dated by the electron spin resonance (ESR) and optically‐stimulated luminescence (OSL) methods at about 103 and 104 ka, respectively. The results indicate that the subtill marine deposits belong to the first Late Pleistocene Boreal transgression that, according to our previous studies of the marginal areas of the Eurasian North, has occurred in the time interval from approximately 145 to 70 ka BP.

Signature and timing of the Kattegat Ice Stream: onset of the Last Glacial Maximum sequence at the southwestern margin of the Scandinavian Ice Sheet

At the end of the Middle Weichselian (30–25 ka BP) a glacier advance from southern Norway, termed the Kattegat Ice Stream, covered northern Denmark, the Kattegat Sea floor and the Swedish West Coast during onset of the Last Glacial Maximum (LGM) at the southwest margin of the Scandinavian Ice Sheet. The lithostratigraphic unit deposited by the ice stream is the till of the Kattegat Formation (Kattegat till). Because morphological features have been erased by later glacial events, stratigraphic control and timing are decisive. The former ice stream is identified by the dispersal of Oslo indicator erratics from southern Norway and by glaciodynamic structures combined with glaciotectonic deformation of subtill sediments. Ice movement was generally from northerly directions and the flow pattern is fan‐shaped in marginal areas. To the east, the Kattegat Ice Stream was flanked by passive glaciers in southern Sweden and its distribution was probably governed by the presence of low permeability and highly deformable marine and lacustrine deposits. When glaciers from southern Norway blocked the Norwegian Channel, former marine basins in the Skagerrak and Kattegat experienced glaciolacustrine conditions around 31–29 ka BP. The Kattegat Ice Stream became active some time between 29 ka BP and 26 ka BP, when glaciers from the Oslo region penetrated deep into the shallow depression occupied by the Kattegat Ice Lake. Deglaciation and an interlude with periglacial and glaciolacustrine sedimentation lasted until c. 24–22 ka BP and were succeeded by the Main Glacier Advance from central Sweden reaching the limit of Late Weichselian glaciations in Denmark around 22–20 ka BP, the peak of the LGM. This was followed by deglaciation and marine inundation in the Kattegat and Skagerrak around 17 ka BP.

Signature and timing of the Kattegat Ice Stream: onset of the Last Glacial Maximum sequence at the southwestern margin of the Scandinavian Ice Sheet

At the end of the Middle Weichselian (30–25 ka BP) a glacier advance from southern Norway, termed the Kattegat Ice Stream, covered northern Denmark, the Kattegat Sea floor and the Swedish West Coast during onset of the Last Glacial Maximum (LGM) at the southwest margin of the Scandinavian Ice Sheet. The lithostratigraphic unit deposited by the ice stream is the till of the Kattegat Formation (Kattegat till). Because morphological features have been erased by later glacial events, stratigraphic control and timing are decisive. The former ice stream is identified by the dispersal of Oslo indicator erratics from southern Norway and by glaciodynamic structures combined with glaciotectonic deformation of subtill sediments. Ice movement was generally from northerly directions and the flow pattern is fan-shaped in marginal areas. To the east, the Kattegat Ice Stream was flanked by passive glaciers in southern Sweden and its distribution was probably governed by the presence of low permeability and highly deformable marine and lacustrine deposits. When glaciers from southern Norway blocked the Norwegian Channel, former marine basins in the Skagerrak and Kattegat experienced glaciolacustrine conditions around 31–29 ka BP. The Kattegat Ice Stream became active some time between 29 ka BP and 26 ka BP, when glaciers from the Oslo region penetrated deep into the shallow depression occupied by the Kattegat Ice Lake. Deglaciation and an interlude with periglacial and glaciolacustrine sedimentation lasted until c. 24–22 ka BP and were succeeded by the Main Glacier Advance from central Sweden reaching the limit of Late Weichselian glaciations in Denmark around 22–20 ka BP, the peak of the LGM. This was followed by deglaciation and marine inundation in the Kattegat and Skagerrak around 17 ka BP.

Was Hardangerfjorden, western Norway, glaciated during the Younger Dryas?

A new site with a shell-bearing till is described from the outer part of Hardangerfjorden. Two shell fragments from the till were dated to 10,870 - 90 and 11,000 - 40 radiocarbon years BP. These dates support the established hypothesis of a major glacial re-advance and an ice-filled Hardangerfjorden during the Younger Dryas. An opposite hypothesis, that Hardangerfjorden remained ice-free during the entire Younger Dryas, was proposed in a recent paper by Helle et al. (1997; Norsk Geologisk Tidsskrift 77, 101-117) and supported by several scientists during the Nordic Geological Winter Meeting in 2000. I argue in the present paper that the latter hypothesis is falsified, not only by the two new dates, but also by previously published dates from shell-bearing tills and sub-till sediments. In my opinion, the data presented by Helle et al. allow for alternative interpretations.

Ice Movements and Till Stratigraphy in the Gudbrandsdal Area. Preliminary Results

The authors have previously described two localities with sub-till sediments from a Weichselian interstadial in the Gudbrandsdal (Bergersen & Garnes 1971). Based on studies of striae and tills, the last glaciation is preliminarily divided into four phases. Most of the till in the investigated area is believed to have been deposited during the two first phases, A and B. During phase A the glaciers advanced from high mountain areas. Entering the valleys, the ice gave rise to ice-dammed lakes, the deposits of which are found as sub-till sediments. Phase B was the main phase of the inland ice with an ice divide northwest of the Gudbrandsdal. Later, during phase C, the ice divide was situated above the central part of the Gudbrandsdal. Consequently, the glacio-geological processes in the investigated area were modest during both phase C and the deglaciation period, phase D. During the latter, a gradual transition from active to stagnant to dynamically dead ice took place. Simultaneously the meltwater drainage changed from being mainly supraglacial to lateral/englacial and finally subglacial.

Evidence of Sub-Till Sediments from a Weichselian Interstadial in the Gudbrandsdalen Valley, Central East Norway

The authors point out several localities with sub-till sediments in the highly ice-eroded Gudbrandsdalen Valley. Two localities are described and discussed. Based upon studies of texture and structure, the sediments are interpreted as proglacial deposits, not removed by a later ice-advance. The assumed permafrost structures (fossil ice wedges) and 6 finds of mammoth remnants, of which one has been C14 dated to 24,400 ± 900 years B. P. (Heintz 1969), indicate that these sediments are of an interstadial age.Tentatively this is named the Gudbrandsdalen interstadial. Out of the total of 15 mammoth finds in Norway, 13 are from the Gudbrandsdalen area. The sub-till sediments were presumably the depositaries for these fossils during the latest glaciation.