Quaternary geology and deglaciation of the continental shelf off Troms, north Norway

The study is based mainly on 4700 km shallow seismic profiling, soil mechanical, and micropaleontological analyses from forty localities, and seven radiocarbon datings. Six foraminiferal assemblages are recognized. The thickness of Quaternary deposits ranges from sparse to more than 200 m. They consist of top sand, soft sensitive clay, and glacial drift divided into four seismic stratigraphic units. The oldest unit, which is the thickest and most extensive, is built up of various sediments and is partly eroded, especially in the southern part. The remaining units occur as three complex linear belts running broadly parallel to the present coast and were deposited during deglaciation. Dating of the outer unit, the Nordvestsnaget Drift, suggests a maximum age of 13,300 years B.P., and datings from the top of the middle unit, the Mulegga Drift, have given a minimum age of about 12,200 years B.P. Seismic stratigraphy shows that the inner unit, the Havbrobakken Drift, is younger than the Mulegga Drift, but no datings have been obtained so far.     

Late Weichselian deglaciation in the Oslofjord area, south Norway

The older 'moraine lines' outside the Ra Moraine in the outer Oslofjord area have been correlated with events in Bohuslän, Sweden. Recent radiocarbon datings in the vicinity of the Ra Moraine and a radiocarbon dated sea-level curve for the Ski area show that the Ra Moraine was formed during the Early Younger Dryas, whereas the Ski Moraine was formed at the end of the Younger Dryas chronozone. An equidistant shoreline diagram together with a large number of marine limit observations have been used to establish the position of the glacier front during Late Younger Dryas and Early Preboreal chronozones. Reconnaissance mapping indicates a fairly regular recession with many short stops during the Bølling, Older Dryas and Allerød chronozones; at least two readvances to the Ra Moraine before 10,600 years B.P.; a rapid recession during the Middle Younger Dryas and a number of ice-front oscillations at the end of the Younger Dryas chronozone.     

Late Weichselian deglaciation and paleoenvironment of the shelf and coastal areas of Troms, north Norway - a review

The present knowledge of the Late Weichselian marginal moraines and paleoenvironment from the coastal and shelf areas of Troms and west Finnmark is reviewed. Diverse opinions exist regarding the position and age of the marginal moraines in the offshore areas.     

Modern and Holocene foraminifera and sediments on the continental shelf off Troms, North Norway

Based on studies of gravity cores from two transverse troughs on the shelf and earlier investigations, the surface sediments are divided into three main facies: bouldery and pebbly sand on the banks and the shelf break; sand on the flanks and outer parts of the troughs and sandy mud in the inner parts of the troughs. Besides a depth control, the distribution must have been influenced by relatively rapidly moving bottom currents in the outer parts of the troughs. The distribution and composition of the modern benthic foraminiferal fauna (e.g. C. lobatulus/T. angulosa in the outer reaches and C. obtusalBolivina spp. in the inner reaches) is mainly controlled by the bottom current regime and sediments. The planktic fauna dominated by N. pachyderma (R) correlates well with the winter surface temperatures. The stratigraphi-cal analysis shows that the 10,000–9,600 years B.P. period experienced high rates of deposition probably due to meltwater runoff from the continental ice sheet. At ca. 9,700 B.P. a minimum in the production of N. pachyderma (R) indicates a temporary cooling of the surface water. During the 9,600–7,800 B.P. period the rate of deposition was reduced. At the end of this period the foraminiferal fauna changed towards one like the modern fauna, reflecting improving ecological conditions. At ca. 7,800 B.P. the sediments became coarser due to reduced input of detrital sediments and an increased production of sand-sized biogenic material. Since then the shelf environment has been fairly stable up to the present time.     

Depositional environments and chronology of Late Weichselian glaciation and deglaciation in the central North Sea

Graham, A.G.C., Lonergan, L. & Stoker, M.S. 2010: Depositional environments and chronology of Late Weichselian glaciation and deglaciation in the central North Sea. Boreas, Vol. 39, pp. 471–491. 10.1111/j.1502‐3885.2010.00144.x. ISSN 0300‐9483. Geological constraints on ice‐sheet deglaciation are essential for improving the modelling of ice masses and understanding their potential for future change. Here, we present a detailed interpretation of depositional environments from a new 30‐m‐long borehole in the central North Sea, with the aim of improving constraints on the history of the marine Late Pleistocene British–Fennoscandian Ice Sheet. Seven units characterize a sequence of compacted and distorted glaciomarine diamictons, which are overlain by interbedded glaciomarine diamictons and soft, bedded to homogeneous marine muds. Through correlation of borehole and 2D/3D seismic observations, we identify three palaeoregimes. These are: a period of advance and ice‐sheet overriding; a phase of deglaciation; and a phase of postglacial glaciomarine‐to‐marine sedimentation. Deformed subglacial sediments correlate with a buried suite of streamlined subglacial bedforms, and indicate overriding by the SE–NW‐flowing Witch Ground ice stream. AMS ¹⁴C dating confirms ice‐stream activity and extensive glaciation of the North Sea during the Last Glacial Maximum, between c. 30 and 16.2 ¹⁴C ka BP. Sediments overlying the ice‐compacted deposits have been reworked, but can be used to constrain initial deglaciation to no later than 16.2 ¹⁴C ka BP. A re‐advance of British ice during the last deglaciation, dated at 13.9 ¹⁴C ka BP, delivered ice‐proximal deposits to the core site and deposited glaciomarine sediments rapidly during the subsequent retreat. A transition to more temperate marine conditions is clear in lithostratigraphic and seismic records, marked by a regionally pervasive iceberg‐ploughmarked erosion surface. The iceberg discharges that formed this horizon are dated to between 13.9 and 12 ¹⁴C ka BP, and may correspond to oscillating ice‐sheet margins during final, dynamic ice‐sheet decay.     

Radiocarbon deglaciation chronology of the Thunder Bay,Ontario area and implications for ice sheet retreat patterns

The sensitivity of ice sheets to climate change influences the return of meltwater to the oceans. Here we track the Laurentide Ice Sheet along a ～400 km long transect spanning about 6000 yr of retreat during the major climate oscillations of the lateglacial. Thunder Bay, Ontario is near a major topographic drainage divide, thus terrestrial ablation processes are the primary forcers of ice margin recession in the study area. During deglaciation three major moraine sets were produced, and have been assigned minimum ages of 13.9 ± 0.2, 12.3 ± 0.2–12.1 ± 0.1, and 11.2 ± 0.2 cal ka BP from south to north. These define a slow retreat (～10–50 m/a) prior to major climate oscillations which was then followed by a factor of ～2 increase during the Bölling–Alleröd, and an additional increase during the early Holocene. When compared to retreat rates in other terrestrial settings of the ice sheet, nearly identical patterns emerge. However this becomes problematic because a key control on retreat rates is the surface slope of the ice sheet and this should vary considerably over areas of so-called hard and soft beds. Further these ice margin reconstructions would not allow meltwater sourced in the Hudson Basin to drain into the Atlantic basin until after Younger Dryas time.     

The effect of metamorphic fluid flow on the nucleation and growth of garnets from Troms, North Norway

A spatial association is observed between the size distribution of garnet porphyroblasts and the size distribution of quartz veins in greenschist facies metapelites from Troms, North Norway. The size distribution of quartz veins reflects the flow regime of metamorphic fluids. The hypothesis that the flow regime of metamorphic fluids is also responsible for the size distribution of garnet crystals was tested by ascribing empirical acceleration parameters to the nucleation and growth rates of garnet crystals.
In regions where fluid flow was interpreted as pervasive', acceleration parameters for nucleation were high, whereas in regions where fluid flow was interpreted as channelled', acceleration parameters for growth were high. Accelerated crystal growth is further implied from the chemical zoning and crystal morphologies of garnets collected near discrete veins.
This spatial association may imply that fluid flow can be instrumental in controlling garnet crystallization. Fluid flow could affect garnet crystallization kinetics by facilitating thermal advection and/or mass transfer. In the study area, rhodochrosite (MnCO₃) veins provide evidence for mass transfer of Mn by fluid flow. An influx of Mn would expand the stability field of garnet to lower temperatures. The resulting thermal overstep could accelerate nucleation and/or growth of garnets.
The corollary of this study is that size distributions and chemical zoning of garnets, or other porphyroblast phases, can be used to study metamorphic fluid flow.     

Glacial geomorphology and chronology of deglaciation,South Georgia,sub-Antarctic

We mapped and dated the glacial geomorphology of north-east South Georgia, in the maritime sub-Antarctic. The aim was to examine the timing of deglaciation of the island in the context of inter-hemispheric phasing of climate change. Former glacier limits are restricted to the inner fjords, and our detailed mapping of them has demonstrated a consistent geomorphological pattern that is similar across several different glacier types and sizes. The pattern comprises three suites of moraines (categories “a–c”), not all of which are represented at every site because the outer suite is often overridden by younger suites. Category “a” is an outer wide, low amplitude moraine ridge, category “b” comprises 2–4 sharp-crested, bouldery moraines that are often located close to or even over-riding “a”, and category “c” is a series of lower amplitude moraines with overprinted streamlined landforms such as flutings. Analysis of in situ cosmogenic ¹⁰Be in boulders on these moraines has allowed us to determine a deglacial chronology for the older two moraine groups. The age of the inner (youngest) group has been estimated from soil development. The cosmogenic nuclide ages show that the outermost moraine was deposited ca 12.2±1.5 ka BP, but that a subsequent readvance in the mid-Holocene (ca 3.6±1.1 ka BP) reached and, in places, over-rode this earlier moraine. This latter advance coincides with the “Mid Holocene Hypsithermal”. A final Late Holocene advance reached closely similar limits to the previous two fluctuations and is estimated from soil data to have an age of ca 1.1 ka BP. We suggest that the close concordance of Late-Glacial and interglacial limits (in this case associated with warming) can be explained by a change in dominant forcing. During glacials, extensive sea-ice limits precipitation availability and so glaciers are restricted to the inner fjords. During interglacials precipitation is not limited in the same way by sea-ice cover and so during warming precipitation increases and tidewater glaciers on the island have responded by advancing. This study emphasises the importance of a clear understanding of geomorphology in order to interpret chronological information.     

Deglaciation chronology and glaciomarine successions in the Malangen-Målselv area, northern Norway

The deglaciation history of the Malangen‐Målselv fjord and valley area proximally to the Tromsø‐Lyngen (Younger Dryas) moraine at Bakkejord, Malangen, northern Norway, is reconstructed based on morphostratigraphic, lithostratigraphic and geophysical evidence, and 25 radiocarbon dates from marine shells and foraminifera. The results show that following the Skarpnes event c. 12 200 ¹⁴Cyr BP, and prior to the Younger Dryas readvance, the area was deglaciated at least as far as Sandmo situated 22 km proximally to the Tromsø‐Lyngen moraine. Two moraine ridges crossing the fjord at Sandmo and buried beneath thick glaciomarine sediments are correlated with this period. The area was subsequently deglaciated between 10 300 and 9200 ¹⁴Cyr BP, following the Tromsø‐Lyngen (Younger Dryas) readvance. Five ice‐front accumulations post‐dating the Tromsø‐Lyngen moraine and situated 19, 27, 42, 55 and 77 km behind it are identified and dated based on radiocarbon dates and correlation of marine limits: Målsnes (c. 10 050 ¹⁴Cyr BP), Kjerresnes (c. 10 000 ¹⁴Cyr BP), Solli (c. 9750 ¹⁴Cyr BP), Bardufoss‐Brentmoen‐Storskogmoen (c. 9600–9700 ¹⁴Cyr BP) and Alapmoen (c. 9200 Cyr BP). The largest of these, at Bardufoss‐Storskogmoen, possibly accumulated as a response to an ice advance. Fourteen dates of apparent late Allerød/Younger Dryas age (11 100–10 000 ¹⁴Cyr BP), obtained from fossils in glaciomarine sediments in the Målselv valley up to 77 km proximally to the Tromsø‐Lyngen moraine, are interpreted as postdating rather than predating this moraine. Several of these are considered to be too old because of uncertain reservoir age, carbon‐dating plateaus and/or contamination. This highlights uncertainties associated with radiocarbon‐dating and the profound effect such uncertainties may have on interpreting geological events.     

Holocene shore displacement and deglaciation chronology in Norrbotten, Sweden

The coastal zone of Norrbotten, northern Sweden, was gradually inundated by the Ancylus Lake following the retreating ice margin and forming a highest coastline approximately 210 m above the present sea level. The succeeding shore displacement is reconstructed based on lithological investigations and radiocarbon datings of identified isolation sequences from 12 cored lake basins. The highest lake basins, along with two basins above the highest shoreline, suggest ice-free conditions already at 10 500 cal. yr BP. This is at least 500 years earlier than previously thought and implies rapid ice-sheet break-up in the Gulf of Bothnia. The shore displacement (RSL) curve represents a forced regression of successively decreasing rate through the Holocene, from 9 m/100 yr to 0.8 m/100 yr. During the first 1000-1200 years, the isostatic uplift is exponentially declining, followed by a constant uplift rate from c. 9500 cal. yr BP to 5500-5000 cal. yr BP. The last 5000 years seem to be characterized by a low but constant rebound rate. The development of the Ancylus Lake stage of the Baltic may also be discerned in the Norrbotten RSL curve, suggesting that the chronology of the Ancylus Lake stages may have to be revised. The Littorina transgression is also reflected by the RSL curve shape. In addition, a series of early to mid-Holocene beach terraces were OSL-dated to allow for comparison with the ¹⁴C-dated shore displacement curve. Interpretations of these ages and their relation to former sea levels were clearly more problematic than the dating of the lake basin isolations.     

Carbonate-orthopyroxenites (sagvandites) from Troms, northern Norway

Massive carbonate-orthopyroxenites occur in tectonic lenses within narrow zones of high-grade regional metamorphism of the Caledonian mountain belt. Typical country rocks are garnet-kyanite-biotite gneisses, amphibolites, or lime silicate marbles. Some of the pelitic country rocks exhibit water-deficient mineral assemblages, others have endured partial melting. Original peridotites (dunites and, possibly, saxonites) are believed to have reacted with highly CO₂-bearing, SiO₂-enriched gas phases emanating from neighbouring silicate marbles, essentially according to the equations forsterite + CO₂ →estatite + magnesite, and forsterite SiO₂ →estatite. This metasomatism as well as the high-grade metamorphism of the country rocks may have taken place under essentially the same pressure temperature conditions, i.e. under at least 6 kb and 630°C.     

Lateglacial and Holocene terrestrial and marine proxies reflecting climate changes in the Malangen fjord area, Norway, northeast North Atlantic

A high-resolution Younger Dryas–late Holocene record of climate and environment from the Malangen fjord has been established on the basis of two marine sediment cores. Five pollen-spore assemblage zones have been defined covering the period c . 11 500 cal. yr BP (10 200 ¹⁴C yr BP) to c . 1600 cal. yr BP (1600 ¹⁴C yr BP) with a hiatus of c . 2000 cal. years between c . 10 200 and 8100 cal. yr BP (9000 and 7300 ¹⁴C yr BP). The Holocene vegetation development from pioneer vegetation to forest development, identified in the marine pollen record, correlates well with pollen records from terrestrial sections of northern Norway. The marine pollen record was also correlated directly with marine proxy records of the bottom water temperature investigated in the same sediment cores. Correlation between the marine and terrestrial proxies suggests that changes in the influx of warm Atlantic Water to the fjord led to an instant change in the vegetation of the surrounding land area. The results thus support a strong link between marine and atmospheric mean climatic states in the North Atlantic region throughout the Holocene.     

Younger Dryas deglaciation at Mt. Billingen, and clay varve dating of the Younger Dryas/Preboreal transition

A clay varve chronology has been established for the Late Weichselian ice recession east of Mt. Billingen in Västergötland, Sweden. In this area the Middle-Swedish end moraine zone was built up as a consequence of cold climate during the Younger Dryas stadial. A change-over from rapid to slow retreat as a result of climatic deterioration at the Alleröd/Younger Dryas transition cannot be traced with certainty in the varve sequences, but it seems to have taken place just before 11,600 varve years BP. The following deglaciation was very slow for about 700 years — within the Middle-Swedish end moraine zone the annual ice-front retreat was only c . 10 m on average. A considerable time-lag is to be expected between the Younger Dryas climatic event and this period of slow retreat. The 700 years of slow retreat were succeeded by 200 years of more rapid recession, about 50–75 m annually, and then by a mainly rapid and uncomplicated retreat of the ice-front by 100–200 m/year or more, characterizing the next 1500 years of deglaciation in south and central Sweden. The change from about 50–75 m to 100–200 m of annual ice-front retreat may reflect the Younger Dryas/Preboreal transition. Clay-stratigraph-ically defined, the transition is dated at c . 10,740 varve years BP, with an error of +100 to -250 years. In the countings of ice layers in Greenland ice cores (GRIP and GISP-2) the end of the Younger Dryas climatic event is 800–900 years older. However, a climatic amelioration after the cold part of the Younger Dryas and in early Preboreal should rapidly be reflected by for example chemical components and dust in Greenland ice cores, and by increasing δ¹³C content in tree rings. On the other hand, the start of a rapid retreat of the inland ice margin can be delayed by several centuries. This can explain at least a part of the discrepancy between the time-scales.     

The deglaciation between Skjerstadfjord and Svartisen, north Norway

Six successive zones with distinctive marginal moraines are recognized. Based on radiocarbon dates the ages of the three oldest zones are believed to be: older than 11,100 years B.P. (Vasdal event), about 10,900 B.P. (Glomfjord-Straumöy events), and between 10,400 and 9500 B.P. (Skjerstad event). The three youngest zones are Preboreal. Corresponding marine sediments, shorelines, glaciation limits and other glacial features are described.     

Deglaciation and palaeoclimate of the Andfjord-Vågsfjord area, North Norway

This paper reviews the deglaciation history and palaeoclimate from 22 to 9.5 ¹⁴Cka BP in the Andfjord-Vagsfjord area. Eight main glacial events are recorded: The Egga-I (>22 ¹⁴Cka BP), the Bjerka, the Egga-II (>14.6 ¹⁴Cka BP), the Flesen (14.5 ¹⁴Cka BP), the D (13.8–13.2 ¹⁴Cka BP), the Skarpnes (12.2 ¹⁴Cka BP), the Tromsø–Lyngen (10.7–10.3 ¹⁴C ka BP) and the Stordal (10.0–9.5 ¹⁴Cka BP). Onset of the final deglaciation occurred about 14.6 ¹⁴Cka BP. Most of the western part of the Fennoscandian and Barents Sea Ice Sheets receded from the outer continental shelf 15–14 ¹⁴Cka BP. The delivery and melting of icebergs at this time to the Norwegian-Greenland Sea resulted in a low oxygen isotope event recorded in a number of cores in the region. Atlantic water intruded the area 13.2 ¹⁴Cka BP, and an atmospheric warming commenced 12.9/12.8 ¹⁴Cka BP. A marked glacial recession occurred before the Skarpnes event. During Allerød time, the glaciers retreated to the fjord heads or even farther inland. The Fennoscandian outlet glaciers readvanced (locally more than 40 km), reached their Younger Dryas outer limit after 10.7 ¹⁴Cka BP and retreated from this position before about 10.3 ¹⁴Cka BP.     

First post-glacial establishment of forest trees: early Holocene vegetation, mollusc settlement and climate dynamics in central Troms, North Norway

Sediments from two limnic basins in a sub-continental region of northern Norway are investigated for pollen, plant macrofossils and bivalves at a high stratigraphical resolution. The basins are located at 280 and 400 m a.s.l. on the S-SW slope of Mt. Skrubben (848 m a.s.l.). The bio- and lithostratigraphical records are interpreted in terms of immigration and establishment of forest tree species and climate. The mountain was deglaciated at both sites at c . 9200-9100 BP ( c . 10200-10100 cal. BP). Betula pubescens forest stands established at c . 8900 BP ( c . 9900 cal. BP). The first Pinus sylvestris individuals may have established at or near the lowermost investigation site at c . 8700-8600 BP ( c . 9700-9600 cal. BP), while P. sylvestris forest stands were present at 400 m a.s.l. c . 7700 BP ( c . 8500 cal. BP) and expanded in area and/or density from c . 7200 BP. Macrofossils of P. sylvestris occur in strata with a pine pollen influx as low as c . 200 grains cm -2 yr -1 . The immigration of P. sylvestris to the western part of northern Fennoscandia may thus have happened earlier than formerly interpreted from pollen analyses alone, where influx values as low as c . 200 grains cm -2 yr -1 would not be considered as indicative of local presence. Alnus incana established at approximately the same time as P. sylvestris . During the period from deglaciation to c . 6600 BP ( c . 7300 cal. BP), calcareous limnic sediments were deposited in the basins, with Chara species and a Sphaeriidae fauna consisting of Pisidium hibernicum , P. lilljeborgii , P. milium , P. nitidum , P. waldeni and P. casertanum . During the period c . 8800-8500 BP, P. nitidum prevailed, and almost pure CaCO 3 sediments were formed. We interpret the environment as dry with a low influx of both minerogenic particles and humic substances. The period 8800-8500 BP (9800-9500 cal. yr BP) is interpreted as a period with warm and dry summers and cold and dry winters.     

北秦岭板山坪岩体锆石U-Pb年代学及岩石成因研究

板山坪岩体是北秦岭二郎坪群中的侵入岩。为了查明该岩体的成因,对该岩体进行了锆石U-Pb年代学、锆石原位Hf同位素研究以及矿物化学分析等方面的研究。研究结果表明,板山坪岩体岩性组成主要为石英闪长岩和花岗闪长岩,花岗闪长岩内部存在暗色包体。本次研究获得板山坪石英闪长岩锆石U-Pb年龄为442.7～432.2 Ma,花岗闪长岩锆石U-Pb年龄为436.8～432.7 Ma,暗色包体锆石U-Pb年龄为437.6 Ma。锆石176Hf/177Hf值为0.282 737～0.282 736,εHf（t）值集中分布在8.4～9.4之间,二阶段Hf模式年龄（TDM2）在876～832 Ma之间。石英闪长岩结晶温压分别为673 ℃～745 ℃和0.19～0.54 GPa,花岗闪长岩结晶温压分别为657 ℃～730 ℃和0.48～0.96 GPa,暗色包体结晶温压分别为680 ℃～734 ℃和0.69～1.65 GPa。综合分析认为板山坪岩体为复式岩体,两期结晶年龄分别为496～487 Ma和442～432 Ma。岩石来源于地幔分离出来的新生下地壳。     

Accuracy limits of palaeomagnetic chronology

The paper deals with palaeomagnetic chronology as a statistical method. Using R. Fisher's statistics, the author derived the mean virtual pole positions, beginning with the Quaternary up to the Devonian, from data published in different countries and laboratories for the Eurasian continent up to the end of the year of 1966. From the mean pole positions the divergence of palaeomagnetic directions was computed and its value for Central Europe versus time was plotted. — Palaeomagnetic research into biostratigraphically dated rocks and mineral deposits in the Bohemian Massif has reached a stage, where it is possible to study the tectonic development of mineral deposits in relation to tectonic development of various basins, grabens and rock blocks. With respect to tectonic deformations, to the natural scattering of palaeomagnetic directions and to the accuracy in deriving the mean values of palaeomagnetic divergence, it is possible to achieve an accuracy in palaeomagnetic dating within the limits of a geological period, i.e. some tens of millions of years. Some other parameters may be used in special instances to reach a higher degree of accuracy.

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Zusammenfassung Die Arbeit behandelt die paläomagnetische Chronologie als statistische Methode im Hinblick auf die Erforschung von Lagerstätten. Mit der statistischen Methode von R. Fisher berechnet der Verfasser die mittleren Lagen der virtuellen Pole vom Quartär bis zum Devon; zur Berechnung wurden die paläomagnetischen Daten ausgenutzt, die für den eurasischen Erdteil bis 1966 veröffentlicht wurden. Von den mittleren Pollagen wurde die Divergenz der paläomagnetischen Richtungen für Mitteleuropa abgeleitet und ihr Wert wurde als Zeitfunktion graphisch dargestellt. Auf diese Weise wurde durch eine objektive Methode eine chronologische Vergleichskala ermittelt, die sich auf ein umfangreiches statistisches Material vom gesamten eurasischen Erdteil stützt. — Die paläomagnetische Erforschung der biostratigraphisch datierten Gesteine und Lagerstätten von Mineralrohstoffen auf dem Gebiet des Böhmischen Massivs ist so weit fortgeschritten, daß es möglich ist, die tektonische Entwicklung der Lagerstätten mit Bezug auf die tektonische Entwicklung verschiedener Becken, Gräben, ganzer Gesteinsblöcke usw. zu untersuchen. Mit Rücksicht auf die tektonischen Deformationen, auf den natürlichen Streubereich der paläomagnetischen Richtungen und auf den Genauigkeitsgrad der Ableitung der mittleren Werte der paläomagnetischen Divergenz besteht die Möglichkeit, bei der paläomagnetischen Datierung die Genauigkeit im Bereich von einigen zehn Millionen Jahren zu erreichen. In speziellen Fällen ist es zweckmäßig, auch andere Parameter zum Erreichen eines höheren Genauigkeitsgrades zu benützen.

     

Quantitative study of the distributary braidplain of the Preboreal ice-contact Gardermoen delta complex, southeastern Norway

This raised delta structure is an ice-contact deltaic complex with a volume of c. 4.4.10⁹ m³, deposited c . 9500 yr BP in a shallow wide 'fjord' during the retreat of the Scandinavian ice cap. The delta plain lies at an altitude of 200–223 m. It aggraded c . 20 m above the contemporaneous sea level during a regional marine regression. The braidplain palaeochannel characteristics indicate a peak meltwater discharge of 7–9 10³ m³/s. Calculations based on a glacial ablation model indicate a mid-summer discharge of c . 5.5 10³ m³/s. However, the fluvial topset of the delta has an erosive base whose altitude decreases upstream and indicates stream incision by more the 6 m below the contemporaneous sea level. The deep scour is ascribed to episodic floods over the relatively short delta plain, which exceeded direct ablation-associated discharges. The depositional time-span of the delta is assessed to have been 70 years, calculated from coastal gradient and shoreline displacement curves. The average sedimentation rate of the delta is thereby inferred to have been extremely high, c . 6. 10⁷ m³/yr. The sedimentation is thought to reflect 'extreme' ice-margin conditions, where the sediment and water discharge was maximized by full-scale ablation, with simultaneous subglacial, englacial and supraglacial sediment and water supply. These conditions might further coincide with an abundant rainfall in the catchment area or the drainage of dammed waters, initiating episodic floods which eroded deep beneath sea level. As a whole, the study illustrates the hydrological conditions of proglacial sedimentation at the front of the rapidly retreating last Scandinavian ice cap.