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Extent and chronology of glaciations in Iceland; a brief overview of the glacial history
Quaternary Glaciations - Extent and Chronology
Editors J. Ehlers and P.L. Gibbard
92004 Elsevier B.V. All fights reserved
Extent and chronology of glaciations in Iceland; a brief overview of the glacial history /kslaug Geirsd6ttir
Deptartment of Geosciences, University of Iceland E-mail: age@rhi,hi. is Introduction
The most direct evidence for the existence of glaciers in the past is terrestrial ice contact sediments. However, because only a very small proportion of ice contact sediments is preserved in the Northern Hemisphere, deductions on the extent of glaciers and periodicity of glaciations are in most cases derive~from indirect proxies such as microfossils and the distribution of ice-rafted detritus (IRD) in the deep sea sediments. Iceland is exceptional in this regard, where Tertiary an d Quaternary terrestrial basal tillites are very common anc~ well preserved. The reason for this unusually well preserved glacial record in Iceland is related to its location within a rifling zone where recurrent volcanic activity with Ithe formation of laterally extensive lava flows provides a ishield to the underlying sediments. The geographical position of Iceland in the midst of shifting positions of cold and warm ocean currents and at atmospheric ~ronts further enhances the island's importance in glacial geqglogical and palaeoclimatic research (Fig. 1). Comparison ,Nith the deep-sea record and studies of glacier cyclicity imply that the late Tertiary and early to middle Pleistocene terrestrial record in Iceland does reflect the large-scale pldaeoclimatic processes in the North Atlantic region (Fig. 2). i
Tertiary to Middle Pleistocene glaciations of Iceland
Geological mapping and sedimentary studies have identified over 20 glacial sediment sequences interbedded with lava flows in the 15 million year old stratigraphy of Iceland (Fig. 2). The oldest indication of the presence of ice is found in the c. 5 My old deeply-eroded rock sequence of the southeast, at the margin of the current Vamaj6kull glacier. Although no sedimentological analysis has been performed, deposits of supposed glacial origin have been reported from this region (Fridleifsson, 1995). These deposits are associated with hyaloclastites and pillow lavas, the characteristic rock types of subglacial eruptions. Helgason & Duncan (2001) identify a 4.6 to 4.7 My old glacial deposit in the Skaftafell area at the southern margin of VatnajOkull. The Vamaj6kull ice cap covers the highest and most humid region of Iceland, with an annual precipitation of 3600 mm. Assuming similar circumstances in the past, local ice caps at the site of the present VatnajOkull may have already existed in the Miocene.
The distribution of glacial deposits and the correlation between rock sequences in Iceland show a fairly distinct trend during the Pliocene-Pleistocene transition indicating a gradually growing ice sheet from the southeast towards the west and north (Geirsd6ttir & Eiriksson, 1994, Geirsd6ttir & Eiriksson, 1996). The oldest glacial deposit identified based on detailed sedimentological analyses is found imbedded within c. 3.8 to 4.0 My old lava flows in Flj6tsdalur, eastern Iceland (Fig. 1). Another glacial bed found higher up in the sequence at the same site has an estimated age of 3.4 My. Since it has not been possible to trace these two glacial beds over a larger area, they are thought to represent only local glacier activity. The first indication of a major ice sheet in Iceland dates back to about 2.9 My, based on a correlation between glacial deposits found in Flj6tsdalur and JOkuldalur, the two main valleys of eastern Iceland. Intensification in glacier growth in Iceland occurred c. 2.7 My ago. Two stratigraphicallyseparated glacial deposits have been identified both in eastern (J6kuldalur and Flj6tsdalur) and western Iceland (Borgarfj6r6ur and Hvalfj6r6ur) interbedded within lava flows dated between 2.7 and 2.5 myr (Geirsd6ttir & Eiriksson, 1994). Recent palaeomagnetic study and K-Ar radiometric dating on the stratigraphy of Skaftafell, southeast Iceland, also suggests an increased glaciation at c. 2.6 Ma (Helgason & Duncan, 2001). More extensive glaciation is indicated by glacial deposits in all parts of the country dating from 2.4 to 2.5 My ago (Fig. 1). Further periods of glacier expansion and full-scale glacialinterglacial cyclicity appear at 2.2-2.1 My and again at 1.6 My (Fig. 2). From the period between 2.9 and 1.6 My at least 6 glaciations and possibly 8 glaciations are indicated, but the geological record implies a decrease in glacial activity in the period between 1.6 to 1.2 My. (Fig. 2). Geological studies of the TjOmes section in northern Iceland (Fig. 1 and 2) suggest an increase in the periodicity of glaciations after 1 My when the 100,000 years icevolume cycle develops (Eiriksson, 1985). This is in accordance with the findings of Helgason & Duncan (2001) from southeast Iceland. However, the TjOmes section is the only site known in Iceland with a complete glacial stratigraphy from the earliest regional glaciations to the last glaciation. The number of glacial deposits identified in different parts of Iceland reflects the growth of the early Icelandic ice sheet from the southeast to the west and north. In southeast Iceland (Skaftafell), Helgason and Duncan (2001) report a 4.6 - 4.7 My old deposit of possible glacial origin. They describe a total of 11 glacial sequences in a
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Aslaug Geirsd6ttir
stratigraphy, which extends from 4.7 -0.4 My. In eastern Iceland (Flj6tsdalur), a total of 11 glacial units are recorded in a stratigraphic sequence that spans 6.5 - 1.0 My. The oldest glacial deposit in this section is c. 4.0 My. old. In
northern Iceland, 14 glacial units are identified in a section which extends back to 9.0 My; here the oldest glacial deposit is correlated with rocks aged c. 2.5 My. A total of 7 glacial horizons preserved in a sequence from western
Iceland
Iceland (Hvalfj6r6ur and Borgart]6r6ur) date from 7.0 - 1.8 Ma, with the oldest glacial deposit correlated with 2.6 My old rocks. Whilst in the Hreppar area, southem Iceland, the oldest glacial deposit identified is between 2.5 - 2.2 My. where the oldest rocks are only c. 3 million years old (Fig. 3) (Geirsd6ttir & Eiriksson, 1994). This figure of glacier distribution through time may reflect the correlation between lava flow formation and preservation potential of glacial deposits where foci of volcanic activity was along the eastern volcanic zone between 6 and 1 million years, but around the Hreppar area not until after 3 My. No Pliocene- Pleistocene glacial deposits have been found within the stratigraphy of the Northwest Peninsula of Iceland, which contains only rocks older than 6 My (Fig. 3).
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the Tj6mes section, which contains 9 stratigraphicallyseparated glacial deposits during this interval (Eiriksson, 1985). The lack of continuous records through the period may be due to a lack of systematic research and radioI~tric dates since old glacial and interglacial deposits of unknown age have been reported from various sites all around Iceland. Tillites from the Middle Pleistocene have been reported from the north (Jancin et aL, 1985), and at Snaefellsnes Peninsula in West Iceland' (Einarsson, 1994; Leifsd6ttir, 1999). The glacial deposits found on the Sn~efellsnes Peninsula were probably formed between 2 and 0.7 m.y based on palaeomagnetic measurements on interbedded lava flows.
From the Last Glacial Maximum to the last deglaciation Middle to Late Pleistocene glaciations in Iceland
The glacial stratigraphy of Iceland from c. 1.5 million years to the last deglaciation is very fragmentary, apart from in
During the Last Glacial Maximum (LGM),:Iceland and the surrounding shelf ~vas presumably ice-6b~,ered (Fig.4), although small ice-flee areas may have existed along the
Aslaug Geirsd6ttir
178
6 - 5 m.y. a g o
2 . 9 - 2.8 m.y. ago
4 - 3 re.y. a g o
Ta
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2 . 7 - 2.5 m.y. a g o
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2 . 2 - 2.0 m.y. ago
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o'"
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.............. "" ~ t F: FIj6tsdal~ J: ~ u i ~
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9 i 0 0 Km '
Fig.3. A hypothetical reconstruction of the Tertiary glaciations in Iceland at certain time slices. OJ6pill moraine; Helg~16Wr PoNible extent of LGM based on 14C dates from & Thorn, 1998 B9-97 cores (Andrews et el., 2000)
BrMdafJ6rdur (LGM??) moraine (61afQd6ttlr, 1975; ~ b M ~ al., 1999)
O Tvleker moraine Boulton et aL, 1988 II0km
Black dots repreeent Inferred minimal glacial extent based on Egl0ff & Johnson (1979)
LGM Inferred from Boulton et al. (1988)
Fig. 4. The Icelandic ice margin during the Last Glacial Maximum (LGM), the Younger Dryas and the Preboreal.
Iceland
coastal mountains, particularly in the northwest, north and east (e.g. Einarsson & Albertsson, 1988; Ing61fsson, 1991; Norddahl, 1991; Ing61fsson & Norddahl, 1994; Andrews et aL, 2000; Rundgren & Ing61fsson, 1999). Although the precise timing of the LGM and the full extent of the icesheet is poorly constrained by observations, ice streams and outlet glaciers are thought to have emanated from ice divides in south-central Iceland, terminating at the shelf edge (Fig. 4). The Vestfirdir peninsula in NW Iceland may have supported dynamically independent ice caps and/or valley glaciers with outlet glaciers originating within an ice-divide near the centre of the peninsula. Recent studies, based on sites from all around Iceland, indicate that glacier retreat began by 15 cal ka, (13.000 BP) and that it was rapid, but step-like (e.g. Ing61fsson, 1991; Norddahl, 1991; Ing61fsson & Norddahl, 1994; Ing61fsson et al., 1997; Andrews et al., 2000; Syvitski et al., 1999). The strongest evidence for step-like retreat (or readvance) of the main glacier during the last deglaciation is found in the Bfl6i morainic complex in south central Iceland (Geirsd6ttir et al., 1997, 2000), and in northern Iceland (Norddahl, 1991). The B66i moraines are 70 km long, extending across the southern lowlands from NW to SE about 25 to 45 km inland from the present coastline. The morainic complex is built up of multiple, discontinuous ridges that parallel the boundary between the interior highlands and the southern lowlands. Most of this morainic complex shows delta characteristics with distinct foreset bedding and sandur accumulation, indicative of a transition between terrestrial and coastal environments (Geirsd6ttir et al., 1997, 2000). Shells found both in front and behind the B66i moraines range in age from 11.5 to 10.1 cal ka (10,075-9055 BP), indicating a Preboreal age for the succession. Noting the paucity of ~4C dates from Iceland that predate the Preboreal, Hjartarson & Ing61fsson (1988) suggested that the ice extended beyond the present coastline during the Younger Dryas (Fig. 4). The marine limit in the area is 110 m a.s.1. Although not dated directly, Ing61fsson and Norddahl (1994) assume that the transgression maximum occurred during the transition from the Younger Dryas to the Preboreal (between 12.2 and 11 cal ka). Geirsd6ttir et al. (1997, 2000) provide an alternative view that implies a much less extensive Younger Dryas ice sheet in south-central Iceland. On the basis of detailed lithofacies analyses of the sedimentary successions in and around the Bfi6i morainic complex they suggest that most of the lithofacies reflect deltaic sedimentation resulting from an interplay of coastal, terrestrial and glacier processes, qhe sediments in front of the morainic system rest on a stratified diamictite deposited in front of a tidewater gl~tcier prior to 11.1 cal ka (9800 BP) (Hjartarson & Ing61fsson, 1988; Geirsd6ttir et al., 1997). The glacier that deposited the diamictite may have been of Younger Dryas age (Geirsd6ttir et al., 1997). This interpretation is supported by radiocarbon dates on foraminifera and the occurrence of the Vedde Ash at the base of a discontinuous 25-m-long sediment core from Lake Hestvatn, (15-20 km
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south of the Bta6i moraines), covering the last 12 cal ka (Geirsd6ttir et al., 2000; Har6ard6ttir et al., 2001). Collectively, the Bfi6i moraines and the Lake Hestvatn sediments provide evidence for an ice-free interval in southern Iceland from at least 12.2 (possibly as early as 14) to 11.1 cal ka. A brief glacier readvance towards the Bfi6i moraines took place at around 11.1 cal ka. during the Pre-boreal. Subsequently, the ice margin retreated rapidly to-wards the highlands, followed by rapid isostatic rebound; Hestvatn (50 m a.s.1) was isolated from the sea shortly after 10 cal ka, a drop in relative sea level of almost 60 m in c. 1 ka. One of the most important findings in northern Iceland regarding the deglaciation history, are four separate ice-lake strandlines attributed to four separate periods of glacial isostatic recovery (Norddahl, 1991). The occurrence of the Sk6gar tephra, correlated with the Vedde Ash in the icelake sediments constraints the age of these ice dammed lakes and shows that during the Younger Dryas Chronozone outlet glaciers in northern Iceland extended at least half-way out EyjafjOr6ur (Fig. 4) (Norddahl & Haflidason, 1990). The reconstructed deglaciation history of North Iceland shows four plausible glacier readvances in the period between 14700-10900 cal. ka (ca. 12,655 and 9,650 BP), where the youngest ice-lakes were formed during the Younger Dryas and the marine limit was reached in late Younger Dryas or early Preboreal time. Early Holoeene thermal maximum and the onset of neoglaciation
By 10 cal ka, the main ice sheet was in rapid retreat across Iceland; pollen studies suggest a climate similar to the present was established by this time (BjOrck et al., 1992; Hallsd6ttir, 1995). Possibly as early as 9 cal ka (although perhaps not until 8 cal ka) the central highlands of Iceland were mostly ice free (Kaldal & Vikingsson, 1991). Gudmundsson (1997) summarizes evidence pertaining to the evolution of Iceland's climate during the Holocene. The expansion of birch shortly after 10 cal ka suggests an early Holocene climate characterized by warm, dry summers; a condition that persisted at least until 7 ka. There is little consensus on the nature of both ice caps and climate between 9 and 5 cal ka, with scattered evidence for glacier activity in the central highlands, but none that is unequivocal. No continuous pollen records span this interval, and the available discontinuous sequences are not conclusive. The pollen record, as summarized by Hallsd6ttir (1995), suggests a maximum expansion of birch forests across southern Iceland between about 8 and 5 cal ka, suggesting a possible thermal maximum through this interval. The onset of Neoglaciation and decline of tree vegetation remains debated: Clear evidence for glacier expansion after 3 cal ka is available, but some authors have suggested glacier growth as early as 5 cal ka BP (cf Gu6mundsson, 1997). In almost all cases, the Little Ice Age moraines
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Aslaug Geirsd6ttir
(1600-1900 AD) represent the most extensive ice margins since early Holocene deglaciation.
Glacial Limits - Quality of Data, Alternative Interpretation
Map 4 shows a clear indication of intense glaciation in Iceland around 2.2 - 2.0 My (Reunion geomagnetic Event). Glacial deposits have been mapped in eastem (Flj6tsdalur, J6kuldalur), northern (Tj6mes), western (Hvalfj6r6ur and Borgarfj6r6ur) and southem (Hreppar area) Iceland. These are the oldest glacial deposits in the Hrepppar area in South Iceland.
Tertiary to Middle Pleistocene glaciations The reconstructed Tertiary glacial history of Iceland is based on the use of multiple sedimentological criteria from two sites in eastern Iceland (Flj6tsdalur and J6kuldalur), two sites in northern Iceland (Tj6rnes and Flatey), two sites in western Iceland (Borgarfj6r6ur and Hvalfj6r6ur) and one site in southern Iceland (Hreppar) (Geirsd6ttir, 1988; Geirsd6ttir, 199 l, Geirsd6ttir et al., 1993; Geirsd6ttir et al., 1994; Geirsd6ttir & Eiriksson, 1994, 1996). This multiproxy approach includes a comparison with modern depositional environments, lithofacies analysis, clast fabric measurements, textural studies and rock magnetic measurements, including both remanent magnetization and analysis of anisotropy of magnetic susceptibility. Major emphasis has been placed on detailed mapping and lithofacies analyses where both vertical and lateral changes in facies arrangements Could be detected over several kilometres for all sections. The results imply that Iceland has experienced over 20 glaciations since c. 4 My ago. This is in reasonable agreement with the number of glaciations retrieved from the 7180 record from the deep sea and suggests that the late Tertiary and early to middle Pleistocene terrestrial record in Iceland may be regarded as an example of global palaeoclimatic processes in the North Atlantic region (Fig. 2). Map 1 shows the first indication of glacier formation in Iceland. It includes the oldest glacial deposits (ca. 4.0 - 3.8, 3.4 My) mapped in eastem Iceland (Flj6tsdalur). Older glacial deposits suggested in southeast Iceland at the margin of the current Vatnaj6kull glacier are not shown because of a lack of information on the location and description of sites. Also shown on map 1 is the oldest glacial deposit (2.9 My) that can be correlated between Flj6tsdalur and J6kuldalur, eastern Iceland. The deposit rests at both sites on reversed magnetized rocks correlated to the Kaena geomagnetic Event. Map 2 shows the first indication of regional glaciation in Iceland. Two glacial deposits are identified within the time period from approximately 2.7 My to 2.5 My (i.e. within the upper part of the Gauss geomagnetic Epoch). Glacial deposits from this time period have been mapped in eastern Iceland (J6kuldalur and Flj6tsdalur) and in western Iceland (HvalfjOr6ur and BorgarfjOr6ur) Map 3 shows the first indication of full glacialinterglacial cyclicity in Iceland at approximately 2.5 My. Glacial deposit lying on normal magnetized lava (presumably Gauss magnetic Event) has been found in eastern (Flj6tsdalur, JOkuldalur), western (Hvalfj6r6ur, Borgarfj6r6ur) and northern (TjOrnes) Iceland.
Middle to Late Pleistocene and early Holocene Only fragmentary information is available about the Middle to Late Pleistocene glaciations of Iceland. Current work on sedimentary cores from the shelf around Iceland is intended to define the Last Glacial Maximum. The deglaciation history of the last glacial in Iceland is better constrained by radiocarbon age determinations. Figure 4 shows the inferred glacier margin during the Last Glacial Maximum, the Younger Dryas and the Preboreal in Iceland.
Dating of glacial limits - reliability of dates
Tertiary glacial deposits Correlations between sections/glacial deposits of Tertiary age are still tentative and mainly based on previous palaeomagnetic work that was done on lava flows in order to reconstruct a palaeomagnetic time scale for the Icelandic lava pile (Wensink, 1964; McDougall & Wensink, 1966; McDougal et al., 1976; 1977; Kristjfinsson et al., 1980; Eiriksson et al., 1990). Because lava flows chosen for K/Ar dating were not necessarily associated with glacial deposits, some constraints are placed on the reliability of the correlations. Stratigraphical and palaeomagnetical work has further revealed some glacial deposits in southeast Iceland (Helgason and Duncan, 2001). Their K-Ar dates on identified glacial strata confirm previous findings of Geirsd6ttir and Eiriksson (1994) on the dating of Tertiary glaciations in Iceland. Although there is insufficient control on individual glacial units, there does appear to be a variation in the frequency of glaciations with time during the last 2.5 My. The author's results show that changes in the ice cover of Iceland correlate reasonably with variations in the deep-sea oxygen isotope records from the North Atlantic (Fig. 2). Middle to Late Quaternary glacial deposits C Contentious interpretation of the glacial history of Iceland reflects the lack of continuous chrono- and biostratigraphical records and relatively low age-resolution of the data. However, new studies, particularly on lake sediments, look promising for establishing an improved and more continuous record of the climatic and environmental changes in Iceland for this critical time period (Bj6rck et
Iceland al., 1992; Rundgren, 1995; Hardard6ttir, 1999; Hardard6ttir et al., 2001.). The two prevailing conceptual models of the last deglaciation history in southern Iceland agree in regard to the Preboreal age glacier advance, but differ in their interpretation of 1) the extent of the Younger Dryas ice, and 2) the sedimentological history of the B66i morainic complex (Figure 4).
Open questions It should be noted that some of the Tertiary glacial deposits studied might have a larger lateral extent indicating more extensive regional glaciation than shown on the maps. Old glacial deposits of possible Tertiary age have been reported from various sites all around Iceland, but no dates are available as yet. This makes it impossible to determine precisely the chronological position of the deposits. However, era-rent work is aimed at increasing the number and accuracy of radiometric dates (Ar/Ar and K/Ar) from the terrestrial sections, with emphasis on dating of the lava flows interbedded with the identified glacial deposits. This will give more accurate dates for the actual climatic events. The absence of Tertiary glacial deposits on the NW peninsula raises questions about the preservation potential of glacial deposits. Does this reflect limited or no glaciation on the peninsula or did subsequent glaciations completely erode the evidence for Tertiary glaciation in the area? The Tj6rnes Peninsula in northem Iceland is the only area in Iceland where we have a complete glacial stratigraphy mapped from about 3.0 My to the Holocene is available. It is however very difficult to draw a map of the extent of the younger glaciations (i.e. younger than c. 1.5 My) due to lack of studied sites and radiometric dating of glacial deposits. In the light of the current knowledge of the deglaciation history of south-central Iceland, there seems to be little doubt about the Preboreal age of some of the Bt~6i moraines. The question that remains is whether some of the sediments in the Bt~di morainic complex represent earlier glacial advances. Although much has been written about the Younger D12r glaciation of Iceland, knowledge of the extent of the Younger Dryas ice sheet in the south is still ambiguous. The main problem with the current hypotheses is that the), are based on indirect evidence (i.e. missing sedimentary records). Those who support extensive Younger Dryas glaciation point to the sea for further evidence. ]-'hose who support the hypothesis of a more limited extent of the Younger Dryas ice emphasise the importance .of the undated basal till in the B66i succession. However, the key to the puzzle is more likely to be the sediment sequence still to be found beneath Lake Hestvatn. Although these problems await future research for their solution, the; importance of detailed sedimentological work and facies analyses in glacial geological studies should be emphasized A comprehensive understanding of the deglaciation history requires a detailed lithofacies analysis in context with good stratigraphic control of the successions.
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Consequently future research should emphasise detailed sedimentological, stratigraphical and chronological studies of both lacustrine and marine shelf sediments
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Geirsd6ttir,/k., Th6rdarson, Th. & Kristjfinsson, L. (1994). /~hrif eldvirkni og loftslagsbreytinga ~i upphledslu jardlaga i Gntipverjahreppi. Agrip a Jardfraedaradstefnu islenska Jardfraedaf61agsins, 21. april 1994. (Abstract vol. Geological Society of Iceland). Geirsd6ttir, A. & Eiriksson, J. (1994). Growth of an intermittent ice sheet in Iceland during the late Pliocene and early Pleistocene. Quaternary Research, 42, 115-130. Geirsd6ttir,/i.. & Eiriksson, J. 1996. A review of studies of the earliest glaciations in Iceland. Terra Nova, 8, 400414. Geirsd6ttir, A., Hardard6ttir, J. & Eiriksson, J. (1997). Depositional history of theYounger Dryas (?) - Preboreal sediments, South Central Iceland. Arctic and Alpine Research, 29, 13-23. Geirsd6ttir, A., Hardard6ttir, J. & SveinbjOmsd6ttir, A.E. (2000). Glacial extent and catastrophicmeltwater events during the deglaciation of Southern Iceland. Quaternary Science Reviews, 19, 1749-1761. Gudmundsson, H. (1997). A review of the Holocene environmental history of Iceland. Quaternary Reviews, 16, 81-92. Hallsd6ttir, M. (1995). On the pre-settlement history of Icelandic vegetation. B~visindi (Icelandic Agicultural. Sciences), 9, 17-29. Hardard6ttir, J. (1999). Late Weichselian and Holocene environmental history of south and western Iceland as interpreted from studies of lake and terrestrial sediments. Unpublished Ph.D. Thesis, University of Colorado, Boulder. Helgad6ttir, G. & Thors, K. 1998. Setlog i Isafjardardjupi, Jokulfjordum og Djupal.Vorradstefna 1998. Agrip erinda og veggspjalda. Jardfraedafelag Islands, p. 32. Hardard6ttir, J., Geirsd6ttir, A. & Thordarson, Th. (2000). Tephra layers in a sediment core from Lake Hestvatn, SIceland: implications for evaluating sedimentation processes andenvironmental impact of tephra fall deposits in the surrounding watershed. In: White, J. & Riggs, N. (eds), Lacustrine Volcaniclastic Sedimentation International Association of Sedimentologists (IAS) Special Publ~ation. Blackwell, Oxford. Hardard6ttir, J., Geirsd6ttir, A. & Sveinbj6msd6ttir, A.E. (2001). Seismostratigraphy and sediment studies of Lake Hestvam, south Iceland; Implication for the deglacial history of the region. Journal of Quaternary Sciences, 16, 167-179. Helgason, J. & Duncan, R.A. (2001). Glacial-interglacial history of the Skaftafell region, southeast Iceland, 0-5 Ma. Geology, 29, 179-182. Hjartarson, A. & Ing61fsson, O. (1988). Preboreal glaciation of Southern Iceland. Jokull, 38, 1-16. Ing61fsson, O. (1991). A review of the late Weichselian and early Holocene glacial andevironmental history of Ice-land. In: Maizels, J. & Caseldine, C. (eds), Environ-mental change in Iceland." Past and present. Dordrecht, Holland: Kluwer Academic Publishers, 13-29.
Ing61fsson, 6. & Norddahl, H. (1994). A review of the environmental history of Iceland, 13000-9000 yr BP. Journal of Quaternary Science, 9, 147-150. Ing61fsson, O., Bj6rck, S., Haflidason, H. & Rundgren, M. (1997). Glacial and climatic events in Iceland reflecting regional North Atlantic climatic shifts during the Pleistocene-Holocene transition. Quaternary Science Reviews, 16, 1135-1144. Jansen, E., Fronval, T., Rack, F. & Channell, E.T. (2000). Pliocene-Pleistocene ice rafting history and cyclicity in the Nordic Seas during the last 3.5 Myr. Paleoceanography, 15, 709-721. Kaldal, I. & S. Vikingsson, S. (1991). Early Holocene deglaciation in Central Iceland. JOkul, 40, 51-66. Kristjfinsson, L., Fri61eifsson, I.B. & Watkins, N.D. (1980). Stratigraphy and paleomagnetism of the Esja, Eyrart]all and Akrafjall Mountains, SW-Iceland. Journal of Geophysics, 47, 31-42. Leifsd6ttir, O.E. 1999. JsaldarlOg ~ nor6anver6u Sn~efellsnesi. Setl6g og skeld~af~inur. Unpublished MS thesis, University of Iceland. 101 pp McDougall, I. & Wensink, H. (1966). Paleomagnetism and geochronology of the Pliocene-Pleistocene lavas in Iceland. Earth and Planetary Science Letters, I, 232-236. McDougall, I., Watkins, N.D. & Kristj~nsson, L. (1976). Geochronology and paleomagnetism of Miocene-Pliocene lava sequences at Bessastada~i, eastern Iceland. American Journal of Science, 276, 1078-1095. McDougall, I., Watkins, N.D., Walker, G.P.L. & Kristj~insson, L. (1976). Potassium-Argon and Paleomagnetic Analysis of Icelandic Lava flows: Limits on the Age of Anomaly 5. Journal of Geophysical Research, 81, 15051512. Norddahl, H. (1991). Late Weichselian and early Holocene deglaciation history of Iceland. JOkull, 40, 27-50. Norddahl, H. & Haflidason, H. (1992). The Sk6gar Tephra, a Younger Dryas marker in North Iceland. Boreas, 21, 23-41. 01afsd6ttir, Th. (1975). Jokulgardar a sjavarbotni ut afBreidafirdi. Ndtt4rufrcedingurinn, 45, 31-36. Jancin, M., Young, K.D. & Voight, B. (1985). Stratigraphy and K&Ar ages across the West flank of the Northeast Iceland axial rift zone, in relation to the 7 Ma volcanoTectonic reorganization of Iceland. Journal of Geophysical Research, 90, 9961-9985. Rundgren, M. (1995). Biostratigraphic evidence of the Allerod-Younger Dryas-Preboreal oscillation in northern Iceland. Quaternary Research, 44, 405-416. Rundgren, M. & Ing61fsson, O. (1999). Plant survival in Iceland during periods of glaciation? Journal of Biogeography, 26, 387-396. Syvitski, J.P.M., Jennings, A.E. & Andrews, J.T. (1999). High-resolution seismic evidence for multiple glaciation across the southwest Iceland shelf. Arctic, Antarctic and Alpine Research, 31, 50-57. Wensink, H. (1964). Paleomagnetic stratigraphy of younger basalts and intercalated Plio-Pleistocene tillites in Iceland. Geologische Rundschau, 54, 364-384.
Editors J. Ehlers and P.L. Gibbard
92004 Elsevier B.V. All fights reserved
Extent and chronology of glaciations in Iceland; a brief overview of the glacial history /kslaug Geirsd6ttir
Deptartment of Geosciences, University of Iceland E-mail: age@rhi,hi. is Introduction
The most direct evidence for the existence of glaciers in the past is terrestrial ice contact sediments. However, because only a very small proportion of ice contact sediments is preserved in the Northern Hemisphere, deductions on the extent of glaciers and periodicity of glaciations are in most cases derive~from indirect proxies such as microfossils and the distribution of ice-rafted detritus (IRD) in the deep sea sediments. Iceland is exceptional in this regard, where Tertiary an d Quaternary terrestrial basal tillites are very common anc~ well preserved. The reason for this unusually well preserved glacial record in Iceland is related to its location within a rifling zone where recurrent volcanic activity with Ithe formation of laterally extensive lava flows provides a ishield to the underlying sediments. The geographical position of Iceland in the midst of shifting positions of cold and warm ocean currents and at atmospheric ~ronts further enhances the island's importance in glacial geqglogical and palaeoclimatic research (Fig. 1). Comparison ,Nith the deep-sea record and studies of glacier cyclicity imply that the late Tertiary and early to middle Pleistocene terrestrial record in Iceland does reflect the large-scale pldaeoclimatic processes in the North Atlantic region (Fig. 2). i
Tertiary to Middle Pleistocene glaciations of Iceland
Geological mapping and sedimentary studies have identified over 20 glacial sediment sequences interbedded with lava flows in the 15 million year old stratigraphy of Iceland (Fig. 2). The oldest indication of the presence of ice is found in the c. 5 My old deeply-eroded rock sequence of the southeast, at the margin of the current Vamaj6kull glacier. Although no sedimentological analysis has been performed, deposits of supposed glacial origin have been reported from this region (Fridleifsson, 1995). These deposits are associated with hyaloclastites and pillow lavas, the characteristic rock types of subglacial eruptions. Helgason & Duncan (2001) identify a 4.6 to 4.7 My old glacial deposit in the Skaftafell area at the southern margin of VatnajOkull. The Vamaj6kull ice cap covers the highest and most humid region of Iceland, with an annual precipitation of 3600 mm. Assuming similar circumstances in the past, local ice caps at the site of the present VatnajOkull may have already existed in the Miocene.
The distribution of glacial deposits and the correlation between rock sequences in Iceland show a fairly distinct trend during the Pliocene-Pleistocene transition indicating a gradually growing ice sheet from the southeast towards the west and north (Geirsd6ttir & Eiriksson, 1994, Geirsd6ttir & Eiriksson, 1996). The oldest glacial deposit identified based on detailed sedimentological analyses is found imbedded within c. 3.8 to 4.0 My old lava flows in Flj6tsdalur, eastern Iceland (Fig. 1). Another glacial bed found higher up in the sequence at the same site has an estimated age of 3.4 My. Since it has not been possible to trace these two glacial beds over a larger area, they are thought to represent only local glacier activity. The first indication of a major ice sheet in Iceland dates back to about 2.9 My, based on a correlation between glacial deposits found in Flj6tsdalur and JOkuldalur, the two main valleys of eastern Iceland. Intensification in glacier growth in Iceland occurred c. 2.7 My ago. Two stratigraphicallyseparated glacial deposits have been identified both in eastern (J6kuldalur and Flj6tsdalur) and western Iceland (Borgarfj6r6ur and Hvalfj6r6ur) interbedded within lava flows dated between 2.7 and 2.5 myr (Geirsd6ttir & Eiriksson, 1994). Recent palaeomagnetic study and K-Ar radiometric dating on the stratigraphy of Skaftafell, southeast Iceland, also suggests an increased glaciation at c. 2.6 Ma (Helgason & Duncan, 2001). More extensive glaciation is indicated by glacial deposits in all parts of the country dating from 2.4 to 2.5 My ago (Fig. 1). Further periods of glacier expansion and full-scale glacialinterglacial cyclicity appear at 2.2-2.1 My and again at 1.6 My (Fig. 2). From the period between 2.9 and 1.6 My at least 6 glaciations and possibly 8 glaciations are indicated, but the geological record implies a decrease in glacial activity in the period between 1.6 to 1.2 My. (Fig. 2). Geological studies of the TjOmes section in northern Iceland (Fig. 1 and 2) suggest an increase in the periodicity of glaciations after 1 My when the 100,000 years icevolume cycle develops (Eiriksson, 1985). This is in accordance with the findings of Helgason & Duncan (2001) from southeast Iceland. However, the TjOmes section is the only site known in Iceland with a complete glacial stratigraphy from the earliest regional glaciations to the last glaciation. The number of glacial deposits identified in different parts of Iceland reflects the growth of the early Icelandic ice sheet from the southeast to the west and north. In southeast Iceland (Skaftafell), Helgason and Duncan (2001) report a 4.6 - 4.7 My old deposit of possible glacial origin. They describe a total of 11 glacial sequences in a
176
Aslaug Geirsd6ttir
stratigraphy, which extends from 4.7 -0.4 My. In eastern Iceland (Flj6tsdalur), a total of 11 glacial units are recorded in a stratigraphic sequence that spans 6.5 - 1.0 My. The oldest glacial deposit in this section is c. 4.0 My. old. In
northern Iceland, 14 glacial units are identified in a section which extends back to 9.0 My; here the oldest glacial deposit is correlated with rocks aged c. 2.5 My. A total of 7 glacial horizons preserved in a sequence from western
Iceland
Iceland (Hvalfj6r6ur and Borgart]6r6ur) date from 7.0 - 1.8 Ma, with the oldest glacial deposit correlated with 2.6 My old rocks. Whilst in the Hreppar area, southem Iceland, the oldest glacial deposit identified is between 2.5 - 2.2 My. where the oldest rocks are only c. 3 million years old (Fig. 3) (Geirsd6ttir & Eiriksson, 1994). This figure of glacier distribution through time may reflect the correlation between lava flow formation and preservation potential of glacial deposits where foci of volcanic activity was along the eastern volcanic zone between 6 and 1 million years, but around the Hreppar area not until after 3 My. No Pliocene- Pleistocene glacial deposits have been found within the stratigraphy of the Northwest Peninsula of Iceland, which contains only rocks older than 6 My (Fig. 3).
177
the Tj6mes section, which contains 9 stratigraphicallyseparated glacial deposits during this interval (Eiriksson, 1985). The lack of continuous records through the period may be due to a lack of systematic research and radioI~tric dates since old glacial and interglacial deposits of unknown age have been reported from various sites all around Iceland. Tillites from the Middle Pleistocene have been reported from the north (Jancin et aL, 1985), and at Snaefellsnes Peninsula in West Iceland' (Einarsson, 1994; Leifsd6ttir, 1999). The glacial deposits found on the Sn~efellsnes Peninsula were probably formed between 2 and 0.7 m.y based on palaeomagnetic measurements on interbedded lava flows.
From the Last Glacial Maximum to the last deglaciation Middle to Late Pleistocene glaciations in Iceland
The glacial stratigraphy of Iceland from c. 1.5 million years to the last deglaciation is very fragmentary, apart from in
During the Last Glacial Maximum (LGM),:Iceland and the surrounding shelf ~vas presumably ice-6b~,ered (Fig.4), although small ice-flee areas may have existed along the
Aslaug Geirsd6ttir
178
6 - 5 m.y. a g o
2 . 9 - 2.8 m.y. ago
4 - 3 re.y. a g o
Ta
Fy
lving.
2 . 7 - 2.5 m.y. a g o
Present
2 . 2 - 2.0 m.y. ago
~" ; ."~
++h
Hr
o'"
.-" ",,,.~-~.. ! / I
.............. "" ~ t F: FIj6tsdal~ J: ~ u i ~
B: Bofgarf//~,~tH: H v ~
TR: T ~ Fy: FlaZey
Hr: Hreppar
D~..~__,~2~.. t,'. Plkx:e~ (0. 7- a,1,~m.y.) r--IMk:k:le aml Lale ~ "-'~(0.01 - 0.7 m.y.)
9 i 0 0 Km '
Fig.3. A hypothetical reconstruction of the Tertiary glaciations in Iceland at certain time slices. OJ6pill moraine; Helg~16Wr PoNible extent of LGM based on 14C dates from & Thorn, 1998 B9-97 cores (Andrews et el., 2000)
BrMdafJ6rdur (LGM??) moraine (61afQd6ttlr, 1975; ~ b M ~ al., 1999)
O Tvleker moraine Boulton et aL, 1988 II0km
Black dots repreeent Inferred minimal glacial extent based on Egl0ff & Johnson (1979)
LGM Inferred from Boulton et al. (1988)
Fig. 4. The Icelandic ice margin during the Last Glacial Maximum (LGM), the Younger Dryas and the Preboreal.
Iceland
coastal mountains, particularly in the northwest, north and east (e.g. Einarsson & Albertsson, 1988; Ing61fsson, 1991; Norddahl, 1991; Ing61fsson & Norddahl, 1994; Andrews et aL, 2000; Rundgren & Ing61fsson, 1999). Although the precise timing of the LGM and the full extent of the icesheet is poorly constrained by observations, ice streams and outlet glaciers are thought to have emanated from ice divides in south-central Iceland, terminating at the shelf edge (Fig. 4). The Vestfirdir peninsula in NW Iceland may have supported dynamically independent ice caps and/or valley glaciers with outlet glaciers originating within an ice-divide near the centre of the peninsula. Recent studies, based on sites from all around Iceland, indicate that glacier retreat began by 15 cal ka, (13.000 BP) and that it was rapid, but step-like (e.g. Ing61fsson, 1991; Norddahl, 1991; Ing61fsson & Norddahl, 1994; Ing61fsson et al., 1997; Andrews et al., 2000; Syvitski et al., 1999). The strongest evidence for step-like retreat (or readvance) of the main glacier during the last deglaciation is found in the Bfl6i morainic complex in south central Iceland (Geirsd6ttir et al., 1997, 2000), and in northern Iceland (Norddahl, 1991). The B66i moraines are 70 km long, extending across the southern lowlands from NW to SE about 25 to 45 km inland from the present coastline. The morainic complex is built up of multiple, discontinuous ridges that parallel the boundary between the interior highlands and the southern lowlands. Most of this morainic complex shows delta characteristics with distinct foreset bedding and sandur accumulation, indicative of a transition between terrestrial and coastal environments (Geirsd6ttir et al., 1997, 2000). Shells found both in front and behind the B66i moraines range in age from 11.5 to 10.1 cal ka (10,075-9055 BP), indicating a Preboreal age for the succession. Noting the paucity of ~4C dates from Iceland that predate the Preboreal, Hjartarson & Ing61fsson (1988) suggested that the ice extended beyond the present coastline during the Younger Dryas (Fig. 4). The marine limit in the area is 110 m a.s.1. Although not dated directly, Ing61fsson and Norddahl (1994) assume that the transgression maximum occurred during the transition from the Younger Dryas to the Preboreal (between 12.2 and 11 cal ka). Geirsd6ttir et al. (1997, 2000) provide an alternative view that implies a much less extensive Younger Dryas ice sheet in south-central Iceland. On the basis of detailed lithofacies analyses of the sedimentary successions in and around the Bfi6i morainic complex they suggest that most of the lithofacies reflect deltaic sedimentation resulting from an interplay of coastal, terrestrial and glacier processes, qhe sediments in front of the morainic system rest on a stratified diamictite deposited in front of a tidewater gl~tcier prior to 11.1 cal ka (9800 BP) (Hjartarson & Ing61fsson, 1988; Geirsd6ttir et al., 1997). The glacier that deposited the diamictite may have been of Younger Dryas age (Geirsd6ttir et al., 1997). This interpretation is supported by radiocarbon dates on foraminifera and the occurrence of the Vedde Ash at the base of a discontinuous 25-m-long sediment core from Lake Hestvatn, (15-20 km
179
south of the Bta6i moraines), covering the last 12 cal ka (Geirsd6ttir et al., 2000; Har6ard6ttir et al., 2001). Collectively, the Bfi6i moraines and the Lake Hestvatn sediments provide evidence for an ice-free interval in southern Iceland from at least 12.2 (possibly as early as 14) to 11.1 cal ka. A brief glacier readvance towards the Bfi6i moraines took place at around 11.1 cal ka. during the Pre-boreal. Subsequently, the ice margin retreated rapidly to-wards the highlands, followed by rapid isostatic rebound; Hestvatn (50 m a.s.1) was isolated from the sea shortly after 10 cal ka, a drop in relative sea level of almost 60 m in c. 1 ka. One of the most important findings in northern Iceland regarding the deglaciation history, are four separate ice-lake strandlines attributed to four separate periods of glacial isostatic recovery (Norddahl, 1991). The occurrence of the Sk6gar tephra, correlated with the Vedde Ash in the icelake sediments constraints the age of these ice dammed lakes and shows that during the Younger Dryas Chronozone outlet glaciers in northern Iceland extended at least half-way out EyjafjOr6ur (Fig. 4) (Norddahl & Haflidason, 1990). The reconstructed deglaciation history of North Iceland shows four plausible glacier readvances in the period between 14700-10900 cal. ka (ca. 12,655 and 9,650 BP), where the youngest ice-lakes were formed during the Younger Dryas and the marine limit was reached in late Younger Dryas or early Preboreal time. Early Holoeene thermal maximum and the onset of neoglaciation
By 10 cal ka, the main ice sheet was in rapid retreat across Iceland; pollen studies suggest a climate similar to the present was established by this time (BjOrck et al., 1992; Hallsd6ttir, 1995). Possibly as early as 9 cal ka (although perhaps not until 8 cal ka) the central highlands of Iceland were mostly ice free (Kaldal & Vikingsson, 1991). Gudmundsson (1997) summarizes evidence pertaining to the evolution of Iceland's climate during the Holocene. The expansion of birch shortly after 10 cal ka suggests an early Holocene climate characterized by warm, dry summers; a condition that persisted at least until 7 ka. There is little consensus on the nature of both ice caps and climate between 9 and 5 cal ka, with scattered evidence for glacier activity in the central highlands, but none that is unequivocal. No continuous pollen records span this interval, and the available discontinuous sequences are not conclusive. The pollen record, as summarized by Hallsd6ttir (1995), suggests a maximum expansion of birch forests across southern Iceland between about 8 and 5 cal ka, suggesting a possible thermal maximum through this interval. The onset of Neoglaciation and decline of tree vegetation remains debated: Clear evidence for glacier expansion after 3 cal ka is available, but some authors have suggested glacier growth as early as 5 cal ka BP (cf Gu6mundsson, 1997). In almost all cases, the Little Ice Age moraines
180
Aslaug Geirsd6ttir
(1600-1900 AD) represent the most extensive ice margins since early Holocene deglaciation.
Glacial Limits - Quality of Data, Alternative Interpretation
Map 4 shows a clear indication of intense glaciation in Iceland around 2.2 - 2.0 My (Reunion geomagnetic Event). Glacial deposits have been mapped in eastem (Flj6tsdalur, J6kuldalur), northern (Tj6mes), western (Hvalfj6r6ur and Borgarfj6r6ur) and southem (Hreppar area) Iceland. These are the oldest glacial deposits in the Hrepppar area in South Iceland.
Tertiary to Middle Pleistocene glaciations The reconstructed Tertiary glacial history of Iceland is based on the use of multiple sedimentological criteria from two sites in eastern Iceland (Flj6tsdalur and J6kuldalur), two sites in northern Iceland (Tj6rnes and Flatey), two sites in western Iceland (Borgarfj6r6ur and Hvalfj6r6ur) and one site in southern Iceland (Hreppar) (Geirsd6ttir, 1988; Geirsd6ttir, 199 l, Geirsd6ttir et al., 1993; Geirsd6ttir et al., 1994; Geirsd6ttir & Eiriksson, 1994, 1996). This multiproxy approach includes a comparison with modern depositional environments, lithofacies analysis, clast fabric measurements, textural studies and rock magnetic measurements, including both remanent magnetization and analysis of anisotropy of magnetic susceptibility. Major emphasis has been placed on detailed mapping and lithofacies analyses where both vertical and lateral changes in facies arrangements Could be detected over several kilometres for all sections. The results imply that Iceland has experienced over 20 glaciations since c. 4 My ago. This is in reasonable agreement with the number of glaciations retrieved from the 7180 record from the deep sea and suggests that the late Tertiary and early to middle Pleistocene terrestrial record in Iceland may be regarded as an example of global palaeoclimatic processes in the North Atlantic region (Fig. 2). Map 1 shows the first indication of glacier formation in Iceland. It includes the oldest glacial deposits (ca. 4.0 - 3.8, 3.4 My) mapped in eastem Iceland (Flj6tsdalur). Older glacial deposits suggested in southeast Iceland at the margin of the current Vatnaj6kull glacier are not shown because of a lack of information on the location and description of sites. Also shown on map 1 is the oldest glacial deposit (2.9 My) that can be correlated between Flj6tsdalur and J6kuldalur, eastern Iceland. The deposit rests at both sites on reversed magnetized rocks correlated to the Kaena geomagnetic Event. Map 2 shows the first indication of regional glaciation in Iceland. Two glacial deposits are identified within the time period from approximately 2.7 My to 2.5 My (i.e. within the upper part of the Gauss geomagnetic Epoch). Glacial deposits from this time period have been mapped in eastern Iceland (J6kuldalur and Flj6tsdalur) and in western Iceland (HvalfjOr6ur and BorgarfjOr6ur) Map 3 shows the first indication of full glacialinterglacial cyclicity in Iceland at approximately 2.5 My. Glacial deposit lying on normal magnetized lava (presumably Gauss magnetic Event) has been found in eastern (Flj6tsdalur, JOkuldalur), western (Hvalfj6r6ur, Borgarfj6r6ur) and northern (TjOrnes) Iceland.
Middle to Late Pleistocene and early Holocene Only fragmentary information is available about the Middle to Late Pleistocene glaciations of Iceland. Current work on sedimentary cores from the shelf around Iceland is intended to define the Last Glacial Maximum. The deglaciation history of the last glacial in Iceland is better constrained by radiocarbon age determinations. Figure 4 shows the inferred glacier margin during the Last Glacial Maximum, the Younger Dryas and the Preboreal in Iceland.
Dating of glacial limits - reliability of dates
Tertiary glacial deposits Correlations between sections/glacial deposits of Tertiary age are still tentative and mainly based on previous palaeomagnetic work that was done on lava flows in order to reconstruct a palaeomagnetic time scale for the Icelandic lava pile (Wensink, 1964; McDougall & Wensink, 1966; McDougal et al., 1976; 1977; Kristjfinsson et al., 1980; Eiriksson et al., 1990). Because lava flows chosen for K/Ar dating were not necessarily associated with glacial deposits, some constraints are placed on the reliability of the correlations. Stratigraphical and palaeomagnetical work has further revealed some glacial deposits in southeast Iceland (Helgason and Duncan, 2001). Their K-Ar dates on identified glacial strata confirm previous findings of Geirsd6ttir and Eiriksson (1994) on the dating of Tertiary glaciations in Iceland. Although there is insufficient control on individual glacial units, there does appear to be a variation in the frequency of glaciations with time during the last 2.5 My. The author's results show that changes in the ice cover of Iceland correlate reasonably with variations in the deep-sea oxygen isotope records from the North Atlantic (Fig. 2). Middle to Late Quaternary glacial deposits C Contentious interpretation of the glacial history of Iceland reflects the lack of continuous chrono- and biostratigraphical records and relatively low age-resolution of the data. However, new studies, particularly on lake sediments, look promising for establishing an improved and more continuous record of the climatic and environmental changes in Iceland for this critical time period (Bj6rck et
Iceland al., 1992; Rundgren, 1995; Hardard6ttir, 1999; Hardard6ttir et al., 2001.). The two prevailing conceptual models of the last deglaciation history in southern Iceland agree in regard to the Preboreal age glacier advance, but differ in their interpretation of 1) the extent of the Younger Dryas ice, and 2) the sedimentological history of the B66i morainic complex (Figure 4).
Open questions It should be noted that some of the Tertiary glacial deposits studied might have a larger lateral extent indicating more extensive regional glaciation than shown on the maps. Old glacial deposits of possible Tertiary age have been reported from various sites all around Iceland, but no dates are available as yet. This makes it impossible to determine precisely the chronological position of the deposits. However, era-rent work is aimed at increasing the number and accuracy of radiometric dates (Ar/Ar and K/Ar) from the terrestrial sections, with emphasis on dating of the lava flows interbedded with the identified glacial deposits. This will give more accurate dates for the actual climatic events. The absence of Tertiary glacial deposits on the NW peninsula raises questions about the preservation potential of glacial deposits. Does this reflect limited or no glaciation on the peninsula or did subsequent glaciations completely erode the evidence for Tertiary glaciation in the area? The Tj6rnes Peninsula in northem Iceland is the only area in Iceland where we have a complete glacial stratigraphy mapped from about 3.0 My to the Holocene is available. It is however very difficult to draw a map of the extent of the younger glaciations (i.e. younger than c. 1.5 My) due to lack of studied sites and radiometric dating of glacial deposits. In the light of the current knowledge of the deglaciation history of south-central Iceland, there seems to be little doubt about the Preboreal age of some of the Bt~6i moraines. The question that remains is whether some of the sediments in the Bt~di morainic complex represent earlier glacial advances. Although much has been written about the Younger D12r glaciation of Iceland, knowledge of the extent of the Younger Dryas ice sheet in the south is still ambiguous. The main problem with the current hypotheses is that the), are based on indirect evidence (i.e. missing sedimentary records). Those who support extensive Younger Dryas glaciation point to the sea for further evidence. ]-'hose who support the hypothesis of a more limited extent of the Younger Dryas ice emphasise the importance .of the undated basal till in the B66i succession. However, the key to the puzzle is more likely to be the sediment sequence still to be found beneath Lake Hestvatn. Although these problems await future research for their solution, the; importance of detailed sedimentological work and facies analyses in glacial geological studies should be emphasized A comprehensive understanding of the deglaciation history requires a detailed lithofacies analysis in context with good stratigraphic control of the successions.
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Consequently future research should emphasise detailed sedimentological, stratigraphical and chronological studies of both lacustrine and marine shelf sediments
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