Jökull


Jökull - 01.12.1987, Side 80

Jökull - 01.12.1987, Side 80
gionally extensive marine terraces at 60-70 m a.s.l. re- late to the relative sea level during stage H. The Mela- bakkar glaciomarine facies was deposited primarily from suspension during this stage. The isostatic rebound is registered in the sequence as glaciomarine sedimenta- tion gives way to sublittoral sand with burrows, finally (stage I) to be truncated by an emergence facies associ- ation (the Melagil gravels and sands), of beach gravels and sands, found above a regionally extensive, time- and space transgressive gravel lag. The glacial stratigraphy and chronology of the Mela- bakkar-Ásbakkar cliffs presented here differ substan- tially from previous interpretations of the lower Bor- garfjördur strata (lngólfsson 1984). According to the deglaciation synthesis of Einarsson (1961,1968,1971) the Borgarfjördur main valley/fjord was not glaciated after ca. 13.000 BP. It has also been maintained that glaciers did not reach the coastal areas of W-Iceland after the Álftanes Stadial, correlated with the Older Dryas of Scandinavia, and that the region was not seriously af- fected by glaciation during the Budi Stadial, correlated with the Younger Dryas of Scandinavia (c/. Einarsson 1968,1979, Andersen 1981). The evidence from the Me- labakkar-Ásbakkar cliffs suggests that glaciers ad- vanced down the Borgarfjördur valley both around 12.000 BP and after 11.400 BP, indicating a more exten- sive glaciation than hitherto assumed for W-Iceland dur- ing the last stages of the Late Weichselian. For explaining the development of the depositional basin with regard to lithofacies distribution and stra- tigraphic associations, I have applied recent models for sedimentation in a subarctic glaciated fjord environ- ment, as outlined by e.g. Powell (1981, 1984), Molnia (1983), Mode et al. (1983), Domack (1983) and Eyles et al. (1985). These models primarily relate to glacial reces- sion sequences, but the Melabakkar-Ásbakkar example indicates that they can also be applied to ice-transgres- sive sequences. The development of glaciotectonic deformations in the Melabakkar-Ásbakkar strata is probably best ex- plained by a combined effect of frontal push, differential ice loading and hydrodynamic mechanisms on pro- and subglacial unfrozen sediments. ACKNOWLEDGEMENTS Thorleifur Einarsson, of the University of Iceland, pro- posed this investigation, and placed his resources at my disposal during the first stages of the work. Christian Hjort supervised the investigation on behalf of the Uni- versity of Lund, and has given valuable comments and advise during all stages ofthe work. I have also benefitted from discussions with Erik Lagerlund and Per Möller at the University of Lund and Hreggvidur Norddahl at the University of Iceland. Lena Adrielsson and two anony- mous reviewers gave valuable comments on the manu- script. My good-humouredfield assistant during twofield seasons, Ingólfur Gíslason, is warmly thanked. The Radiocarbon datings were carried out by Sören Hákans- son at the Radiocarbon Dating Laboratory in Lund. Lena Barnekow and Gun Hansson did the laboratory work on my sedimentsamples. My English was improved by Ingela Bergenrud. Financial assistance for the in- vestigation was received from the Swedish Society for Anthropology and Geography, the Royal Swedish Aca- demy ofSciences and the Arctic Institute ofNorth Amer- ica, which is gratefully acnowledged. Since 1983 the study has been supported by a research scholarship at the Uni- versity of Lund. REFERENCES Aber, J.S., 1982: Model for Glaciotectonism. Bulletin of the Geological Society of Denmark, 30:79-90. — 1985: The character of glaciotectonism. Geologie en Mijn- bouw 64: 389-395. Andersen, B.G. 1981: Late Weichselian Ice Sheets in Eurasia and Greenland. In: G.H. Denton and T.J. Hughes (eds.). The Last Great Ice Sheets. John Wiley and Sons, Inc., New York: 1-65. Ashwell, I. Y. 1975: Glacial and Late Glacial processes in West- ern Iceland. Geografiska Annaler 57: 225-245 Banham, P.H. 1975: Glacitectonic structures: a general dis- cussion with particular reference to the contorted drift of Norfolk. In: A.E. Wright and F. Moseley (eds.). Ice Ages: Ancient and Modern. Geological Journal, Special Issue No. 6. Steel House Press, Liverpool, pp. 69-94. Bárðarson, G.G. 1923: Fornar sjávarminjar vid Borgarfjörð og Hvalfjörð. Vísindafélag íslendinga, Rit 1: 116 pp. Berthelsen, A. 1978: The methodology of kinetostratigraphy as applied to glacial-geology. Bulletin of the Geological Society of Denmark 27: 25-38. — 1979: Recumbent folds and boudinage structures formed by subglacial shear: an example of gravity tectonics. Geologie En Mijnbouw 58: 253-260. Blatt, H., G. Middleton and R. Murray 1972: Origin of Sedi- mentary Rocks. Prentice Hall Inc., Englewood Cliffs. 634 p. Boothroyd, J.C. and G.M. Ashley 1975: Process, Bar Morphol- ogy, and Sedimentary Structures on Braided River Outwash Fans, Northeastern Gulf of Alaska. In: A.V. Jopling and B.C. McDonald (eds). Glaciofluvial and Glaciolacustrine Sedimentation. Society of Economic Paleontologists and Mineralogists, Special Publication No. 23. Tulsa, pp. 193- 222. Boulton, G.S. 1968: Flow tills and related deposits on some Vestspitsbergen glaciers. Journal of Glaciology 7: 391—412. — 1976: A genetic classification of tills and criteria for dis- tinguishing tills of different origin. In: W. Stankowski (ed.). Till, its genesis and diagenesis. Univ. Mickiewicza W. Poz- naniu. Ser. Geografia 12: 65-80. 78
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