Jökull


Jökull - 01.01.2012, Page 184

Jökull - 01.01.2012, Page 184
Í. Ö. Benediktsson sections, as exemplified by ductile and brittle defor- mation structures and horizontal shortening ranging from 20% to 76%. Ductile deformation was favoured by high pore- water pressure and dominated the formation of the 1890 end moraine, developing from open folding to multiple overfolding. A substantial drop in porewater pressure at the very end of the surge induced brittle deformation as the final phase of the end-moraine for- mation. Folds are generally rooted indicating high friction along a basal décollement, which lies across bedrock or outwash. This suggests that the applied glacio- tectonic stress was absorbed within a narrow zone, as also suggested by steeply dipping faults and axial planes of folds. Orientation of faults and axial planes of folds pri- marily reflects local and marginal ice-flow direction (e.g. in an outward-spreading surge lobe) which is perpendicular to the moraine strike and not necessar- ily coinciding exactly with the main ice-flow direction of the 1890 surge. In general, though, structural data coincide with the ice flow from southerly directions, as expected. Acknowledgements Jaap van der Meer is thanked for collaboration on the 1890 end moraine in Kringilsárrani. Thanks are furthermore due to other members of the Brúarjökull field campaigns for great company and collaboration. Constructive comments by reviewers Jonathan R. Lee and Jón Eiríksson and editors Helgi Björnsson, Bryn- dís Brandsdóttir and Leó Kristjánsson, improved the manuscript and are gratefully acknowledged. ÁGRIP Rannsóknir á innri gerð jökulgarða (Hrauka) sem mynduðust við aflögun fínkornóttra setlaga í fram- hlaupi Brúarjökuls 1890 sýna að garðarnir urðu til á síðasta degi framhlaupsins, þegar jökullinn ýtti fram efsta og ysta hluta setfleygs sem myndast hafði und- ir jökulsporðinum dagana á undan. Vatnsþrýstingur í setlögunum undir og framan við jökulsporðinn var hár er þau aflögðuðust undan fargi hans; fyrst í opnar fell- ingar sem síðan steyptust yfir sig við aukna framrás jökulsins. Undir lok framhlaupsins þrýstist vatn út úr setlögunum og þau stífnuðu. Fellingarnar brotnuðu þá um skerfleti og þrýstimisgengi uns álaginu frá jöklin- um var létt. Þetta ferli einkennir aflögun fínkornóttra setlaga við framhlaup jökla og varpar ljósi á myndun margra fornra jökulgarða. REFERENCES Aber, J. S. and A. Ber 2007. Glaciotectonism. Elsevier, Amsterdam. Developm. Quaternary Sci. 6, 1–246. Benediktsson, Í. Ö. 2010. End moraine and ice-marginal processes of surge-type glaciers – Brúarjökull and Eyjabakkajökull, Iceland. Unpublished PhD. thesis, Faculty of Earth Sciences, University of Iceland, 26 pp. + 4 app. Benediktsson, Í. Ö., P. Möller, Ó. Ingólfsson, J. J. M. van der Meer, K. H. Kjær and J. Krüger 2008. Instan- taneous end moraine and sediment wedge formation during the 1890 surge of Brúarjökull, Iceland. Quater- nary Science Reviews 27, 209–234. Benediktsson, Í. Ö., Ó. Ingólfsson, A. Schomacker and K. H. Kjær 2009. Formation of submarginal and proglacial end moraines: implications of ice-flow mechanism during the 1963-64 surge of Brúarjökull, Iceland. Boreas 38, 440–457. Benediktsson, Í. Ö., A. Schomacker, H. Lokrantz and Ó. Ingólfsson 2010. The 1890 surge end moraine at Eyjabakkajökull, Iceland, a re-assessment of a classic glaciotectonic locality. Quaternary Science Reviews 29, 484–506. Benn, D. I. and D. J. A. Evans 2010. Glaciers and Glacia- tion. 2nd ed. Hodder Education, 802 pp. Bennett, M. R. 2001. The morphology, structural evolu- tion and significance of push moraine. Earth-Science Reviews 53, 197–236. Bennett, M. R., D. Huddart, R. I. Waller, N. Cassidy, A. Tomio, P. Zukowskyj, N. G. Midgley, S. J. Cook, S. Gonzalez and N. F. Glasser 2004. Sedimentary and tectonic architecture of a large push moraine: a case study from Hagafellsjökull-Eystri, Iceland. Sedimen- tary Geology 172, 269–292. Björnsson, H., F. Pálsson, M. T. Guðmundsson and H. H. Haraldsson 1998. Mass balance of western and north- ern Vatnajökull, Iceland, 1991–1995. Jökull 45, 35– 38. Björnsson, H., F. Pálsson and O. Sigurðsson 2003. Surges of glaciers in Iceland. Ann. Glaciology 36, 82–90. 182 JÖKULL No. 62, 2012
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