Jökull


Jökull - 01.12.1987, Side 78

Jökull - 01.12.1987, Side 78
water in reducing shear strength of pro- and subglacial sediments (e.g. Banham 1975, Sharp 1985). But se- quences also exist where a combination of high pore- water pressure and differential permafrost in the strata subject to glacial pressure are invoked to explain large- scale deformations (e.g. Thomas 1984b). Recently Aber (1985, p. 389) stated that “glaciotectonic features may . . . affect materials that were either frozen or thawed at the time of deformation”. There are no structural methods available yet with which to get objective information from deformed sedi- ments on the basal conditions of a glacier at the time of glaciotectonic deformation. The information is sparce from recent glaciers on the effects of different combina- tions of basal temperature, different substratum, hydro- dynamic situation, differential loading, compressive ice flow etc. on the development of glaciotectonic deforma- tion. Thus at this stage, the models are somewhat cir- cumstantial and should be used carefully. In the case of the Melabakkar-Asbakkar glaciotectonics, the com- bined effect of frontal push, differential ice loading and hydrodynamic mechanisms is to me the most attractive explaination. The two ice advances occured into a sub- merged fjord basin, where the presence of any perma- frost is very unlikely. There is also evidence of abundant meltwater in connection with the glacial events. SUMMARY AND DISCUSSION It is my conclusion, that the Melabakkar-Ásbakkar se- quence, bounded by a lower surface of glacial erosion and by an upper surface of wave erosion, contains a fairly continuous record of glacial episodes in the lower Borgarfjördur region for the later part of the Late Weichselian, after ca. 12.500 BP. The sequence was deposited in a glacio-isostatically depressed fjord basin, where the major controls of lithofacies distribution and stratigraphical associations were waterdepth and the proximity to an ice margin and a source of meltwater input. The total thickness of the exposed strata is about 145 m, of which the glaciomarine sequences constitute about 85 m, ice-proximal/ice contact outwash sedi- ments, debris flows and tills 40-45 m, and an emergence facies of sand and gravel about 15 m. A composite verti- cal section is shown in Fig. 3. The striated bedrock, which constitutes the lower boundary of the sequence, bears witness to a glaciation event when the ice reached beyond the present coast — some time prior to about 12.500 BP. The development of the Melabakkar-Ásbakkar sequence can be divided into nine stages (Fig. 19): During the first stage (stage A), mollusc-bearing glaciomarine sediments (the Ásbakkar diamicton) accumulated from suspension, random un- derflows and ice-rafted debris. Around 12.300 BP, the relative sea level was at least 70 m above the present sea level, possibly reaching the marine maximum level at 80-90 m a.s.l. Around 12.000 BP a glacial advance down the Borgarfjördur valley/fjord (stage B) caused the mol- lusc populations to disappear, and subaqueous ice-mar- ginal/ice-proximal stratified sediments (the Ás beds) were deposited from subglacial meltwater streams and - .V, ' 'T- ' . s ' ‘ Y\‘ - Fig. 17. A composite oblique photograph of alarge fold- synclinal (trough) hinge zone. The deforming force has ed structure developed in the Ásbakkar diamicton at acted sub-parallel to the outcrop, from left to right. The around 160 m. The structure is truncated by a gravel lag section is about 50 m long. horizon and overlain by littoral gravels. The arrow 17. mynd. Samsett Ijósmynd afhögguðum jarðlögum við points at axial plane foliations developed close to the 160 m. 76
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