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Jökull - 01.12.1987, Qupperneq 66

Jökull - 01.12.1987, Qupperneq 66
water could explain the disappearance of molluscs from the sediments. I interpret the sandy matrix of facies Dmu to have accumulated through a combination of settling of suspended material from turbid owerflows (the silt fraction), icerafting (the sand and outsized clast fraction) and possible density flow input of sand. The interbedded units will be treated separately below. There is some controversy in the scientific literature as to the proximity of ice-rafted glaciomarine facies to a glacier margin (Boulton and Dreynoux 1981, Molnia 1983, Powell 1983b, 1984, Mackiewich et al. 1984). Though iceberg dropped coarse detritus enters the prox- imal glaciomarine environment, it can be masked when incorporated in the fjord floor sediments due to the dilution effect of the much greater volume of finer grain sizes. The relative magnitude of iceberg dropped mate- rial, compared to other genetic components in the ice- proximal environment, is a function of calving activity, meltwater activity and debris content of the ice, i.e. it varies in time and space for individual depositional ba- sins, and given favourable conditions should be recog- nizable very close to the ice front. In the case of Borgar- fjördur, the high and steep mountains on the fjord/ valley eastern flank, and their easily weathered volcanic rocks, could have introduced large quantities of debris onto the surface of a passing valley glacier. In a few instances in the cliffs, isolated mounds, pods or lenses of material, on the whole much coarser than the surround- ing sediments, were observed. These fit well to descrip- tions given by Thomas and Connell (1985) and Domack and Lawson (1985) of dump structures resulting from release of debris aggregates from overturning icebergs. Powell (1983b, 1984), Domack (1983) and Domack and Lawson (1985) have modelled ice-rafted diamicton dep- osition in a proglacial marine environment. I suggest that facies Dmu, with a peak in dropped detritus, was deposited in an environment similar to Powell's (1983b) glaciomarine environment in front of a slowly advanc- ing, actively calving tidewater glacier, and corresponds to the dispersed meltwater and ice-rafted facies of Do- mack (1983). I interpret this environment as an ice- intermediate (molluscs disappear) to ice-proximal depo- sitional environment (Fig. 5). The intrabedded units Description: Intrabedded in the Asbakkar diamicton are units of interbedded sand (Ss, Sl) and diamictons (Dms, Dmu). These are most conspicuous in the southernmost part of the section (4900- 5250 m in Fig. 2). They usually occur in poorly defined lenses or tongues, randomly stacked or superimposed on each other, often with a lamina or thin bed of silt on the bounding surfaces (Fig. 8). Occasionally, the intrabedded units have a iobate-like appearance, with sharp to erosive base and slightly convex upper contacts (Fig. 6B). There is a tendency towards concent- ration of coarser clasts at the base of some of the units. The diamicton lenses (Dms, Dmu) differ from the surround- ing Asbakkar diamicton in having a coarser texture, with sub- angular to rounded gravel clasts, floating in a sandy-silty matrix. The stratified diamicton lenses have weakly stratified silt, sand and gravels in the outer parts, and a core of either silty diamicton or massive silty sand. The stratification is an expression of intraformational discontinuous sand and silt bandings and lami- nae, which sometimes appear to be sheared. I could not decide whether this deformation is syndepositional or due to later glaciotectonic deformation. Some of the interbedded facies also show loaded structures, interpreted as convolute structures (Fig. 7B) and small scale folds. The stratified sand (Ss) is poorly sorted and sometimes pebbly. The stratification is due to normal grading of the 5-10 cm thick sand beds, intralaminae of silt and occasional rippled surfaces. Isolated stratified sand units also occur, both as interbedded, 10-40 cm thick units, ranging lat- erally up to 50 m, and as isolated bodies, resembling large scale ball and pillow structures. The upper contact of interbedded sand and diamicton units with facies Dmu is usually sharp, but appears to be conformable. Interpretation and discussion: The Asbakkar diamicton is glaciotectonically deformed (see later) and the intrabedded units are often displaced so that it is difficult to reconstruct their primary sedi- mentary geometry. A glaciotectonic origin of some of the structures observed cannot be ruled out. I propose that the interbedded sand and diamicton facies are formed due to resedimentation processes and meltwater influx, where semi-plastic to liquefied glaciogenic sedi- Fig. 8. A close up of an interbedded diamicton and sand body in the Ásbakkar diamicton at 5100 m: (1) silty- sandy, stratified diamicton, (2) pebbly, massive diam- icton. Black arrows point at the sharp but conformable upper contact with the Ásbakkar diamicton. White ar- row points at an intraformational silt lamina. 8. mynd. Seteining í Asbökkum. 64
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