Jökull


Jökull - 01.12.1987, Side 76

Jökull - 01.12.1987, Side 76
zone of the fold, wing shaped, drawn-out boudins (Fig. 12-3) have developed on the boundary between the As beds and the Ásbakkar diamicton. The boudins are 0.3-1.0 m long, 0.2-0.4 m thick, separated by a distance of approximately half their width. The isoclinal fold is a 8-10 m high anticline. Its limbs consist of the lowest lithofacies of the Ás beds, while the core is of Ásbakkar diamicton. The fold axis trends 75°-255° and the axial plane dips 28° towards 345°, indicating a glacial push from NNW. The Ás beds above the fold are also sheared and folded. Immediately above the fold, in facies Ss, there are boudins and small-scale overturned to recumbent folds (Fig. 12-2). Higher up in the interbedded gravel- and diamicton facies, larger overturned sheared folds occur (Fig. 13). At 3850 m, a overturned folded structure has developed in the mid-section of the Ás beds, at the junction between the lowest sandy lithofacies and the gravelly upper part (Fig. 14). Its fold axis trends 90°- 270°, and the axial plane dips 20° towards 360°, in- dicating a deforming push from approximately north. The uppermost part of the Ás beds is so heavily sheared and mixed that it is often difficult to recognize primary textures, fabrics and geometries. Boulton and Deynoux (1981) suggested that such non-till glacially deformed sediments should be called “deformation till”. A large-scale (> 50 m long) recumbent structure of concentric folded strata, with almost horizontal axial plane, is exposed at around 4700 m in the cliffs, affecting the Ásbakkar diamicton (Fig. 15). The inverted strata and part of the anticlinal hinge zone are exposed in the section. The upper part of the structure has been re- moved by erosion along a horizon approximately coinci- ding with the axial plane. The fold axis appears to trend 100°-280°. As the exposure trends 135°-315°, the SSW facing hinge strata appear as a succession of concave clastic dykes, stacked sub-parallel to the exposure. The folded strata are transected by normal- and reverse faults as well as overthrusts who converge towards north. The dislocating force has operated from a north- erly direction. The folded structure is discordantly over- lain by heavily sheared gravel-diamicton admixture (“deformation till"). At 1525 m, in the immediate substratum of the Melar diamicton there is an example of a small scale, asym- metrical, angular fold which has been transformed into a thrust fault (Fig. 16A). The sharp trough hinge has frac- tured and the inverted limb has been replaced by a slide plane with an apparent dip of 26° towards NNE. At 2400 m there is a folded structure affecting the glaciomarine facies of the Látrar beds (Fig. 16B). It is a 5-6 m high structure of open (interlimb angle: 115°), asymmetrical anticlinal strata. The fold axis trends 80°- 260° and the axial plane dips 50° towards 350°, indicating a push from approximately NNW. At 160 m, a synclinal part of a large overturned fold. Fig. 15. Structural outline of heavily sheared and folded strata of the Ásbakkar diamicton at 4650-4700 m, drawn from photographs. The deformed strata are truncated by a glacial discordance, approximately coinciding with the axial plane of the folded structure. They are overlain by a heavily sheared gravel-diamicton admixture (“deforma- tion till”, probably belonging to the Ás beds. The deforming shear has been from left to right. Height of section about 20 m. 15. mynd. Óþyrmilega haggaður jarðlagabunki við 4650-4700 m. 74
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Jökull

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