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Jökull - 01.12.1987, Qupperneq 68

Jökull - 01.12.1987, Qupperneq 68
grains in the matrix to be altered into palagonite, which then can act as a cementing substance. The sediments lack other diagnostic characteristics of palagonite, such as a brownish color or precipitation of opal or zeolites. The relative sea level was high during the deposition of the Ásbakkar diamicton, possibly at the marine maxi- mum limit at 80-90 m some time during its deposition. A shell sample from the Skorradalur valley, dated to about 12.300 BP (Ashwell 1975), implies that at that time the relative sea level was above 62 m a.s.L, which is the elevation of the valley threshold. THE ÁS BEDS: A MORAINAL BANK FACIES ASSOCIATION The Ásbakkar diamicton is overlain by a sequence of gravels, sands and diamictons, the Ás beds. They are exposed in the cliffs below the Ás farm (Fig. 1), between 3550 m and about 4700 m in the section (Fig. 2 and logs F and G in Fig. 3). The Ás beds have four major character- istics: (1) Their lower contact with the Ásbakkar diamic- ton is Iocally gradational and conformable, indicating continued submarine deposition, and locally a glaciotec- tonic thrust plane. (2) Except for the lowest part of the strata, beds are discontinuous and laterally variable in thickness. (3) There is a clear coarsening upwards trend in the unit, from silty sands to coarse gravels. (4) The unit is bounded upwards by a surface of glacial erosion and shear. Its uppermost part is at places so sheared and mixed that it is difficult to recognize primary textures, fabrics and geometries. The Ás beds gravel facies Description: The gravel facies are most conspicuous in the Ás beds sequence, especially in its upper part between 3550 m and 3900 m in the cliffs, and constitute about half its total thickriess. Four major lithofacies of gravel were recognized: normally graded gravel (Ggn), reversely graded gravel (Ggr), massive to weakly strati- fied gravel (Gu) and planar cross stratified gravel (Gp). The gravel facies are interbedded with units of sands and diamictons described below. The gravel facies are discontinuous and lat- erally variable in thickness. Typical dimensions for individual units are thicknesses of less than one m and a lateral extent of less than 40 m. Unit contacts are sharp and sometimes erosional. The gravel is unconsolidated and poorly sorted, and the graded facies show variations from pebble-cobble gravels to sandy peb- ble gravels or vice versa. Lenses of sorted grain-to-grain contact pebbles, sometimes showing imbrication, occur in all gravel facies. The imbricated fabrics indicate a depositing stream or current from a northerly direction. Laminae and lenses of silt, thin beds of crudely stratified to laminated sand and massive to stratified diamicton occur in all gravel units, except in facies Gp. Facies Gp is a 2-3 m thick isolated set of planar, tangential- based cross stratified sandy gravel with foreset bed thicknesses of 10-30cm. Individual beds extend over the full thickness of the unit and dip at an angle of 8-17° towards SW. The Ás beds sand and silt facies Description: The lowest part of the Ás beds, which can be seen at intervals between 3650 m and 4700 m, ís up to about 8 m thick unit of stratified sand (Ss). Its lower part is poorly sorted and pebbly (samples 14 and 15, Fig. 4), with stratification due to normal grading of individual 5-10 cm thick beds from gravelly sand to poorly sorted sand. Upwards the unit grades to stratified silty sand, with laminae and thin intrabeds of sandy silt (Fl) and coarse to fine sand (S1 ,Su,Ss). Random rippled bedding surfaces and isolated structures such as sand lags and rip-ups of sandy silt laminae also occur. The unit is heavily sheared, and its lower gravelly part is partly incorporated in a large fold (Fig. 12). The silty upper part is lithified, at least partly due to palagonitization (precipitates of opal present). The contact with the underlying Ásbakkar diamicton is of two types: between about 3560 m and 4000 m it is obscure due to glaciotectonic shear, but beyond 4000 m the contact is gradational. Incorporated in the lowest gravelly sand are pitted and worn shell fragments, probably derived from the Ásbakkar diamicton. The lowest stratified sand facies is conformably overiain by the only major silt facies in the Ás beds sequence, a laminated sandy silt (Fl), deformed due to loading. The thickness of facies F1 ranges from a few cm to about 30 cm. Silt laminae and thin siltbeds were also found coating bounding surfaces of many of the overlying lithofacies in the sequence. Fig. 9. The Látrar beds stratified diamicton: The con- tact (broken line) between the cross stratified esker fan sediments (Sp) and the stratified undermelt/ice-rafted diamicton (Dms). Here, the diamicton is very rich in outsized clasts. 9. mynd. Jakabornir hnullungar á mörkum áskeilu og jökulsjávarsets. 66
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