Jökull


Jökull - 01.12.1963, Blaðsíða 5

Jökull - 01.12.1963, Blaðsíða 5
the latter. To explain the gradual encroach- ment of H5 on H4 it is here suggested that the deposition was very slow, and agitation in the water swept the fine material into deeper water, keeping pace with the deposition. Whether these strata are marine or lacustrine is uncertain, as no fossils have been found. He/7 is a coarse deposit containing much material derived from Hr, and deeper sediments, even traces of the Lower Sediments on western Tjörnes (Bárdarson, Strauch). The deposit no doubt corresponds to a major erosional interval. The previous deposits were now til- ted slightly to the east, and to the west of Breiðavík they were subjected to erosion, per- haps completely wiped out, so that the Lower Sediments were laid bare. But as east- or north- eastward tilting and erosion also occurred later, the exact position of the earlier erosional sur- face is difficult to reconstruct. The lowest layers in Hg/7, just east of Foss- gil, have steep northeast dip (20°) but the dip decreases gradually upwards and eastwards, and blocks and pebbles of basalt become more pre- dominant. Towards the top, material derived from the brown sediments is nearly absent. This change in composition, which has not been noticed earlier, will be seen to be of stratigraphic value. H7 was distinguished from H6 by Péturss and Bárðarson by its content of large boulders and greyish colour, which suggested glacial influ- ence. But the two members are in other re- spects difficult to distinguish from each other, whence Strauch combines them into He/7. But also a sandstone resting on H7 is difficult to distinguish from it. Near the top of the cliff a conglomerate bank is prominent by its resi- stence to weathering. On the E—W section seen, it has only a very slight eastward dip. At the west end this bank is hardly distinguish- able from H7 while at the east end it rests in part on the sandstone while also it ends against a wall of the liigher part of the sandstone. This is apparently not a fault, it may be slumping or a temporary sea cliff, but this is not too clear; for a detailed study in the steep cliff, working in ropes would be necessary. What is clear is that the conglomerate bank rests on the sandstone and the latter seems to be a direct continuation of H7, although it is per- haps not excluded that during a short lowering of sea-level H7 was eroded a little before a new submergence that brought the sandstone. These strata are cut by a fault, on the east side of which is found the thick layer Hs- This is a muddy sandstone with some pebble bands at the base. Althougli it is not possible to see directly the sense of the movement at the fault, it seems most likely that the east side was thrown down and that the base of Hg is re- presented by the finer sediments in the upper part of the wall on the west side of the fault, whence these are also marked IL in the dia- gram. In my former study I also discussed an- other possibility but this can now be dropped on the basis of new observations at Svarthamar, farther east. Svarthamar is an isolated mass of conglomer- ate whose stratigraphic position lias formerly not been ascertained. Bárðarson put it as No. 11 in his section, while he, Áskelsson and Ein- arsson suspected some lower position, and Strauch identified Svarthamar with H2a. There can now be no doubt that Svarthamar belongs to H6/ 7. The Svarthamar conglomerate is a delta consisting of stratified, well-worn river gravel, with a dip of 20—30° NNE. Most com- mon are pebbles of 2—3 cm diam. but blocks of 30—40 cm occur, and a part is sandy. The pebbles consist very largely of brown sandstone to siltstone. According to my estimate such pebbles are at least 50—60% of the total volume. North of the centre of Svarthamar we can observe liow IIs rests on the conglomerate. There is no sharp limit, the gravel becomes grádually finer and the dip decreases as we come higher, and the percentage of the brown sediment material decreases at the same time. This is the same relationship as is found in Hö—H7—Hg, and the composition of Svartham- ar sliows very clearly that, like He/7, it was formed during a breaking down of the sedi- ments Hg to Hatl. Strauch mistakenly states that brown sediment pebbles are nearly absent from Svarthamar. Perhaps he only saw the con- glomerate in a wet state, when the sediment pebbles look rather similar to basalt. Cementation in Svarthamar is the same as noted for H2a and He/7. Pieces of peat and carbonized wood chips are found occasionally in Svarthamar. We see now that the coarse material of He/7 JÖKULL 1963 3
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