Náttúrufræðingurinn - 1963, Side 34

174
NÁTTÚRUFRÆÐINGURINN
SUMMARY
On Submarine Geology around Iceland
by Trausti Einarsson
Department of Engineering, University of Iceland.
In a general introduction it is stated that the theory o£ guyots implies the
possibility of the sinking of an Early Tertiary extensive land in the area
between Greenland and Britain, in spite o£ the great difficulties inherent in
such a hypotliesis from the point o£ view of isostasy. The main features of
the Middle Atlantic Ridge may be as old as Lower Tertiary and the outpouring
of the lower plateau basalts of Iceland are then possibly related to the forma-
tion of the Ridge. In general, the continental sltelf, where formed of sediments
cleposited during much o£ the Tertiary time, is probably of complex structure
owing to oscillations of sea-level. It rnight contain a number of erosional plains
as suggested in Fig. 1, and a last one, íormed in the late Tertiary, might in
sorne areas exist today, only slightly modified by Pleistocene deposition and
erosion. This might be an explanation of the great width of the shelf in
many areas.
The main features of the shelf around Iceland are shown in Fig. 2, which
is based on existing charts. The continental slope is mostly about 11/2°,
steepening to 4° where it rises from the greater depths, G00—1200 m. On a
30 km stretch near Dyrhólaey (Portland) the steepness is 12—15°; this is
probably due to tectonic disturbances while elsewhere the slope seems to be
a natural result o£ the advance o£ sediments from the land. It is therefore
assumed that the outer part of the shelf consists of sediments. Their volume
is estimated as 36.000 km3 on the assumption that the outer half o£ the shelf
forms a wedge of sediments thickening linearly outwards.
This volume corresponds to a layer of average thickness 360 m covering the
area of Iceland and may correspond to the denudation of a 300—400 m thick
rock layer from the country. Topograpliic analysis indicates that denudation
since the Upper Tertiary tectonic uplift and isolation of the present country
amounts to about 400 m. These figures, in spite of the roughness of the
estimate of tlie shelf volume, suggest that the sediments of the shelf corre-
spond to the denudation of the country since isolation. This transport of
material would lead to isostatic readjustment. This is calculated by using a
method givcn earlier (Trausti Einarsson 1953), taking T = 20 km. The country
is schematized Ity a circular area of' radius 200 km, and the denudation is in
one case evenly distributed (400 m o£ rock of density 2.7) in an other case it
rises parabolically írom 300 m in the centre to 500 m on the periphery. About
70% of this material is in both cases distributed evenly on a circular zone
with inner and outer radii 250 and 300 km (giving a thickness ol 400 m by
density 2.7). The results are presented in Fig. 3. Greatest crustal stress is
developed near the periphery of the country and if the crust has yielded at