Jökull - 01.12.1978, Page 46

450 m apart, positioned so as to give complete
sonar coverage of the area (Fig. 2). The
instruments used were an Atlas surveying
echo-sounder and an EG&G Mk II side-scan
sonar system. Positioning was by Raydist.
Secondly, a Shipek grab was lowered at 264
stations in the area in order to sample uncon-
solidated sediments. Thirdly, Boomer profi-
les were obtained by the use of EG&G
equipment operated by engineers from the
National Energy Authority, Reykjavík. The
Boomer tracks are shown in Fig. 7.
The echo-sounding profiles were processed
by the Hydrographic Survey and made avail-
able to the author in the form of a sounding
sheet (Fig. 2).
Isometric reproduction of the sonographs
was made manually in the usual manner (see
e.g. Flemming, 1976), originally in the scale
1:25000.
Some 150 sediment samples were analysed
for carbonate content by the Cement Works
laboratory, using standard titration methods,
and sieve analyses of grain size were made at
the Marine Research Institute.
RESULTS
Bathymetric and Sonar maps
The sounding sheet (Fig. 2) broadly illus-
trates the bathymetry of the area. A compar-
ison with the sonar map (Fig. 3) shows how the
consolidated rocks of the “hraun” areas rise
from the surrounding sediment-covered floor.
The contrast between the rough topography
of the “hraun” areas and the smooth topo-
graphy of the surrounding floor is also well
brought out. The relatively steep slopes into
the channels on the northeastern and
southeastern flanks of the area are indicated.
The sonar map (Fig. 3) illustrates that the
sea-bed in the area may be divided into four
map units on the basis of difference in cha-
racter of the sonographs. The four broad sub-
divisions are:
Unit 1. Sea bed characterised by dark,
blotchy appearence on the sonographs, map-
ped as “rock” in Fig. 3. The limits of this unit
are defined by steep slopes in many places,
such as near both “hraun” areas which occur
within unit 1. Rock floor is not limited to the
“hraun” areas, however, but occupies the
main portion of the central and western parts
of the map. Small “islands” of rock occur on
the northeastern and eastern sides of the
otherwise contiguous rock floor. Their
elongate shape and often pointed southeastern
and northwestern terminations are suggestive
of glacial erosion. Glacial striae on the coast
also show the same northwesterly trend
(.Kjartansson, 1960). Two sets of lineations or
structural trends observed in sonographs from
unit 1 and outlined on the map are thought to
represent strike and fault trend (SW—NE, cf.
Jónsson, 1965) and ice scour (SE—NW). Most
depressions in the rock floor contain
sediments.
Thirteen dredge samples taken in 1974 by
the author, from the rock floor of Sydra Hraun
and Vestra Hraun were all made up of
sedimentary rocks, siltstones, sandstones, and
conglomerates.
Umt 2. An area of gravel (dark tone on
sonographs) bordering the rock. The low
ground between the two “hraun” areas has
extensive cover of gravel, but otherwise
gravels occur as a continuous border around
unit 1, interrupted only at one place in the
south, where a strip of finer sediment appears
to dissect it. The boundaries of this strip are
not well defined on the sonographs. The
sonographs show that much of the gravel sur-
face is in the form of ripples with a wavelength
of 1—2 metres. The rippled gravel is shaded
on Fig. 3. Long, narrow strips of light tone
(sand) occur within this map unit.
Grab samples from unit 2 show that the
gravels are made up almost entirely of shell
material, complete and broken shells mainly
of bivalves, gastropods, and barnacles, along
with calcareous worm tubes. Living individu-
als were commonly found.
Unit 3. On the downslope side of the
44 JÖKULL 28. ÁR