Jökull


Jökull - 01.12.1978, Side 46

Jökull - 01.12.1978, Side 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
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