Jökull - 01.12.1989, Page 13

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Fig. 6. b) North-south cross-section along seismic line 2. Reflections ’c’ and ’d’ on Fig. 5b correspond to the
two uppermost interfaces below the lakefloor. Reflection ’e’ corresponds to the deepest interface. The open
water undir the southem caldera wall is at the site of the 1983 eruption.
Myndó. b) Þversnið frá suðri til norðurs gegnum meginöskjuna eftir endurkastsmœlUínu 2. Vatnið undir
Vestari-Svíahnúk er á gosstöðvunum frá 1983.
reflection suggests, that each horizon only exists
over a limited area. The reflections are only seen in
sediment covered parts of the lake bottom. The
reflections are cut off fairly abruptly over the step
which marks the edge of the lava flow in line 2. The
roughness of the lava surface could be partly
responsible for masking the predominantly high fre-
quency signal from the underlying Iayers.
The deeper reflections are fragmentary and weak
everywhere except in the northem half of line 2.
However, as far as can be seen from the data, the
general trend in these reflections is of dip towards
north. Furthermore, there are indications that steps,
similar to the ones observed in the present caldera
floor, can be seen in places (line 1, 3.5 km; line 2,
5.0 km, below 350 ms.; line 3, 3.0 km). It is there-
fore believed that these reflections arise from buried
lava flows and possibly sills intruded at shallow
depths into sediments.
The existence of these reflections shows that sub-
stantial amounts of material have accumulated
within the caldera since its formation. The caldera
infill is made up of lavas and sediments. The sedi-
ments are probably mostly hyaloclastites from erup-
tions within the caldera, but sediments from glacial
erosion may also be deposited on the caldera floor.
The deepest reflections are 100-150 ms below the
lakefloor which corresponds to a 75-150 m thick pile
of sediments and lavas.
COMPARISON WITH PREVIOUS SURVEYS
In 1951 and 1955, seismic reflection surveys were
carried out in the Grímsvötn Caldera (Joset and
Holtzscherer, 1954; Þórarinsson, 1965). These sur-
veys were essentially point soundings with one shot-
point and 3-12 geophones at an offset of 300-
JÖKULL, No. 39, 1989 11