Jökull - 01.12.1978, Síða 51

Three lines of evidence from the present
survey may be used to make inferences about
sediment transport in the study area. These
are a) grain size of the sediments, b) the dis-
tribution of carbonate and c) the bedforms
observed on the sonographs.
a) A general guide to sediment grain size in
the area is the sonar map (Fig. 3), which
broadly shows a pattern of decreasing grain
size downslope, away from unit 1, with
coarser material appearing again in the south.
b) The carbonate distribution illustrated in
Fig. 6 is roughly parallel to grain size trends in
that carbonate content appears, broadly, to
decrease with water depth.
Taken on their own, these two properties,
viz. grain size and carbonate content, might
be taken to indicate downslope sediment
transport radially away from the central shal-
low areas. The carbonate distribution would
then indicate high carbonate productivity in
and around the “hraun” areas, and the out-
wards decrease in carbonate content would
represent decreasing productivity or mixing
with terrigenous material. This suggestion is
not supported by the bedforms, as discussed
below.
c) Bedforms observed in the study area are
defined by relief patterns on the one hand,
and variations in grain size on the other.
Those bedforms defined by relief are the
ripples of map unit 2. The fact that these
ripples occur in gravel, and their wavelength is
of the order of 1 m, suggest kinship with ripp-
les described by Flemming and Stride (1967) and
Channon and Hamillon (1976) off the south-
western coast of England, which were ascribed
to the action of oscillatory wave currents.
The orientation of the gravel ripples found
here is generally near N—S (i.e. parallel or
subparallel to the survey tracks). They are
thus indicative of E—W (oscillatory) motion.
A piece of evidence indicating that the rippled
gravel is indeed in motion is provided by some
dredge marks observed to the east of Sydra-
Hraun. At the time the survey took place, the
suction dredger SANDEY was at work in the
area on her annual 1-month dredging oper-
ation for the State Cement Works. The marks
left by the dredger were the only ones observed
in this area where dredging has taken place for
a number of years. It seems clear that dredge
marks from previous years have been filled in
by the movement of the bottom sediment.
Bedforms defined mainly by variation in
grain size make up the rest of bedforms ob-
served in the area and were found in map units
2, 3 and 4. They are perhaps best brought out
where they occur on the boundaries of these
units. The bedforms include the elongate sand
and gravel bodies along the northeastern
border of the map, the crescentic forms at the
boundary between units 2 and 3 in the
southeast, the striped and criss-cross pattern of
unit 4 and the irregular boundary between
units 3 and 4 in the south. All these bedforms
have been identified as falling in the category
of sediment patches. Analogous bedforms have
been found to be widespread on the shelf of
the British Isles, for example (Belderson, Kanyon
and Stride, 1971), and to occur in areas of weak
tidal currents (less than about 50 cm/sec).
Linearity of sand patches has been found to be
parallel or transverse to peak tidal currents.
Although sand patches occur in areas of low
transport rates, suggestions of transport direc-
tion may be made on the basis of the nature of
the sand/gravel boundary, mainly its sharp-
ness. Sharp, well defined boundaries often
represent a steep edge of the sand body over-
lying the gravel (Channon and Hamillon, 1976),
whereas a diffuse, or gradational, boundary
suggests a more gradual thinning of the sand.
A sharp boundary is thought likely to
represent the downcurrent side of a sand
patch.
A direct comparison of the Faxaflói sand
patches with British examples shows a degree
of similarity. Some of the longitudinal,
NW—SE-trending features in the northern
part of the area resemble features found by
Belderson, Kenyon, Stride and Stubbs (1971, Fig.
71) in the Celtic Sea, although there is a
transverse component to the shape of the
Faxaflói patches. This transverse trend is not a
right angles to the elongation of the sand and
JÖKULL 28. ÁR 49