Jökull - 01.12.1993, Blaðsíða 39

Figure 4. The Bakkaflói area with raised shoreline
features and major sediment terraces. Contour interval
is 100 m. —Fomfjörumörk, sethjallar og jökulgarð-
ar við Bakkaflóa. Hœðarlínur eru á 100 m bili.
THE REGIONAL MARINE LIMIT AT ABOUT 30 m
More or less continuous raised shoreline features,
representing a regional marine limit at about 30 m
a.s.l. can be followed between the Rauðanes penin-
sula and Lónafjörður in the Þistilfjörður area (Fig-
ure 3), and at 30-35 m a.s.l. between Finnafjörður
and the Digranes peninsula in the Bakkaflói area (Fig-
ure 4). At two locations in the Bakkaflói area, at
Hölkná river and in the inner parts of Bakkafjörður,
prominent beach ridges that reach about 35 m a.s.l.
(Figure 4) are superimposed on lower beach terraces,
indicating a sea-level transgression during their forma-
tion. Heavily dissected fluvioglacial and marine ter-
races are found north-east of the mountain Flautafell,
above the Alandsvík and Tumavík coves, and south of
Lónafjörður in the Þistilfjörður area (Figure 3), and
at Miðfjörður and Bakkafjörður in the Bakkaflói area
(Figure 4).
The surface of the Miðfjörður terrace, rising to-
wards the south-west from about 25 m and up to
55 m a.s.l. (Figure 4), displays a prominent pattern
of braided channels that can be followed down to the
regional marine limit at about 30 m a.s.l. In a coastal
section of the terrace (Figure 5) we observed an ap-
proximately 20 m thick sedimentary sequence resting
on top of the local bedrock (Figure 6A). The lower
part of the terrace between 4 m and 16 m a.s.l. consists
of laminated silt and fine sand, which grades upward
into cross-bedded sand. The upper part of the ter-
race, between 16 m and 25 m a.s.l. consists of trough
cross-bedded sand and gravel.
An interpretation of this succession suggests a de-
position of the silt and fine sand sediments in rea-
sonably deep water subsequently to a glacial retreat.
The fine grained sediments that constitute the lower
parts of other similar terraces in the Þistilfjörður and
Bakkaflói areas (Figures 3 and 4) have probably been
deposited in a similar manner. The following 5 m of
cross-bedded sand indicate a shallowing environment,
whereas the uppermost 9 m of the Miðfjörður section
is a sandur formation representing an aggradation con-
tinuously adjusted to a rising relative sea-level (cf. Ed-
wards, 1986). According to the extent of the braided
channels on top of the terrace, the transgressing sea-
level eventually reached the regional marine limit at
about 30 m a.s.l.
A comparable development is probably also seen
in the coastal cliffs at river Saurbæjará, about 6 km
north-west of Miðfjörður (Figure 4). Here a 3-
4 m thick sandy till is resting on a glacially abraded
bedrock with striae indicating a north-easterly direc-
tion of the ice-flow. The till is overlain by a few metres
of coarse grained sand and gravel outwash sediments
containing cobbles and lenticular beds of sorted sand.
Further upstream a several metres thick succession of
littoral sand and gravel represents the build-up of a
beach ridge that reaches about 35 m a.s.l. (Figure 4).
The surface of other sedimentary terraces in the Þistil-
fjörður and Bakkaflói areas (Figures 3 and 4) has also
been adjusted to the 30 m regional marine limit.
Towards the south-west the Miðfjörður terrace
ends at a terminal moraine zone (Figure 4) with NE-
SW orientated morainic ridges. The morainejust north
JÖKULL, No. 43, 1993 37