Jökull - 01.12.1993, Page 42

Figure 7. An exposure in
a wedge-shaped sediment
body at the edge of the Mið-
fjörður terrace, containing
the light coloured rhyolitic
Miðfjörður Tephra (photo
Hreggviður Norðdahl).
— Jarðlög og Ijósleita Mið-
fjarðargjóskan í brún set-
hjallans við Miðfjörð.
Table I: Microprobe analyses of samples from the MiðfjörðurTephra in the Bakkaflói area and the S-Tephra in
the Krafla area in northeast Iceland. All analyses are expressed in weight %. Total iron is expressed as FeO*.
— Niðurstöður efnagreininga með örgreini á sýnishornum af Miðfjarðargjóskunni og svonefndu S-lagi í ná-
grenni Kröflu.
Si02 tío2 AI2O3 FeO* MnO MgO CaO Na20 K20 P2O5
Miðfjörður tephra 72.7 0.24 12.4 2.57 0.09 0.24 1.64 4.33 2.47 0.00
S-Tephra (76.7)* 0.30 12.4 2.28 0.09 0.23 1.68 4.59 2.60 0.04
* The SÍO2 value for the S-Tephra is too high according to Karl Grönvold (personal communication, 1994).
west of the study area (Figure 9). This situation prob-
ably prevailed until the general Lateglacial climatic
improvement in the North Atlantic Region, coupled
with the eustatic rise in sea-level, caused the glaciers
in northeast Iceland to retreat to grounding positions
close to the present coastline. Following this initial
glacial retreat, areas on the seaward facing sides of
high coastal mountains became ice-free. During short-
lived readvances or standstills of the glacier margin
scattered shoreline features were formed, possibly as
high as 65 m and certainly between 50 m and 40 m
a.s.l. (Figure 9).
A subsequent glacial retreat to positions well in-
side the present coastline, coupled with a lowering of
the relative sea-level, is demonstrated by the progra-
dational shoreface sequence in the Miðfjörður terrace.
Furthermore, the Miðfjörðurterrace sediments also il-
lustrate a subsequent change from an ongoing regres-
sion of sea-level to a transgression, which eventually
reached the regional marine limit at about 30 m a.s.l.
when the glacier margin was situated some 5 km in-
land (Figure 4). During a concurrent glacial readvance
other glaciers within the Þistilfjörður and Bakkaflói
drainage areas probably terminated at ice-marginal
features near Flautafell and south of Lónafjörður in
the Þistilfjörður area, i.e. inside and above the 30 m
regional marine limit (Figure 9). The apparently con-
sistent altitude of this marine limit indicates that the
40 JÖKULL, No. 43, 1993