Jökull - 01.12.1979, Page 21

HENGILL- SOG -VALLE Y
Post-
glacial
Last
Glacial
(Wiirm)
Last
Inter
glacial
Glacial
2. <5’ 4
~tZ ' o
Lavg (Nesjahraun, Hellisheidi)
marme terraces
lake terraces/moraine
interglacial lava of Bitra - Arstadafjall
hyaloclastite "j y0ung ridges
pillow lava, breccia > north of
J Hengill
supraglacial lava cop
breccia
piliow lava
tillite
Subglacial acidic rocks
of Sleggja
supraglacial lava cap
Hengill tuya
hyaloclastite
pillow lava
Hœdir lava shield
supraglacial lava cap
subglacial ridges of
Grafningshólsar
Hahryggur
Stangarhóls
units
/ /*.- /\

iQl 'ím
Glacial A A £
Iql m
A
Xgl
tillite
basalts underneath
Graf ningshólsar
fluvial and lake beds
lava
lignite/conglomerate
hyaloclastite
tillite
Slope behind
Stórihóls farm
basalt lava
foreset breccia
Exposed in
Sog valley
below Stóriháls
conglomerate/sandstone
bosalt lava
volcanic ash/with organic matter
moraine
basalt lava
Temporarily
exposed in
tunnel
Fig. 13. Composite section through the Upper
Pleistocene of Hengill and the Grafningshálsar
volcanic systems of SW-Iceland. The rocks are
primarily hyaloclastite forming thick units of small
lateral extent (no thickness scale!). Intercalated
lava flows formed when the area was ice free either
during interglacials or as lava cap above the in-
fluence of glaciers. The lowest part of the section is
from T. Tryggvason, 1955.
or less continuous from the Holocene well back into
the Upper Pleistocene and includes 4 inter-
glacial/glacial cycles. No radiogenic ages are
available. The microflora of a single thin lignite
bed occurring in the section resembles that of
Svínafell (base of Öraefajökull) which has been
tentatively dated at about half a million years.
The high proportion of subglacial rocks relative
to subaerial lavas within the Upper Pleistocene
series especially towards southwestern and Central
Iceland indicates that the glacial periods were of
longer duration in Upper Pleistocene time than
during build up of the Plio-Pleistocene series.
Glacial erosion features within the Upper
Pleistocene series or neovolcanic zones in general
are, however, rather insignificant; apparently vol-
canic accumulation proceeded there at a much
faster rate than denudation by glacial or fluvial
agencies. Nevertheless subglacial eruptions are
known to produce large quantities of fragmental
material that is carried away with the meltwater
and redeposited downstream from the site of erup-
tion. Glacial erosion became very effective outsidé
areas of volcanic accumulation. In western Iceland
about 800—1000 m deep valleys have been carved
out in less than 1.8 m.y. and on the Snaefellsnes
Peninsula even in less than 1 m.y. In southeastern
Iceland erosion was even more rapid exceeding 1
km in a million years.
Postglacial time.
The Postglacial series comprises lava flows and
pyroclastics, unconsolidated marine clays, fluvio-
glacial and fluvial outwash and soil formed after
deglaciation of the land area. The oldest Post-
glacial strata occur in western and northern Ice-
land which became uncovered from glaciers before
the Younger Dryas period. The Younger Dryas
glaciation still covered about half of the land area
of Iceland, mainly its southeastern part (Fig. 14).
The span of time represented by the Postglacial is
thus quite different for e.g. the Reykjanes Pen-
insula (11.000—13000 years) and the highlands
southwest of Vatnajökull, perhaps around 9000
years.
Postglacial volcanism continued along the same
pattern as during the last glacial and interglacial
periods. Some 24 volcanic systems have been active
in Postglacial time. Their total lava production has
been estimated as 400—500 km3, and the lavas
cover about 12.000 km2, i.e. over 10% of the surface
of Iceland. About 90% of the Postglacial volcanics
are basaltic and 10% intermediate and acidic, a
similar ratio as found in the older series. Volcanism
was by no means evenly distributed within the
neovolcanic zones. Lava production was highest in
JÖKULL 29. AR 19