Jökull - 01.12.1983, Side 63

The volcanic Rocks of the Sólheimajökull Area,
southern Iceland
D.A. CARSWELL
Department of Geology, University of Sheffield,
Mappin Street, St. George’s Square, Sheffield SIO 2TN, U.K.
ABSTRACT
A succession of six volcanic units has been identified and
mapped in the area adjacent to Sólheimajökull. Most of these
units are thought to be the products of essentially single
eruptive episodes which at least in their initial stages were
sub-glacial. Hence palagonite luffs and breccias dominate
the succession. The volcanic units are separated by major
erosion surfaces characlerised by conglomerates (tillites) on
ice striated pavements, together with local preservation of
sub-aerially deposited tuffs and limited evidence of sub-
aerial weathering. Chemical analyses of a selected suite of
rocks from this volcanic succession show a trend towards
increased alkalinity with time.
INTRODUCTION
This paper describes the results offield mapping
and subsequent petrographic and geochemical
studies of the volcanic rocks exposed in close vicin-
ity to Sólheimajökull. This work was done by the
geology team members of a multidiscipline Earth
Sciences Expedition from the University of Shef-
field which spent some four weeks in the area in the
summer of 1973. The full expedition report, which
also includes the results of botanical, geomorpho-
logical and sedimentological studies, is available on
request.
GENERAL GEOLOGICAL LOCATION
The Sólheimajökull area lies near the southern
end of the eastern limb of the main active volcanic
belt which trends NE-SW across Iceland and out
into the Atlantic via Vestmannaeyjar. The large ice
cap of Mýrdalsjökull and its outlet glacier Sól-
heimajökull covers an area previously mapped
(Kjartansson 1962) as Upper Pleistocene Palagonite
or “Moberg” Formation. However, late Tertiary-
Pleistocene “Old Grey Basalts” were indicated to
outcrop just to the west on the southern side of
Eyjafjallajökull ice cap and somewhat younger
„New Grey Basalts“ in the col area (Fimmvörðu-
háls) between the two main ice caps. Hcnce the area
studied lies between, and slightly to thesouthof, the
Postglacial active volcanic systems of Eyjafjöll
(Eyjafjallajökull) and Katla as outlined byJakobsson
(1979).
THE VOLCANIC STRATIGRAPHY
The rocks in the area are Iargely distinctive yell-
ow-brown palagonite tufTs and breccias with inter-
calated, irregularly shaped, dark grey basalt lobes.
No lava flows were recognised which could be used
as mappable horizons of enable correlations across
the flood plain of the glacier. The recognition and
correlation of major erosion levels over the area is
therefore important in any attempt to work out the
detailed stratigraphy of the volcanic rocks in the
area.
The erosion levels were usually characterised by
conglomerates, interpreted as tillitesor at least fluv-
ioglacial deposits, but sometimes by limited sub-
aerial weathering as evidenced by haematite forma-
tion or by small pockets of sub-aerially deposited
black tuffs, occasionally with distinct Iapilli.
Six volcanic units separated ffom each other by
major erosion levels have been recognised and
mapped (Fig. 1). Details of these units, from oldest
to youngest, are as follows:
Unit A: This volcanic unit outcrops in a small
area at the foot of Jökulhaus North adjacent to the
northern edge of the glacier snout. The exposure
consists of both yellow brown palagonite breccia
and large irregular lobes of more coherent lava.
JÖKULL 33. ÁR 61