Jökull - 01.12.1990, Síða 5

Diamictites of late Pliocene age in westem Iceland
Áslaug Geirsdóttir
Science Institute, University oflceland,
101 Reykjavík, Iceland
ABSTRACT
The sedimentology and stratigraphic relations of
several late Cainozoic diamictites from localities in
the Borgarfjörður and the Hvalfjörður areas, western
Iceland is reviewed. Attention is drawn to the impor-
tance offacies analysis of sediments in environmental
interpretation of ancient rock successions. A multi-
criteria approach was used to determine the origin
°f the diamictites. The diamictites represent diverse
environments of deposition that can be attributed not
only to glacial activity, but also to volcanism and mass
flow deposition.
Important questions about the successions con-
cern the number and stratigraphical arrangement of
glacial horizons. Lithological analysis identifies five
glacial units in the upper Borgarfjörður area and at
least six glacial units are recognized in the Hvalfjörð-
ur area. The oldest glacial deposit has an approximate
oge of 2.6 Ma and the five glacial units in the Borg-
orfjörður area were deposited during a time-span of
500,000-600,000years. Two ofthe glacial deposits in
the Hvalfjörður area were presumably formed during
late Gauss magnetic epoch, all others are correlated
with the Matuyama epoch. A tentative correlation is
made between the glacial deposits in Borgarfjörður
and the glacial deposits in Hvalfjörður area. The re-
sults imply more varied depositionalprocesses for the
origin of the diamictites in Borgarfjörður and Hval-
fjörður area and fewer and perhaps less extensive
glaciations during late Pliocene than previously as-
sumed.
INTRODUCTION
Geologic mapping and geochronologic studies
suggest that many stratigraphic sections within the
late Pliocene rocks of Iceland (ca. 3.1 Ma to 1.8 Ma)
arenearly continuous (e.g. Sæmundsson, 1980,1985).
They consist of alternating basaltic lava flows in-
terbedded with sediments of diverse appearance and
origin which contain evidence of physical processes
responsible for their formation, climate and deposi-
tional setting. Such stratigraphic successions offer a
great opportunity to work out a detailed paleoenviron-
mental reconstruction. However, until quite recently,
the principal emphasis has been on the geochronology,
the mineralogy, geochemistry and magma genesis of
the lava flows, with a simple interpretative description
of the interbedded sediments. Based on their non-
sorted appearance, the interbedded sediments have fre-
quently been regarded as subglacial in origin implying
more frequent major glaciations than found elsewhere
on land in the northem hemisphere.
The differentiation of diamictites is a particular
problem in Iceland where glacial and volcanic pro-
cesses work both interactively and separately. Due to
the ubiquitous volcanic source rock, diamictites such
as till, tillites, and other coarse-grained but non-glacial
deposits (e.g., fluvial gravels, lahars and autoclastic
and epiclastic volcanic sediments) may have similar
field appearance. A further complication is the inter-
action of volcanic activity with glacial processes. Var-
ious deposits can be ascribed to either one or a combi-
nation of volcanic and glacial processes. In addition,
diamictite units may be taken as either representative
of an ice cap cover or a succession of mountain glacia-
JÖKULL, No. 40, 1990 3