Jökull - 01.12.1990, Side 11

noted by Sæmundsson and Noll (1974) was the pres-
ence of extensive hyaloclastites among two of the in-
ferred glacial horizons. This suggested to them that
most of the glaciations were of short duration, perhaps
over an interval of 5,000-10,000 years. Their criteria
for identifying the diamictites as glacial deposits was
based largely on the unsorted nature of the deposits,
and their association with fluvial sediments.
Pjetursson (1904) describedthe stratigraphy north-
east of Hvalfjörður and the interbedded diamictites.
He described all of the diamictites as ”moraines“, on
the basis of striations on the underlying basalts, stri-
ated boulders within the diamictites, and the unsorted
nature of the sediments. Einarsson (1946) argued
against Pjetursson’s interpretation. He pointed out
numerous features, such as volcanic matrix, rounded
clasts and conformable bed-contacts, which he thought
presented evidence against a glacial origin. In-
stead he favoured a high-temperature mudflow ori-
gm. Rutten (1958) disputed both previous explana-
fions, but pointed out that the structures of some of the
sediments were not explicit as to their origin. They
might be tillites, but such deposits could also form as
mudstreams or lahars (Rutten, 1958). He proposed
a fluvial origin for some of the sedimentary units and
mterpreted others as re-cemented scree deposits. Both
Einarsson (1946) and Rutten (1958) were unsuccess-
ful in finding glacial striations on the contacts between
diamictites and basalt-flows, they thus favoured either
a fluvial or a mass-flow origin.
In the Hvalfjörður area a glacial origin for the di-
amictites has been favoured because of their unsorted
appearance and association with sheet-like hyalo-
dastite deposits (Gunnlaugsson et al., 1972; Jónas-
son et al., 1973; Arason et al., 1974; Guðmundsson,
1976). Altogether eleven glacial horizons were de-
scribed, and the existence of two more were suggested
further south in the Akrafjall-Esja area (Friðleifsson,
1973, 1985; Kristjánsson et al., 1980). The four low-
ennost units are described as ill-sortedconglomerates,
while all the other units represented tuffaceous hyalo-
clastite and or pillow breccias. Kristjánsson et al.
(1980) correlated the lowest diamictite horizon with
the 3.1 Ma diamictite unit in the Borgarfjörður area.
THE SEDIMENTOLOGY OF DIAMIC-
TITES IN THE UPPER BORGARFJÖRÐ-
UR AREA AND THE HVALFJÖRÐUR
AREA
The principles of sedimentology imply that un-
der similarprocess conditions, similar deposits should
be formed. Similar process conditions must be inter-
preted as a function of at least two independent vari-
ables. One of these is the physical environment of the
area where deposition takes place, and the other is the
nature of the sedimentary material supplied. However,
various process conditions may also form similar de-
posits especially where the nature of the sedimentary
material supplied is from a fairly homogeneous lithol-
ogy as in Iceland. In such situations it is necessary to
apply several criteria in order to detect differences in
the deposits and to distinguish between the sedimen-
tary processes.
METHODS
In the study of the diamictites of the upper Borg-
arfjörður area and the Hvalfjörður area, a distinction
was made between deposits of two environments in
particular, the glacial environment and the mass flow
environment. The differentiation was based on litho-
facies analyses, pebble fabric measurements, textu-
ral studies, rock magnetic measurements, both nor-
mal remanent magnetism (NRM) and anisotropy of
magnetic susceptibility (AMS), as well as a compar-
ison with analogues from modern depositional envi-
ronments (Geirsdóttir, 1988).
The application of lithofacies analyses to sedimen-
tary deposits and investigation of facies associations
has recently gained considerable attention. Such an
approach can provide the basis for the genetic study of
both glacial and volcaniclasticrock (Eyles et al., 1983;
Anderson, 1983; Eyles and Miall, 1984; Shultz, 1984;
Shaw, 1987; Smith, 1987; Eiríksson et al., 1987).
Emphasis is most often placed on vertical logging of
sedimentary sections where the associations of certain
facies types are thought to be the best evidence for the
origin of the unsorted sediment. In both the Borgar-
fjörður and the Hvalfjörður areas, investigation of the
interstratified diamictites permits the recognition and
JÖKULL, No. 40, 1990 9