Jökull - 01.12.1990, Page 12

analysis of vertical changes, and lateral changes in fa-
cies arrangement over several km. Detailedlithofacies
logging was carried out on 23 sections within the ten
sedimentary units of the Borgarfjörður area, and 19
sections within 9 sedimentary units of the Hvalfjörður
area. Each lithofacies is interpreted as an individual
depositional event defined on the basis of its distinc-
tive lithologic features, including grain size, sorting,
sedimentary structure and composition.
The fabric of sedimentary deposits refers to the
spatial arrangement and orientation of constituentpar-
ticles within the sedimentary unit. The orientation and
dip of the long axes (a-axes) was used as a measure of
preferred orientationof clasts within each sedimentary
deposit and when applicable it was used to reconstruct
pattems of stream flow, ice-flow direction, or mass-
flow direction. Lithologic analysis of diamictite matrix
is another criterion that was used for environmental
and genetic classification of the sediments. The phys-
ical characteristics of a sediment, such as texture and
structure, are directly influenced by local conditions,
whereas the mineral composition is a reflection of
local conditions but also affected by distant and re-
gional influences, depending on the depositional pro-
cesses. Petrographic characteristics of the sediment
matrix can thus give some clues as to the source of the
material, whereas grain size, sorting, grain shape and
grain orientation can be useful criteria to distinguish
between depositional processes. Simple point count-
ing of diamictite matrix was conducted to delineate
gross mineralogy or petrographic differences between
sedimentary units. In addition, a two dimensional ori-
entation analysis of minerals and rock fragments was
compared with clast fabric measurements.
Finally, both remanent magnetization measure-
ments (NRM) and a study of the anisotropy of mag-
netic susceptibility (AMS) was made on core samples
from each stratigraphic section. A combined analysis
of these two magnetic techniques provided a valuable
accessory data set for discriminating shear and flow
mechanism and could also be used to derive flow pat-
terns in conjunction with the fabric data.
Fig. 4 shows schematized the main patterns de-
rived by using each of these methods on samples of
diamictites from Borgarfjörður and Hvalfjörður. The
first example shows the characteristics of a basal till
formed by basal melt-out of ice or by release of de-
bris when pressure-melting point is reached. The di-
amictite is unstratified, but strongly sheared with a
bimodal pebble fabric pattern. It is characterized by
abundant minerals in thin sections and scattered re-
manence magnetic directions (NRM). The orientation
of the anisotropy of magnetic susceptibility (AMS)
shows a preferred direction along the direction of the
first vector calculated for the pebble fabric. The sec-
ond example describes a glacial deposit formed as a
passive melt-out and flow tillite formed at the margin
of the glacier. This deposit is differentiated from the
first example by its coarser and more stratified appear-
ance, greater percentage of rock fragments in the ma-
trix, unimodal fabric pattem and magnetic properties
that display a well clustered magnetic remanence. The
third example shows the characteristics of mass flow
deposits formed on alluvial fans. These diamictites
are in general characterized by thick units of massive
to faintly graded diamictite and conglomerate facies.
The matrix contains abundant rock fragments and vol-
canic glass fragments. The pebble fabric is unimodal
but highly irregular with both high and low dips of
pebble long axes. The magnetic remanence is fairly
well clustered and the anisotropy of magnetic minerals
shows a preferred orientation. The fourth example is
a volcanically induced mass flow and fluvial deposits.
These deposits may consist of thin, unstratified sand-
stone and coarser units of variable grain sizes, which
may be distinguished from the other diamictites by
the abundancy of glass fragments. The pebble fabric
strength is fairly high with the strongest mode suppos-
edly perpendicular to the flow direction. Both the re-
manence and the anisotropy of magnetic susceptibility
measurements are inconsistent and irregular, probably
due to alteration.
THE DIAMICTITES, DEPOSITIONAL AND STRATI-
GRAPHIC SETTING
Most of the late Pliocene strata exposed at Borgar-
fjörður and Hvalfjörður lack direct evidence for depo-
sition in a glacial environment. Classical glacigenic
features such as striated pavement, roches mouton-
nées or striated pebbles are rare. This may be due
10 JÖKULL, No. 40, 1990