Jökull - 01.12.1990, Page 16

older debris flows. The pebble fabric is unimodal and
the remanent magnetization is well clustered.
The upper bed (diamictite unit 6) shows texture
and structure different from that of the lower bed. It
display s a clear fissility structure particularly in the up-
permost 1 m and more rounded clasts than the lower
bed. The pebble fabric is bimodal and the remanent
magnetic pattem is very dispersed. These are two fea-
tures that are related to pressure melt-out and lodge-
ment of debris from ice. Glaciofluvial deposits are
also associated with this glacial unit. This upper bed
marks the second glacial unit in the Borgarfjörður sec-
tion (Fig. 6b).
Additional 100-150 m of olivine tholeiite lavas
formed prior to the deposition of the next glacial unit
(diamictite unit 7) in the Borgarfjörður section. This
is a basal tillite, with bimodal pebble fabric, dispersed
remanent magnetization and heterogeneous lithology.
This tillite, however, is in places less well preserved
than the stratigraphically lower glacial units, but it
is followed at one outcrop by cross-bedded fluvial
sediments and at another by both fluvial sediments
and up to 15 m thick plane bedded lake sediments (in
Ásgil, Fig. 6c).
The next diamictite unit (unit 8), which is sep-
arated from the third glacial unit by olivine tholei-
ite lava flows, is only found in one gully (Ásgil,
Fig. 2) as a thin, approximately 2 m thick debris bed.
This sedimentary unit has previously been correlated
with hyaloclastite formation within the central volcano
(Sæmundsson and Noll, 1974).
The fourth glacial deposit (diamictite unit 9) in the
upper Borgarfjörður area marks the culmination of the
central volcano (Sæmundsson and Noll, 1974). It is
associated with an extensive hyaloclastite sheet in the
E and SE, but shows all characteristics of lodgement
tillite with strong shear features, bimodal pebble fabric
and dispersed remanent magnetization (Fig. 6d).
Between the fourth and the fifth glacial units there
is an unconformity of 300,000- 600,000 years dura-
tion (Sæmundsson and Noll, 1974; Albertsson, 1976).
The fifth glacial unit marks another glacial advance
accompanied by the formation of basal tillites and
hyaloclastites more extensive than any of the strati-
graphically lower deposits.
THE DIAMICTITES OF THE HVALFJORÐ UR AREA
Although the stratigraphic sequence of Hvalfjörð-
ur is not as well known as that of Borgarfjörður, its
identical location relative to the rifting zone suggests
a similar depositional history. However, the two ar-
eas differ in the amount and lateral extent of hyalo-
clastites which are very prominant in the Hvalfjörður
area. Acid volcanism and ignimbrite deposition are
more characteristic for the Borgarfjörður area. The al-
temating hyaloclastite sheets, lavas and sedimentary
rocks in the Hvalfjörðurstratigraphy suggest repetitive
eruptive and sedimentary depositional events.
The lowermost diamictite beds in Hvalfjörðurbe-
longing to the time interval discussed here, indicate a
huge amount of sediment supply within the area. The
formation of extensive normally graded beds, current
formed sedimentary structures (mainly cross-bedding)
and rounded cobbles are the principal evidence for
supporting a fluvial or even mudflow origin for the
first diamictite unit in Hvalfjörður. A thickly stratified
and cross-bedded diamictite preserved in the Akrafjall
mountain implies that this region was at least partially
ice-free and received large amounts of waterlain de-
bris. The common occurrence of heterogeneous as-
semblages of diamictite, mudstone and siltstone in-
tercalations and frequent local cross-bedding, indicate
that sedimentation was not continuous.
Approximately 120 m of tholeiitic and olivine
tholeiitic lavas separate the first and second diamic-
tite units in Hvalfjörður. This second unit forms an
impressive cliff along the coastline of the fjord Hval-
fjörður, and is interpreted as a localized succession of
flow units based on its overall stratigraphic framework
and composition. The diamictite which in places rests
on stratified fine-grained sandstone and conglomer-
ate, displays a facies association consisting of mas-
sive, matrix supported diamictite overlain by clast
supported diamictite intercalated with finer grained
waterlain sediments and sediments of fluvial origin.
The overall appearance of this section suggests con-
tinuing sedimentation with occasional interruption of
single lava flows. Pebble fabric from this exposure is
inconsistent and in general shows a unimodal pattern.
Post-depositional tectonic and igneous activity is indi-
cated by basaltic dykes and mud dykes that cut through
14 JÖKULL, No. 40, 1990