Jökull - 01.12.1983, Page 47

material and methods
Profile descriþtions
Interbasalt sediments, and where possible, over-
and underlying lavas, were collected in vertical pro-
files from localities representing geological ages
from Middle Miocene to Late Pliocene (Fig. 1,
Table 1). The profiles were selected away from
penetrating dikes and geothermal fields. The
basalts are little altered by weathering, except for
the upper parts of the flows are often stained red by
weathering.
Approximately 70 samples have been subjected
to detailed mineralogical and chemical investiga-
tions.
The Óshlíð þrofile (Fig. 2 A), of 14-15 m.y. age,
belongs to the oldest laterite-lignite horizon ofVest-
firðir (Fig. 1, zone 1) (Kristjánsson 1973, 1979, pers.
comm.) The lower basalt is deeply weathered with
gravel and core stones in the upper parts. The red
and white colours of the sediment layers reflects
primary basaltic and rhyolitic tephra or welded
tuffs. Other localities within this zone have been
described by Thoroddsen (1896), Bárðarson (1918)
and Pflug (1959).
The Húsavikurkleif þrofile (Fig. 2 B) dates to the
lower part of Anomaly 5 which gives the age 9-10
m.y. The section is associated with a regional sedi-
ment horizon (zone 4, K. Stemundsson 1979, pers.
comm.) and consists of alternating beds of lignite
and argillized tephra. A 5 m thick lava lies on top of
the layered sediments. The locality is classical for
the Tertiary of North Atlantic fossil flora (Ólafsson
1772, Winkler 1863, Heer 1868, Windish 1886, Thor-
oddsen 1896, Bárðarson 1918, Schwarzbach 1955, Pfiug
1959, Friedrich 1968, and Friðriksdóttir (1978, loc.
94). Pseudomorphs of tree trunks buried in the
overlyine lava have been reported by Símonarson et
al. (1975).
The Hreðavaln þrofile (Brekkuá), Borgarfjörður,
consists of more or less weathered tephra, lignite
and limno-fluviatile sediments that deposited 7-6.5
m.y. ago (McDougalletal. 1977). Theprofilesection,
the lignite beds and the plant fossils have been
described in earlier works (Ólafsson 1772, Kierulf
1950, Winkler 1863, Heer 1868, Thoroddsen 1896,
Schwarzbach 1956,Pfiug 1959, Friedrich andSimonarson
1976, Fnðriksdótlir 1978, loc. 118).
The Sámsstaðir þrofile, Hvítá valley, Borgarfjörður
(Fig. 2 C) is located within McDougall et. al. (1977)
profile XII, between lava 258 and 259, ofwhich the
lower has been dated to 3.45 ± 0.3 m.y., i.e. the
upper part of the Gilbert Epoch. The geology of the
area is reported by Jóhannesson (1972), Semundsson
and Noll (1974). Volcanism in the Borgarfjörður
area was virtually continuous between about 7 m.y.
to 2 m.y. ago (McDougall et al. 1977). The sediments
make up about 15% of the column and consist of
thin red beds, fluviatile and lacustrine sediments.
Methods
By combining morphological (optical and elec-
tron scanning microscopical (SEM), X-ray differ-
action (XRD), infra red spectroscopy (IR), chem-
ical analysis of minerals by microprobe and of bulk
samples by X-ray fluorescence (XRF), a series of
minerals and some major chemical changes were
recorded.
The specific weights were determined pyknome-
trically. The water contents (H20+) were determ-
ined as the weight loss by heating the powdered
sample from room tempreature to 110°C over night,
and the loss on ignition (LOI) as the weight loss by
heating from 110°C to 1050°C for 2 hours.
Mineralogical analyses were performed by Phil-
lips XRD for the total sample and the finest frac-
tions (<4/xm). Each sample was analysed airdried,
after ethylene glycolation, and after heating to
450°C, 2 hours. The methods of mineral identific-
ation by XRD followed standard criteria (Brown
1961, Brindley and Brown 1980) with additional refer-
ence to the zeolite d — spacings given by Deer et al.
(1962,1963).
The concentrations of Si, Ti, Al, total Fe, Mn,
Mg, Ca, Na and K in bulk samples were determined
by X-ray fluorescence spectrometry after fusion of
the powdered sample in a lithium tetra borate oxide
flux (Kraeft 1972). Organic carbon was determined
by a „Leco gravimetric carbon determinator“.
All mineral and glass analyses were carried out
on a automatic ARL — SEMQ microprobe. The
sediments and soils were impregnated with epoxy
resin and polished sections made.
The texture and composition of the sediment
were analysed by a Jeol scanning electro micro-
scope.
The IR-spectra was obtained using a Perkin
Elmer 283 infra red spectrophotometer. The identi-
fication was based on characteristic absorption
bands with reference to Farmer (1974), Farmer et al.
(1977, 1978).
By means of IR spectroscopy, one can different-
iate between magnetite, maghaemite, hematite,
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