Jökull - 01.01.2014, Page 56

J. Helgason and R. Duncan
Borgarfjörður. The Borgarfjörður sequence, W-
Iceland is unique in its thorough recording of mag-
netic reversals where the study of McDougall et al.
(1977) added the Þverá and Síðufjall subchrons to the
geomagnetic time scale. The Gauss sequence in Borg-
arfjörður is about 435-m-thick but only 234-m-thick
in Hafrafell. The Borgarfjörður sequence consists of
subaerially erupted lava flows intercalated with 2 to
3 glacial horizons. In Borgarfjörður pillow basalts
first occur well above Gauss or in strata with an age
of about 1.6 Ma (McDougall et al., 1977) wheras in
Hafrafell the oldest pillow basalts have an age over 3.2
Myr. In the Borgarfjörður sequence the first 8 glacials
have an age range of between 2.8 to 1.6 Myr and ap-
pear sharply intercalated between lava flows without
any noticeable relief. In Hafrafell, strata belonging to
Gauss time have a total thickness of only 234 m com-
posed of: subaerially erupted lavas (169 m, 26 flows),
sediments (34 m), and pillow basalts (31 m) with just
two clear examples of glaciations and clear erosion
surfaces that cut unconformably the underlying lava
sequences.
What has caused the great difference in Gauss-age
volcanic stratigraphy between these two regions? At
least two factors could contribute to the observed dif-
ferences, namely:
(a) different accumulation rates (or volcanic pro-
duction). The comparison shows that the magne-
tostratigraphy in Borgarfjörður is more complete in
containing a number of subchrons that are not present
in Hafrafell, namely the reverse Kaena and Mammoth
subchrons within Gauss in addition to the normal
Jaramillo and Reunion subchrons of the Matuyama
chron higher up in the composite section. This would
suggest that even if intervals of intense erosion had
occurred in Hafrafell, some remains of the subchron
strata would still be present as erosion tends to be het-
erogeneous in valleys and thus leaving behind the val-
ley walls. As this is not the case it is reasonable to
conclude that the two times thicker Gauss section in
Borgarfjörður is due to higher accumulation rates, be-
ing closer to a mature rift zone and more complete
recording of the geomagnetic polarity time scale.
b) axial rift zone versus rift flank environment.
The strata in Hafrafell (Figure 9) consist of lenses of
lavas, sediments and breccias that are of much smaller
dimensions, than seen at Borgarfjörður. There is a
strong indication that strata in Hafrafell were built
by progradational oblique lenses toward south during
Gauss time. Clearly, considerable relief was present
in Hafrafell while gentle lava plains with rivers and
lakes appear to have characterized the Borgarfjörður
section.
Considering these two factors we conclude that
volcanic production/accumulation rates (McDougall
et al., 1977) were indeed much higher during Gauss
time in Borgarfjörður than in Hafrafell. In Borgar-
fjörður the greater distance from the main ice sheet
favoured greater preservation of the stratigraphic
record compared to Hafrafell. Subsidence and burial
within the accreting rift in Borgarfjörður also con-
tributed to better preservation of the strata, a charac-
teristic that did not apply to the Hafrafell region. Most
likely, a large portion of the magmatic production in
SE Iceland during Gauss time, and other intervals as
well, formed intrusions (Walker, 1975) whereas in
Borgarfjörður the magmas reached the surface and
produced lavas.
The Gauss-age sequences in Hafrafell and Borgar-
fjörður both have glacial strata. At Hafrafell we ob-
serve a well-defined glacial deposit near the mountain
base, of age close to ∼3.6 Ma. On the other hand the
oldest glacial in Borgarfjörður is no older than ∼2.8
Myr. We note that for both areas only two to three
glacials are found in the Gauss sequence. Three fac-
tors may explain the different stratigraphic thickness
of the two Gauss sequences, namely that much greater
erosion took place in the Hafrafell area that caused
hiatuses there and smaller volcanic production caused
slower accumulation rates. Thirdly, the early onset of
glaciation in Hafrafell, at ∼3.60 Ma, compared with
∼2.80 Ma in Borgarfjörður, led to greater erosion dur-
ing Gauss time in Hafrafell.
Fljótsdalur. Geirsdóttir et al. (2007) show five
glacials during the Gauss magnetic chron in Fljóts-
dalur, E Iceland, some 120 km NNE of Hafrafell.
Thus it is likely that glaciers there would simultane-
ously have occupied the Hafrafell area. However, the
relatively thin stratigraphic Gauss sequence and as-
sociated erosion surfaces in Hafrafell, with only two
56 JÖKULL No. 64, 2014