Jökull - 01.12.1983, Page 67

like Unit D, this unit was erupted over an irregular
topography in which Skógafjall was already a prom-
inent ridge. In the area mapped this unit only out-
crops on the high ground of Skógafjall and due to
the difficulties of access to this ground, the Skóga-
fjallsgil gorge being quite impassable, was not stud-
ied in any great detail.
The “Top” Conglomerate: Considerable erosion
must have occured after the eruption of Unit E
during which the main outline of the present topo-
graphy and drainage pattern was established. In
the area mapped, outcrops of Unit E are only pre-
served on Skógafjall whereas originally this unit
must certainly have extended west of the present
course of the Skógafjallsgil river.
A further glacial advance and retreat over the
area appears to have then led to the deposition and
consolidation of a thick bed of conglomerate, the
so-called “Top” conglomerate, over the area. This
conglomerate is an excellent glacial tillite with
smooth striated boulders, some ofconsiderable size,
in a fine grained, and remarkably hard, rock flour
matrix. The thickest conglomerate is preserved
where it has filled gullies or depressions in the pre-
existing land surface. On the tops of the ridges
between the canyons to the west of the Jökulsá river
and on Eystriheiði, east of the river, the conglomer-
ate forms a fairly uniform flat lying sheet, but it dips
down ofT the southern ends of these ridges and can
be found underneath the thick sandur gravel de-
posits, just south of the present mouths of the
canyons. Thus the present boundary betweeen the
coastal plain and the Moberg Formation mountains
inland must have already been essentially establish-
ed at the time of deposition of the “Top” conglom-
erate.
Unit F - The “Ringing” Ash: A remarkably thick
volcanic ash fall deposit referred to as the “Ring-
ing” Ash, becauseofits resonance when struck with
a geological hammer, is locally preserved directly
above the „Top“ conglomerate on either side of
Hofsárgil and in a large strip between that river and
the Skógafjallsgil gorge. This unwelded volcanic
ash bed is in places 30-40 feet thick, and is perfectly
fresh in its lower portions with dark brown glassy
shards and sparse pumice bombsup toafootacross.
In its present waterlogged state it is rapidly weath-
ering to a red brown colour from its top surface.
There is no direct evidence for the precise site of
eruption ofthis tephrabed but its thicknessand the
size of some of the pumice bombs indicate that it
cannot have been more than a few kilometres away
and doubtless it was connected with the neighbour-
ing Eyjafjöll volcanic centre.
Postglacial volcanic activity: F’or comparative pur-
poses outcrops of more recent Postglacial lavas,
TABLE 1. Chemical Analyses of Lavas and Tephra from the Sólheimajökull Area.
TAFLA 1. Efnagreiningar.
S4 S5 S7 S8 S12 S20 S21 S30 S33 S39
sío2 45.65 47.50 47.08 50.30 45.49 47.98 47.33 48.08 47.37 68.16
TiOj 4.04 3.61 2.76 3.14 1.30 2.72 4.36 4.03 3.89 0.38
ai2o3 12.86 14.99 14.85 14.29 8.16 12.86 12.44 12.79 13.31 13.79
Fe203 5.11 3.56 6.69 1.29 2.60 3.57 4.72 2.74 3.09 1.82
FeO 10.26 9.73 6.83 11.53 7.83 8.34 11.03 12.50 11.47 2.65
MnO 0.23 0.19 0.19 0.24 0.16 0.18 0.23 0.24 0.21 0.06
MgO 5.02 5.33 5.11 4.69 18.49 9.59 4.94 4.72 5.64 0.29
CaO 9.57 10.88 10.82 8.15 11.04 10.00 9.44 9.33 10.20 1.53
Na20 2.59 2.85 2.82 4.18 1.18 2.49 3.23 3.39 3.00 5.27
k2o 0.56 0.66 0.64 1.25 0.34 0.89 0.81 0.98 0.80 3.48
p2o5 0.46 0.36 0.34 0.11 0.13 0.34 0.69 0.69 0.54 0.06
h2o- 1.39 - 0.66 - 0.81 - 0.22 0.04 0.19 -
h2o+ 1.97 0.55 0.55 0.43 2.55 1.07 0.19 0.28 0.36 2.47
Total 99.71 100.21 99.34 99.54 100.18 100.03 99.63 99.77 100.07 100.03
Analyses kindly performed by Mr. V.A. Somogyi in the Department ofGeology of The University of ShefTield, using a
combination of atomic absorption, spectrophotometric, ílame photometric and gravimetric techniques.
JÖKULL 33. ÁR 65