Jökull - 01.11.1998, Page 6

naissance field observations and measurements from a
single soil profile, containing historic and pre-historic
tephra layers. This paper presents new tephro-
chronological and radiometric age data that show un-
equivocally the age of the Leiðólfsfell cone group and
thereby its origin. A full account on the geology of the
cone group will be presented elsewhere.
LEIÐÓLFSFELL CONE GROUP AND
LAKI LAVA FLOW
The Laki lava flow almost completely covers the
floor of the river valley to the north and west of Mt.
Leiðólfsfell. It is composed of two distinct morpholog-
ical lava types; the ‘a‘a-like main lava stream which
branches into an association of relatively small slab
pahoehoe flow units (for definition see Wentworth and
Macdonald, 1953). The boundary between the ‘a‘a and
pahoehoe is a 10-20 m high barrier, delineated by the
largest scoria cones and ramparts of the Leiðólfsfell
cone group, now forming a 300-400 m wide stretch of
discontinuous outcrops along the edge of the main lava
stream. These outcrops extend from the NW foot of
Leiðólfsfell towards Hólahaft in the SW, where they
turn lo the NNW across the valley in the direction of
Sæmundarsker (Fig. 1). The outcrops reveal erratic as-
sociations of partly overlapping scoria cones and ram-
parts (50-300 m in basal diameter) and numerous small
explosion craters (5 to 30 m in diameter), which are the
surface manifestation of a larger cone field, which now
is largely buried by the lava. The main lava stream is
piled up against the up-valley flank of the cone field
with a main outflow channel in the north. Elsewhere at
points of low elevation it barely extends over the crest
of the cone field. The slab páhoehoe flow units, howev-
er, formed as multiple breakouts of lava from the edge
of the main lava stream. These almost completely
cover the down-flow side of the cone field with the ex-
ception of several kipukas (i.e., óbrynnishólmi) which
are mantled by the scoria fall deposit produced by the
explosive activity of the Leiðólfsfell cones (Fig. 1).
Outcrops along the edges of these kipukas, and at a re-
cent quarry, show that the Laki lava rests directly on
the scoria (Fig. 2). Nowhere did we find any evidence
of soil, other sediments, or remains of vegetation be-
tween the two.
THE HISTORIC TEPHRA SEQUENCE IN
SOIL PROFILES AROUND
LEIÐÓLFSFELL
The Leiðólfsfell scoria deposit and other historic
tephra layers were examined at outcrops within the
kipukas and in soil banks in the area to the south of
the Laki lava flow field. Fifteen profiles were mea-
sured, documenting thicknesses and other distin-
guishing features of the tephra deposits (Fig. 1). With-
in the largest kipuka in the center of the flow field
(Location 1 on Fig. 1), the Leiðólfsfell scoria fall de-
posit is 1-3 m thick and rests unconformably on an
eroded soil horizon containing prehistoric tephra lay-
ers (see Fig. 3 in Jónsson, 1985). However, further
south at Hólahaft (Location 2 on Fig. 1), the scoria
deposit is 1.1-1.7 m thick, and resting directly on soil
contaning numerous tephra layers and is covered by
several cm of topsoil containing the 1918 Katla tephra
layer. The soil below the Leiðólfsfell scoria contains
both historic and pre-historic tephra layers, including
the Settlement layer (AD -870; Grönvold et al., 1995)
which marks the boundary between the two time peri-
ods (Fig. 3).
At Location 2 the Leiðólfsfell scoria deposit con-
sists of eight 10-35 cm thick fall units of dark gray to
black tephra, although the top two units are reddish-
brown due to high contents of oxidized clasts. Each
unit consists of poorly sorted, fine to coarse scoria
lapilli and are inversely graded (Fig. 4). Scattered 6-
30 cm poorly vesicular scoria and spatter bombs
occur near the top of each unit and these bombs often
have breadcrust surfaces. The clast population is pre-
dominantly comprised of dense (<30 vol.% void
space) juvenile lapilli (85%) and hypocrystalline lava
crust fragments (15%). Armored bombs are common,
featuring a core of vesicular páhoehoe crust frag-
ments and an armor of poorly vesicular glassy lava.
The major element concentrations of the core and
armor are identical (Table 1) and the composition of
the Leiðólfsfell scoria is cognate to that of the Laki
lava (Þórðarson et al., 1996). The Leiðólfsfell scoria
rests on a 0.3-0.5 cm thick strombolian ash layer con-
taining Pele's hair and glassy clasts with fluidal outer
surfaces and highly vesicular interior (60-90 vol.%
voids). Chemical analyses by electron microprobe
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JOKULL, No. 46, 1998