Jökull - 01.12.1988, Blaðsíða 12

Highway 1. The sediments are massive to stratified,
silty to sandy diamicton with erratics up to 1-2 m3.
The outcrop is rich in shells of many species (Table
IV). The surface above the section contains a
number of large erratics, probably washed out from
the diamicton by wave action during the postglacial
marine regression. The diamicton could be a till
deposit. We have not collected any shells from this
locality, and it needs to be studied in more detail.
YTRI-RANGÁ AT BJARG (Lu-2406)
The Ytri-Rangá river has eroded a shallow
depression in the Rangárvellir sandur. Sedimentary
sections are only exposed at few locations in its
banks, and outcrops with shell-bearing sediments
are rare. Kjartansson (1958) described a shell local-
ity near the waterfall Ægissíðufoss, but this we were
not able to find. Another section with shell-bearing
sediments was discovered in the riverbanks below
the farm of Bjarg, opposite to Hella at 30-35 m a.s.l.
(Fig. 8). The following stratigraphy was observed
(Fig. 9):
A. Basaltic bedrock that can be traced to the
Ægissíðufoss waterfall.
B. Laminated silt and fine sand, loose to compact.
We interpret this to be a glaciomarine sediment.
The sediment input was probably from meltwater
streams and sedimentation has been from suspen-
sion and bed-load transport. The lamination could
indicate periodic influx of sediment or reworking
of the sediments themselves.
C. Incoherent unit of fine to coarse sand lenses,
resembling ripple drift lamination. This unit con-
tains fragmented shells (Table IV). The shell frag-
ments make up 10-20% of the volume of this unit.
Individual lenses are generally 20-50 cm long and
5-15 cm thick. The shell-bearing horizon can be
followed some 100 m along the exposure near the
water well of the Bjarg farm.
We interpret this horizon to indicate energy
input in the depositional environment and rework-
ing of the sediments. The radiocarbon age of the
shell fragments is 10.015+90 BP (Table II: Lu-
2406), which is a maximum age for the sediments.
ES! 2
mnD 5
ÍLu-2601| 7
© 8
88.09.0520
Fig. 9. The stratigraphy at Ytri-Rangá at Bjarg,
opposite to the Hella village. 1) Soil. 2) Aeolian
sand. 3) Glaciofluvial sand. 4) Gravel. 5) Basaltic
bedrock. 6) Laminated silt. 7) Dated sample.
8) Shells.
Mynd 9. Jarðlagasnið við Bjarg hjá Ytri-Rangá.
1) Jarðvegur. 2) Foksandur. 3) Vatnsborinn sandur.
4) Möl. 5) Berggrunnur. 6) Silt. 7) Aldursgreint
sýni. 8) Skel.
D. Laminated silt and fine sand as in unit C.
E. LFnit D is conformably overlain by a 4 m thick
unit of stratified to laminated silty sand. We inter-
pret this unit as marine pro-delta sediments. The
unit is compact to semi-lithified. It conducts
groundwater and several springs appear at its sur-
face.
F. Unit E is unconformably overlain by a 4 m thick
bed of cross-stratified gravel and sand, interpreted
as a glaciofluvial delta deposit.
G. This unit is a horizontally stratified gravel depo-
sit, probably topsets to the cross stratified delta
sediments.
H. The section is topped by aeolian sand and
covered by up to 2 m thick soil.
This section reflects the postglacial isostatic
rebound of the lowlands and the marine regression.
10 JÖKULL, No. 38, 1988