Jökull - 01.12.1988, Blaðsíða 6

TABLEIII. Geological timescale. The traditional
Icelandic names are given. They need reevaluation
if the conclusions of this article are right.
TAFLAlll. Jarðsöguleg tímatafla. Islensku nafn-
giftirnar í töflunni þarfnast endurskoðunar ef niður-
stöður þessarar greinar reynast réttar.
Age BP (millenium) (þús. ára) English name Icelandic name Climate
13-12 12-11.8 11.8-11 11-10 10-9 Bölling Older-Dryas Alleröd Younger-Dryas Preboreal Kópaskersstig Alftanesstig Saurbœjarstig Búðastig Birkilausa sk. warm cold warm cold warm
(Partly based on Mangerud and Berglund, 1978)
The lowest sedimentary unit in the profile is a semi-
lithified diamicton (unit B). Its matrix is silty to
sandy with angular to rounded pebbles and cobbles.
It is exposed at the water level of Þjórsá at the river-
bend, where it makes up the ledge of a small cas-
cade in the river at low waters. In this unit we have
observed irregular stratification of silt laminae with
gravel clasts. We interpret unit B to be a till deposit.
Its surface is compact and lithified and rich in
Balanus and bivalve fragments. This fauna has
lived on an unconsolidated till surface when the
Búðaberg diamicton began to accumulate. A
radiocarbon date of a Balanus sp. sample from this
stratigraphic position gave the value of 9.855±90 BP
(Table II: Lu-2404).
The till is overlain by an up to 25 m thick silty
diamicton (unit C), which makes up the bulk of the
Búði moraines proper at this locality. We call this
unit the Búðaberg diamicton. The Búðaberg diamic-
ton is a massive to weakly stratified silty to sandy
deposit, rich in volcanic glass, with random gravel
clasts. Occasional lenses of laminated silt and
stratified sand and gravel occur. Kjartansson (1943)
reported shell fragments in the diamicton, but we
only found shells at the contact with the underlying
till deposit. We interpret the Búðaberg diamicton to
Fig. 2. Kjartansson’s (1943) diagrammatic profile
section from Búðaberg. I Basaltic bedrock, II marine
silt with deformed layering, III till, IV stratified silt,
sand and boulders, V soil.
Mynd2. "Jarðlagaskipun Búðaraðarinnar (óná-
kvœmt riss). I basaltklöpp, II sjávarleir með
böggluðum lögum, III jökulruðningur, IV leir (með
óhögguðum lögum), sandur og hnullungar, V mold"
(Guðmundur Kjartansson, 1943).
be an iceproximal/near-shore glaciomarine deposit.
Random dropstones indicate icerafting. The sandy
and gravelly lenses indicate periodic input of melt-
water into the depositional basin.
The Búðaberg diamicton has partly been overrid-
den by a glacier and is intensely deformed and frac-
tured. The deforming push appears to have come
from an easterly direction. It is discordantly overlain
by unit D, an up to 3.5 m thick diamicton, which we
interpret to be till. In the till angular to rounded cob-
bles and boulders are observed in a silty to sandy
matrix. Unit D is compact and semi-lithified. The
sea has transgressed the ridge after the glacier
retreated, and the uppermost stratigraphic units in
Fig. 3 (units E and F) are stratified littoral sand and
gravel. Large erratics on the ridge crest are residuals
from the till.
ÞRÁNDARHOLT (Lu-2403)
The Þrándarholt section is in the northem bank of
Þjórsá, some 2 km downstream of the Búðaberg sec-
tion, at about 60 m a.s.l. The following stratigraphy
was observed in this section (Fig. 4):
The lowest unit (unit A), is exposed in the central
and westemmost part of the section. It is a lithified,
crudely stratified diamicton. Its matrix is sandy to
silty with numerous angular to rounded gravel to
boulder clasts. Its lower contact could not be
observed. It can be traced c. 100 m in the central
part of the section (at around 750 m in Fig. 4). There
4 JÖKULL, No. 38, 1988