Jökull - 01.12.1987, Side 67

ment debris flows or slumps off an ice front and/or
submarine morainal bank were deposited as vaguely
defined lobes or lenses together with stratified sands
from meltwater streams.
The deposition by flow mechanisms of glacigenic de-
bris in the ice-proximal glaciomarine and glaciolacus-
trine environments has been discussed by various au-
thors during the past few years, and the existence of
subaqueous deposits of an origin similar to Boulton’s
(1968) “flow tills” and Lawson 's (1979) “sediment flows”
is well established (May 1977, Evenson etal. 1977, Drei-
manis 1979, Hicock et al. 1981, Cheel and Rust 1982,
f>oive//1983a,1983b,Z)omacZ:1983, Gravenoretal. 1984,
Broster and Hicock 1985, Eyles et al. 1985). The charac-
teristics of glaciogenic subaqueous debris flow deposits
include unit geometries, deformation structures result-
mg from penecontemporaneous slumping and liquefac-
tion, flow structures formed during deposition, and con-
tent of glacially abraded clasts. Gravenor et al. (1984)
stated that in most cases it is difficult to determine if a
debris flow originated directly from ice surface or from a
break in the paleoslope or an unstable pile-up of debris
in the proglacial environment. They suggested that such
flows should be called “glaciogenic subaquatic debris
flows”, indiscriminately of flow origin.
The Asbakkar diamicton, a discussion:
The maximum thickness of the Ásbakkar diamicton is
35-40 m. Radiocarbon dated shells place its deposition
between about 12.500 and 12.00014C years BP (Table III,
samples R, to Rs). The radiocarbon dated shells give
decreasing ages upwards in the sequence, and from
them sedimentation rates of 0.54 x 10 m • y'1 to
3.6 xL0': m • y'1 can be calculated for the lower, fossili-
ferous part of the sequence. This can be compared to
average Holocene glaciomarine sedimentation rates for
the Gulf of Alaska shelf, 0.45 x 10': m • y'1 (Molnia
1983), and 5 x 10'2 m ■ y'1 to 1 x 10'1 m ■ y--1 for the inner
basin of Kongsfjorden, W-Spitsbergen (Elverhöi et al.
1983). The Ásbakkar diamicton is bounded upwards by
a glacial unconformity except in the southern part of the
cliffs, from ca. 3650 m, where it is partly conformably
overlain by the Ás beds (see later). The thickness of the
Ásbakkar diamicton in the section ranges from a few
cm to about 25 m. It is compact and sometimes lithified.
A thin section study showed a tendency of sideromelane
table III.
Radiocarbon dates of subfossil marine shells collected from the Melabakkar-Ásbakkar sequence.
Tafla III. Geislakolsaldur fornskelja úr Mela- og Ásbökkum.
Location no. (Fig. 2) Sample name m above sea level 14C date, years BP Species Reference no of date
R, Melabakkar-Melaleiti 2 12.465±110 Chlamys islandica (Muller) Lu-2193
R, Melabakkar-Melaleiti 2 4 12.095 ±120 Chlamys islandica (Muller), Mya truncata (Linné) Lu-2192
r3 Ásbakkar 1 2-3 12.505±110 Chlamys islandica (Muller) Lu-2195
r3 Ás 23-25 12.015± 110 Chlamys islandica Lu-2379
r5 Ásbakkar 3 3-4 11,945± 110 Chlamys islandica (Muller), Hiatella arctica (Linné), Balanus ssp. Lu-2377
r6 Ásbakkar-Ásgil 1 2-3 11.615± 130 Chlamys islandica (Muller), Mya truncata (Linné), Buccinum spp., Balanus spp. Lu-2196
r7 Ásbakkar-Ásgil 2 3-4 11.715 ±120 Chlamys islandica (Muller), Mya truncata (Linné), Hiatella arctica (Linné), Astarte elliptica (Brown) Lu-2372
r8 Ásbakkar-Ásgil 3 3-4 11.545 ±140 Chlamys islandica spp., Balanus spp. Lu-2373
R9 Ásbakkar 2 6-8 11.465±100 Mya truncata (Linné), Hiatella arctica (Linné) Lu-2376
rio (Scattered fragments) Melabakkar-Melar 1 3-20 11.985±120 Chlamys islandica (Muller), Mya truncata (Linné), Balanus balanus (Linné), + un- identified fragments. Lu-2375
All samples are referred to 0.95 NBS oxalic acid standard, using the value of 5568 for the half life of 14C. The base year is 1950.
Corrections for I3C/12C ratios and apparent age of living marine organisms have been made for all samples, using the reservoir age 365
± 20 years (Hákansson 1983). Samples R„ R,, R3 and R6 were described and discussed by H&kansson (1984).
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