Jökull - 01.01.2004, Side 49

Sediment thickness and erosion rates within Hvítárvatn, central Iceland
reflectors in the seismic record (e.g., Unit 3, Figure
8). The lowest 0.2 m of 02HVT-04 is markedly dif-
ferent from the overlying laminated sequence. The
sediment in this interval is characterized by poorly to
faintly stratified, mottled gray to greenish-gray sedi-
ment. The contact between the gray, finely stratified
stratigraphic unit and the more massive lower unit is
distinct. The lithostratigraphic break corresponds to a
change in the seismic reflections, from a series of thin
reflectors at higher levels, to poorly defined, moder-
ately strong reflectors at deeper levels. We use the
lithostratigraphic break near the base of core 02HVT-
04 to define the Unit 2/Unit 3 boundary. Character-
istic acoustic properties of each unit allow us to trace
this boundary across the sediment fill.
Seismic Unit 2 is characterized by a series of
poorly defined reflectors, bounded by strong reflec-
tors. This unit is the thinnest of the four seismostrati-
graphic units, ranging between 0.5 and 5.9 m. Unit 2
has a volume of 2.6x107 m3 and a mass of 4.4x1013
g (Table 1). Unit 2 thickens toward Fúlakvísl (Fig-
ure 9b), the main stream feeding the Hvítárnes Delta,
suggesting this was the dominant sediment delivery
system during deposition of Unit 2. This interpreta-
tion is supported by the observed thinning of Unit 2
both north and south of the delta.
Seismic Unit 3 is composed of a series of thin re-
flectors, interrupted at random intervals by stronger
reflectors (c.f., Figure 8). Its thickness (1.4 m to
21.3 m) is similar to that of Unit 1. Unit 3 has a vol-
ume of 9.7x107 m3 and a mass of 16.5x1013 g (Table
1). Unlike the underlying units, the sediments of Unit
3 (Figure 9c) are thickest at the north end of the North-
ern Flats, where the inflow stream Fróðá enters the
lake, and thin rapidly toward the south. The streams
feeding the Hvítárnes Delta do not appear to be im-
portant sources of sediment at this time.
Core 02HVT-05 has two main stratigraphic units:
a thickly laminated, brown unit that overlies a more
finely laminated gray unit, similar to the upper por-
tions of 02HVT-04. Ice Rafted Debris (IRD) is
present visibly and in x-rays in the upper thickly lam-
inated, brown unit. The contact between the upper
brown unit and the lower gray unit is distinct. The
change in lithostratigraphic character between these
two units corresponds with a change in the acoustic
signature from weak reflectors with scattered stronger
reflectors, to a dominance of very weak reflectors.
This forms the basis of the subdivision between seis-
mic Units 3 and 4.
Seismic Unit 4 is characterized by well-defined
reflectors, typically more reflective than underlying
Unit 3. Parabolic reflectors, possibly indicative of
dropstones, are locally common in Unit 4, but are not
present in underlying units (Figure 8). This suggests
outlet glaciers were calving into Hvítárvatn during de-
position of Unit 4, but not earlier. This interpretation
is supported by x-radiography of cores 02HVT-04 and
02HVT-05 in which small stones (IRD) are apparent
only in the upper 3 m of 02HVT-05. The thickness
of Unit 4 ranges from 1.4 m to 21.0 m, similar to
Unit 3; it has a volume of 8.1x107 m3 and a mass
of 13.6x1013 g (Table 1). The isopach map of Unit 4
shows a very different distribution than for underly-
ing Unit 3 (Figure 9c,d). The major sediment source
shifted to the Hvítárnes Delta, with sediment deliv-
ered to the lake along the entire length of the delta
front, presumably indicating a period of active delta
progradation. Sediment thickness drops quickly both
north and south of the delta. The river Fróðá was not a
major source of sediment at this time. Sediment deliv-
ery also was not from a point source as it was during
deposition of Unit 2. The Central Deep received a
larger input of Unit 4 sediment than of Units 2 or 3.
The isopach map of Unit 4 sediment is used to
infer the rapid progradation of the Hvítárnes Delta.
To test this interpretation we positioned sediment core
02HVT-03 near the base of the delta front, where the
acoustic signal suggested common slope failure along
the slip face of the delta (Figure 7). The upper 2.3 m
of the 5.1 m long core are characterized by disturbed,
finely stratified sediment. This level of disturbance is
consistent with frequent, thin, sediment-gravity flows.
The lower 2.8 m of the core are finely laminated with
minimal disturbance. We interpret the core to reflect
the progradation of an unstable delta slip face over
the core site during deposition of Unit 4, consistent
with the seismic interpretation of a rapidly prograding
delta with heavy sediment loads and regular instabil-
ity along the delta front.
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