Jökull - 01.01.2012, Síða 170

Í. Ö. Benediktsson
described the sediment distribution in the marginal
zone of the 1890 surge and the architecture and sed-
imentary composition of the end moraine, and con-
cluded that the moraine was an inseparable part of a
marginal sedimentary wedge that was formed during
the last few days of the surge. Despite their detailed
descriptions, the structural evolution of the actual end-
moraine ridge remains to be described. The aim of
this paper is therefore to describe the glaciotectonic
architecture and the structural evolution of the 1890
end moraine where it consists of fine-grained sedi-
ments in Kringilsárrani in the central forefield of Brú-
arjökull (Figure 1), and thereby linking the moraine
directly into the ice-marginal landsystem presented by
Benediktsson et al. (2008).
SETTING
Brúarjökull is a surge-type outlet of the northern
Vatnajökull ice cap in Iceland (Figure 1). It descends
from c. 1500 to 600 m a.s.l. and terminates with an
approximately 55 km long ice margin (Björnsson et
al., 1998). Historical surges occurred in 1625, ∼1730,
1775?, 1810, 1890, and 1963-64, giving a surge cycle
of 80–100 years, whereof the active surge phase dura-
tion is only about 3 months (Eythorsson, 1963, 1964;
Thorarinsson, 1964, 1969; Björnsson et al., 2003).
During the last two surges, the glacier advanced 10
and 9 km, respectively, in Kringilsárrani in the cen-
tral glacier forefield, with maximum ice-flow veloci-
ties of at least 120 m/day (Kjerúlf, 1962; Thorarins-
son, 1964, 1969; Guðmundsson et al., 1996). During
the surges in 1810 and 1890, Brúarjökull advanced
further than during previous surge cycles overriding
and deforming a sediment sequence of loess, peat
and tephra (Benediktsson et al., 2008). The area of
Kringilsárrani, in which the central forefield occurs, is
a triangular area bounded by the glacier to the south,
and the glacial rivers Jökulsá á Dal and Kringilsá to
the east and west, respectively (Figure 1).
The Brúarjökull forefield is glacially streamlined
with a 6–7 m thick sediment sequence overlying
basaltic bedrock. The most prominent landforms
of the forefield are end-moraine ridges, ice-cored
landforms and ice-free hummocky moraine, crevasse-
fill ridges, eskers, concertina eskers, and flutings
(Evans and Rea, 1999, 2003; Kjær et al., 2006,
2008; Schomacker et al., 2006; Evans et al., 2007;
Schomacker and Kjær, 2007; Benediktsson et al.,
2008, 2009) (Figure 1). At present, there is negligible
ice movement in the marginal 1–2 km of Brúarjökull
and the snout is rapidly retreating and downwasting
(Kjær et al., 2008).
METHODS
The sedimentology and glaciotectonic architecture of
the 1890 end moraine in Kringilsárrani were investi-
gated in four natural cross-sections that were cleaned
and enlarged by hand. Sediment lithologies and struc-
tures were documented on the basis of the data chart
by Krüger and Kjær (1999) and deformation struc-
tures were described according to the terminology of
Twiss and Moores (1992) and Evans and Benn (2004).
The glaciotectonic architecture was mapped at a scale
of 1:20 and structural elements, such as strike and dip
of primary bedding and fault planes and plunge and
direction of fold axes, were plotted and statistically
analysed in a Schmidt equal-area net with the Spheri-
Stat 2.2 software.
Line and area balancing were applied to the four
cross-sections to calculate the horizontal shortening
of the sedimentary strata and the depth to the dé-
collement plane (Marshak and Mitra, 1988; Bene-
diktsson et al., 2010). By tracing tephra marker hori-
zons through the sections and measuring the horizon-
tal (shortening) distance which they occupy, the dif-
ference (∆L) between their length in the undeformed
(Lu) and deformed (Ld ) states could be determined.
Then the total shortening is described as: s = Ld–Lu
/ Lu. By calculating the total area of the deformed
section (A), the décollement depth can be estimated
by: h = A / ∆L. Then it is assumed that the area of
the body subjected to stress remained constant before
(Au) and after (Ad) deformation. The tephra marker
horizons were visually traced through each section on
the basis of their general appearance and properties
(particularly grain size and colour) and stratigraphi-
cal position. It should be noted that tracing the tephra
markers through the sections often requires inferences
as to where and how different marker segments con-
nect. The inferences made are conservative and there-
168 JÖKULL No. 62, 2012