Jökull - 01.01.2012, Síða 173

Structural evolution of the 1890 Brúarjökull end moraine
Table 1. Sedimentary facies identified in the 1890 end moraine in Kringilsárrani. – Setásýndir í jökulgarðinum
frá 1890 í Kringilsárrana.
Facies Location and description Interpretation References
F1 Section 1
– up to 60 cm thick but discontinuous massive,
silty-sandy to sandy, matrix- supported, clast-rich,
friable diamict. Contact to underlying facies 2 is
sharp and possibly erosional. Clasts are <10 cm
with similar shapes as in underlying facies 2 and
rare or non-preferred striae orientation. The
diamict interfingers with facies 4.
Till
– pre-surge till of unknown age, deposited at the
interface between the ice and an under- lying
sandur surface (facies 2). Clasts from the sandur
were incorporated into the diamict in a
deforming-bed.
Eyles et al., 1983; Krüger and
Kjær, 1999; Benn and Evans, 2010.
F2 Section 1
– massive, clast-supported gravel, with pebbles of
3–8 cm although outsized pebbles of 15–20 cm
occur. Clasts are sub-rounded to rounded,
elongated in shape and often imbricated.
Sandur
– deposited prior to the 1890 surge during
moderate meltwater discharge. Represents a
low-lying terrace formed during a down-cutting of
the Kringilsá river. Genesis is evident from the
surface morphology of sandur terraces around the
section.
Maizels, 1993; Benn and Evans,
2010.
F3 Section 2
– faintly stratified, dark-brown, dark-gray and
ligh-brown layers of fine to coarse sand,
interbedded with thin (1-10 mm) silty layers of
facies 4 and containing some organic matter
(which is often rusty and give yellowish hue).
Contacts to facies 4 are sharp, erosive and defined
by normal faults on distal side.
Shallow pond or stream sediments
– resemble sediments found in channels and
circular lakes (collapsed palsas) proximal and
distal to section 2, respectively, where organic
matter is abundant.
French, 1996; Maizels, 2002.
F4 Sections 0–4
– 0.1–2 cm thick clay, silt and fine-sand laminae of
yellowish to orange and red-brown peat and
light-brown to dark-brown loess interbedded with
tephra. Laminae thickness and strength varies
greatly within different tectonic regimes. The
facies constituents are totally mixed in places.
Contacts between laminae are usually sharp but
may be gradational.
Loess-peat-tephra sequence (LPT)
– cohesive sequence of loess, peat and tephra. Peat
formed from the submergence of vegetated terrain
(mosses and grasses) during loess aggradation.
The vertical accretion of loess and peat was
frequently interrupted by depostion of tephra. The
sequence was formed prior to the 1890 surge.
Brady and Weil, 1999.
F5 Sections 0–4
– generally 0.5–3 cm thick layers of black basaltic
and white to yellowish rhyolithic tephra with <2
mm angular grains. Thickness varies grately and is
the most (<10 cm) in association with folds. The
tephra either occurs in continuous or
discontinuous layers, or as pathces or lenses,
particularly within facies 4.
Tephra
– primary air-fallen (not redeposited), transported
by wind from various Icelandic volcanoes. White
tephra marker is from Öræfajökull AD1362.
minor meltwater outlet of the 1890 surge (Figure 3).
In this area, the 1890 end moraine is generally 70 m
wide and 10 m high with a steep frontal slope and a
hummocky backslope. However, the particular ridge
segment in which section 0 occurs is 15–20 m wide
and approximately 5 m high. Section 0 is 3 m wide
and 2.7 m high and covers the core of the ridge but
is orientated at a 45◦ angle to the ridge strike (Figure
3). Two sediment facies were identified in the sec-
tion: deformed LPT (F4) and tephra (F5) (Table 1).
The section is described from bottom up.
The base of the section, i.e. the lowermost ∼80
cm, is characterized by sub-horizontal, continuous
tephra layers and LPT beds which develop upwards
into discontinuous lenses and patches as they ap-
proach the core of a north-verging recumbent syn-
cline. This is signified by the white Öræfajökull AD
1362 tephra marker. The core of the fold contains oc-
casional tectonic foliation. The upper limb of this syn-
cline forms the lower limb of a north-verging recum-
bent anticline, as revealed by the LPT beds and tephra
layers which are overall thin on the limbs but thicken
JÖKULL No. 62, 2012 171