Jökull - 01.01.2016, Page 55

The marginal zone of Tungnaárjökull, Iceland
Figure 2. The northern study area. A: Geomorphological sketch; B: Panoramic view. – Jarðfræðikort og yfir-
litsmynd af nyrðra rannsóknasvæðinu, vestan Kerlinga.
The sedimentary sequence deposited within the
ice-dammed lake was examined in a 2.5 m deep ex-
cavation dug into the flat upper surface of a bench
marking a former palaeoshoreline approximately 3 m
above the present lake level (Figure 3A). The base
of the sedimentary sequence is marked by a layer of
massive, matrix-poor, coarse gravel (Gm), showing
the characteristics of a boulder pavement (Figure 3B).
The gravel is overlain by a c. 0.95 m thick sequence
of fine-grained, horizontally bedded silty sands (Sh,
SFh), which contains a c. 10 cm think bed of silt (Fw)
of wavy lamination. Between 0.9 and 1.55 m depth,
the sequence is represented by gravel–sand–silt litho-
facies (GSi, SGi, Si, SFi) characterised by a well-
developed inclined stratification which dips at c. 30◦
towards 310◦ to 330◦ and is overlain by a c. 20 cm
thick layer of horizontally stratified coarse gravel and
sand (GSh) (Figure 3B) and an erosive base which
can be clearly seen to truncate the inclined stratifica-
tion within the underlying sediments. The gravels are
overlain by a several-cm-thick layer of massive to hor-
izontally laminated silt (Fh, Fm) which coarsens up-
wards. The upper 60 to 70 cm of the sequence is com-
posed of interbedded sand and silt (Sl, Fl) which pos-
sess a low-angle (c. 10◦) cross-stratification recording
a palaeoflow direction towards 290◦. The top of the
sedimentary sequence is truncated by a marked ero-
sional surface.
JÖKULL No. 66, 2016 55