Jökull - 01.12.1983, Blaðsíða 108

Fig. 7. Tephra layer in section
B. The topmost part of rhyth-
mic bed X is seen at the base
of the figure. Note the over-
turned folds in the rhythmic
bed and in the tephra bed.
Mynd 7. Líparítgjóska í sniði B.
Efsti hluti syrpu X sést neðst á
myndinni. Yfuteygðar fellingar
sjást vel í hvarfleirnum og í gjósku-
laginu.
an important factor in the formation of summer
layers. This has been confirmed by several authors
(Mathews 1956, Gustavson 1975, Gilbert 1975). The
turbidity currents are primarily generated by sedi-
ment laden meltwater plunging beneath lake water.
Gradational transition from the silts to clay layers is
caused by settling from suspension at the end of the
melt season.
Agterberg and Banerjee (1969, p. 647) provided a
summary on the genesis of glacial varves and noted
that deposition of fine sediment characterizes the
winter season:
“The clay (winter) layer consists of two parts:
Part 2a deposited by the turbidity current after
stagnation, and part 2b deposited by slow, con-
tinuous settling from suspension.”
Shaw et al. (1978) have noted that the winter
season is not only characterized by slow settling
from suspension because thin silt laminae some-
times occur in the winter component of varves.
They conclude that slope failure at the delta front is
favored as the cause of interfingering coarse layers
in the clay parts. Possible triggers for such failures
include periods of high runoff related to winter
storms or jökulhlaups, disturbance by glacier or
lake ice, and reduced sediment strength related to
groundwater seepage under low stage ofwinter.
These descriptions indicate that the rhythmites
of the Skagafjall deposits are in most cases of an
annual nature (that is, they are varves). Thin clay
laminae occasionally occur in the silt (summer)
layers. They have probably formed in the melting
seasons on occasionally cold days when little or no
meltwater was entering the lake. Thin silt laminat-
106 JÖKULL33. ÁR