Tidewater glaciers

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21 Φεβ 2014 (πριν από 3 χρόνια και 3 μήνες)

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A2.3GQ3 Glacial and Quaternary Geology


LECTURE 5



TIDEWATER GLACIERS

2

SUMMARY



Introduction


Dynamics of tidewater glaciers


Fjord
-
based deposition


Ice frontal sedimentation



Proximal sedimentation


Distal sedimentation



Fjord
-
based facies associations


Introduction

4


At the present day many glaciers terminate at
sea
-
level in fjords. This creates a distinctive
series of sediment associations, partly marine
and partly terrestrial.

5

Kongsfjord

Spitsbergen

Photo: J.D.Peacock

6

Kongsvegen: Spitsbergen

Norsk Polarinstitutt photo

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Kongsfjord

Spitsbergen

Photo: M.A.Paul

8


Marine and lacustrine fronts display many
similarities; differences arise from the relative
densities of lake and seawater and from the
absence of tidal motion in lakes.


This influences the behaviour of
plumes

(underflow vs overflow) and the drift of
icebergs.


In turn, this controls the distribution of so
-
called
rain
-
out
deposits and of
ice
-
rafted

deposits.

Dynamics of tidewater glaciers

10


Tidewater glacier margins are defined by the
position of the
grounding line
, and thus by
ice dynamics and bed topography.


Ice will float in a water depth about 90% of
the ice thickness. Thus water depth
determines the position of the grounding line.

11


From the grounding line, the ice will advance
until the rate of loss by calving equals the rate
of ice discharge.


This typically occurs at a distance in front of
the grounding line about equal to the ice
thickness.


This defines the position of the
calving line
,
which is thus slightly in advance of the
grounding line.

12


The
calving rate

is roughly proportional to
water depth. Thus the overall position of the
glacier margin is strongly dependent on
seabed topography.



In fjords, constrictions in the sidewalls and
bed, termed
pinning points
, are likely to
define possible stationary positions of the
margin.

Fjord
-
based deposition

Ice frontal sedimentation

14


Ice
-
front depositon in fjords is very similar to that
seen at water
-
based margins in lakes.


If deposition occurs near to the grounding line, a
mixture of sediment will be received from basal
debris, supraglacial debris and meltwater.


The assemblage of grounding sediments is thus
produced by a mix of subglacial, ice contact, gravity
driven and water column processes.

15

16

17

Fjord
-
based deposition

Proximal sedimentation

19


Proximal sediments are deposited near to the ice
-
margins by deposition through the water column.
They show some similarities to ice
-
frontal sediments.


They are often till
-
like sediments that contain layers
of sorted materials, which often show evidence of
disturbance.


These have sometimes (misleadingly) been referred
to as ‘water
-
lain tills’.

20


Sediment may fall directly into the water
column by the slumping of supraglacial debris
or by the rolling of icebergs.


This releases a shower of variously sized
particles that settle through the water under
gravity and produce a graded deposit.


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Kongsfjord

Spitsbergen

Photo: J.D.Peacock

22

23

24

Proximal glaciomarine sediments

Dicksonfjord, Spitsbergen

Photo: M.A.Paul

25


Deposition can also occur by rain
-
out from
hyperconcentrated plumes around the ice
-
front.


Some glaciomarine sediments are
rhythmically graded due to the pulsed input of
sediments.


Rhythmic proximal sediments are usually
composed of sand and silt and are termed
cyclopsams
;

26

Probable glaciomarine cyclopsams

Inverness area

Photo: J.W.Merritt

Fjord
-
based deposition

Distal sedimentation

28


The zone of distal sedimentation can extend
for tens of kilometres from the ice margin.


It is dominated by suspension rain
-
out and
ice rafting.


There is normally a proximal to distal fining of
the bulk sediment, with a relative increase in
the proportion of the fine material.

29


Continued rain
-
out can occur from sediment
plumes that are generated at the inflow points
of waterfalls and jets.


These plumes consist of a hyper
-
concentrated, fine
-
grained suspension that
moves from the entry point due to density
-
,
wind
-

and tide
-
driven currents.


30


During this movement the plume loses
material by rain
-
out until it can mix freely with
the surrounding water and lose its coherent
identity.


Rainout is assisted by the processes of
flocculation and by biogenic pelletisation
within the water column.

31


The rained out material forms a sediment
drape over the seabed.


In shallow water, where bioturbation is
common, the sediment is typically a massive,
distal
-
fining sandy diamicton or mud.

32


Where biogenic activity is suppressed, silts
and clays separate to give upwards
-
fining
units with sharp bases. These are often
rhythmic and are termed
cyclopels
.

33

Probable
glaciomarine
cyclopels

Inverness area




Photo:
J.W.Merritt

34


Ice rafting involves the transport of debris by
floating ice, followed by release through the
water column.


The distribution of this debris will follow the
tracks of bergs, which are often constrained
by bathymetric features and well
-
defined
currents.

35

Kongsfjord

Spitsbergen

Photo: J.D.Peacock

36

Kongsfjord

Spitsbergen

Photo: M.A.Paul

37

Dropstones

Vibrocore 60
-
05/51

Faeroe
-
Shetland Channel

38


Icebergs will often impact the seabed close to
a calving margin, leading to ploughing and
possible debris release.

39

40

Woodworth
-
Lynas & Guigné 1990

Fjord
-
based facies associations

42


Powell (1981) has described five fjord
-
based
sediment associations based on examples
from Alaska.


The key concepts are


Depth of water at the ice front


Speed of ice
-
front retreat


Whether the glacier actually enters the water

43


Association 1

is formed in deep water in
which the ice front is likely to retreat rapidly.


It is dominated by so
-
called morainic bank
deposits produced by oscillation sages,

44

45


Association 2

is formed in shallow water in
which the ice front is likely to retreat slowly.


It is dominated by grounding line deposits
produced by meltwater
-
driven input and
subsequent disturbance and redistribution.

46

47


Association 3

is formed in very shallow
water with little or no iceberg calving.


It is dominated by ice
-
contact deltaic beds
(mainly foresets) composed of sand and
gravel.

48

49


Associations 4 and 5

are formed when the
ice no longer enters the water. In this case a
true Gilbert
-
type delta is formed.


The distinction between associations 4 and 5
depends on the distance from the ice to the
sea:


if close, the sediments will be relatively coarser;


if distant, the sediments will be relatively finer

50

51


In principle, a succession of associations from
1 to 5 will be seen as a glacier retreats up the
fjord.


The associations will replace one another
both laterally and vertically.

52

53

SUMMARY



Introduction


Dynamics of tidewater glaciers


Fjord
-
based deposition


Ice frontal sedimentation



Proximal sedimentation


Distal sedimentation



Fjord
-
based facies associations


THE END

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