Sedimentologi
Kamal Roslan Mohamed
GLACIAL
ENVIRONMENTS
INTRODUCTION
Glaciers are important agents of erosion of
bedrock and mechanisms of transport of
detritus in mountain regions.
Deposition of this material on land produces
characteristic landforms and distinctive
sediment character, but these continental
glacial deposits generally have a low
preservation potential in the long term and
are rarely incorporated into the stratigraphic
record.
Glacial processes which bring sediment into
the marine environment generate deposits
that have a much higher chance of long-term
preservation, and recognition of the
characteristics of these sediments can
provide important clues about past climates.
DISTRIBUTION OF GLACIAL ENVIRONMENTS
Ice accumulates in areas where the addition
of snow each year exceeds the losses due to
melting, evaporation or wind deflation.
The climate is clearly a controlling factor, as
these conditions can be maintained only in
areas where there is either a large amount of
winter snow that is not matched by summer
thaw, or in places that are cold most of the
time, irrespective of the amount of
precipitation.
There are areas of permanent ice at almost
all latitudes, including within the tropics, and
there are two main types of glacial terrains
- temperate (or mountain) glaciers
- polar ice caps.
Temperate or mountain glaciers
Form in areas of relatively high altitude
where precipitation in the winter is
mainly in the form of snow. These
conditions can exist at any latitude if
the mountains are high enough.
Accumulating snow compacts and
starts to form ice especially in the
upper parts of valleys, and a glacier
forms if the summer melt is insufficient
to remove all of the mass added each
winter.
A valley glacier in a temperate mountain
region partially covered by a carapace of
detritus.
Once formed, the weight of snow accumulating in the upper part of the
glacier (the accumulation zone of the glacier) causes it to move downslope,
where it reaches lower altitudes and higher temperatures.
The lower part of the glacier is the ablation zone where the glacier melts
during the summer
Temperate or mountain glaciers
Snowfall adds to the mass of a glacier in the accumulation zone and as the glacier advances
downslope it enters the ablation zone where mass is lost due to ice melting. Glacial advance
or retreat is governed by the balance between these two processes.
Polar glaciers
Polar glaciers occur at the north and
south poles, which are regions of
low precipitation (Antarctica is the
driest continent): the addition to the
glaciers from snow is quite small
each year, but the year-round low
temperatures mean that little
melting occurs.
Permanent ice in the polar
continental areas forms large ice
sheets and domed ice caps
covering tens to hundreds of
thousands of square kilometres.
Hills and ridges of bare rock (known as
nunataks) surrounded by glaciers and ice sheets
in a high-latitude polar glacial area.
Erosional glacial features
Cirques, U-shaped valleys and hanging valleys are
evidence of past glaciation, which, in the framework
of geological time, are ephemeral, lasting only until
they are themselves eroded away.
Smaller scale evidence such as glacial striae
produced by ice movement over bedrock may be
seen on exposed surfaces, including roche
moutone´e.
Pieces of bedrock incorporated into a glacier by
plucking may retain striae, and contact between
clasts within the ice also results in scratch marks on
the surfaces of sand and gravel transported and
deposited by ice.
These clast surface features are important criteria for
the recognition of pre-Quaternary glacial deposits.
Transport by continental glaciers
Debris is incorporated into a moving
ice mass by two main mechanisms:
supraglacial debris, which
accumulates on the surface of a
glacier as a result of detritus falling
down the sides of the glacial valley,
and basal debris, which is entrained
by processes of abrasion and
plucking from bedrock by moving
ice.
Supraglacial debris is dominantly
coarser-grained material with a low
proportion of fine-grained sediment.
Basal debris has a wider range of
grain sizes, including fine-grained
rock flour produced by abrasion
processes.
Deposition by continental glaciers
The general term for all deposits directly deposited by ice is till if it is
unconsolidated or tillite if it is lithified.
The terms diamicton and diamictite are used to describe unlithified
and lithified deposits of poorly sorted material in an objective way,
without necessarily implying that the deposits are glacial in origin.
Till deposits result from the accumulation of debris above, below and in front of a glacier.
Deposition by continental glaciers
Tills can be divided into a number of different types depending on
their origin. Meltout tills are deposited by melting ice as
accumulations of material at a glacier front.
Lodgement tills are formed by the plastering of debris at the base of
a moving glacier, and the shearing process during the ice movement
may result in a flow-parallel clast orientation fabric. Collectively
meltout and lodgement tills are sometimes called basal tills.
Till deposits result from the accumulation of debris above, below and in front of a glacier.
Characteristics of glacially transported material
Glacial erosion processes result in a
wide range of sizes of detrital
particles.
As the ice movement is a laminar
flow there is no opportunity for
different parts of the ice body to mix
and hence no sorting of material
carried by the glacier will take place.
Glacially transported debris is
therefore typically very poorly
sorted.
CONTINENTAL GLACIAL DEPOSITION
Glacial landforms and glacial deposits in continental glaciated areas.
MARINE GLACIAL ENVIRONMENTS
At continental margins in polar areas, continental ice feeds floating ice sheets that eventually
melt releasing detritus to form a till sheet and calve to form icebergs, which may carry and
deposit dropstones.
Characteristics of glacial deposits
• lithologies – conglomerate, sandstone and
mudstone
• mineralogy – variable, compositionally immature
• texture – extremely poorly sorted in till to poorly
sorted in fluvio-glacial facies
• bed geometry – bedding absent to indistinct in
many continental deposits, glaciomarine deposits
may be laminated
• sedimentary structures – usually none in tills,
crossbedding in fluvio-glacial facies
• palaeocurrents – orientation of clasts can indicate
ice flow direction
• fossils – normally absent in continental deposits,
may be present in glaciomarine facies
• colour – variable, but deposits are not usually
oxidised
• facies associations – may be associated with
fluvial facies or with shallow-marine deposits
SEKIAN