Sedimentologi

1
Sedimentologi Kamal Roslan Mohamed
GLACIAL ENVIRONMENTS
2
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.
3
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.

4
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
5
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.
6
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.
7
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
moutonee. 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.
8
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.
9
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.
10
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.
11
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.
12
CONTINENTAL GLACIAL DEPOSITION
Glacial landforms and glacial deposits in
continental glaciated areas.
13
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.
14
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

15
SEKIAN