Sedimentologi

Kamal Roslan Mohamed

GLACIAL ENVIRONMENTS

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.

INTRODUCTION

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.

DISTRIBUTION OF GLACIAL ENVIRONMENTS

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.

Temperate or mountain glaciers

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

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.

Temperate or mountain 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.

Polar glaciers

Hills and ridges of bare rock (known as nunataks) surrounded by glaciers and ice sheets in a high-latitude polar glacial area.

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.

Erosional glacial features

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.

Transport 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.

Deposition by continental glaciers

Till deposits result from the accumulation of debris above, below and in front of a glacier.

Till deposits result from the accumulation of debris above, below and in front of a glacier.

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.

Deposition by continental glaciers

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.

Characteristics of glacially transported material

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.

• 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

Characteristics of glacial deposits

SEKIAN