Box 83 Architectural Components Of A Subglacial Till

Boyce and Eyles (2000) used an architectural element analysis to unpick the components present within a subglacial till (Northern Till) in Ontario, Canada. The method works by breaking down the individual components present (elements or lithosomes), while also focusing on the characteristics of the boundaries between these elements. The approach is to establish the individual building blocks that make up a complete facies and recognise a hierarchy of bounding surfaces that separate these building blocks. They used a range of different data sources to assist with this analysis, from detailed borehole logs and associated geophysics to the analysis of outcrop data. A range of architectural elements are identified within the till along with their bounding surfaces. The importance of this work is the recognition that subglacial tills are heterogeneous facies and that their sedimentation involves a range of subglacial processes that varies both in space and time. For example, the presence of boulder lines represent local episodes of erosion and reworking, and clastic dykes result from the local release of overpressured groundwater. Thin beds of sand and gravel may represent the location of subglacial drainage routes or short periods of bed-ice separation, and soft-sediment rafts result from the assimilation of sediment via deformation. The bounding surfaces between elements record the shifting pattern of erosion and deposition beneath the former ice sheet, as well as the lateral movement of different depositional processes through time. This level of detail is not always possible in the field and Boyce and Eyles (2000) used a superb three-dimensional data set, but the work illustrates the dynamic nature of subglacial sedimentation and the range of processes involved within it.

Source: Boyce, J.I. and Eyles, N. (2000). Architectural element analysis applied to glacial deposits: Internal geometry of a late Pleistocene till sheet, Ontario, Canada. Geological Society of America Bulletin, 112, 98-118.

-S^b9lacQ~ ~ - -^Sfecfe," ~ ~ ~ -Non - glacial

Vertical profile or

Ice advance

Vertical profile or

Figure 8.11 Illustration of Walther's law in the context of glacial environments.

To understand the relationship of one facies to another we must understand Walther's law. Walther studied recent sedimentary facies and their relationship to the environment in which they were deposited. From these observations he deduced that environments are not static through time and that as environments shift position, so the respective sedimentary facies of adjacent environments or processes succeed each other in a vertical profile (Figure 8.11). Therefore, in sequences where there is no apparent break in the sedimentary record the vertical profile of sedimentary facies is equivalent to the lateral variation of facies at any one time. In other words, if one was to turn a vertical profile on its side then it would give a picture of the lateral variation in the depositional environments present during the period of time represented by the vertical profile. In this way a vertical profile or section of glacial sediments can be translated into a picture of the particular glacial environment in which they were deposited. The formation of individual sediment units can then be explained. This is the principle by which facies analysis works. Vertical logs through sediment sections are constructed and the component facies described (Table 8.2); in this way the whole sequence is documented and then compared to modern depositional environments. Emphasis is therefore placed on interpreting the whole sequence of sediments and the environment in which they formed and not simply on the interpretation of specific units or components. The concept is illustrated with respect to a subglacial till in Figure 8.12.

The use of facies analysis provides a powerful field-based tool with which to interpret glacial sediments and the origin of till sequences. It does not, however, replace the need to examine the internal evidence present within

Table 8.2 Diagnostic criteria for recognition of common diamict lithofacies.

Code Facies

Dmm Matrix supported, massive

Dmm(r) Dmm with evidence of resedimentation

Dmm(c) Dmm with evidence of current reworking

Dmm(s) Matrix-supported, massive, sheared

Dms Matrix supported, stratified diamicton

Dms(r) Dms with evidence of resedimentation

Dms(c) Dms with evidence of current reworking

Dmg Matrix-supported, graded

Dmg(r) Dmg with evidence of resedimentation

Description

Structureless mix of mud, sand and pebbles

Initially appears structureless but careful cleaning reveals subtle textural variability and fine structure (e.g., stringers of silt or clay with small flow noses). Stratification less than 10% of the unit thickness

Initially appears structureless but careful cleaning reveals subtle textural variability and fine structure produced by water flow (e.g., isolated ripples). Stratification less than 10% of the unit thickness

Initially appears structureless but careful cleaning reveals shear planes, foliation and orientated clasts. Breccitated clasts may be present

Obvious textural differentiation or structure within the diamict. Stratification more than 10% of the unit

Flow noses frequently present; diamict may contain rafts of deformed silt/clay laminae and abundant silt/clay stringers. May show slight grading. Often contain high clast contents, which often form clusters. Clast fabric random or parallel to bedding. Erosion along the base of the unit may be present

Diamict often coarse, due to removal of fines. May be interbedded with sand, silt and gravel beds showing evidence of flowing water (e.g., ripples and cross-bedding). Abundant stringers within the diamicton. Units may have channellised base

Diamict exhibits variable vertical grading in either matrix of clast content

Clast imbrication common

Key to common terms: matrix-supported, the matrix dominates the sediment and the clasts are set within it; clast-supported, clasts dominate and matrix infills spaces between the clasts; massive, structureless; grading, variation in particle size; flow noses, small folds or curved beds indicating sediment flow; stringer, thin discontinuous layer of sand, silt or clay. [Modified from: Eyles et al. (1983) Sedimentology, 30, table 3, p. 397]

Interpretation

Upper till layers may show evidence of weathering and sediment reworking by solifluction.

Discontinous units of glaciofluvial sediment, may contain lenses of till deposited by the collapse of channel walls.

Concentrations of boulders / clasts may occur locally forming boulder pavements.

Discontinous units of glaciofluvial sediment deposited in subglacial channels. Upper contact may be eroded by overriding ice.

Shear laminations caused by the shearing out of soft clasts. Brecciated clasts may also be present. Foliations and shear planes occur throughout.

Deformation of the rockhead. Inclusion of blocks and rafts of bedrock. Injection of till into bedrock joints.

Dmms(r) Sm

Gm/Sm

Dmms(r) Sm

Gm/Sm

Gm/Sm

Figure 8.12 Facies model for a warm-based glacier moving over a rigid bed. [Modified from: Eyles (1983) Glacial Geology, Pergamon Press, figure 1.8, p. 16]

each sediment unit. Interpreting glacial sediments and distinguishing between different tills and depositional environments is difficult and success is based largely on experience.

8.1.4 Till Facies and Basal Thermal Regime

Glaciers with different thermal regimes deposit sediment in different ways. They give rise to different depositional environments and therefore facies patterns or architecture. We can recognise three main types of thermal regime, each of which is associated with a different facies pattern. They are: (i) warm- or wet-based glaciers; (ii) cold- or dry-based glaciers; and (iii) glaciers with mixed thermal regimes. The type of glacial deposition and facies patterns typical of each is discussed below.

1. Deposition by warm-based glaciers. Where warm-based glaciers move over rigid beds most of the debris is transported in the basal layers. Lodgement of basal debris occurs over most of the glacier bed. The lodgement facies will vary from areas associated with direct particle lodgement to those deposits laid down in cavities, where the sediment will be locally more diverse and may not possess a strong particle fabric. The process of lodgement may be interrupted locally by subglacial rivers, which rework the sediment into units of sand and gravel. These subglacial rivers are usually ephemeral and flow may switch on and off suddenly. The location of these rivers also varies through time. Changes in ice flow will also affect the continuity of the depositional processes and may cause erosional breaks. For example, different units of till, perhaps with different lithological clast contents, may be superimposed on top of one another. This occurs where different ice streams or ice lobes, from different source areas, compete with one another in a lowland area. As one lobe of ice waxes or wanes in strength its extent may vary with respect to other lobes, and therefore over time the lithological content of a till at any one point may vary depending on which lobe was dominated at that time. Sediment that is lodged is likely to experience subglacial deformation if the pore-water pressures are sufficiently high and the sediments may become overprinted by a range of tectonic characteristics. Figure 8.12 provides a generalised facies model for this type of environment.

In areas where a warm-based glacier moves over a soft substrate, subglacial deformation may occur as the underlying sediment is assimilated into the deforming layer. The thickness of the deforming layer will depend on the properties of the deforming sediment and on the shear stress applied to it by the glacier. As shown in Figure 8.4 low-levels of deformation are associated with simple overturning and folding, high-levels of deformation may involve intense folding and the development of tectonic laminations and boudins, whereas very high levels of deformation produce a homogeneous diamict. Sediment is transported within the deforming layer from areas of extending flow up to areas of compressional flow (Figure 8.6). Beneath areas of extending ice flow, deformation involves excavation and is defined by a sharp erosional surface along which new material is added to the deforming layer by downcutting of the deformation base. In contrast, in areas of compressive flow, the deforming layer thickens by the accumulation of till from up-ice areas. As a consequence, the deforming pile thickens by the addition of sediment at the top of the sequence and each successive tectonic state is preserved vertically in the sequence. At the base of the section there will be undeformed sediment, above which there will lightly folded and deformed sequences, followed by highly deformed sediment with tectonic laminations, boudins and other evidence of intense shear, and ultimately a homogenised diamict may occur at the top of the sequence if the level of deformation is sufficient. Figure 8.13 shows a model of the different deformation facies that might exist beneath an ice sheet. In practice, however, the sedimentary facies produced by subglacial deformation may be extremely complex depending on the deformation history and the character of the sediment that is being assimilated into the deforming layer.

2. Deposition by cold-based glaciers. Cold-based glaciers do not usually possess well-developed basal debris layers due to the absence of widespread glacial erosion and basal debris entrainment. Basal debris is derived in one of two ways: (i) by overriding of the frontal apron of fallen ice blocks and debris in front of the glacier snout; and (ii) freezing-on of water and debris draining from warm-based areas elsewhere in the ice sheet. Debris is usually frozen on in layers and highly folded and attenuated (Figure 8.14A). During glacier recession, the stratified basal-debris layers decay in situ and englacial debris is lowered onto

A: Constructional deformation (deposition)

Homogenised diamicton

Fold attenuation, tectonic laminations and boudins

Folding

No deformation

Fold attenuation, tectonic laminations and boudins

Folding

No deformation

B: Excavational deformation (erosion)

B: Excavational deformation (erosion)

Homogenised diamicton

No deformation

Homogenised diamicton

D├ęcollement surface

No deformation

Figure 8.13 Variation in subglacial glaciotectonic (deformation) facies beneath an ice sheet. Constructional deformation occurs in areas of compression, whereas excavational deformation is common under extending flow. [Modified from: Hart (1990) Earth Surface Processes and

the basal substrate. Interstitial ice is lost in cold arid areas by sublimation. Large areas of sublimation or subglacial meltout till will result, which often retain the crude stratification and folded structure of the englacial debris from which it is derived. A typical vertical profile of the facies associated with cold-based glacier is given in Figure 8.14B.

Deposition by mixed-regime glaciers. Many glaciers exhibit very thick basal and englacial debris zones in response to repeated freezing-on of subglacial meltwater draining from warm- to cold-based areas of the glacier and to thrusting at the warm-cold interface. A mixed thermal regime is, however, not the only way in which this type of sediment assemblage may develop, because intense

A Advancing

Debris

A Advancing

Debris

Retreating

Thick debris layers due to sustained freezing-on

Retreating

Folding of debris layers

Only minor reworking

Folding of debris layers

Moulded drumlin-like layers

Only minor reworking

Moulded drumlin-like layers

Subglacial Erosion
Sublimation till englacial structures preserved

Interpretation

May contain some evidence of till flow, sliding/ slumping or free fall. Occasional signs of sediment reworking by meltwater.

Fold structures may be picked out by the stratified till. Clast orientation reflects structure.

Stratified till, may be picked out by alternating patterns of clast size or density.

Dms(r)/Sm Dms Dcs Dms

Dms(r)/Sm Dms Dcs Dms

Figure 8.14 Facies model for a cold-based glacier. (A) Debris structure within a cold-based glacier that has experienced extensive freezing-on. Compression during glacier retreat causes the deformation of debris-rich ice stratification. (B) Typical vertical log of the associated sedimentary facies within this environment.; [Modified from: (A) Shaw (1977) Canadian Journal of Earth Sciences, 14 figure 1, p.1241. (B) Eyles (1983) Glacial Geology, Pergamon

folding of a thin horizon of basal debris may generate thick debris layers within both warm- and cold-based ice. If this is also associated with glacial thrusting, thick englacial sequences may result. This occurs not only when warm-based glaciers have a thin frozen ice margin, but also where ice flows against steep bedrock highs or escarpments. Where the ice is cold-based, deposition at the base of the glacier occurs through basal meltout. If this sediment is saturated it may become prone to remobilisation and subglacial flow. Thickness variations in the meltout till reflect the fold and thrust structures within the englacial ice. Where the ice is warm-based, lodgement till may be deposited. Surface meltout of the englacial debris produces a large thickness of supraglacial meltout till, which is frequently resedimented as flow tills. A complex ice-surface topography results from differential debris insulation and surface melting (Figure 8.15). Fluvial action on this topography is common and much of the supraglacial debris may be reworked and deposited over the ice surface. A complex irregular topography of depositional landforms results when all the buried ice melts. One of the main characteristics of this type of depositional environment is the diverse nature of tills present and their intimate association with fluvial deposits. Multiple till sequences, often separated by layers of sand and gravel are common, produced by a single glacial episode (Box 8.4). A typical vertical profile of the facies associated with glaciers with mixed thermal regimes is given in Figure 8.16.

Figure 8.15 Topography of ice-cored moraines or debris ridges, the structure of which reflects the presence of thrusts and debris bands within the glacier. A complex depositional environment is produced involving fluvial deposition, flow tills and supraglacial meltout till. [Modified from: Edwards (1986), in (ed. H.G. Reading) Sedimentary Environments and Facies, Blackwell, figure

Debris ridges: debris-rich ice along thrust plane retards melting

Debris ridges: debris-rich ice along thrust plane retards melting

Bands Glacial Deposits

Figure 8.15 Topography of ice-cored moraines or debris ridges, the structure of which reflects the presence of thrusts and debris bands within the glacier. A complex depositional environment is produced involving fluvial deposition, flow tills and supraglacial meltout till. [Modified from: Edwards (1986), in (ed. H.G. Reading) Sedimentary Environments and Facies, Blackwell, figure

Supraglacial meltout till

Buried stagnant ice

Supraglacial meltout till

Buried stagnant ice

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