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Geology 370: Glacial Geology

GLACIAL DEPOSITION & TILL CHARACTERISTICS

A. Introduction

Deposit characteristics affected by:

• mode of transport

  • high & low level transport by ice

  • transport by meltwater

• mode of deposition

  • lodgment

  • meltout

  • subglacial deformation

  • meltwater deposition (covered earlier)

Till: sediment deposited by ice

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B. Lodgment

Lodgement processes

• Boulton's model: lodgment is part of a continuum with erosion

• Hallet's model: lodgment is independent of normal pressure; occurs with low flow velocities & high rates of basal melting

Characteristics of lodgement till (basal till)

• particle shape: subrounded, striated & faceted

• particle size: fine grained matrix with larger clasts

• fabric: strong

• particle packing: densely compacted

• particle lithology: dominated by local rock types

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C. Meltout

Direct release of debris by melting of ice

Characteristics of subglacial melt-out till (another type of basal till)

• associated with stagnant ice

• subsequent modification of deposit due to flowage

• characteristics:

  • similar to lodgment till, but:

  • fabric not as strong,

  • packing not quite as dense

Characteristics of supraglacial melt-out till (ablation till)

• driven primarily by solar radiation

• supraglacial debris affects surface albedo and insulation

• characteristics:

  • particle shape: typical of high level transport

  • particle size: coarse; possibly some sorting

  • fabric: poorly developed

  • packing: poorly consolidated

  • lithology: variable

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D. Basal Deformation

Process

• transportation & accumulation of sediment within subglacial deforming layer

• assimilation of new material into deforming layer

Characteristics of deformation till

• shape, size & lithology: same as subglacial sediments

• fabric: may be strong, but not always

• packing: densely consolidated

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E. Distinguishing Tills in the Field

Internal sedimentary properties

• particle fabric - most useful

  • compass orientation of elongated particles

  • dip or angle at which particles are inclined within the deposit (imbrication)

• particle size & shape

• particle composition (lithology)

External relationships

• often more useful than internal characteristics

• local scale

  • facies analysis - examine individual units

  • facies: body of sediment with a distinct combination of properties that distinguish it from neighboring sediment bodies, and that are a product of a particular depositional environment or process

  • lithofacies: objective physical characteristics with no reference to depositional process

  • genetic facies: imply a mode of formation

  • e.g. diamicton is a lithofacies - a body of non-sorted, non-stratified sediments;
    till is a genetic facies - glacial origin

• intermediate scale

• interpret complete depositional sequence, not just individual units

• Walther's principle: in a depositional sequence with 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

• large scale: interpret entire landform assemblages

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F. Summary

• Ice deposition processes: lodgment, melt-out, subglacial deformation

• Till types:

  • lodgment till (basal till)

  • subglacial melt-out till (basal till)

  • supraglacial melt-out till (ablation till)

  • deformation till

• Distinguishing tills in the field

• internal sedimentary properties

• external relationships