Page 119 - Sedimentology and Stratigraphy

106 Glacial Environments

7.3.2 Transport by continental glaciers 7.3.3 Deposition by continental glaciers
Debris is incorporated into a moving ice mass by two The general term for all deposits directly deposited by
main mechanisms: supraglacial debris, which ice is till if it is unconsolidated or tillite if it is lithified.
accumulates on the surface of a glacier as a result of These terms are genetic, that is, they imply a process
detritus falling down the sides of the glacial valley, of deposition and should therefore not be used as
and basal debris, which is entrained by processes of purely descriptive terms: for example, a bed may be
abrasion and plucking from bedrock by moving ice. described as a matrix-supported conglomerate (2.2.2),
Supraglacial debris is dominantly coarser-grained but because a deposit of this description could be
material with a low proportion of fine-grained sedi- formed by a number of different mechanisms in differ-
ment. Basal debris has a wider range of grain sizes, ent environments (e.g. on alluvial fans, 9.5 and asso-
including fine-grained rock flour (6.5.4) produced by ciated with submarine slumps, 16.1.2), the beds may
abrasion processes. or may not be interpreted as a tillite. To overcome this
This basal debris of very fine to coarse material problem, the terms diamicton and diamictite are
tends to be most abundant in polythermal glaciers commonly used to describe unlithified and lithified
because the alternation of pressure melting and freez- deposits of poorly sorted material in an objective
ing of the ice in contact with the bedrock exerts a way, without necessarily implying that the deposits
strong freeze–thaw weathering effect (6.4.1). Melt are glacial in origin. (It is noteworthy, however, that
water between the glacier and the bedrock forms a these terms, along with diamict for both unlithified
lubrication zone allowing the ice to move more freely and lithifed material, are rarely used by sedimentolo-
and there is less erosion by the ice. Cold glaciers move gists for deposits of pre-Quaternary age, and hence
only by internal deformation and hence do not erode their use tends to be associated with glacial facies.)
bedrock. Cold and temperate glaciers therefore carry Tills can be divided into a number of different types
mainly coarser-grained supraglacial debris (Hambrey depending on their origin (Fig. 7.6). Meltout tills are
& Glasser 2003). deposited by melting ice as accumulations of material
During movement of a glacier the ice mass under- at a glacier front. Lodgement tills are formed by the
goes deformation, internal folding and thrust faulting plastering of debris at the base of a moving glacier,
that can mix some of the basal and supraglacial and the shearing process during the ice movement
debris into the main body of the glacier. In addition, may result in a flow-parallel clast orientation fabric.
the merging of two or more glaciers brings detritus Collectively meltout and lodgement tills are some-
from the margins of each into the centre of the com- times called basal tills. Flow tills are accumulations
bined glacier. Some modification of the debris occurs of glacial sediment reworked by gravity flows.
where it is carried along in the basal layer, with
abrasion and fracturing of clasts occurring: water in
channels within and at the base of the ice (englacial 7.3.4 Characteristics of glacially
channels and subglacial channels) may also sort transported material
sediment carried in temperate glaciers. Supraglacial
detritus is usually unmodified during transport and Glacial erosion processes result in a wide range of sizes
retains the poorly sorted, angular character of rock- of detrital particles. As the ice movement is a laminar
fall deposits. flow there is no opportunity for different parts of the

Fig. 7.6 Till deposits result from the
accumulation of debris above, below and
in front of a glacier.