Page 126 - Sedimentology and Stratigraphy

Further Reading 113

sea level changes on sedimentation are covered in commonly preserved, including dropstones which
Chapter 23. may provide a record of periods of glaciation in the
past. The volume of continental ice in polar areas is
closely linked to global sea level, so the history of past
7.7.2 Glacial rebound – isostasy glaciations is an important key to understanding var-
iations in the global climate.
During periods of glaciation the ice layer on the con- Characteristics of glacial deposits
tinents may be hundreds to thousands of metres thick. . lithologies – conglomerate, sandstone and mud-
This mass of ice creates an extra load on the crust that stone
forces the base of the crust down into the mantle. . mineralogy – variable, compositionally immature
When the ice melts and the ice is removed, there is . texture – extremely poorly sorted in till to poorly
an isostatic uplift of the crust (6.7). The rate of melt- sorted in fluvio-glacial facies
ing is typically much faster than the isostatic uplift . bed geometry – bedding absent to indistinct in
and consequently the crust continues to go up for many continental deposits, glaciomarine deposits
thousands of years after the ice has melted. This effect may be laminated
is seen in many areas, such as Scandinavia, which . sedimentary structures – usually none in tills, cross-
were covered during the last ice age and are still
bedding in fluvio-glacial facies
undergoing uplift of a few millimetres a year. The
. palaeocurrents – orientation of clasts can indicate
effects of this so called glacial rebound are most
ice flow direction
clearly seen around coasts, where raised beaches
. fossils – normally absent in continental deposits,
provide evidence of the position of the land relative
may be present in glaciomarine facies
to the sea thousands of years ago, prior to uplift of
. colour – variable, but deposits are not usually
the land.
oxidised
. facies associations – may be associated with fluvial
7.8 SUMMARY OF GLACIAL facies or with shallow-marine deposits
ENVIRONMENTS
Glacial deposits are compositionally immature and FURTHER READING
tills are typically composed of detritus that simply
represents broken up and powdered bedrock from Benn, D.I. & Evans, D.J.A. (1998) Glaciers and Glaciation.
beneath the glacier. Reworked glacial deposits on out- Arnold, London.
wash plains may show a slightly higher compositional Dowdeswell, J.A. & Scourse, J.D. (Eds) (1990) Glaciomarine
and textural maturity. There is a paucity of clay Environments: Processes and Sediments. Special Publication
53, Geological Society Publishing House, Bath.
minerals in the fine-grained fraction because of the
Hambrey, M.J. 1994. Glacial Envionments. UCL Press, London.
absence of chemical weathering processes in cold
Knight, P. (Ed.) (2006) Glacier Science and Environmental
regions. Continental glacial deposits have a relatively
Change. Blackwell Science, Oxford.
low preservation potential in the stratigraphic record, Miller, J.M.G. (1996) Glacial sediments. In: Sedimentary
but erosion by ice in mountainous areas is an impor- Environments: Processes, Facies and Stratigraphy (Ed.
tant process in supplying detritus to other deposi- Reading, H.G.). Blackwell Scientific Publications, Oxford;
tional environments. Glaciomarine deposits are more 454–484.