Page 125 - Sedimentology and Stratigraphy

112 Glacial Environments

Marine glacial facies Extensive ice sheets are today confined to the polar
regions within the Arctic and Antarctic circles. Dur-
Scale Lithology MUD SAND GRAVEL Structures etc Notes ing the glacial episodes of the Quaternary the polar ice
caps extended further into lower latitudes. The sea
clay silt vf m vc gran pebb cobb boul level was lower during glacial periods and many
c
f
parts of the continental shelves were under ice.
Upland glacial regions were also more extensive,
with ice reaching beyond the immediate vicinity of
the mountain glaciers. The growth of polar ice caps is
known to be related to global changes in climate, with
Laminated muds
with ice-rafted sands the ice at its most extensive when the globe was
and gravels as beds several degrees cooler. Other glacial episodes are
and isolated
dropstones known from the stratigraphic record to have occurred
in the late Carboniferous and Permian (the Gond-
10s metres wana glaciation in the southern hemisphere), in the
early Palaeozoic and in the Proterozoic.

7.7 ICE, CLIMATE AND TECTONICS

7.7.1 Glaciation and global climate

The continental ice caps at and near the poles contain
the vast majority of the ice on the planet. The Ant-
arctic ice cap covers almost all of the continent and
has a fringe of floating ice shelves; much of the ice in
the Arctic is sea ice in the Arctic Ocean, but Green-
Fig. 7.14 Glaciomarine deposits are typically laminated land also has a large ice cap. Compared with these
mudrocks with sparse coarser debris derived from icebergs. polar regions, the ice in the mountain glaciers is of
little global significance, although individual conti-
similar deposits can also result from sediment released nental ice masses are important parts of local envi-
from floating vegetation. ronments.
Evidence from the distribution of glacial sediments
and sedimentary rocks indicates that there have been
7.6 DISTRIBUTION OF GLACIAL a number of periods during Earth history when the
DEPOSITS polar ice caps covered much larger areas than at
present. The best documented glacial periods are
Quaternary valley and piedmont glaciers form distinc- from the Quaternary, a time of fluctuating global
tive moraines but are largely confined to upland areas temperatures that has experienced advances and
that are presently undergoing erosion. In these retreats of the polar ice caps a number of times over
upland areas glacial and periglacial deposits such as the past few hundred thousand years. The causes of
moraines, eskers, kames, and so on have a very poor the global changes in climate that lead to the ice caps
preservation potential in the long term. Of more inter- growing and shrinking are complex and are consid-
est from the point of view of the stratigraphic record ered further in Chapter 23. When the polar ice melts
are the tills formed in lowland continental areas and the water released adds to the volume of water in the
in marine environments as these are much more oceans, and the sea level rises worldwide: it is esti-
likely to lie in regions of net accumulation in a sedi- mated that complete melting of the Antarctic ice sheet
mentary basin. The volume of material deposited by would result in a global sea level rise of over 50 m,
ice sheets and ice shelves is also considerably greater while the Greenland ice cap would add 7 m to world
than that associated with upland glaciation. sea levels (Hambrey & Glasser 2003). The effects of