Page 123 - Sedimentology and Stratigraphy

110 Glacial Environments

Quaternary age (loess – 8.6.2) are thought to be tinent extend hundreds of kilometres out to sea form-
sourced from periglacial environments. Clasts exposed ing areas of floating ice which cover several hundred
on the outwash plain may be abraded by wind-blown thousand square kilometres (Drewry 1986). These ice
sediment to form ventifacts (8.2). shelves partially act to buffer the seaward flow of
continental ice: melting of the floating ice of an ice
shelf does not add any volume to the oceans, but if
7.4.4 Periglacial areas they are removed then more continental ice will flow
into the sea and this will cause sea level rise. Ice
In polar regions the areas that lie adjacent to ice shelves such as those around the Antarctic contain
masses are referred to as the periglacial zone relatively small amounts of sediment because there is
(6.6.2). In this area the temperature is below zero little exposed rock to provide supraglacial detritus, so
for much of the year and the ground is largely frozen the main source is basal debris. Ice shelves break up at
to create a region of permafrost. Only the soil and the edges to form icebergs and melt at the base in
sediment near the surface thaws during the summer, contact with seawater. Ice in a marine setting also
and to a depth of only a few tens of centimetres, below occurs where temperate or poythermal valley glaciers
which the ground remains perennially frozen. The flow down to sea level: these tidewater glaciers can
thin layer of thawed material is often waterlogged contain large amounts of both supraglacial and basal
because the water cannot drain away into the frozen debris. Sea ice is frozen seawater and does not contain
subsurface. This upper mobile layer can be unstable any sedimentary material except for wind-blown dust.
on slopes and will slump or flow downslope. Other
features of regions of permafrost are patterned
ground (Fig. 7.11), which is composed of polygons 7.5.1 Erosional features associated
of gravelly deposits formed by repeated freezing and with marine glaciers
thawing of the upper mobile layer, and ice wedges,
which are cracks in the ground formed by ice that Where continental ice from an ice sheet or valley
subsequently become filled with sediment. glacier reaches the shoreline of a shallow shelf the
ice may be grounded on the sea floor. The movement
of the ice mass and drifting icebergs may locally scour
7.5 MARINE GLACIAL ENVIRONMENTS the sea floor, resulting in grooves in soft sediment that
may be metres deep and hundreds of metres long.
Where a continental ice sheet reaches the shoreline the Meltwater flowing subglacially may be under consid-
ice may extend out to sea as an ice shelf (Figs 7.12 & erable pressure and can erode channels into the sea-
7.13). Modern ice shelves around the Antarctic con- floor sediment beneath the ice, forming tunnel val-
leys that subsequently may be filled with deposits
from the flowing water. The tunnel-valley deposits
and the glacial scours features are preserved within
shallow-marine strata in places such as continental
shelves that have been covered with ice.

7.5.2 Marine glacial deposits

The terms till and tillite are also used to describe
unconsolidated and lithifed marine glacial, glacio-
marine, deposits (Fig. 7.14). The primary character-
istics of the material are the same as the glacial
sediment associated with continental glaciation. The
detritus released from the bottom of an ice shelf forms
Fig. 7.11 In periglacial areas, freeze–thaw processes in the till sheets (Fig. 7.12), which may be thick and exten-
surface of the permafrost form polygonal patterns. sive beneath a long-lived shelf (Miller 1996). These