Page 124 - Sedimentology and Stratigraphy

Marine Glacial Environments 111

Fig. 7.12 At continental margins in polar areas, continental ice feeds floating ice sheets that eventually melt releasing detritus
to form a till sheet and calve to form icebergs, which may carry and deposit dropstones.
icebergs. The more distal glaciomarine deposits are
subject to reworking by shallow marine processes
(Chapter 11): waves and currents produce a grain-
size sorting of material, sand may be reworked to form
wave and current bedforms and the finer-grained
material may be transported in suspension to be
deposited as laminated mud. Mixing of the glacially
derived material with other sediment, such as bio-
genic material, can also occur.
The edges of ice shelves break up to form icebergs
that can travel many hundreds of kilometres out into
the open sea, driven by wind and ocean currents, but
they often carry relatively little detritus. Icebergs
formed at the front of tidewater glaciers are generally
Fig. 7.13 An ice shelf at the edge of a continental glaciated
area. small, but may be laden with sediment. As an iceberg
melts, this debris will gradually be released and depos-
ited as dropstones in open marine sediments. Drop-
deposits may be divided into those deposited close to stones can be anything up to boulder size and their
the ice front (ice-proximal glaciomarine sedi- size is in marked contrast to surrounding fine-grained,
ments), which are typically poorly sorted diamictons pelagic deposits (16.5.1). Although rarely found in
with little or no stratification or other sedimentary deep marine strata, dropstones are important indica-
structures, and ice-distal glaciomarine sediments, tors of the presence of ice shelves and hence provide
which are composed mainly of sediment released from evidence of past global climate conditions. However,