 |
| Titel |
Glacier surges past and present |
| VerfasserIn |
D. Benn, L. Kristensen |
| Konferenz |
EGU General Assembly 2009
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250026382
|
|
|
|
|
|
| Zusammenfassung |
| Investigations of the glacial geologic record commonly use modern
analogues to underpin reconstructions of former environments. While
this approach is very powerful, it is not without problems and is
vulnerable to changing paradigms (or fashions) in glaciology. A
well-known example is the ascendancy of the \'deforming bed\' model in
the 1980s and 1990s, which was subsequently challenged by the
\'ploughing bed\' model. These models have contrasting implications for
till genesis and subglacial sediment transport, but rigorous testing
is hampered by the difficulty of directly observing modern glacier
beds and the lack of unambiguous diagnostic criteria for interpreting ancient tills.
We address this issue by examining sediment-landform assemblages
formed by surging glaciers in Svalbard. Surges leave a distinctive
imprint on fjord floors, including fluted subglacial till,
crevasse-fill ridges, thrust block moraines, and extensive proglacial
mud flows. The latter have been interpreted as either masses of
extruded subglacial till or the collapsed fronts of oversteepened
thrust moraines. The \'extrusion hypothesis\' implies significant
subglacial sediment flux towards the margin, consistent with a
metres-thick deforming layer, whereas the \'moraine failure\' hypothesis
implies dominantly proglacial transport.
We show that both fjord-floor and terrestrial \'mud aprons\' consist of
masses of marine sediment which were pushed in front of the advancing
glacier, while undergoing more or less continuous gravitational
failure. The subaqueous moraines and mud flows are therefore
interpreted as end-member glacitectonic landforms, formed by similar
processes to thrust-block moraines. These results indicate highly
episodic glacial sediment transport in Svalbard fjords, accomplished
largely by ice-push during surges. The survival of transverse
(moraine) ridges below megaflutings in some fjords suggests that subglacial sediment transport is relatively unimportant, and that the \'ploughing model\' best describes the behaviour of the ice-bed interface during surges. We suggest that similar glacitectonic processes may have been important for delivering sediment to the margins of Pleistocene marine ice sheets. |
|
|
|
|
|
|