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Thursday 12 January 2017

Putorana Piedmont Glaciers?

Now and then, when one is trundling cross-country, courtesy of Google Earth, one spots something rather peculiar.  Above, we see the northern edge of the Putorana plateau in Central Siberia.  The plateau edge is quite sharp, running tight to left across the photo.  In the mountain area we see the modified dendritic pattern of troughs that were transporting glacier ice northwards at various times during the Quaternary.  To the north we see the tundra of the North Siberian Plain, with patchy woodland and many small meltwater lakes in the permafrost.

But the really impressive features are the bulbous lobes pushing out from the mountain front onto the plain.  What on earth are they?  The most logical explanation is that they are morainic loops formed by piedmont glaciers pushing northwards from the ice sheet -- rather like the features we see in Greenland, Ellesmere Island, Alaska and elsewhere today.  The top photo is from Peary Land (Greenland), the middle one is the Malaspina Glacier in Alaska, and the lower one is from Ellesmere Island.

It's often stated that pedmont glaciers form most readily in high Arctic situations where conditions are quite arid, with continuous permafrost and very little glacier bed melting.  This means that basal melting and bed sliding are minimal, and that most glacier movement is byb internal deformation.  This does not alwats hold -- but it is a fair assumption that those conditions would have prevailed in the Weichselian (Devensian) glacial episode on the northern flank of the Putorana Plateau.

According to Svendsen and his colleagues (2004) there were some early glaciations during which both the plateau and the adjacent coastal plain were heavily inundated beneath the Eurasian Ice Sheet.  The Late Saalian Glaciation (160 ka - 140 ka) is the one about quite a lot is known.  During the Weichselian things got complicated.  There appear to have been three glacial episodes, each one less extensive than the last.  In the early Weichselian (related to the Onyoka Moraine) -- about 90 ka ago -- the Putorana ice sheet was incorporated into the Eurasian ice sheet on its western edge, but on the northern and north-eastern flanks of the plateau the local segment of the ice sheet terminated just beyond the plateau edge.  After an ice retreat there was another expansion -- referred to as the mid-Weichselian glacial phase -- where the glaciers in this area terminated in more or less the same positions.  This is called the "Norilsk Glaciation" by Russian workers.  After that there was another retreat, and during the LGM "last glacial maximum) dated to c 20,000 years ago, there was apparently no ice sheet on the Putorama Plateau -- just an assortment of small cirque glaciers, local ice caps and small valley glaciers.

Therefore it is possible that the piedmont glaciers decanting onto the North Siberian Plain were active twice, in the early Weichselian and again in the mid Weichselian. This means that the features seen in the satellite image at the top of this post might be composite in age.

Nonetheless, they are pretty impressive -- and I know of hardly any other examples of Weichselian piedmont glacier maraine loops that are so well exposed in an area currently devoid of glacier ice.  It will be interesting to see whether field observations tally with what the satellite image suggests!


Late Quaternary ice sheet history of Northern Eurasia
Quaternary Science Reviews 23 · January 2004, pp 1229-1271
John Inge Svendsen et al


This appears to be a crucial paper, if only one could get at it:

Isayeva, L.L., Kind, N.V., Kraush, M.A., Sulerzhitsky, L.D., 1976.
On the age and structure of the marginal formations along the northern foot of the Putorana Plateau. Bulleten Komissii poizucheniyu chevertichnogo perioda,
USSR Academy of Sciences
45, 117–123 (in Russian)


BRIAN JOHN said...

Message from John Inge Svendsen: Yes, I know about these features. As you say, they are lobes (piedmont glaciers) from an ice cap sitting on the Putorana Plateau. Most likely they are 60-70,000 yrs old, but they may also be 90,000 yrs.

Good to get corroboration from somebody who knows the territory!

Dave Maynard said...

They kind of look like positive versions of the cirques shown earlier.


BRIAN JOHN said...

Yes, nature seems to like semi-circles.........