Mittwoch, 18. März 2009

Mysterious boulder ?

The term „Geschiebe“ in german is unfortunately ambiguous, or at least used so. It can indicate sediment transported by a river, or by a glacier too. Geologists use the name traditionally for glacially transported sediments, limnologist and water engineers use it in the second meaning of the word, confusion – even indicating if the term is used in the glacial or fluvial sense, is secured.
But what´s now the mysterious boulder? It was found in an outcrop that at first looks seems a diamict with no apparent internal structure, single larger polimict clasts in a loamy to sandy matrix. A morain?

But a more precise observations of the larger clasts shows a distribution in single clusters, and an suggested imbrication (note granite-clasts, ca. in the middle and extreme right of the picture).

Also the landscape is very flat, or has only a gentle dip, not so typical for morain deposits, but commonly found in fluvial or limnic deposits deposited in large braided river systems.

So I tend to attribute this deposits to a fluvial system of high energy, with sporadic floods capable to transport even large boulders. The granite was first transported surely by glaciers (the actual catchment area don´t possess outcrops of this kind of lithology), but the final deposition occured by a river.
The disaggradation of the rocks must be happened after this second deposition – the boulder couldn´t possibly survive a transport in this conditions. The lack of color alteration of the feldspat, and the „freshness“ of the mica, let´s assume mechanical weathering, maybe in a colder climate with strong thaw and freeze cycles.

So a proglacial sediment of a glacier feed braided river system still seems possible to explain this sediment..


jolo hat gesagt…

If you have hints that the deposits belong to some sort of glacier related braided river... Why should't it be possible that the granite was transported in an ice block floating on the river, ice melted and the rather fragile block was deposited - sort of how drop stones come into place... What would you think about this explanation?

David Bressan hat gesagt…

this is a very solid objection - I will check in the field for some evidence in this directon...