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Wednesday, 9 November 2016

Erratic clusters and super-erratics

I have been revisiting this old article by Joakim Donner, which I have read before,  and have found within it something really rather interesting.  In describing the downstream transport of Jotnian sandstone boulders and cobbles during the Devensian glaciation of SW Finland, he shows us this map:


 The map shows that downstream of the outcrop, there is quite a wide spread in the erratic trail or apron, attributable to frequent swings in ice movement directions.  As you would expect, the greatest concentrations of erratics from the sandstone outcrops lie downglacier to the SE, with some concentrations of erratics above 40% in a zone about 25 km wide. Some of the erratics are more than 200 km away from the source, well off the area covered by the map.

But Donner's attention is drawn to two anomalies or sudden increases in erratic concentrations on the Salpausselka moraines II and III.  In one place there is a peak of over 30% and in another the peak is over 40%.  He explains in the text that he first considered these "clusters" or "anomalous concentrations" to be down to entrainment on, and transport away from, unknown outcrops of the parent rock in the vicinity.  But that line of reasoning was difficult to sustain, and then he noticed that the erratics in these concentrated zones were more angular and less weathered than those in adjacent areas where less than 5% of erratics were made of Jotnian sandstone.

His explanation?  The concentrations have come from the breaking up of super-erratics, maybe hundreds or thousands of tonnes in weight, which have initially been transported intact by the overriding ice and have then been subjected to sufficient stresses adjacent to these big morainic ridges to cause them to be broken up.

Sounds perfectly reasonable to me.  If in Finland, why not elsewhere too?






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DONNER, JOAKIM, 1996. On the origin and glacial transport of erratics of Jotnian sandstone in southwestern Finland. Bull. Geol. Soc. Finland 68, Part 2, 72-83.

Abstract

Late Proterozoic Jotnian sandstone erratics were transported during the last Quaternary glaciation from the source area in Satakunta at the coast of south- western Finland and the bottom of the Bothnian Sea to the southeast as far as Estonia, Latvia and Russia. The frequencies of the sandstone erratics show that they were transported greater distances than indicators of other rocks in the southern parts of Finland. In addition, high frequencies in small areas, south of Salo and in Bromarv, indicate that there are or were small separate source areas of Jotnian sandstone outside the main area. This is supported by the distribution of erratics of Cambrian sandstone and Ordovician sedimentary rocks in the same area. The tracing of possible small occurrences of Jotnian sandstone or Palaeozoic rocks is, however, difficult in an area with numerous faults and fracture zones in the Precambrian bedrock, where the depressions are covered by thick Quaternary drift.

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