Databank

Workreport 2007-34

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Name:

Studies of Quaternary Deposits of Investigation Trench OL-TK13 at the Olkiluoto Study Site, Eurajoki, SW Finland

Writer:

Huhta, P.

Language:

English

Page count:

49

Summary:

The Quaternary deposits in investigation trench OL-TK13 were studied by the Geological Survey of Finland in autumn 2006. The investigations were carried out in three phases as the excavation progressed. Samples for grain size determinations were taken from vertical profiles, placed 25 m apart along the trench. The profiles extended from the soil surface down to bedrock or, if the bedrock was covered with water, to the water table. Between 6 and 7 kg of sediment was taken in a bucket with a shovel and placed in plastic bags for transport to the laboratory. The samples were first dried in the laboratory after which they were sieved. In addition, grain size distribution of the < 63 µm fraction was analyzed with the Sedigraph 5100 instrument.

Sedimentological observations of the sampling profiles were documented in field by drawing on a field observation form and the profiles were photographed using a digital camera except profiles 7 and 8. In addition, the excavated section was photographed along their whole length.

The length of OL-TK13 is approximately 250 m and 11 profiles were studied. The height of the profiles varied from 7.3 m to 2.3 m, with an average of 4.6 m. The till cover in OL-TK13 consist of two parts. The upper till has three layers. Surface layer is oxidized, brown sandy till, which is underlain by grayish brown intermediate layer of sandy till. This layer represents the grounwater table, which is moving up and down. The lower layer is unoxidized, grey sandy till. The thickness of the upper till varies from 2.3 to 5.5 meters. The upper till in OL-TK13 is classified as having a normal amount of cobbles and boulders with the size of the larger clasts varying from 6 to 60 cm. The edges of clasts have been eroded and rounded. The upper till has normal compactness.

The lower till has been spared in bedrock depressions. Part of it’s material has ascended the upper till. The lower till is compact, dark grey silty till with over 15 % of fine material (under 0.002 mm). The lower till is classified as having a normal amount of cobbles and boulders with the size of the larger clasts varying from 6 to 60 cm. The edges of clasts have been eroded and rounded. Part of the clast material is weathered.

The upper till in OL-TK13 was deposited in the last flow phase of the continental ice. Bedrock striations indicate that the ice moved in a WNW-ESW direction. The lower till bed has been deposited in flow phase from N to S according to one till fabric analysis. The till beds level out the bedrock topography.

No signs of postglacial faults were detected either in the tills or in the exposed bedrock. The tills showed no signs of disturbance related to bedrock movements.

Keywords:

overburden, glacial geology, till, stratigraphy, trench, grain size, Quaternary, Finland; Western Finland Province; Satakunta; Eurajoki; Olkiluoto

File(s):

Studies of Quaternary Deposits of Investigation Trench OL-TK13 at the Olkiluoto Study Site, Eurajoki, SW Finland (pdf) (5.5 MB)


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