Geology of Gotland

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A Silurian reef complex in Gotland. The reef belongs to the Sundre formation, and overlies the Hamra.

Gotland is made up of a sequence of sedimentary rocks of a Silurian age, dipping to the south-east. The main Silurian succession of

bioherm detritus, and terrestrial sediments, filled the basin. Reef growth started in the Llandovery, when the sea was 50–100 m (160–330 ft) deep, and reefs continued to dominate the sedimentary record.[1] Some sandstones are present in the youngest rocks towards the south of the island, which represent sand bars deposited very close to the shore line.[5]

Fossils from Gotland beaches (placed on notebook paper with 7 mm (0.28 in) squares).

The lime rocks have been weathered into characteristic

sea-stacks are preserved in places.[6]

The rocks of Gotland display signals of global extinction events, which take their name from parishes on the island: the Ireviken, Mulde and Lau events.

Stratigraphy

Burgsvik beds
are highlighted in blue.

The earlier Paleozoic of the island comprises the following formations, listed from youngest to oldest (i.e. from south to north).[7][8]

Ludlow

Wenlock

Llandovery

Quaternary geology

Ancylus Lake around 8 700 years BP. The relic of Scandinavian Glacier in white. Note that most of Gotland was submerged at this stage.
See also: Alvar and Baltic Klint

Gotland has mostly a subdued relief composed of flat erosion surfaces. Higher areas usually correspond to those of more-less pure limestone while lower areas have commonly a geology of marl.[2] The reason for this is that limestone is more resistant to erosion than marl.[9] Another general relief feature is that northwestern margin of the island is higher than the southeastern parts.[2] This higher northwestern coast is in part result of the southeast tilt of the sedimentary strata.[10] The northwest coast is straight and contain active cliffs.[2][10] On the other side the eastern coastline is irregular and sinuous.[10]

During large

glacial erosion are not as visible in Gotland as they are in mainland Sweden because of its sedimentary basement. Glaciation did however left thin blankets of fine-grained boulder-clay till. The boulder include rocks of Archean age transported from far-away regions of Fennoscandia. Where the till blanket is absent the bare rock surfaces are exposed.[2]

At the end of the last glaciation Gotland was fully summerged in the waters of the proto-Baltic Sea. The

transgressions left marks in form of beaches. These beaches are now located above current sea level due to post-glacial rebound and changes in sea level. Compared to the rest of Sweden Gotland host good examples of active coastal processes.[2]

There are few traces of river and stream erosion in Gotland. Some drainage in the island occurs through karstic systems, including caves.[2] The soils of Gotland are thin with calcareous till clay being the main parent material.[10]

Economic geology

The particular geology of Gotland has conditioned many aspects of human life and economic activity. The economic activities influenced by geology include forestry, farming, cement production and quarrying building stones.[11]

city wall of Visby
built from local limestone.

During the industrial age demand for Gotland limestone came from

iron works. People working in the quarries of Gotland have diminished from about 600 in the mid-1930s to 350 by 2010.[12] The intended opening of a new limestone quarry in the 2010s in northern Gotland has led to a substantial conflict between environmentalists and those who support the project.[13] In April 2015, the legal process regarding the allowance of the project was put on hold by Swedish courts until the Swedish government takes a decision or, if not, until August 31 of 2015.[14]

Exploratory wells have revealed the existence of petroleum oil of Lower Paleozoic age beneath Gotland.[15]

References

  1. ^ a b Laufeld, S. (1974). Silurian Chitinozoa from Gotland (PDF). Fossils and Strata. Universitetsforlaget.
  2. ^
    JSTOR 520895
    .
  3. ^ Creer, K. M (1973). Tarling, D. H.; Runcorn, S. K. (eds.). "A discussion of the arrangement of palaeomagnetic poles on the map of Pangea for Epochs in the Phanerozoic". Implications of Continental Drift to the Earth Sciences L. London, New York: Academic Press: 47–76.
  4. S2CID 22967281
    .
  5. ISSN 0016-786X
    .
  6. ^ Laufeld, Sven; Martinsson, Anders (22–28 August 1981). "Reefs and ultrashallow environments. Guidebook to the field excursions in the Silurian of Gotland". Project Ecostratigraphy Plenary Meeting.
  7. doi:10.1017/S0263593302000093
    .
  8. ISBN 978-91-44-05847-4
  9. ^ Eliason et al. 2010, p. 11
  10. ^ a b c d Behrens, Sven. "Gotland: Terrängformer". Nationalencyklopedin (in Swedish). Cydonia Development. Retrieved November 30, 2017.
  11. ^ Eliason et al. 2010, p. 5
  12. ^ Eliason et al. 2010, p. 41
  13. ^ Liljebäck, Lars-Erik. "Kalkbrottet som delar Gotland". Naturvetarna (in Swedish). Naturvetarna. Retrieved July 13, 2015.
  14. ^ "Mål om kalkbrytning på Gotland skjuts upp". Dagens Nyheter (in Swedish). April 10, 2015.
  15. ^ Zdanaviciute, O.; Lazauskiene, J.; Khoubldikov, A.I.; Dakhnova, M.V.; Zheglova, T.P. (2013). The Hydrocarbon Potential of the Baltic Basin: Geochemistry of Source Rocks and Oils of the Lower Paleozoic Succession. Beijing, China: AAPG Hedberg Conference.

Bibliography

  • Eliason, Sara; Bassett, Michael G.; Willman, Sebastian (2010). Geotourism highlights of Gotland. Tallinn. pp. 5, 41.
    ISBN 978-9985-9973-4-5.{{cite book}}: CS1 maint: location missing publisher (link
    )

Further reading

For a reconstruction of the facies of Gotland, presented as an east–west section, see page 25 of:

External links
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