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 limestones and shales comprises thirteen units spanning 200-500 m of stratigraphic thickness, being thickest in the south, and overlies a 75-125 m thick Ordovician sequence.[1] It was deposited in a shallow, hot and salty sea, on the edge of an equatorial continent.[2] The water depth never exceeded 175–200 m,[3] and shallowed over time as bioherm detritus, and terrestrial sediments, filled the basin. Reef growth started in the Llandovery, when the sea was 50–100 m 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.[4]

The lime rocks have been weathered into characteristic karstic rock formations known as rauks. Fossils, mainly of rugose corals and brachiopods, are abundant throughout the island; palæo-sea-stacks are preserved in places.[5]

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[edit]

Geological map of Gotland. The Burgsvik beds are highlighted in blue, the Klinteburg in dark grey.

The island is composed of the following formations, listed from youngest to oldest (i.e. from south to north).[6]

  • Sundre Formation - Ludlow (upper Silurian)
  • Hamra Formation
  • Burgsvik Formation - terrestrial input; deposited during regression
  • Eke Formation
  • Hemse Formation
  • Klinteburg Formation
  • Fröjel Formation - terrestrial input; deposited during regression and topped with erosional sequence boundary.
  • Halla Formation
  • Silte Group
  • Lower & Upper Visby Formation
  • Tofta Formation
  • Högklint Formation - Ordovician[verification needed]

Holocene events[edit]

After the retreat and thinning of the Baltic Ice Sheet about 14,000 BP the island begun to rise slowly due to post-glacial rebound. The isostatic effects of the rebound coupled with the Baltic Sea's changes in sea level created a series of old shorelines and coastal cliffes in Gotland that are now out of the reach of the sea.

Further reading[edit]

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

  • Samtleben, C.; Munnecke, A.; Bickert, T. (2000). "Development of facies and C/O-isotopes in transects through the Ludlow of Gotland: Evidence for global and local influences on a shallow-marine environment". Facies 43 (1): 1–38. doi:10.1007/BF02536983. 

References[edit]

  1. ^ a b Laufeld, S. (1974). Silurian Chitinozoa from Gotland. Fossils and Strata (5). Universitetsforlaget. 
  2. ^ Creer 1973
  3. ^ Gray, Laufield & Boucot, 1974
  4. ^ Long, D.G.F. (1993). "The Burgsvik beds, an Upper Silurian storm generated sand ridge complex in southern Gotland". Geologiska Föreningens i Stockholms Förhandlingar (GFF) 115 (4): 299–309. ISSN 0016-786X. 
  5. ^ 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. 
  6. ^ "The Silurian Mulde Event and a scenario for secundo–secundo events". Transactions of the Royal Society of Edinburgh: Earth Sciences 93 (02): 135. 2002. doi:10.1017/S0263593302000093.