Geology of Gotland

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

The geology of Gotland is made up of a sequence of sedimentary rocks of a Silurian age, dipping to the south-east. Gotland is the largest island of Sweden, and is located in the Baltic Sea. The main Silurian succession of limestones and shales comprises thirteen units spanning 200–500 m (660–1,640 ft) of stratigraphic thickness, being thickest in the south, and overlies a 75–125 m (246–410 ft) thick Ordovician sequence.[1] Precambrian shield rocks that underlie these sediments are found 400 to 500 meters sea level.[2] Sedimentary rocks cropping out in Gotland were deposited in a shallow, hot and salty sea, on the edge of an equatorial continent.[3] The water depth never exceeded 175–200 m (574–656 ft),[4] 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 (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 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.[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.

  1. ^ a b Laufeld, S. (1974). Silurian Chitinozoa from Gotland (PDF). Fossils and Strata. Universitetsforlaget.
  2. ^ Cite error: The named reference Rudberg1967 was invoked but never defined (see the help page).
  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. ^ Gray, Jane; Laufeld, Sven; Boucot, A.J. (19 July 1974). "Silurian Trilete Spores and Spore Tetrads from Gotland: Their Implications for Land Plant Evolution". Science. 185 (4147). Science: 260–263. Bibcode:1974Sci...185..260G. doi:10.1126/science.185.4147.260. PMID 17812053. S2CID 22967281.
  5. ^ Long, D.G.F. (1993). "The Burgsvik beds, an Upper Silurian storm generated sand ridge complex in southern Gotland". Geologiska Föreningen i Stockholm Förhandlingar. 115 (4): 299–309. doi:10.1080/11035899309453917. 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.