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
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]
Further reading
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:.
References
- ^ a b Laufeld, S. (1974). Silurian Chitinozoa from Gotland. Fossils and Strata. Universitetsforlaget.
- ^ Creer 1973
- ^ Gray, Laufield & Boucot, 1974
- ^ 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.
- ^ 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.
- ^ "The Silurian Mulde Event and a scenario for secundo–secundo events". Transactions of the Royal Society of Edinburgh Earth Sciences 93: 135. 2002. doi:.
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