Geology of Gotland

Geology of Gotland

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.cite book
author = Laufeld, S.
year = 1974
title = Silurian Chitinozoa from Gotland
publisher = Universitetsforlaget
series = Fossils and Strata
number=5 ] It was deposited in a shallow, hot and salty sea, on the edge of an equatorial continent. [Creer 1973] The water depth never exceeded 175–200 m, [Gray, Laufield & Boucot, 1974] 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. 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.cite journal
author = Long, D.G.F.
year = 1993
journal = Geologiska Föreningens i Stockholms Förhandlingar (GFF)
title = The Burgsvik beds, an Upper Silurian storm generated sand ridge complex in southern Gotland
volume = 115
number=4
pages = 299–309
issn = 0016-786X ]

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. [cite journal |author = Laufeld, Sven | coauthors = Martinsson, Anders | title= Reefs and ultrashallow environments. Guidebook to the field excursions in the Silurian of Gotland | journal = Project Ecostratigraphy Plenary Meeting | date = 22–28 August, 1981 ]

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.

tratigraphy

The island is composed of the following formations, listed from youngest to oldest (i.e. from south to north). [Cite journal
doi = 10.1017/S0263593302000093
title = The Silurian Mulde Event and a scenario for secundo–secundo events
year = 2002
journal = Transactions of the Royal Society of Edinburgh Earth Sciences
volume = 93
pages = 135]

*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 - OrdovicianVerify source|Not certain about this age|date=July 2008

Further reading

For a reconstruction of the facies of Gotland, presented as an east-west section, see page 25 of:
*cite journal
author = Samtleben, C.; Munnecke, A.; Bickert, T.
year = 2000
title = 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
journal = Facies
volume = 43
issue = 1
pages = 1–38
doi = 10.1007/BF02536983

References


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