The Silurian of Gotland (Sweden): facies interpretation based on stable isotopes in brachiopod shells |
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Authors: | Chr Samtleben A Munnecke T Bickert J Pätzold |
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Institution: | 1. Geologisch-Pal?ontologisches Institut, Universit?t Kiel, D-24118, Kiel, Germany 2. Fachbereich Geowissenschaften, Universit?t Bremen, D-28334, Bremen, Germany
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Abstract: | The Silurian of Gotland, Sweden, consists of 440 m of carbonate deposits. Repeatedly, uniform sequences of micritic limestones
and marls are interrupted by complex-structured reefs and by adjacent platform sediments. Generally, the alteration of facies
is interpreted as the result of sea-level fluctuations caused by a gradual regression with superimposed minor transgressive
pulses. The purpose of this study is a facies interpretation based on both field observations and stable isotope measurements
of brachiopod shells. Approximately 700 samples from stratigraphically arranged localities in different facies areas have
been investigated. The carbon and oxygen isotopes show principally parallel curves and a close relationship to the stratigraphic
sequence. Lower values occur in periods dominated by deposition of marly sequences. Higher values are observed in periods
dominated by reefs and extended carbonate platforms. The oxygen isotope ratios are interpreted to reflect paleosalinity changes
due to varying freshwater input, rather than to paleotemperature. Carbon isotope ratios are believed to have been connected
to global changes in the burial of organic carbon in black shales during periods of euxinic deep water conditions. Consequently,
the facies succession on Gotland results from global paleoclimatic conditions. Changes in terrigenous input due to different
rates of weathering and freshwater runoff, rather than sea-level fluctuations, control the carbonate formation of the Silurian
on Gotland. |
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Keywords: | Silurian Gotland stable isotopes brachiopods carbonate facies |
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