The sedimentology of Lower Palaeozoic black shales from the shallow wells Skelbro 1 and Billegrav 1, Bornholm, Denmark

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Previous studies of the Lower Palaeozoic shales on Bornholm have mainly been based on the outcrops
along the streams. The outcrops provide data on the lateral continuity of the facies while the selection of
(2-3 cm wide) cores for thepresentstudy focused the attentionon the vertical sequence offacies. Cores of
high quality were obtained and have been found to provide an excellent basis for a study of structures,
sediment composition and diagenesis. In the outcrops of shale fossils are locally present in high numbers
and such levels are also recognized in the cores.

Seven sedimentary facies, ranging fromblack mudshale to greyish mudstone and silt-streakedshale, are
distinguished in the Middle Cambrian to lowermost Silurian shales. Gamma-ray logs were run in the two
wells and a convincing correlation to the cored sequence can be demonstrated. Gamma-ray logs provide thus a good means of correlation to wells where no cores have been cut. The shales are all interpreted as deposited in an epicontinental sea due to their geological setting i.e. the association with shallow water carbonates (Andrarum Limestone and KomstadLimestone) and their stratigraphicalposition above the shelf to shoreface sandstones of the Læså formation.

Three facies associations are distinguished: The mudshale association comprises black organic-rich
shales which represent a low-energy anoxic depositional environment which prevailed in the Middle Cambrian to Lower Ordovician. The mudstone association istypical of the Middle andUpper Ordovician and represents a continuation of low-energy environments though mottling indicates that ventilationimproved in certain periods. The siltshale association represents higher energy environments which were dominant in the lowermost Silurian.

A well defined Upper Ordovician CU sequence probably reflects the global eustatic fall in sea level
caused by the extensive glaciation in Gondwanaland. In the late Silurian the average rate of deposition
increased in response to the approaching Caledonian orogeny.
Sider (fra-til)151-173
Antal sider23
TidsskriftBulletin of the Geological Society of Denmark
StatusUdgivet - 1989
Udgivet eksterntJa


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