The Lower Silurian Osmundsberg K-bentonite is a
widespread ash bed that occurs throughout
Baltoscandia and parts of northern Europe. This paper describes
its characteristics at its type locality in the
Province of Dalarna, Sweden. It contains mineralogical and
chemical characteristics that permit its regional
correlation in sections elsewhere in Sweden as well as Norway,
Estonia, Denmark and Great Britain. The
<2 μm clay fraction of the Osmundsberg bed contains
abundant kaolinite in addition to randomly ordered
(RO) illite/smectite (I/S). Modelling of the X-ray
diffraction tracings showed the I/S consists of 18% illite
and 82% smectite. The high smectite and kaolinite content is
indicative of a history with minimal burial
temperatures. Analytical data from both pristine melt inclusions
in primary quartz grains as well as whole
rock samples can be used to constrain both the parental magma
composition and the probable tectonic setting
of the source volcanoes. The parental ash was dacitic to
rhyolitic in composition and originated in a tectonically
active collision margin setting.
Whole rock chemical fingerprinting of coeval beds elsewhere
in Baltoscandia produced a pronounced
clustering of these samples in the Osmundsberg field of the
discriminant analysis diagram. This, together
with well-constrained biostratigraphic and lithostratigraphic
data, provides the basis for regional correlation
and supports the conclusion that the Osmundsberg K-bentonite is
one of the most extensive fallout ash beds
in the early Phanerozoic. The source volcano probably lay to
the west of Baltica as part of the subduction
complex associated with the closure of Iapetus.