Name: Orsten Location: Sweden Age: Upper Cambrian

Geological Setting and Age

The Orsten fossils are found in rich calcareous nodules, which are enclosed in dark, organic-rich alum shales. The Orsten rocks were deposited in the Upper Cambrian and they are collected in quarries and lying loose, on the Baltic shore (Tang, 2001).

Orsten, a term that means 'smelling or stinking stone', is a sulphurous anthraconitic limestone, occurring as concretions of about 0.1 - 2 m in diameter, or as large flat lenses within the Alum Shale (Muller, 1990). The limestone is rather carbonaceous and often petroliferous. Most of the sediment deposition was under very low energy conditions.

There are six types of limestone lithology in the region (Tang, 2001):

1. Black-gray micritic to sparry thin-layered limestones with Orsten and generally sorted, slightly compressed megafossils.
2. Beige sparry limestones with fossil preservation similar to 1.
3. Black fine-grained limestones with high organic content and lower fossil concentration.
4. Light gray micritic to sparry limestones composed of coarse fossil hash.
5. Black micritic thin-layered fossil hash limestones.
6. Whitish-gray weathered limestone with fine fossil hash.

Petrological and sedimentological analyses of the shales suggest they were deposited slowly offshore under anoxic conditions. The distribution of pyrite in the limestones and a high organic content indicates reduced levels of oxygen on the seafloor during deposition. The carbonates were formed during times of increased productivity and oxygenation (Tang, 2001).

For pictures of the Orsten rock formation, see Dieter Walossek's homepage.