WN3 - Unaltered biomicrite
Click the microscope button to view a thin section for this sample.
Click the microscope button to view a thin section for this sample.

Fact sheet

WN3 - Unaltered biomicrite

This sample is a muddy limestone of Silurian (Wenlock) age, with numerous fossil fragments (bioclasts) distributed heterogeneously in fine-grained, brown micrite (very fine particles of recrystallized lime mud). This sample was collected near WN1 and WN2, but as it was further away from the the fault zone it escaped alteration by the hot fluids associated with that feature. There are still some sinuous, dark brown seams in this section representing deposits of insoluble iron oxy-hydroxides; these may have been formed during the initial stages of burial of the sediment, during diagenesis. In the lower left corner, a thick section of one of these seams seems to eat into a large, pale echinoderm plate and truncates curved brachiopod shells, testifying to the dissolution that occurred at the same time as the insoluble iron oxy-hydroxides were precipitated.

The limestone was laid down in a shallow tropical sea south of the equator around 428 million years ago. The area of Wren's Nest featured small coral reefs teeming with life, separated by shallow banks and troughs of lime mud; this sample represents the latter environment.

The limestone reefs were home to compound and solitary corals, bryozoans, crinoids, brachiopods, gastropods, cephalopods and trilobites. Fossils visible in this thin section indicate that shell debris was periodically washed into muddy troughs alongside the reefs. Bioclasts in this sample are much clearer than in WN1 and WN2. They include fragments of crinoid stem, echinoderm plates, fibrous and ribbed brachiopod shells, bryozoans, gastropods and corals. The thin, three-bumped shell of a trilobite can be found at (25.3, 3.3), near a bulbous gastropod. Some shell fragments have dark rims that mark where the shell margins have been micritised by endolithic algae eating into the shell.

There are few voids infilled with sparry calcite and the colour variation in the fine-grained matrix of this sample is less marked than in WN1 and WN2.

This sample was collected as part of the 'Macro to Micro' project.

Additional images
  • Hand specimen of limestone on black background
52.520756, -2.093354
About this collection

This Collection showcases the geodiversity of a classic geological site: the Wren's Nest National Nature Reserve in the West Midlands.

As well as displaying thin section and hand specimen views along with information setting them in the context of their landscapes, we also include perspectives and creative responses to the geological heritage of the sites from the local community.

Explore the stories of the rock layers at Saltwells and Wren's Nest NNRs, designed by students at King Edward VI School, Stourbridge:

This Collection was made possible by funding awarded to the 'Macro to Micro' project by the Natural Environment Research Council (NERC) under their 'Growing Roots' scheme.


Sample details

Collection: Wren's Nest
Rock-forming mineral
Category guide  
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Accessory minerals
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We would like to thank the following for the use of this sample: