L341 - Calcareous mudstone

Lower Carboniferous

Location

Very dark grey to black mudstone. 
Faint, fine-scale laminations visible on cut surface. 
Not fissile - does not readily split along bedding planes.
Contains fossils and fossil imprints. 
Some glinting flakes of mica. 
This rock fizzes gently with acid, indicating that it contains some carbonate. 
Weathered to light brown-grey in places.

Laminations are very clear when the thin-section is held up to the light. They are mainly planar, but some wrap around lighter regions in the rock.

Under the microscope, the rock is brown, with fine dark brown streaks, laminated.
Mainly carbonate, some white mica, rare quartz.
Bioclasts, mainly along bedding surfaces, probably gastropods and bivalves. Some of the shells have been dissolved. The spaces left have been filled by secondary calcite.
Sparry carbonate cement.

Fine-grained, so formed in a very low energy environment. 
Shells are too large to have been swept in by such low energy currents, so the shells are in life position. 
If the shells are indeed gastropods and bivalves the rock could be marine or non-marine. The presence of brachiopods would indicate a marine environment.

calcareous mudstone