L350 - Argillaceous limestone

L350 hand specimen. Note arthopod skeleton.

L350 thin section, plane-polarised light. Note angular quartz.

L350 thin section, cross-polarised light. Note carbonate bioclasts.

L350 thin section, plane-polarised light. Note fine calcite mosaic.

L350 thin section, cross-polarised light. Note brown (organic) impurity.

L350 thin section, plane-polarised light. Note argillaceous (clay grade) limestone.

L350 thin section, cross-polarised light. Note high brown (organic) content.

Age

Jurassic

Location

Purbeck
Near Salisbury

Hand Specimen

Pale yellow-grey mudstone containing the fossil remains of a small arthropod with a segmented exoskeleton.
The rock is banded and shows some surface discolouration due to weathering.
Rock fizzes with acid, so contains carbonate.
This is a soft, though well consolidated limestone (the surface has been scratched).

Thin-section

Moving across the slide, perpendicular to the laminations, there are alternations (gradual transitions) between calcareous mudstone and argillaceous limestone. One band is rich in angular quartz grains (0.1 to 0.2 mm), and contains carbonate shell fragments.

Calcite mudstone – fine-grained mosaic of calcite, small proportion of transparent brown impurity and a few small grains of quartz.

Argillaceous limestone – even finer grained calcite mosaic, more abundant brown material (probably organic) than in the calcite mudstone.

Rock History

Low energy, aqueous environment.

Advanced Notes

The fossil remains are those of Archaeoniscus, a small crustacean (which belongs to the arthropods), of supposed freshwater origin. With this extra knowledge it may be inferred that the rock was deposited in fresh water.

Rock Name

argillaceous limestone