Slab of algal mound limestone from lower Worland Limestone

Polished slab of algal mound limestone from the Worland. Most of
the skeletal material is platy calcareous algae, with only scattered
gastropods and brachiopods. The algal plates became fragmented
readily, providing insight into origin of the tiny rectangular algal fragments
that form a major fraction of skeletal material in Altamont and Bandera
limestones. In this rock, several once-continuous algal encrustations
are broken into short segments, but they have not been displaced from
their original positions. Fragmentation therefore probably occurred after
burial. Some extend almost entirely across the photo.

Some modern calcareous encrusting algae (rhodophyte family
Peyssonneliaceae) form an aragonitic coating on the undersides of
their photosynthesizing leafy fronds and mats, forming calcareous
plates. The algal plates in this limestone may have had a similar origin.
These plates are now composed of calcite, however. So, if originally
aragonite, they were replaced.

The algal plates did not closely encrust each other or the substrate,
but were separated by open space in which finely fragmented skeletal
material and lime mud accumulated as internal sediment. Some of the
spaces between calcareous plates were wide enough for burrowing
animals to inhabit and churn up the sediment.

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