Crinoid Geopetal Infilling

Crinoid: Geopetal Infilling

A geopetal is in fact a fossil spirit level and is extremely useful for mapping purposes

On death the crinoid sinks to the sea bed. The skeleton is broken down sometimes allowing mud held in suspension to fill in available cavities. Sometimes the stem and / or the crinoid calyx collects mud as it settles out of suspension. Ideally any remaining space would need to be filled with sparry calcite. If subsequently the rocks were tilted, the geopetal will preserve the horizontal of when it was formed.

The crinoid calyx in the photograph is a good example of a fossil spirit-level. The top half of the cavity has been completely recrystallised with sparry calcite, whilst the bottom half contains sediment.

Use of geopetal fabrics i.e. the partial sediment infills of shell cavities (in this case a crinoid calyx)

1. Geopetal orientations in horizontally bedded limestone: 1.(a) as deposited; 1.(b) after tilt. Note that the geopetals are always parallel to the bedding of the enclosing limestone.

2. Geopetal orientations in limestone deposited with a depositional dip:
2.(a) as deposited; 2.(b) after tilt. Note that there is always an angular variance between the geopetals and the bedding of the enclosing limestone.

3. Geopetal orientations in a boulder bed. Each block contains consistent geopetal orientations within the blocks, and differing orientations between the blocks. Good examples of these can be seen in the Cracoe Reef Belt near Rylstone, Skipton, North Yorkshire.