Please read the definition carefully. That’s for the entire site, not a point inside that you want.
geological= may not be the best. It’s for the geologic formations themselves. As an analogy, natural= isn’t used for bedrock benchmarks.
At most, an attribute should be used to show the natural= or geological= feature is a GSSP, eg designation= or protection_title= . For comparison, there exists geopark=yes , but I personally don’t like it much. The scalability of having a *=yes for everything is questionable. GSSP is even more niche than that. Unesco Global 'Geopark' tagging
For the spike itself, Seemingly valid objects mentioning it:
- Node: Golden Spike (1854687198) | OpenStreetMap
- Way: GSSP Lemme Carrosio (559733648) | OpenStreetMap
- Node: GSSP Fågelsång (4432783538) | OpenStreetMap
- Way: Elatina Formation-Nuccaleena Formation Boundary (1065385349) | OpenStreetMap (next to it is a invalid
board_type=without=boardor any feature tag)
Key:marker - OpenStreetMap Wiki may be the safest choice, describing the structure without complication. The described uses aren’t exhaustive.
I strongly disagree with using utility= for something that’s not a “utility” at all. It’s abused to show what the marker= is for. There was a discussion on alternatives viz message= . Proposal talk:Markers subject refinement - OpenStreetMap Wiki
boundary=marker is not suitable . There’s no boundary= feature for the geologic “boundary” , and it may be inappropriate for OSM. Don’t interpret them literally.
In general, all geologic markers should be considered together for a solution. Are there markers for faults, plate boundaries, etc? geologic=plate_boundary could be showing anything unmarked, and it may be a linear feature that’s furthermore topologically distinct from markers.