In a stratigraphic context, what is a “golden spike?” Explain why the choice of the Ludlow Bone Bed in the Welsh Borderland was rejected as the Golden Spike for the Silurian / Devonian boundary. What alternate section was chosen and why was it selected?
Although definitions are not strictly science, being able to communicate clearly is essential to make scientific progress. The Global Boundary Stratotype Section and Points (GSSP) are human-selected strata sections to mark boundaries between major time periods, usually defined by the appearance or disappearance of organisms. Like type sections used to define formal units in the stratigraphic code, the GSSP should be the best reference section available, a surface section with visible top and bottom contacts, and be complete and continuous with no faults and no long covered intervals. The exposure representing a period boundary needs to cover a range of facies and rich in fossils, to allow correlations between different types of fossils. It should be accessible, and protected (or at least unlikely to be destroyed) for future research.
The official boundary between the Silurian and Devonian is marked by the first occurrence of graptolite. Graptolites are pelagic creatures, so have a global distribution in marine sediments. Sections in the Czech Republic, Australia, Italy, Poland, Spain, Nevada, Quebec, Algiers, and Morocco were considered, with outcrops in the former Soviet Union, Morocco, Nevada, and the Czech Republic as the final candidates. The Golden Spike is the final selected outcrop near Klonk in the Czech Republic, a section with an uninterrupted succession of fossils, facies changes, and extensive exposure.