Why has it been difficult to select a complete rock section for the Cambrian-Ordovician boundary?

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Cambrian paleogeography — Cambrian Period Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the late Cambrian Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

Selecting a complete rock section for the Cambrian-Ordovician boundary has proven difficult primarily due to the presence of minor unconformities, which are interruptions in the deposition of sedimentary rock.1

An ideal Global Standard Section and Point (GSSP) requires an uninterrupted stratigraphic section to provide a clear, continuous record of the transition between these two periods. The existence of these gaps in the geological record complicates the identification of such a site. Furthermore, while the boundary is formally marked by the first appearance of the conodont Iapetognathus fluctivagus and specific trilobites, the lack of a perfectly continuous, undisturbed sequence makes the selection of a definitive reference point challenging.123