APPEARANCES AND EXTINCTIONS OF UPPER CAMBRIAN-LOWER ORDOVICIAN SPECIES COMMONLY COINCIDE WITH SEQUENCE-STRATIGRAPHIC BOUNDARIES

Cambrian-Ordovician strata in the House Range-Ibex Area of Millard County, Utah are ~15,000 ft (4575 m) thick and were deposited on a rapidly subsiding miogeoclinal platform. Miller et al. (2003, BYU Geology Studies) studied ~2700 ft (825 m) of carbonates within this interval and divided it into 13 sequences and smaller divisions (called packages) of variable thickness. This interval has no major hiatuses in Utah because of rapid deposition, although minor truncations and microkarst surfaces occur at some sequence and package boundaries. Larger hiatuses exist at correlative horizons in a cratonic section in Central Texas. There is no evidence that sea-level changes at these sequence boundaries are directly related to bolide impacts. Some sequence stratigraphic boundaries can be correlated to other continents. These strata contain abundant conodonts, trilobites, and less abundant brachiopods (both calcitic and organophosphatic). Detailed biozonations exist for conodonts and trilobites, and strata can be correlated among 18 measured sections in western Utah and to other parts of North America. Conodont correlations extend globally.

The upper Cambrian Millardan Series records the first major diversification of euconodonts. Sequence boundaries are at drops in sea level; the subsequent rises in sea level are characterized by appearances of new species of conodonts in evolving lineages. The first major extinction event is associated with the start of a major sea-level drop at the base of the Ibexian Series. Thin packages of strata, each more regressive than the previous package, record faunal extinctions at the sea-level drops and new appearances of taxa at the subsequent sea-level rises. Other sequence-stratigraphic boundaries within the Skullrockian Stage show a similar pattern of new taxa appearing just above package boundaries (used for decades as conodont subzone boundaries) and extinction events occurring at sequence boundaries that mark important drops in sea level (used for many years as conodont zone boundaries). Trilobite zonal boundaries are not so closely tied to sequence stratigraphic boundaries compared with conodonts. Ranges of organophosphatic brachiopods are also rather closely tied to sequence-stratigraphic boundaries, as are some calcitic brachiopods (but the latter are rather rare).