A REVISED TRILOBITE ZONATION FOR THE LOWER ORDOVICIAN OF WESTERN LAURENTIA

The lettered sequence of trilobite zones devised by Ross in 1949 has remained the standard for western Laurentian Lower Ordovician biostratigraphy for over half a century with only minor modification. The zonation was the basis for the formal establishment of the Ibexian Series and its four stages in 1997, but little new data have been added since the classic monographs by Ross (1951) on faunas from southeastern Idaho and Hintze (1953) on those from western Utah.

Extensive new field sampling of silicified fossils on a meter scale from sections in Idaho, Utah, and Nevada, demonstrates that there are many more completely distinct, stratigraphically successive trilobite assemblages than recognized in the traditional Ross/Hintze zonation. Where the traditional scheme recognized five zones for the combined Tulean and Blackhillsian Stages, for example, we recently expanded this to 16 on the basis of our new collections.

Here, we extend this revised and more detailed scheme to the Skullrockian and Stairsian Stages, as well as the lowermost part of the western Laurentian Middle Ordovician. Similar increases in stratigraphic resolution result: four Stairsian zones are replaced with a new nine zone scheme, and a single upper Skullrockian zone with six new zones. All of the new zones are recognizable across the Great Basin region where relevant faunas are encountered.

Although the benthic trilobite faunas are highly endemic to western Laurentia, considerable wider correlation - in some cases even intercontinental - is possible due to the presence of common, widely distributed pelagic trilobites.