RECOGNITION OF THE BASE OF THE TULEAN STAGE (LOWER ORDOVICIAN) IN SOUTHERN LAURENTIA

The Tulean Stage, the third of four stages (ascending order) of the Ibexian Series, was based upon silicified trilobite faunas of western Utah and southern Idaho. Bases of the underlying Skullrockian and Stairsian Stages are traceable into eastern North America. However, elevated levels of endemism within the mid-Ibexian have impeded recognition of the Tulean outside its type area and led to the retention of regional stratigraphic equivalents (the Jeffersonian Stage of Flower) for Laurentia south of the paleoequator.

Adrain et al. (2009) redefined the base of the Tulean and selected a new boundary stratotype to coincide with a widespread mass extinction in the trilobite fauna. The newly defined base of the Tulean is recognizable in the El Paso Group of southern New Mexico and westernmost Texas on a similar succession of trilobite taxa comprising 1) extinction of a fauna that includes the Stairsian genus Hillyardina, 2) appearance of the asaphid Aulacoparina, and 3) appearance slightly higher of bathyurid trilobites along with the richardsonellid genus Menoparia. Within the El Paso Group, the Hillyardina fauna occurs in the basal few meters of the Jose Member of the Hitt Canyon Formation but is replaced upward by Aulacoparina in roughly the uppermost two thirds of that member. The occurrence of cross-bedded oolite and ooids as burrow-fill in the Jose is indicative of a significant eustatic sea-level change. The final bathyurid-rich fauna is associated with the lower McKelligon Formation.

In addition to the trilobite succession, the base of the Tulean is also closely constrained in the El Paso Group by a large (3 per mil) latest Stairsian negative carbon isotope excursion (the José excursion) only a meter or two below the base of the Jose and by the base of the Acodus deltatus – Oneotodus costatus conodont Zone at or only a few meters above the base of the overlying McKelligon Formation.