TRILOBITE FAUNAS OF THE LOWER ORDOVICIAN JOSE MEMBER (HITT CANYON FORMATION, EL PASO GROUP) IN SOUTHERN NEW MEXICO AND WEST TEXAS

The Jose Member of the Hitt Canyon Formation is a thin (>20m), regionally extensive unit characterized by dark (medium-gray), oolitic packstone to grainstone that contrasts with the light-gray carbonates above and below it. Correlation of the Jose between New Mexico and Texas has been problematic due, in part, to the paucity of faunal control available. New carbon isotopic data reveals the presence of a significant negative excursion coincident with the base of the member in several sections, confirming that the Jose was deposited synchronously over its outcrop area. Although the onset of ooid production historically has been considered the result of a regressive event within the El Paso Group, we interpret it as evidence of a transgression and suggest that the member records maximum water depth. The dark color, strongly bioturbate fabric in at least some sections, and thin intervals of ribbon limestone consisting of thinly bedded lime mudstone intercalated with organic shales near the base of the unit in Texas all suggest deeper, rather than shallower conditions than the bounding lithologies.

The depauperate trilobite fauna of the Jose comprises 3 asaphid species (including Aulacoparia ? huygenae Flower), 2 species of the bathyurid genus Jeffersonia, 1 hystricurid species, and a few specimens of Leiostegium. Intriguingly, these elements vary in relative abundance across the region. In the Franklin Mts., TX, the fauna includes only uncommon hystricurids in association with rostroconchs and gastropods in fairly coarse intraclastic, bioclastic grainstone. A second assemblage numerically dominated by Jeffersonia with less common Aulacoparia ? and other asaphids was recovered from bioturbated, oolitic packstone in the Caballos and Florida Mts., NM. A third assemblage, dominated by A. ?huygenae with minor Jeffersonia, characterizes the fauna in the Cookes Range and Big Hatchet Mountains, NM.

Conodonts of the Macerodus dianae conodont Zone have been recovered from within the Jose and species of the Acodus deltatus – Oneotodus costatus conodont Zone were recovered from less than 10m above the top of the member. These conodont data suggest correlation of the Jose with member 1of the Fillmore Formation in the upper Stairsian Stage (Rossaspis superciliosa trilobite Zone) of the Ibexian Series standard in Utah.