Redeposited Elvinia Zone (Upper Cambrian) trilobites from the Collier Shale, Ouachita Mountains, west-central Arkansas

Abstract

Trilobites assigned to the Elvinia Zone (Lower Franconian, Upper Cambrian) have been recovered from the Collier Shale in the Ouachita Mountains of west-central Arkansas. The trilobites are preserved as disarticulated exuviae, and are found in both dark-colored limestone clasts and dark, thinly-bedded carbonate layers. Field observations and petrologic/petrographic evidence indicate that these trilobites were originally deposited in a deep-water, outer shelf setting, and that shortly after deposition they were transported by sediment gravity flows to the continental slope or basinal plain.The faunal assemblage is very similar taxonomically and taphonomically to faunas of the Elvinia Zone previously reported from North American shelf environments. Two distinctive trilobite assemblages occur in the Collier Shale. Most common is a diverse fauna that is correlative with the middle to upper part of the Elvinia Zone in Missouri, Oklahoma, and Texas. One locality yielded numerous trilobites characteristic of the Irvingella major Subzone, which occurs at the top of the Elvinia Zone in these areas. These Elvinia Zone trilobites belong to the North American Faunal Province, a largely endemic group restricted to shelf environments around the perimeter of the North American craton. Their occurrence in the Collier Shale strongly indicates that the original site of Collier deposition was proximal to the Upper Cambrian shelf margin and that the Ouachita sedimentary section is not an exotic terrane.More than 7,900 trilobites were identified during this study; they are assigned to 26 genera and 27 species. The family Cliffidae is proposed for a group of trilobites that includes species of Cliffia Wilson, Aphelotoxon Palmer (junior synonym Ponumia Hu), Drabia Wilson, and possibly Elyaspis Kurtz. Three new genera are erected (Kymagnostus, Pyttstrigis, and Jessievillia) and 10 new species are described and illustrated (Apachia genaitholix, Cliffia magnacilis, Dellea planafrons, Iddingsia hapsis, Jessievillia radiatus, Kymagnostus harti, Neoagnostus? dilatus, Aphelotoxon lumaleasa, Pulchricapitus fetosus, and Pyttstrigis dicilia).