2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 3
Presentation Time: 8:00 AM-12:00 PM


KARIM, Talia S., Department of Geoscience, Univ of Iowa, 121 Trowbridge Hall, Iowa City, IA 52242 and ADRAIN, Jonathan M., Department of Geoscience, The University of Iowa, 121 Trowbridge Hall, Iowa City, IA 52242, talia-karim@uiowa.edu

The Cow Head Group comprises a unique series of shelf-slope sediments, obducted onto the craton and representing a platform margin biofacies otherwise entirely lost to subduction. The rich trilobite faunas of the group (mostly from the more proximal Shallow Bay Formation) have been critical to understanding Cambrian and Middle Ordovician trilobite evolution. The Lower Ordovician (Ibexian) faunas, however, have never been studied.

Trilobites occur in boulders in shelf-slope derived megaconglomerates throughout the Shallow Bay Formation. Large collections made in the 1950s and 1970s, along with recent field sampling, provide a prolific series of collections spanning the Ibexian. Lithologies include trilobite grainstones and wackestones, with occasional algal buildups. The faunas confirm that the rich and diverse shelf margin "illaenid-cheirurid" biofacies known from the Middle Ordovician (Whiterockian) was well established by the mid-Ibexian. Although taxa characteristic of this biofacies in younger rocks (e.g., illaenids, styginids, cheirurids) occur in the faunas, they also contain early representatives of a variety of taxa (particularly trinucleoideans) which were distributed mostly in deeper water biofacies during the Middle and Upper Ordovician. The Ibexian collections contain representatives of the families Alsataspididae, Asaphidae, Bathyuridae, Cheiruridae, Dimeropygidae, Harpetidae, Hystricuridae, Illaenidae, Leiostegiidae, Nileidae, Olenidae, Pilekiidae, Pliomeridae, Raphiophoridae, Remopleurididae, Shumardiidae, Styginidae, and Telephinidae.

The faunas are strongly trilobite dominated (other taxa include plumulitid machaeridians, gastropods, and rare cephalopods). They include several of the earliest known occurrences (and in some cases the only known Lower Ordovician occurrence) of taxa assigned to the Whiterock Evolutionary Fauna. This fauna comprises families which diversified rapidly during the Ordovician Radiation and includes all trilobite survivors of the end-Ordovician mass extinction. Recent work has indicated that the Whiterock Fauna had its origins in Laurentian platform margin biofacies. Data from the Shallow Bay Formation suggest that the Whiterock Fauna was established in these settings considerably earlier than has been realized.