BIOSTRATIGRAPHIC AND PALEOECOLOGIC SIGNIFICANCE OF LATE CAMBRIAN (STEPTOEAN) TRILOBITE FAUNAS FROM THE COW HEAD GROUP, WESTERN NEWFOUNDLAND

The Cow Head Group of western Newfoundland provides a glimpse of the faunas and facies of a Cambrian-Ordovician shelf margin that was destroyed during the Taconic Orogeny. It consists of a sequence of shelf margin-derived debris flows that accumulated in a continental slope setting. This was then thrust onto an autochthonous shelf sequence as part of the Humber Arm Allochthon. Limestone boulders recovered from the debris flows provide a unique record of shelf-margin trilobite fauna. Steptoean boulders yield at least three distinct trilobite faunas. An assemblage dominated by Aphelaspis, Blountia and agnostoids such as Innitagnostus is comparable to other low diversity faunas that appear across North America following the trilobite extinction event at the base of the Steptoean Stage. Most of the boulders yield a more diverse assemblage that includes such genera as Glyptometopsis, Pterocephalops, Bathyholcus and Dunderbergia. This fauna most likely correlates with the Dunderbergia Zone of the Great Basin. A few boulders contain a younger assemblage that includes Irvingella, Buttsia and an undescribed pterocephaline that probably correlates with the Elvinia Zone of cratonic sequences. Faunal similarities are greatest with other Steptoean shelf-margin sequences, including the Frederick Limestone of Maryland, the Gorge Formation of Vermont, and the Jones Ridge Limestone of Alaska. Together, the trilobite assemblages of the Cow Head and these other formations document the distribution of a widely-distributed, shelf-margin biofacies that is quite distinct from those of coeval facies of the craton and the Cordilleran miogeocline.