PALEOECOLOGIC SIGNIFICANCE OF LATE IBEXIAN (LOWER ORDOVICIAN) TRILOBITES FROM THE SHALLOW BAY FORMATION (COW HEAD GROUP), WESTERN NEWFOUNDLAND

The Shallow Bay Formation (Cow Head Group), western Newfoundland, is composed of a series of allochthonous massive limestone conglomerates and autochthonous interbedded limestones and shales. This sequence was obducted onto the craton and represents a nearly complete platform margin biofaces otherwise lost to subduction. As such, it provides a unique opportunity to sample shelf margin environments, which are virtually absent elsewhere in Laurentia during the Ibexian. A large and highly diverse late Ibexian (likely Blackhillsian) trilobite fauna from the Shallow Bay Formation was collected by H.B. Whittington and C.H. Kindle from the Back Cove section, Cow Head Peninsula and has remained largely undescribed until now. Trilobites, which occur as disarticulated sclerites, were recovered from 29 large boulder-size clasts in the conglomerates. A total of 73 species from 46 genera representing 20 families were identified. Of these, 15 species and three genera are new and will be formally described in several publications currently in progress. Data from the Back Cove fauna were included in a Q- and R-mode cluster analysis in order to quantitatively illustrate biofacies patterns during the late Ibexian in Laurentia for the first time. A presence-absence dataset for this time slice was compiled using data from five of the largest collections from Back Cove as well as previously published studies of late Ibexian trilobites from Laurentia. Four biofacies are discernible on the resulting two-way dendrogram. These correspond well with respect to environment, but not necessarily taxonomic composition, with previously published studies for the earliest Whiterockian.