MARJUMAN-STEPTOEAN (CAMBRIAN) TRILOBITE BIOSTRATIGRAPHY OF THE COW HEAD GROUP, WESTERN NEWFOUNDLAND

The Cow Head Group of western Newfoundland provides the most complete record of Cambrian shelf-margin trilobite faunas of Laurentian North America. The trilobites, collected through a 40-year period by C.H. Kindle, are from allochthonous shelf boulders in debris flows that accumulated in a continental slope setting. Marjuman and Steptoean assemblages are diverse, yielding at least 190 species that represent more than 90 genera. The faunas are most similar to less completely known assemblages from Alaska, Greenland, Quebec, Vermont and Maryland, and demonstrate the long-term persistence of a distinct shelf-margin biogeographic region. Marjuman trilobites occur in 60 boulders and fall into four distinct faunas. Agnostoids, including Ptychagnostus atavus (Tullberg) and Hypagnostus parvifrons (Linnarsson), are common and facilitate correlation both with other parts of Laurentia and with other Cambrian continents. The Steptoean is represented by 23 boulders that yield three distinct faunas. More than half of the boulders are from an interval that correlates with the mid-Steptoean Dunderbergia Zone. The distribution of boulders is related to Laurentian sea level history. Boulders from intervals of relative sea level rise (Early Marjuman; Early Steptoean; Late Steptoean) are rare, but both highstands (e.g., Late Marjuman) and the mid-Steptoean lowstand are well represented by boulders.