THE EARLY MIDDLE ORDOVICIAN FAUNAL SUCCESSION OF THE COW HEAD GROUP, WESTERN NEWFOUNDLAND AND ITS POTENTIAL FOR INTERNATIONAL CORRELATION

The Cow Head Group of western Newfoundland has the potential to produce an integrated biostratigraphic scheme based on numerous fossil groups and also is suitable for geochemical and magnetostratigraphy as a consequence of the relatively minor tectonic and metamorphic alteration that the sections have experienced. Recently we have also discovered an altered volcanic ash bed near the base of the Middle Ordovician that may provide an opportunity for radioisotopic dating. Graptolites, especially the isograptids and pseudisograptids, represent the most useful tool for the international correlation in this time interval and the Cow Head Group offers one of the most complete graptolitic successions of early Middle Ordovician age. In particular, this slope sequence, with its mixed shales and carbonates, produces an uninterrupted evolutionary succession in the Isograptus, Pseudisograptus, and Arienigraptus lineages that can be correlated precisely among several Paleozoic continents. This graptolite succession can be carried into a range of environments based on correlations using other fossil groups (trilobites, brachiopods, acritarchs, radiolarians, scolecodonts, sponges) as a result of their occurrence in limestones intercalated in the graptolitic shales in the Cow Head Group. Diverse and biostratigraphically significant chitinozoan faunas occur commonly together with graptolites at many levels in the Lower to Middle Ordovician strata. These occurrences provide the basis for a high resolution chitinozoan biozonation that can be used to correlate into non-graptolitic successions with confidence worldwide. Conodonts are also common and diverse in Cow Head Group. The evolution of the Periodon conodont lineage, in particular, permits precise correlation of the proposed base of the Middle Ordovician Series at the two major stratotype candidates into other conodont bearing successions, including both cool and warm water platforms. Hence, although not a stratotype candidate succession, the integration of conodont, chitinozoan, and graptolite zonations and the potential for other means of correlation demonstrate that the Cow Head Group is key source of insight into the international correlation of Lower and Middle Ordovician strata.