SILICEOUS MICROFOSSIL AND GEOCHEMICAL EVENTS RELATED TO THE LUNDGRENI (GRAPTOLITE) EXTINCTION, EARLY SILURIAN OF THE CANADIAN ARCTIC

Siliceous microfossil, graptolite, and geochemical data from measured sections in the Cape Phillips Formation, Canadian Arctic, provide a robust data set that allows for increased calibration of radiolarian faunal turnovers, radiation events, and recognition of paleoceanographically related changes during the early Wenlock through early Ludlow (Silurian). In particular, we recognize a sequence of events associated with a major and globally recognized extinction event in the graptolite community, the Lundgreni Event, which decimates the entire cyrtograptid lineage, and most of the monograptids and retiolitids. In the radiolarian community a major turnover corresponding to the first abundance of Inanigutta tarangulica group occurs within the early Homerian Cyrtograptus lundgreni-Testograptus testis Zone, prior to the Lundgreni Event. The I. tarangulica assemblage continues after the Lundgreni Event with no observed radiolarian extinctions. However; radiolarians occurring at the extinction horizon appear stressed, followed by a reduction in abundance that lasts until the end of the Wenlock. The Lundgreni Event is accompanied by both a geochemical and sedimentological signature. A +2.5 d13C_org excursion occurs at the Lundgreni Event and is interpreted to be paleoceanographic in nature, as it is seen in two sections of differing lithofacies. Sedimentologically, an increase in allodapic limestones and shallow water fossil debris into basinal sections indicates progradation, possibly due to a lowstand. These data indicate that there is no obvious relationship between radiolarian and graptolite faunal turnovers in the mid Wenlock, irrespective of their coexistance in surface waters. Although radiolarians appear stressed at the Lundgreni Event, they recover by the early Ludlow. Decreased radiolarian abundance directly following the Lundgreni Event appears to be related to a shoaling event shifting radiolarian-rich facies basinward.