A NEW TRILOBITE FAUNA AND ASSOCIATED CARBON ISOTOPIC EXCURSION AT THE TOP OF THE PTYCHASPID BIOMERE IN EASTERN ALASKA: WHITTLING DOWN A GAP IN THE LATE CAMBRIAN TIME SCALE

Reconstruction of the environmental conditions that prevailed during the faunal turnover at the top of the Ptychaspid Biomere near the end of the Cambrian has been hampered by a stratigraphic gap created in most places by a third-order drawdown of sea level that peaked during deposition of the Apoplanias trilobite Zone. High-resolution (sub-meter-scale) sampling of basal Skullrockian Stage successions throughout Laurentian North America has revealed the presence of an unconformity at this level, confirmed by the absence of the underlying Tangshanaspis Zone and/or the basal strata of the Apoplanias Zone, where the eponymous genus is most abundant. This thin interval, the Apoplanias Epibole, marks the point of minimum diversity attained in Laurentian platform faunas after extirpation of the fauna of the Tangshanaspis Zone but prior to the appearance of other genera, such as Parakoldinioidia, higher in the Apoplanias Zone. Even in areas where both the Tangshanspis Zone and Apoplanias Epibole have been documented in shallow marine facies, such as the southern Oklahoma aulacogen and northern Rocky Mountains (Wyoming/Montana), there is physical evidence that the contact between the two zones is unconformable.

Recent sampling of a shelfbreak to upper slope succession at Jones Ridge in easternmost Alaska yielded a new fauna and prominent, previously unreported negative Carbon isotopic excursion that identify several meters of strata at that locality as an uppermost segment of the Tangshanaspis Zone that has never been documented elsewhere. An isotopic excursion closely associated with the extinction horizon raises interesting questions regarding the potential cause(s) of the faunal turnover, and also might prove useful for identification of precisely age-equivalent strata/events on other paleocontinents, but only if similar and coeval Shelf Margin or Lowstand Systems Tracts are found and sampled with sub-meter-scale precision. Associated basal Ibexian trilobite and conodont (Hirsutodontus hirsutus Subzone of the Cordylodus proavus Zone) faunas are essential to avoid confusion of the new isotopic excursion with the only slightly older HERB event near the base of the Eoconodontus Zone in the subjacent Sunwaptan Stage.