NEW INSIGHTS INTO STEPTOEAN (LATE CAMBRIAN) SEDIMENTATION AND STRATIGRAPHY OF THE NORTHERN U.S. APPALACHIANS FROM SMITHSONIAN TRILOBITE COLLECTIONS

Pre-Elvinia Zone Steptoean (Late Cambrian) trilobites are known from two localities in the northern U.S. Appalachians: 1) from the Schodack (Germantown or Hatch Hill) Formation at the Elizaville locality of Bird and Rasetti (1968) in Dutchess county of southeastern New York; and 2) from the lower Gorge Formation at Highgate Gorge in northwestern Vermont (Gilman Clark and Shaw 1968). The Steptoean age of the trilobite-bearing strata can be confirmed by analyzing carbon isotopes of their micritic carbonate matrix for the record of a well-documented, global in scope, large Steptoean positive carbon isotope excursion (a.k.a. SPICE). We have previously examined exposures at both of these field localities (Glumac and Spivak-Birndorf 2002; Glumac and Mutti 2007), and have recently also analyzed the matrix of the trilobite specimens from these sites stored at the Smithsonian National Museum of Natural History in Washington, D.C.

Both field and museum specimen data confirm the presence of Steptoean carbonate debris flows and olistoliths in poorly exposed and less than 20 m thick continental slope deposits in New York. This was the first documentation of the Steptoean excursion in the northern U.S. Appalachians. Fieldwork in Vermont, on the other hand, did not document any elevated carbon isotope values characteristic of the Steptoean excursion, but the museum trilobite specimens from this locality have δ¹³C values of up to +5.4 ‰ VPBD confirming their Steptoean (Dunderbergia Zone) age. The likely reason for the absence of the trilobite-bearing strata in the field today is that the fossiliferous layer, which was only about 1 meter thick and probably lenticular in shape, was extensively sampled and subsequently completely eroded away from the succession of continental slope deposits exposed underneath a dam along the banks of the Mississquoi River. The results of this study further support highly reduced carbonate production and condensed sedimentation in response to a sea-level fall corresponding to the Sauk II-III sequence boundary and to the maximum Steptoean carbon isotope excursion. This can also explain the absence of documented Steptoean age carbonate strata from shallow marine successions in the northern U.S. Appalachians.