GSSP for Drumian Stage
Definition:The base of the Drumian Stage (= middle stage of the third series of the Cambrian System) is defined at the base of a dark-gray thinly laminated limestone (calcisiltite) layer, 62 m above the base of the Wheeler Formation, along the ridge crest of a section known informally as "Stratotype Ridge" in the Drum Mountains, northern Millard County, western Utah, USA. This level coincides with the lowest occurrence of the agnostoid arthropod Ptychagnostus atavus. Secondary indicators near the GSSP include the DICE negative δ13C excursion, the onset of a long monotonic 87Sr/86Sr isotopic shift, and a transgressive phase of an eustatic event.
Sedimentology:The GSSP occurs within the Wheeler Formation, which consists of a succession of light- to dark-gray, and lavender-gray, thin-bedded calcareous shales, interbedded with medium- to dark-gray, thin-bedded calcisiltite beds. The GSSP is defined at the base of a dark-gray thinly laminated limestone (calcisiltite) layer, 62 m above the base of the Wheeler Formation.
Primary Markers:Agnostoid arthropods:
The GSSP coincides with the lowest occurrence of the agnostoid trilobite Ptychagnostus atavus.
Just below the GSSP is the base of the Laurentian polymerid trilobite Bolaspidella Zone. Just above the GSSP is the highest occurrence of the cosmopolitan agnostoid arthropod Ptychagnostus gibbus.
Just above the GSSP is the base of the Baltic-Gondwanan conodont Gapparodus bisulcatus-Westergaardodina brevidens Zone.
72m above the base of the Wheeler Formation and 10m above the GSSP occurs the DICE negative δ13C excursion.
Correlation Events:Agnostoid arthropod FAD Ptychagnostus atavus
Other Locations around the World:The first occurrence of Ptychagnostus atavus is recognized worldwide. This species is used as a zonal guide fossil in Australia, South China, Kazakhstan, and Laurentia.
Notes on Derivation of Age:Estimated from trilobite-zone scalings
Babcock, L. E., Robison, R. A., Rees, M. N., Peng, S., and Saltzman, M. R., 2007. The Global boundary Stratotype Section and Point (GSSP) of the Drumian Stage (Cambrian) in the Drum Mountains, Utah, USA. Episodes 30/2, p.85-95.
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