25-9 Unfortunate omission—a “Cordylodus” andresi Zone between the legacy Eoconodontus and C. proavus (emended) zones is required for latest Cambrian stratigraphic analysis and global correlation

Session: Refining the Cambrian—Biotas, Multiproxy Correlations, Workable Global Divisions, and Paleogeography: Discussions in Honor of Fred Sundberg

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Terminal Cambrian (Stage 10/Lawsonian Stage) euconodont diversification and carbon isotope excursions allow greater correlation reliability and calibration, particularly in oxic shelf facies, than coeval trilobites and agnostids. Stage 10’s base is best defined by onset of the HERB carbon isotope excursion just above the Eoconodontus notchpeakensis LO (lowest occurrence; i.e., Eoconodontus Zone base [E. Z., below]). Higher in Stage 10, utility of the oldest strongly denticulate euconodonts (cordylodans) in correlation and understanding stratigraphic continuity is compromised by definition of the legacy C. proavus Zone [C. p. Z.] base at a euconodont turnover purportedly at the C. p. Z. base in SW Laurentia (west-central Utah). But, the lowest C. p. specimens are actually “C.” andersi, which persists up into the C. p. range. The lower “C.” andresi interval has been arbitrarily retained in the C. p. Z. for “stability.” This decision has confused Stage 10 biostratigraphic and depositional syntheses. Indeed, a locally unconformity-bound “C.” andresi Zone with “C.” andresi and the highest euconodonts otherwise limited to the E. Z. has long been documented below the C. p. LO in temperate shelf facies in Baltica (mainland Sweden, Öland, Estonia, St. Petersburg area) and west Gondwana (Oaxaca State, Mexico). Similarly, slope and shelf sections on tropical paleocontinents (NE Laurentia, NW Vermont; Iran; North China; Sibiri, Mali Karatau, Kazakhstan) show “C.” andresi overlaps upper E. Z. conodont ranges below the C. p. LO. This relationship appears in western Utah, where replacement of the E. Z. by the legacy C. p. Z. features a monospecific “C.” andresi assemblage with Eurekia apopsis (“symphysurinid trilobite biomere”) at a facies change—a possible cryptic unconformity with early onlap suggested by a thin (15 cm) bed with “C.” andresi and E. Z. taxa. The “C.” andresi Zone is longer ranged in Baltica (middle Peltura–lower Acerocarina trilobite superzones). This is compatible with the thin lens of a “C.” andresi Zone with E. Z. taxa in western Utah either marking a significant hiatus at the E. Z.–legacy C. p. Z. contact or a very diachronous “C.” andresi LO. For “stability” in correlations, the C. p. Z. should be divided into a lower “C.” andresi Zone (locally 2–6 m) and higher C. p. Z. (emended), per the North American Stratigraphic Code.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-5537

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