NUMERICAL CALIBRATION OF WESTERN LAURENTIAN CAMBRIAN STRATIGRAPHIC SUCCESSIONS INTO THE GLOBAL GEOLOGIC TIME SCALE

In the newly published Geologic Time Scale 2020, no Cambrian radioisotopic ages are sourced from Laurentian stratigraphic sections despite the archetypal and well-studied Cambrian stratigraphy exposed throughout the western United States. To address this issue, we are employing tandem U-Pb geochronology on detrital zircons (DZs) from key stratigraphic sections of the Tonto Group of Grand Canyon to place maximum depositional ages (MDAs) on Laurentian trilobite zones which can be correlated to stage and series boundaries of the global Cambrian timescale. This approach was recently applied to the Tapeats Sandstone, the basal member of the Tonto Group (Karlstrom et al, 2020, Geology, v. 48, p. 425-430), which allowed placement of the first direct radioisotopic age constraints on the Laurentian Dyeran Stage and demonstrated the efficacy of using MDAs to refine the Cambrian timescale in western Laurentia. We extend this work, by combining precise DZ MDAs with biostratigraphy to place numerical age constraints on Tonto Group trilobite zones from Peachella iddingsi (upper Laurentian Dyeran Stage) to Bolaspidella (Laurentian Drumian Stage). The estimated ages of these zones range from 507 to 499 Ma; however, none of them have been verified with precise radioisotopic dating using chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-IDTIMS). Here we report the first preliminary DZ MDAs of approximately <503 Ma for the Ehmaniella Zone (Laurentian Topazan Stage) from the upper Muav Formation in the Tonto Group. We will further discuss how DZ MDAs can constrain the timing of two globally recognized yet numerically undated δ¹³C excursions, the Drumian Isotope Carbon Excursion (DICE) and the Steptoean Positive Isotope Carbon Excursion (SPICE), which correlate to the global Drumian and Paibian Stages, respectively. By combining CA-IDTIMS DZ MDAs with bio- and chemostratigraphy, we will more accurately integrate Laurentian stratigraphic successions into the global timescale and enhance our understanding of the timing and tempo of major Cambrian biotic and biogeochemical changes.