TANDEM U-PB DETRITAL ZIRCON DATING TO CALIBRATE THE TEMPO AND DYNAMICS OF THE CAMBRIAN TONTO GROUP TRANSGRESSION IN WESTERN LAURENTIA

A type example of the Sauk transgression is the Cambrian Tonto Group of the Grand Canyon (Tapeats Ss., Bright Angel Sh., Muav Ls.). New tandem U-Pb detrital zircon (DZ) dating is underway to calibrate the tempo and dynamics of this transgression. Our first step involves LA-ICPMS DZ dating to identify Tonto Group samples with young detrital grains. This is a non-trivial step as shown by an uneven distribution of 620 Cambrian grains discovered amidst the >10,000 dated Precambrian grains from Cambrian rocks in the region. The LA-ICPMS ages of the young grains are useful for general bracketing and correlations, but are inadequate for precisely dating key horizons. Thus as a second step we have begun applying CA-IDTIMS dating to a subset of these young grains to reduce analytical uncertainty from 3-15 Ma to 0.5 Ma. Combining such constraints stratigraphically allows maximum depositional ages (MDAs) to be approximated, and where MDA's decrease upward in a section, to test and calibrate key biostratigraphic and chemostratigraphic transitions. For example, we used this approach to show that the Grand Canyon’s Sixtymile Formation is Cambrian rather than Precambrian; it yielded DZ-based MDAs that decrease upsection from 527.4 ± 0.4 to 508.6 ± 0.5 Ma. The overlying Tapeats Ss. in the eastern Grand Canyon also has an MDA of 508.5 ± 0.3 Ma. The third step of our approach is using these U-Pb constraints to test hypotheses for globally synchronous biochronology that uses trilobite zones calibrated at dated GSSPs and reference sections. For example, the Bright Angel Sh. in the eastern Grand Canyon contains Glossopleura Zone trilobites thought to have lived ~507-506 Ma and the Muav Ls. contains ~502-499 Ma Bolaspidella Zone trilobites. Thus, the ~250 m thick Tonto Group in the eastern Grand Canyon was deposited between 507 and 499 Ma. The ~830 m thick western Grand Canyon Tonto Group is slightly older as the Bright Angel Sh. there contains ~514-512 Ma “Olenellus Zone” trilobites. Paradoxically, our LA-ICPMS Tapeats Ss. MDA (506 ± 3 Ma; n = 28/680) conflicts with the hypothesized Olenellus Zone ages. CA-IDTIMS dating of these grains is underway to resolve this apparent conundrum and further address research questions on Cambrian systems such as drivers for global punctuated evolution and marine transgression.