MAXIMUM DEPOSITIONAL AGE: A CASE STUDY OF THE BOOK CLIFFS, UT USING RAPID (1,200 ANALYSES/HOUR) U-PB GEOCHRONOLOGY (Invited Presentation)

Detrital zircon maximum depositional ages (MDAs) can provide age control for siliciclastic strata that lack volcanic horizons or biostratrigraphy. However, there is debate over how best to calculate MDA, with methods involving averaged clusters of youngest grains, Gaussian fitting, likelihood modeling, and relying on the youngest single age. MDA methods generally benefit when more young grains are identified. To this end, we applied rapid (3 s/analysis, 1,200 analyses/hr) U-Pb geochronology by laser ablation ICP-MS to Campanian (83.6–72.1 Ma) strata of the Book Cliffs, Utah. We gathered 12,448 concordant ages from 11 samples collected from Price, Horse, Tusher, and Thompson canyons immediately below and above the lower Castlegate unconformity. The Campanian subset of these detrital zircons were reanalyzed using a more traditional rate of ~30 s/analysis. In total, 23 Campanian detrital zircons were identified. Most ages overlap within uncertainty for rapid and traditional methods, with uncertainties on average four times higher using rapid acquisition (~4%). Although this case study is not necessarily ideal to establish the merits of different MDA methods due to the paucity of young grains in Book Cliffs strata, it highlights several challenges with current MDA methods and represents a common situation for many researchers. For individual age distributions, several MDA methods are reduced to the youngest single grain age when there is a single young outlier, and clustering approaches either do not produce an MDA or reduce to inappropriate averages. Youngest single ages underestimate true depositional ages. Composite ages of the Blackhawk and Castlegate formations show that rapid acquisition ages produce a range in MDAs of ~4 Myr with some stratigraphic age inversions, whereas the MDA range reduces to 1.5–2.2 Myr using traditional (i.e., 30 s/analysis) and has no age inversions. Rapid acquisition is best used to initially scan samples for young grains, followed by re-analysis of with higher precision methods; however, logistical hurdles remain in streamlining the logistics of low and high precision analytical work. How best to ascribe MDA will always remain in the hands of the researcher. As such, we offer an expandable MATLAB-based software package, DZmda, that calculates commonly used MDA methods.