Measured stratigraphic section in the upper Schrader Bluff Formation (Late Campanian-Maastrichtian?), Ivishak River, Alaska

Preliminary Interpretive Report 2021-3, Measured stratigraphic section in the upper Schrader Bluff Formation (Late Campanian-Maastrichtian?), Ivishak River, Alaska, presents results of U-Pb zircon geochronology analysis of an airfall-tephra deposit sampled from approximately 755.5 m above the base of the measured section. Zircon grains were separated and analyzed by Apatite to Zircon, Inc. via laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). The U-Pb age is the first published robust radioisotopic age constraint for Upper Cretaceous rocks on the east-central North Slope. Thirty zircons were analyzed, yielding a range of dates (and uncertainties) that exceed the amount of dispersion expected for a single eruptive event. In order to derive a statistically valid and geologically reasonable eruption age, a weighted mean was calculated using a subset of the youngest grains. Grain date selection criteria for this subset is modeled after the detrital zircon literature where a variety of techniques have been proposed to derive maximum depositional ages (MDAs) for strata based on the youngest grain or grains within a sample. In this case, we favored a method for MDA calculation that selects a subpopulation based on the youngest cluster of three or more dates that overlap at +/- 2 sigma analytical uncertainty. A subpopulation of 20 out of 30 grains satisfied this selection criteria and resulted in a weighted mean age of 71.13 +/- 1.24 Ma. The reported weighted mean age 2 sigma uncertainty reflects both analytical uncertainty and an additional (generic) 1.5 percent (2 sigma) systematic uncertainty component. The new U-Pb age indicates the upper part of the measured section was deposited in the Early Maastrichtian. All files are available from the Alaska Division of Geological & Geophysical Surveys (DGGS) website: http://doi.org/10.14509/30693.