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

Description

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.

Resources

Name Format Description Link
0 Spreadsheets with sample locations, geochronology lab results, summary ages, and data dictionaries. https://doi.org/10.14509/30693
0 https://doi.org/10.14509/30693
0 https://doi.org/10.14509/15758
0 https://doi.org/10.14509/30156
0 https://doi.org/10.14509/29138
0 https://doi.org/10.14509/30099
0 https://doi.org/10.14509/16090

Tags

  • siltstone
  • zircon
  • methods
  • lowstand
  • prince creek formation
  • rocks
  • outcrop samples
  • geochronology
  • canning formation
  • ivishak river
  • transgressive
  • stratigraphy
  • sedimentology
  • measured stratigraphic section
  • bentonite
  • geologic
  • measured sections
  • energy resources
  • slope
  • nanushuk formation
  • shale
  • maastrichtian
  • schrader bluff formation
  • marine
  • correlation
  • sentinel hill member
  • barrow trail member
  • regressive
  • north slope
  • petroleum
  • u-pb
  • highstand
  • formations
  • channel
  • petrology
  • brookian
  • sandstone
  • unconformity
  • clay
  • exploration
  • campanian
  • sedimentary
  • subsurface
  • surface
  • national petroleum reserve-alaska
  • tephra
  • energy
  • shelf
  • interpretation
  • rogers creek member

Topics

  • Local Government

Categories