Application of 40Ar/39Ar Dating of Illite to Understanding the Geologic Evolution of the Brooks Range and Colville Basin

By

W. Munly and P. Layer (University of Alaska, Fairbanks)

We have applied the 40Ar/39Ar method to fine-grained shale samples from central Brooks Range outcrops and North Slope wells to provide additional information about the geologic evolution of northern Alaska. This method requires that samples be encapsulated to retain 39Ar produced by sample irradiation. Without encapsulation, 39Ar can be lost by recoil from fine-grained material. We have modeled these encapsulated age spectra to identify both detrital and neoformed illite components.

Analyses of age spectra from Brooks Range shales suggest that the Toyuk thrust separates rocks with differing tectonic or burial histories. Samples from south of the thrust are characterized by relatively flat age spectra (less detrital illite), and ~225 Ma argon retention ages, slightly older than zircon fission track ages. Samples from north of the thrust generally have more detrital illite and older retention ages (233–391 Ma). We also see evidence of episodic cooling/uplift at ~140 Ma. Coarser-grained sample splits yield older ages, indicative of an increasing detrital component. Illite crystallinities indicate that all samples underwent shallow burial (<250 degrees C), and based on the preservation of detrital illite, shallowest burial was north of the thrust.

Early Cretaceous Pebble Shale samples from Colville basin wells contain neoformed illite, smectite, vermiculite, and depotassicated illite. Age spectra from these samples are dominated by detrital illite (ages up to 475 Ma). Paleocene samples from the Exxon Alaska State 1-A well show detrital ages of ~205 Ma with a cooling/uplift age of ~40 Ma. All samples have high illite crystallinites, suggesting low burial temperatures.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.