Recent advances in 40Ar/39Ar methodology allow for dating of very fine-grained samples by their encapsulation in quartz tubing prior to irradiation. This technique has been applied to tectonic problems in a number of geologic settings. In our investigation clay (illite) samples from several Brooks Range shale formations have been dated by the 40Ar/39Ar encapsulation method. Our data complement existing fission track and other information to help constrain the tectonic history of pre-Brookian sedimentation and burial, Brookian uplift and subsequent unroofing. Our investigation also illustrates the problems with interpretation of age spectra from fine-grained material where mixtures of detrital and diagenetic illite may be present.

Samples were collected from exposures of the Beaucoup, Hunt Fork, Noatak, and Kayak formations in the Atigun Pass-Atigun Gorge region and then disaggregated and separated into <0.2μm, <1 μm, and <2μm size splits by application of Stokes settling and centrifugation principles. X-Ray diffraction of these splits indicates that illite crystallinity (IC) values correspond to either diagenetic or anchizonal burial conditions. Sampled units comprise depositional components of a Devonian-through-Permian major mixed marine-fluvial clastic wedge. The Beaucoup is a heterogeneous mixture of phyllites, shales, and siltstones with scattered Late Devonian recrystallized limestone bodies. The Hunt Fork is an Upper Devonian sequence of prodelta thin-bedded shales, siltstones, and sandstones. The Noatak is a Late Devonian marginal-marine series of coarsening-upward fine-to medium-grained calcareous sandstones separated by dark siltstone and shale units. The Mississippian Kayak is part of a northward-transgressing mostly shale unit deposited in brackish water and shallow marine environments (Moore et al, 1994).

Fine-grained materials including clays and shales may undergo argon loss due to recoil during irradiation. To retain escaping argon, we encapsulated sample splits prior to irradiation by placing them in quartz tubes, evacuating the tubes, and then sealing them off using a propane torch. A 9 watt Coherent argon laser and VG3600 mass spectrometer were used in automated 45 sec step heats of samples from 100 through 8500 milliwatts of laser power. Immediately before encapsulated step heat analyses, the quartz tubes housing encapsulated samples were crushed and proportions of recoil argon gases measured. Overheating during encapsulation or bakeout may have resulted in release of 40Ar from several preliminary samples. Separate unencapsulated sample splits were also prepared for Ar-Ar analysis to better estimate the affect of recoil on age determinations. Based on the argon retentivity-IC model of Dong et al. (1995), encapsulated total gas ages should be too young, while unencapsulated total gas ages should approximate depositional-diagenetic ages. Retention ages (ages measured after excluding recoil fractions) should also approximate depositional-diagenetic ages, assuming potassium has not been stripped out of nonretentive sites

Beaucoup and Noatak shale samples show similar retention and encapsulated total gas ages of about 200Ma. These ages are considerably younger than their Devonian depositional ages. Hunt Fork samples collected from immediately south of the Toyuk thrust show similar retention ages, but there is a wide variation between encapsulated and unencapsulated ages. However these young ages suggest a cooling event may have occurred around 200Ma. A sample from within the Toyuk thrust shows very similar retention and unencapsulated ages, fitting a Kayak depositional age. Retention and unencapsulated ages from the Kayak immediately north of the Toyuk differ. Diagenetic illite may be present in this sample, as ages vary from depositional to much younger across increasingly finer fractions. This pattern is repeated in other sample sets. Samples collected from a thrust separating Okpikruak and Otuk show apparent ages which fit Otuk depositional time, suggesting the fault is composed primarily of Otuk-aged material. In most cases, encapsulated total gas ages are younger than unencapsulated and retention ages, which may be due to 40Ar loss in nature. The pattern of a number of age spectra indicate diffusional loss associated with cooling (uplift) events at about 50-150Ma.