A Characterization of the Pebble Shale Unit and Lower Hue Shale within the Northeast Brooks Range, Alaska: What's with the High Radioactive Zone?

van der Kolk, Dolores A.¹, Michael T. Whalen², Marwan A. Wartes³, Rainer J. Newberry² (1) University of Alaska Fairbanks, Fairbanks, AK (2) University of Alaska Fairbanks, Fairbanks, (3) Alaska Division of Geological and Geophysical Surveys, Fairbanks, AK

Cretaceous starved basin shale serves as both the cap rock to the Prudhoe Bay oilfield and a potential source for K-T hosted oil in Northern Alaska.  The contact between the lower part, the pebble shale unit (PSU), and the higher part (Hue Shale), is drawn at the base of the ‘high radioactive zone’ (HRZ).  Although widely employed, the origin and variability of the HRZ is unknown.  Three detailed (~20-40 m thick) successions of the PSU, including 5 m of overlying lower Hue Shale (LHS) were measured with a gamma-ray spectrometer (GRS) for total radioactivity, K, U, and Th.   We analyzed samples from both units by XRF.  Both PSU and LHS are dominantly fine-grained heterolithic mudstone punctuated with minor siltstone and concretions.  The LHS also contains bentonite layers.

GRS vs. XRF concentration R² of 0.3 (Th) to 0.7 (K) indicate that much of the variation in gamma ray signature is due to measurement error.  GRS and XRF measurements indicate that the HRZ is due to high U.  Compared to a hemipelagic mud standard (MAG-1), the PSU is slightly enriched (~2x) in Ba, S, As, Cu, V and depleted (~0.5-0.1x) in Na, Mn, Ni, and Mo; U, Th and most other element are comparable.  In contrast, the LHS and bentonite are enriched (>5-100x) in U, Se, Mo, As, Ba, and V relative to hemipelagic mud.  It is likely that the HRZ is due to diagenetically redistributed metals from felsic tuff  (bentonite).