Uranium, thorium, and potassium in black shales of the Bazhenov Formation of the West Siberian marine basin

Abstract

Contents and distribution of U, Th, and K in rocks of the so-called normal and anomalous sections of the Upper Jurassic–Lower Cretaceous Bazhenov Formation in the West Siberian marine basin are examined. In normal sections of this unit, the most widespread clayey–siliceous rocks deposited in the course of slow “background” sedimentation, are characterized by high (relative to black shales) contents of U (66.5 ppm) but moderate contents of Th (5.07 ppm) and K (0.81%). In mudstones identified as turbidite sediments with high sedimentation rate, contents of these elements are 11.68 ppm, 11.00 ppm, and 2.70%, respectively. In rare clayey–silty sediments of anomalous sections marked by maximal deposition rates, contents of these elements are 2.15 ppm, 3.78 ppm, and 2.15%, respectively. Uranium is associated mainly with organic matter and partly with apatite (phosphate clasts of fish skeletons) and calcite; thorium, with clay minerals; potassium, with clay minerals and K-feldspar. The U content is 345 ppm at high apatite concentration (7%).