URANIUM SPECTRAL GAMMA-RAY RESPONSE AS A PROXY FOR ORGANIC RICHNESS IN BLACK SHALES: APPLICABILITY AND LIMITATIONS
S. Lüning 1* and S. Kolonic 1
1 University of Bremen, Department of Geosciences - FB5, PO Box 330 440, 28334 Bremen, Germany.
In many organic-rich, low-carbonate hemipelagic shales, there is a stable and close correlation between the uranium and TOC contents. In this paper, we present a number of case studies using our own data and that from previous publications to investigate black shales with (1) good, (2) fair-to-good and (3) poor U-TOC correlations. U-TOC ratios in the different black shale units are compared to each other, and possible reasons for the observed variations are discussed.
In general, the U-TOC ratio in a black shale is controlled by a number of factors which include for example the primary uranium content of the water body, the carbonate content and the sedimentation rate. The development of a stable U-TOC ratio may be inhibited by the presence of phosphate, by a high carbonate or sand content, by dissolution ("burn-down") of uranium during intermittent oxic periods, and by large-scale, diagenetic remobilisation of uranium. In suitable black shale systems, vertical variations in organic-richness can be approximated by measuring the uranium content using spectral gamma-ray measurements. This may be especially important in outcrop studies because gamma-ray logging is a straightforward field technique. Before the uranium content can be used as a proxy for TOC content in a black shale system, however, a thorough calibration of uranium and TOC is necessary, in order to determine the stratigraphic and regional limits of the derived U-TOC ratios and to establish the presence of a stable U-TOC correlation.