Origin and significance of the toolebuc gamma-ray anomaly in parts of the eromanga basin

The Toolebuc Formation, which is a potentially important source of oil shale in the Eromanga Basin of NW Qld, exhibits a marked gamma-ray response that has long beeen used as a stratigraphic marker. At Julia Creek, a large area of the Formation contains low-grade oil shale that is accessible to mining. Detailed investigation, by gamma-ray spectrometry and neutron activation analysis, of drill core samples from this deposit and from the type section of the Toolebuc Formation at Boulia (BMR Boulia 3A stratigraphic drill hole) show that the Toolebuc gammaray anomaly in the northern Eromanga Basin is due to the presence of uranium. Analyses for organic carbon and phosphorus show that the uranium is associated primarily with organic matter in the oil shales and with phosphatic skeletal fish debris in the accompanying coquinites. Drill core samples from Tibooburra, N.S.W. were also analysed (BMR Urisino-1 stratigraphic drill hole) to investigate whether the gamma-ray anomalies observed in the Bulloo Embayment on the southern margin of the Eromanga Basin (possibly correlative with the Toolebuc Formation) might also be indicative of oil shale. The results show that the much weaker anomaly at this locality is due principally to potassium and reflects the presence of an argillaceous silistone within glauconitic sandstones.