Continued investigations confirm conventionally defined rock units in the Cooper Basin generally exhibit characteristic trace element signatures. In Big Lake 10, the Nappamerri Formation may be subdivided into 3 chemostratigraphic units; the Toolachee Formation into 2 units; the Daralingie Formation into 2 units; and the Epsilon Formation into at least 2 units. The distribution of chemostratigraphic units from the top of the Roseneath Shale to the bottom of the Epsilon Formation (as sampled) suggests a reappraisal of formation boundaries may be justified. Analytical methods used included emission spectroscopy, atomic absorption spectroscopy, organic carbon analysis, elemental carbon analysis and trace element analysis.