ON RECOGNITION OF PHOSPHORITE-BEARING FORMATION BY THE USE OF TRACE ELEMENTS IN CLAYEY FRACTION OF ROCKS (exemplified by Karatau Cambrian basin)

Phosphorites often occur in thick and monotonous carbonate sequences. Recognition of phosphorite-bearing portions was attempted by nonlithological means, including geochemical ones, e. g., by trace element associations. This paper reports some results obtained when studying geochemistry of productive formation in the standard phosphorite-bearing Karatau basin of carbonate sedimentation (Kazakhstan). Distribution of 14 elements – Zr, Sr, Ga, V, Cu, Ba, Co, Zn, Ni, B, Ti, Mn, Pb, Cr – in clayey fraction from rocks of Chulaktau phosphorite-bearing suite (Lower Cambrian) and host members was studied. Clayey fraction was chosen as the most homogeneous component for all types of rocks. Though the distribution of the trace elements (TE) through the sequence in rocks and clayey fraction (cf) are irregular, it is possible to distinguish a phosphorite-bearing interval and to individualize separate members of the productive formation. The productive formation has more variegated and anomalous contents of some TE both in rocks and in cf. The correlation of twins of TE has revealed individuality of all members of the formation. The most informative set of TE is established for character of the horizons making up the formation. Cluster analysis by TE contents reveals some individuality of the correlative groups for every member of the formation and a possibility for subdividing separate areas of the basin on the basis of ties between groups of TE in cf. These ties are more stable in the members which are formed in stages of sinking and widening of paleoaquatories, and unstable for the members which are formed during shall owing stages. The method of major components provided the most sensible element for division of the members – Zr, and the most informative group for such division – Pb, Ti, Cr, V, Zr. By providing representative samplings, investigations of this kind can improve predicting regionalization, stratigraphic correlation and make study of paleogeographic features more efficient.