TRACE ELEMENT STUDY ON THE MIXED CARBONATE-PYROCLASTIC SEDIMENTS (LOWER MIOCENE, CENTRAL BASIN, IRAN)

Abstract

Geochemical investigations have permitted the refinement of the environmental
interpretation of the f-Member of the Qum formation, Central Basin, Iran, which has
been established through detailed microfacies (MF) analysis. The analysis revealed
that the f-Member carbonates consist of eight distinct microfacies designated by the
letters A through H. On the basis of petrographic characteristics and environmental
significance, the studied MF have been interpreted in terms of depositional
environments as follows: MF-A (basin); MF-B (lower slope); MF-C (reef flank); MFD
(proximal talus), MF-E (organic reef); MF-F (lagoon); MF-G (restricted marine
shoal), MF-H (restricted shelf). The concentrations of Ti, V, Fe, Co, Ni, Cu, Sr, Zr,
U and Rb have been measured in limestones from outcrop samples of this formation.
The analysis revealed a wide range of concentration both within and between
different microfacies. A volcanic eruption in the nearby area located in the northeastern
part of the sedimentary basin exerted an influence on trace element composition by
supplying pyroclastic materials to carbonate sediments. In general, the basin
microfacies is characterized by having the highest concentrations of Ti and Zr. The
lower slope microfacies has the highest values of V, Fe, Ni, Cu, Sr, U and ratios of
Ni /Co, Cu/Co and Ni/V, whereas the reef flank microfacies have only elevated
concentrations of Co. All of these traces and elemental ratios are depressed in both
proximal talus and reef rnicrofacies. The restricted marine shoal microfacies possesses
elevated Rb values and the restricted shelf microfacies is characterized by a high
concentration of Sr and elevated Cu , U and Rb values and show a slight increase in
the Ti. Fe, V, Co, Ni, Zr values.