Multi-phase oxygen isotopic analysis as a tracer of diagenesis: The example of the mishash formation, cretaceous of Israel

Abstract

Isotopic analysis of oxygen in several coexisting phases has two complementary aspects: (a) the analysis of two or more cogenetic phases may yield information about prevailing conditions at definite points in time assuming equilibrium fractionation; and (b) the analysis of paragenetic assemblages of phases unravels the sequence of conditions from sedimentation, through early diagenesis to late diagenesis. δ 18O has been analysed in several coexisting phases from a continuous sedimentary section of the Mishash Formation of Campanian age in central Israel. Three isotopically concordant assemblages were identified: (a) A depositional assemblage, formed in normal seawater at temperatures between 23° and 30°C is reflected by benthic foraminifera ( Nodosaria) skeletons (δ 18O SMOW= +27.5 to +29.5‰) and apatiticbio-detritus (δ 18O= +18 to +19.5‰). (b) An early diagenetic assemblage formed in equilibrium with a marine solution is recorded by calcite of micritic cement ( +26 to +27.5‰), silica in opal-CT (porcellanite) (+31.5 to +33.3‰), silica as quartz in homogeneous cherts ( +29 to +32‰), silica as quartz in fragments of chert breccias ( +31 to +33‰), and some quartzose matrix in silicified phosphorites. (c) A later diagenetic assemblage contains calcitic spar infilling of fossils ( + 20 to + 26.6‰), silica in “matrix” of chert breccias ( +21 to +33‰) and coarse quartz matrix in silicified phosphorites ( +20 to +30‰). The phases of this assemblage can be interpreted as all being deposited in equilibrium with a late freshwater fluid. Comparison of the record of siliceous rocks to that of coexisting phosphates and carbonates suggests a time constant of 10 5–10 6 yr. for the closure of the chertification system in a shallow marine environment.