Multi-phase oxygen isotopic analysis as a tracer of diagenesis: The example of the mishash formation, cretaceous of Israel
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. δ18O 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: 1. (a) A depositional assemblage, formed in normal seawater at temperatures between 23° and 30°C is reflected by benthic foraminifera (Nodosaria) skeletons (δ18OSMOW = +27.5 to +29.5‰) and apatiticbio-detritus (δ18O = +18 to +19.5‰). 2. (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. 3. (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 105-106 yr. for the closure of the chertification system in a shallow marine environment. © 1987.
Vengosh, A; Kolodny, Y; Tepperberg, M
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