Early formation of K‐feldspar in shallow‐marine sediments at near‐surface temperatures (southern Israel): evidence from K‐Ar dating

Authigenic K‐feldspar was investigated in two Albian to Turonian sections in Israel using K‐Ar and Ar‐Ar dating, X‐ray diffraction, scanning electron microscopy and chemical analysis. Both sections comprise a similar succession of shallow‐marine limestones, dolomites and marls, with some sandstone and shale beds of continental origin. The HCl‐insoluble residue fraction of the studied samples consists of clays, quartz, feldspars, goethite and trace amounts of heavy minerals. Most of the insoluble residues have a relatively high K‐feldspar content that has an adularia habit and is concentrated in the 4–7 µm size fraction. The authigenic origin of the K‐feldspar in the fine silt fraction is indicated by its high content relative to quartz, the uniform and idiomorphic shape of the crystals and its limited size range. Of the fine silt (4–7 or 4–10 µm) separates, 40% have ages that are similar to stratigraphic ages within the analytical and biostratigraphic errors. Ar‐Ar dating of a fine silt fraction with 94% K‐feldspar (4–10 µm, sample GYP 7) yields a plateau age identical to the total gas age and similar to the stratigraphic age. These results indicate that the K‐Ar age is not a mixture between detrital and late diagenetic K‐feldspar ages, but is rather an age of formation within a few million years after deposition. It is suggested that the early formation of the K‐feldspar was associated with dolomitization and was induced by residual brines as part of a reflux process.