Silicification of Upper Albian Limestones in Southern Istria, Croatia

Thoroughly silicified carbonates appear in Istria (SW Croatia) in three horizons (0,5-6m thick) within the transgressive-regressive megasequence of Upper Albian-Lower Campanian age (Tisljar et al., 1995). High SiO2 content (92-98%) makes these sediments a good raw material for glass industry. While the silicified carbonates vary from well lithified to loose, sand-like sedimentary rocks, they were often incorrectly called “quartz-rich sandstones” (Gabric et al., 1995). Sedimentological setting of silicified horizons with interbedded stromatolites, mudstones and pelloidal grainstones, suggest peritidal depositional environment. Abrupt decrease of SiO2 content in the underlying and superimposing beds can be noticed. Several generations of quartz within the silicified horizons, as well as entirely or partly silicified fossils (foraminifers, ostracods) can be observed. Some relicts of micritic limestones have been noticed. Although a micropetrographic structure pointed out to a late diagenetic silicification, the abrupt decrease of SiO2 content at the base, as well as in the superimposed layers puzzled. Microcrystaline dolomite structures found in underlying and superimposing beds, suggest complex diagenetic history of the investigated section. REE distribution found in insoluble residue from silicified horizons differs to that in the surrounding limestones and might indicate their different origin. The source of quartz remains unknown, but it can be speculated about volcanic material, brought to carbonate platform by wind. Similar appearances of silicified beds in Upper Albian-Lower Campanian limestones that were found in some distant Adriatic islands indicate regional significance and importance of this phenomenon.