The relation of a coastal environment to early diagenetic clinoptilolite (zeolite) formation - New data from the Late Cretaceous European Basin

Abstract

Abstract Clinoptilolite, an authigenic mineral of the zeolite group, is commonly considered as a product of rhyolitic volcanic glass transformation during diagenesis of carbonate pelagic/hemipelagic sediments of Cretaceous-Paleogene age. Another, subordinate way of clinoptilolite formation that has been suggested in the literature is its precipitation during diagenetic transformation of carbonate sediments enriched with biogenic silica. The observations concerning transformation of the latter into zeolite are scarce and mostly based on Cretaceous pelagic, hemipelagic and shallow marine deposits, and post-Cretaceous sediments in which there was no volcanic material. The mechanism of diagenetic clinoptilolite formation in volcanic-free sediments, as well as the role of the depositional setting in its precipitation, is not well understood. In carbonates of the NW part of the European Basin (west side of the Łysogory-Dobrogea Archipelago) studied here, the clinoptilolite is associated with opal-CT (from the dissolution of sponge spicules) and a detrital clays are documented for the first time in a Cretaceous coastal environment. The carbonates were deposited in a coastal environment which was under the influence of weak currents delivering sponge spicules (biogenic opal) from the offshore zone and the terrestrial input was transported by rivers from a neighboring island. The lush plant vegetation, with numerous pteridophytes and conifers, within a warm (sub-tropical or tropical) and humid climate enhanced the terrestrial weathering of clays and feldspars. This unique combination of elevated silica concentrations (from sponge spicules dissolution) and aluminum availability (from terrestrial weathering of feldspars in a subtropical climate) during the early diagenesis combined with alkalinity triggered by organic matter decomposition, enhanced the clinoptilolite formation a few centimeters below the seabed. The investigation demonstrates a novel route by which pore-fluids enriched with silica and aluminum initiate clinoptilolite formation during the very early stage of diagenesis, a few centimeters below the seafloor. The clinoptilolite distribution in Late Cretaceous silica rich deposits (with no volcanic input) of the European Basin is always associated with opal-CT, from sponge spicules and elevated terrestrial input (clays). Taking into account the distinctive Campanian-Maastrichtian sedimentation in the epicontinental European Basin, composed almost exclusively of pure pelagic carbonates, the depositional settings in which the silica and aluminum were available in pore waters was restricted to the part of the basin influenced by terrestrial input. This leads to the conclusion that clinoptilolite formation in volcanic-free sediment is an indicator of not only enhanced sponge silica availability but also the close proximity of the source area.