Palaeoenvironmental reconstruction of a condensed hardground-type depositional sequence at the Cretaceous-Tertiary contact in the Parnassus-Ghiona zone, central Greece

Abstract

Phosphatic hardgrounds at the K/T boundary in the Parnassus—Ghiona zone reflect a major stratigraphic break. The hardgrounds developed on top of the uppermost Maastrichtian beds, which represent a condensed pelagic carbonate sequence of seamount type. Synsedimentary lithification and hardground formation took place on shallow, periodically emerged, seafloors. Phosphate cementation includes fillings of intra- and inter-particle porosity and occludes passive framework open spaces, as well as rim cement fabrics, developed through mineralization by non-phototropic microbial communities. Within the hardground superstructure a highly condensed phosphate sediment was precipitated, post-dating the hardground formation. Phosphate cementation was developed under well-oxygenated conditions, involved in a “low-productivity” phosphogenic system. Phosphate ions were released into pore waters through bacterial decomposition of organic matter. Glauconite developed due to prolonged contact of lithified chalk with seawater. The irregular, bored, impermeable and phosphate-cemented pavements are covered by stratiform and columnar laminated phosphatic stromatolitic crusts trapping Early Paleocene pelagic sediments, deposited during the subsequent Danian transgression. The source of phosphorus was external, either marine or continental. Phosphatization occurred during the bacterial degradation of the organic sheaths. Pyrite crystals within the stromatolitic laminae suggest a sulphidic environment beneath the mat-forming communities. Successive phases of intraformational reworking resulted in “mature” hardground development. Lags from eroded subjacent hardgrounds were accumulated in neighbouring submarine depressions. Marly material rich in quartz has been concentrated along discontinuities. On former topographic heights both the phosphatic hardground and stromatolites have been assimilated by a cryptocrystalline phosphatic material, rich in clay-iron oxides and quartz, considered to originate from subaerial weathering (phoscretization).