Phosphorites, hardgrounds and syndepositional solution subsidence: A palaeoenvironmental model from the miocene of the Maltese Islands

Abstract

Two major phosphorite conglomerate horizons occur throughout the central Mediterranean Maltese Islands within the Lower-Middle Miocene, Globigerina Limestone Formation. The lower conglomerate lies directly upon a hardground and both conglomerates include lithoclasts of hardground material. Variation in clast size and bed thickness indicates a source area to the west and north of the present-day islands of Malta and Gozo. Particularly thick phosphorite conglomerates occur in solution subsidence structures in western Gozo, being developed as a result of synsedimentary seafloor collapse. These structures indicate that phosphorite clasts were deposited during several discrete pulses of turbulence, punctuated by periods of non-depositional hardground formation or normal marine pelagic sedimentation. Geochemical analysis of the contained phosphorite clasts within the conglomerates shows that P 2O 5 generally is concentrated around the outer margins of the pebbles and contributes up to 17% by weight of some samples. Some 654,580 m 3 and 837,870 m 3 of phosphorite-bearing strata are contained within the Xlendi and Qawra subsidence structures, respectively. The Maltese Islands represent part of a Miocene carbonate shelf, whose margin lay nearby to the west and which was subject to periodic erosion by strong currents transporting phosphorite pebbles eastwards to their present sites. These currents further contributed phosphorus to semi-autochthonous phosphorite developments located within the present Maltese area. The Maltese phosphorites are related to other phosphorite areas in Sicily. A major province of Miocene phosphogenesis is proposed to account for these occurrences. It extends along the western margins of the Malta-Ragusa Rise.