DEVELOPMENT OF PHOSPHATIZED HARDGROUNDS IN THE MIOCENE GLOBIGERINA LIMESTONE OF THE MALTESE ARCHIPELAGO, INCLUDING A DESCRIPTION OF GAMOPLEURA-MELITENSIS SP-NOV (GASTROPODA, EUTHECOSOMATA)
In the Maltese Islands two phosphorite layers occur in the Globigerina Limestone Formation (?Aquitanian to Langhian). These layers, labeled C1 and C2, display a multi-stage development with a two-stage hardground development on top (labelled lower and upper hardground).
In the lower hardground, lithification and mineralization followed a sedimentary framework between Thalassinoides burrows, resembling the Cretaceous modular chalks' which were marginally phosphatized when they became exposed to the sea floor. In Phosphorite Layer C2, development of this lower hardground has been superimposed by small-scale cycles. It is underlain by one or more omission surfaces each followed by phosphate-rich, bioturbated biomicrites.
Thalassinoides burrows have been filled with phosphatized particles floating in a chalk-like biomicritic matrix. Particles comprise invertebrate and vertebrate fossils, nodules and intraformational clasts, which represent the same composition as in the superjacent, upper hardground.
Biogenes consist of a diverse, mostly primary aragonitic fauna, among them assemblages of holoplanktonic gastropods of at least 25-30 species. Especially in northwest Gozo and northern Malta pteropods of the hardground layer C1 may accumulate to ooze-like concentrations, dominated by the relatively large species Gamopleura melitensis sp. nov., exclusively known from this area so far.
Phosphatization must have taken place before dissolution of the aragonite, since many fossils are preserved as phosphatized internal moulds and in many instances the formerly aragonitic shell itself has been replaced by phosphorous minerals. Phase analyses by XR-diffraction show replacement by calcite (30-40%), by carbonate-hydroxylapatite (50-60%), and fluorellestadite (less than 10%). Lateral changes in faunal assemblages and their extraordinary good preservation suggest an autochthonous origin of the phosphatized components.
The lower hardgrounds of C1 and C2 represent the terminal product of prolonged times of non-deposition, probably caused by strong bottom currents during sea level lowstands. Phosphatized particles and sediment were trapped in burrows but have been prevented from settling on the sea floor. The upper hardground, instead, developed at times of waning bottom currents when particles could again accumulate. Only the finer chalky biomicritic sediments were winnnowed, leaving behind a phosphatized bio-rudstone which has been intensively cemented, possibly as a result of the dissolved aragonite. These sediments were deposited during a renewed sea level rise. Phosphorite Layer C1 shows little lateral facies changes except for the faunal assemblages.
Phosphorite Layer C2, instead, shows considerable lateral variations which is related to different degrees of condensation due to a more differentiated relief and/or bottom currents.
During phosphorite development oxic conditions with low input of organic carbon prevailed. This was due to increased bottom currents during times of sea level lowstands. This does not meet the conditions in an upwelling region. A possible mechanism triggering phosphogenesis is presented for the Maltese deposits.
In the appendix a description of Gamopleura melitensis sp. nov. is given,representing the major characteristic pteropod species in C1, including details on the formerly aragonitic microstructure of the exceptionally well preserved shells.