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The stratigraphy of four Members within the Lower Coralline Limestone Formation (Oligocene: Rupelian-Chattian) on Malta and Gozo is redescribed, and three Beds are formally defined within the Attard Member. Correlation between these Beds of foraminiferal-coral wackestone, algal rhodolite, and mollusc-echinoid facies is effected by means of a horizon rich in the siphonal tubes of the burrowing bivalve Kuphus cf. polythalamnia. The tube mineralogy is shown to be calcite, rather than aragonite as in other Pholadacea. Eight species of crustose coralline algae, and two of articulating are described and illustrated. From field observations, detailed petrology, fauna and algal flora, it is inferred that this Formation represents a transgressive sequence of shallow-water, sub-tropical, marine, platform carbonates grading from lagoonal muds in southern Malta to platform edge sands in northern Gozo, overlain by typical foreslope large benthonic foraminiferal wackestones, and terminated by a ubiquitous hardground of Paleogene-Neogene boundary age. The succeeding Globigerina Limestone (Miocene: Aquitanian-Langhian) is redescribed; its three Members redefined; three Beds formally defined within the Lower Member, two within the Middle, four within the Upper. Computer-based isopachyte studies confirm that a positive feature in the Malta-Gozo straits area influenced sedimentation pattern. Seventeen trace fossils are described, and six ichnofacies recognised within the Formation. From field observations, petrology, fauna, isopachytes and trace fossils, this Formation is interpreted as a sequence of shelf-carbonates, with deposition at 50-300m for the Lower (planktonic foraminiferal wackestones) Member, deepening during deposition of the Middle (coccolith lime mudstone) Member, with slight and intermittent shallowing in the Upper Member. early lithification. A single phase of submarine diagenesis produced A ubiquitous hardground terminates the Lower Member,and pre-dates a single period of neptunian dyke emplacement. A single, late stage and minor period of subaerial diagenesis is associated with post Miocene (? Messinian) uplift and erosion. Ubiquitous phosphorite conglomerates, the clasts derived from a region to the west-northwest of Gozo, initiate the Middle and Upper Members. Phosphate occurs as dahl lite, precipitated as rims on clasts and hardgrounds, and as francolite, a replacement of limestone. Analyses, principally by X-ray fluorescence and Instrumental Neutron Activation, for major,trace, and rare earth elements detail phosphorite geochemistry, reveal a fractionation of low over high REE, and indicate initial phosphatisation of limestone in a reducing environment (low Eu) prior to erosion and redeposition as clasts. |
