Syntectonic deposition of an oligo-miocene phosphorite conglomerate bed in Malta

Abstract

A succession of Oligo-Miocene sediments at Sliema, Qawra and Migra Ferha includes a 1O-20cm phosphorite conglomerate bed capping the terminal hardground of the Lower Coralline Limestone Formation (Oligocene) which consists of carbonate platform sediments. The conglomerate bed always occurs in areas of significant thinning of the overlying Lower Globigerina Limestone. These palaeohighs have been linked to NNE-SSW trending lineaments. At Sliema, allochthonous phosphatised conglomerate infill NW-SE trending Neptunian dykes that dissect the platform sediments. These Oligo-Miocene syntectonic deposits were later cemented and vertically displaced by minor faulting trending NW-SE. Tectonic features at Sliema are linked to the regional N-S extensional regime and tentatively interpreted to have developed from stresses caused by displacement along the western margin of a NNE-SSW trending strike-slip fault. East of this fault, synclinal subsidence created the Valletta Basin and set conditions for current upwelling. Phosphatogenesis occurred along the basin margin swept by the prevailing westward currents. Phosphatised pebbles and ahermatypic corals were transported westward of palaeohigh margins in central and western Malta and deposited on the terminal hardground of the drowned Oligocene carbonate platform.