Middle Cambrian phosphatic and calcareous depositional environments, the Undilla region of the Georgina Basin, Australia

Abstract

A sedimentological study has been conducted on the phosphorites that occur within the predominantly calcareous rocks in the northeastern parts of the Georgina Basin, Australia. A variety of synsedimentary phosphatic nodules, pavements and profiles have been recognised and are used to determine the physico-chemical conditions under which phosphate was fixed within the sediments. The presence of similar surfaces, or similar ages, in Europe and Asia suggests that the processes of phosphogenesis interpreted for the Georgina Basin deposits may be applicable on a global scale. Middle Cambrian phosphorites of Australia accumulated in shallow in shallow submergent and semi-emergent depositional environments, within a facies mosaic of calcareous and mixed-mineralogy sediments. Calcareous sediments formed and accumulated on a stable shelf with negligible relief. Shoaling resulted in the restriction of water circulation and the development of hypersaline conditions. Mixed-mineralogy rocks (i.e.) those of carbonate, phosphate and minor glauconute composition) accumulated and formed in areas of tidal influence where nutrient-rich predominantly oxygenated waters resulted in an abundant fauna. In sheltered embayments and lagoons the influx of large quantities of organic matter resulted in the local development of anoxic conditions within and beneath a layer of suspended organic matter. Phosphogenesis took place in a variety of subenvironments. Storms periodically dumped large quantities of organic matter on semi-emergent flats. The bacterial degradation of this material released phosphorus to the underlying sediments where it became trapped in the interstitial waters. Anoxic conditions were maintained below the sediment surface so that as solar heating took place the organic matter was gradually desiccated, the pH and temperature of pore waters rose and phosphate precipitated in suitable microenvironments. Phosphate preferentially precipitated on organic templates. This preference was caused by the microenvironmental conditions associated with the gradually decomposing organic matter. Phosphogenesis in semi-emergent settings resulted in the formation of phosphatic nodules, hardgrounds, phoscrete profiles, stromatolites and mudstone phosphorites, whose subsequent erosion produced a variety of structureless and coated phosphate particles. Phosphatic internal moulds of skeletal grains formed in semi-emergent and submergent settings. These phosphate particles were associated with the microenvironmental conditions that prevailed within the test or the pore spaces enclosed by the skeletal framework. In protected and submergent, lagoonal and embayed environments phosphogenesis took place beneath a transient layer of suspended organic matter. During periods of reduced organic matter influx the suspended layer decayed and the phosphate particles and beds were eroded, reworked and locally sorted so that in some cases they accumulated to form an economic phosphate deposit. The global phosphogenic episode of latest Proterozoic/Cambrian age, or more specifically the Georgina Basin phosphorites, are related to a number of coincident variables which facilitated the formation, during transgressions, of warm shallow seas characterised by high levels of organic productivity. During transgressions the irregular former landscape assisted in the formation of protected environments (embayments) into which the organic matter was dispersed, oxidised and the organically-bound phosphorus recycled to the biomass. Show more