Ediacaran–Cambrian phosphorites from the western margins of Gondwana and Baltica

The late Ediacaran–mid Cambrian occurrence s of phosphorites in the wes-tern Mediterranean region (West Gondwana) and southern Sweden (north -west Baltica) are related to the poleward drift of West Gondwana and thenorthern drift of Baltica. As a result, these regions crossed subtropical andtemperate palaeolatitudes of the Southern Hemisphere, in which oceanicupwelling and high organic productivity were the probable sources of phos-phorus along their western margins. A parallel south-west/north-east migra-tion trend of evaporites and delayed, time transgressive phosphorites pointto distal upwelling environments associated with shallower arid conditions.Concentration of hardground-derived phosphate accumulations took placeby repeated alternations of low sedimentation rates and condensation,in situ early diagenetic and microbially mediated precipitation of amorphousand cryptocrystalline, carbonate-bearing francolite, winnowing and poly-phase reworking of previously phosphatized skeletons and hardground-derived clasts. Thicker phosphorites of economic importance display aspiculate sponge-thromboid consortium that points to a distinct benthiccommunity developed on suboxic, slope to basinal substrates. For mostphosphorite samples an up to ten-fold enrichment in rare earth elementconcentration relative to post-Archaean average Australian shale reflects sig-nificant incorporation of rare earth elements and yttrium after deposition.Considerable scavenging took place from anoxic pore waters, followingreductive dissolution of iron oxides as evidenced from the medium rareearth element enrichment in most samples. This contrasts with the rare pre-servation of primary sea water features, such as cerium depletion andyttrium enrichment, in phosphatized thrombolites from the shallower realms(for example, Montagne Noire). The data are consistent with phosphogenesisafter upwelling and transgression of nutrient-rich, suboxic–anoxic, ferrugi-nous waters over more oxygenated, rifting settings and inland portions of anevolving passive margin.