Abstract

Middle Cambrian phoscrete profiles occurring within phosphatic carbonates that surround the phosphate deposits of the Georgina Basin, Australia are described in detail. The profiles occur within a mixed-mineralogy suite of rocks and are characterized by fabrics and textures identical to those developed in calcretes. Phoscrete profiles are subdivided into an overlying laminar phoscrete crust and an underlying phoscrete modified substrate. Diagenesis in the substrate results in the elimination of primary structures, producing a coated to vaguely pelleted texture. Micritic phosphate cements dominate the phosphate profiles; these cements occur as irregular coatings on 1) grains, and 2) thread-like structures which criss-cross interparticle spaces. Scanning electron microscope examination of these cements indicates that they are coatings on tubular structures herein interpreted as chasmolithic and epilithic filaments. The preferential occurrence of phosphate on organic substrates suggests that organic matter plays a dual role in phosphogenesis. It provides both a source of phosphate to the pore waters (via bacterial degradation), and it also provides a substrate on which phosphate preferentially nucleates. Phosphate particles derived from the erosion of phoscrete profiles vary from friable aggregates and lumps to peloids and coated grains. Whereas the composite particles were unlikely to survive sediment transport, peloids and coated grains were coherent particles able to survive transport and sorting. This suggests that some of these particles may occur in the phosphate deposits.

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