Phosphogenesis in the immediate aftermath of the Great Oxidation Event: Evidence from the Turee Creek Group, Western Australia

Phosphatic peloids and pebble-sized microbially-bound sediment clasts have been identified within dolomitic sediment that separates centimetric columnar stromatolites in a 20–150 cm thick horizon from the 2.4–2.2 Ga Turee Creek Group in Western Australia. Petrographic examination of the phosphatic clasts reveals complex internal textures that are defined by different mixtures of fine-grained apatite, medium-grained dolomite, irregular domains of partly euhedral micro-quartz, and carbonaceous matter. Phosphatic sand-sized peloids observed within microbially-bound sediment clasts are similar in size, texture, and composition to the individual peloids in the intercolumn sediment. Both occurrences have finely dispersed kerogen throughout and zoned cores that are packed with both microscopic apatite granules and larger, euhedral quartz crystals. The percentage of apatite vs quartz vs kerogen varies between studied peloids. Similar-sized, white peloids in the intercolumn sediment and in the microbially-bound sediment clasts contain illite and are lined internally by euhedral apatite crystals, 20–100 µm in size. Some peloids are kerogenous in one domain and illite-bearing in another. The phosphatic microbially-bound sediment clasts and peloids are interpreted to represent fragments of a peritidal phosphorite, redeposited in an offshore setting during high energy events. Peloids formed within the microbially-bound sediment and were subsequently eroded out and redeposited in the intercolumn sediment. White peloids are interpreted to represent peloids affected by surficial weathering during temporary exposure, when the fine-grained kerogenous interior was replaced by clays. These weathered peloids were then re-sedimented and captured both within microbially-bound sediment and in the intercolumn sediment. Euhedral apatite crystals that line the insides of illite-bearing white peloids were dated to 2104 ± 70 Ma and 2041 ± 33 Ma, using U-Pb and Th-Pb dating methods, respectively. This age is interpreted to represent the time of metamorphic fluid circulation during a period of known regional metamorphism, and provides a minimum age of the reef complex.