GENESIS OF UPPER TRIASSIC PHOSPHATIC OOID BEDS FROM WILLISTON LAKE, CANADA: MICROBIAL OR ABIOTIC MINERALIZATION?

The genesis of phosphatic ooids in ancient and modern deposits remains poorly understood. Three sections (Ne Parle Pas Point, Pardonet Creek, and Black Bear Ridge) at Williston Lake, British Columbia, Canada record the presence of phosphatic ooids in the interval immediately preceding the end-Triassic mass extinction. A veneer of phosphatic ooids occurs on the sequence boundary in the more proximal Black Bear Ridge section, where most of the Rhaetian succession is missing. Distal sites (Ne Parle Pas Point and Pardonet Creek) record a more complete representation of the Rhaetian Stage. The upper 13 m of the Ne Parle Pas section of the upper Rhaetian sequence consists of of calcareous siltstone interbedded with 5 to 25 cm thick phosphatic ooidal grainstone beds representing a transition from offshore to lower shoreface depositional environments. The Pardonet Creek sequence records 10.5 m of the Rhaetian Stage with siltstone/calcareous siltstone beds at the bottom overlain by calcareous siltstone with dispersed phosphatic ooids and pebble-size phosphatic angular grains in places interbedded with phosphatic ooid grainstone beds. Phosphatic ooid samples were examined from the three localities using Raman, SEM, XRF, cathodoluminescence, and petrographic analyses. In all localities, phosphate ooids are mainly spheroids and ellipsoids that are fragmented due to reworking. The ooids are 50 – 500 mm in diameter, and contain nuclei generally composed of quartz, feldspar, and calcite. Raman and XRF show that ooid cortices are composed of Ca-phosphate. Preservation of concentric layers suggests primary origin. Raman spectroscopy reveals peaks in D-G bands within ooidal cortex at all localities, which likely indicates the presence of organic matter (kerogen). SEM examinations show branching-filaments ~1-3 mm in diameter resembling intracellular structures of filamentous bacteria. In places, ooids are covered by botryoidal grains that may represent the remains of microbial colonies with a botryoidal surface. However, further investigation of these structures is required. Our study suggests that phosphatic ooid formation was microbially mediated with anoxic conditions within the ooid cortex and low-oxic conditions at the outer surfaces of ooids.