ABSTRACT

Both fabric and geochemical evidence suggest that the oolite units of the Late Precambrian Trezona Formation had both aragonitic and calcitic precursors. Ooids inferred to be originally aragonitic occur as ooid molds filled by blocky, ferroan calcite and as calcitized ooids of neomorphic spar. The former are occassionally deformed, forming irregular shapes, and the calcitized ooids have elevated Sr concentration of between 3,000 ppm to 5,000 ppm. Some of the interparticle cements associated with these ooids have high Sr concentrations of between 2,000 ppm-5,000 ppm. The fabrics of these cements include clear calcite mosaics and poikilotopic spar. The precursor mineralogy and genesis of these cements remain uncertain, and various possibilities are evaluated.

The majority of ooids have micritic and microsparitic fabrics with clear, sparry calcite replacing the central cortical parts of the ooids. The precursor mineralogy of these ooids is not interpretable with the confidence of the aragonitic ooids. In some of the micritic ooids the replacing spar has microdolomite inclusions. This might indicate a high-Mg calcite precursor, but the process responsible for the retention and concentration of the Mg is not known. The Sr content of these micritic ooids is between 600 and 1,500 ppm. The micritic ooids invariably have cloudy, bladed to subgranular isopachous cements with Sr content of 800-1,200 ppm.