Siliceous coated grains (oncoids) are common on parts of Primrose Terrace, which is a large sinter apron below Champagne Pool in the Waiotapu geothermal area of North Island, New Zealand. Some oncoids are discoidal with smooth exteriors (Group I), whereas others have clusters of microstromatolites growing upon their surfaces (Group II). The microstromatolites normally began to form when the upper surfaces of the discoidal oncoids had reached mean water level. Growth of the cortical laminae in the oncoids and microstromatolites was mediated by a diverse biota that is dominated by fungi, with subsidiary bacteria (including cyano-bacteria) and diatoms. Pollen grains from Pinus radiata, which are common in some cortical laminae, nourished some of the fungi. Collectively, the microbes mediated growth of the oncoids by providing templates for silica precipitation. Most silica oncoids formed in shallow (<1 cm) flowing water that probably had a temperature of approximately 15-20 degrees C and pH of 3-6. Seasonal variations in the water temperature and pH were probably responsible for the cyclic variations in the fabrics of the cortical laminae. Laminae rich in microbial remains formed during the spring and summer months when the warmer waters allowed the microbes to flourish. Cooler waters in the fall and winter led to the demise of some microbes and the development of thinner cortical laminae with few microbial remains.

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