CAN CHEMICAL WEATHERING OF VOLCANIC ASH RESULT IN THE FORMATION OF CHERT?

Chert is generally considered to be marine biogenic in origin and is not thought to have formed as a result of chemical weathering of volcanic rocks. However, chert lenses and nodules have been observed in the volcanic ash horizon of the Reid's Mistake Formation of Permo-Triassic age at Swansea Head, Boolaroo Subgroup of Newcastle Coal Measures of the Sydney Basin, Australia. Owing to their mode of occurrence and their close association with clay layers within the same volcanic unit, this research suggests that they formed as a result of chemical weathering of volcanic ash. This hypothesis was supported by the data gathered from thin-section petrography, x-ray diffraction analyses (XRD) and chemical study of samples, using the inductively coupled plasma-atomic emission spectroscopic analysis (ICP-AES), collected from the volcanic ash horizon. Thin-section studies indicated precipitation of microcrystalline to cryptocrystalline quartz and the dissolution/replacement/alteration of volcanic quartz, feldspar, mica, and glass shards. XRD analyses showed the presence of clay minerals, such as kaolinite and illite, which are products of chemical weathering. Owing to the absence of the unweathered parent rock, a reference rock, similar in characteristics to the volcanic ash horizon, consisting of an average of eight samples (two from Springbrook and six from Binna Burra of Tertiary volcanic complexes in Southeastern Queensland and Northeastern New South Wales) was selected. The mass balance calculations performed using chemical composition of the samples gathered from the Reid's Mistake Formation indicated an increase in SiO2 ranging from almost 4 to 9% and a higher loss on ignition and, a lower weight percentage of the mobile oxides when compared with the reference rock. This is what is expected when the rocks are chemically weathered. The evidences gathered from all of the above mentioned methods indicated chemical weathering to be responsible for the formation of chert in the volcanic ash horizon of the Reid's Mistake Formation. Recommendations for future studies involve reconstruction of the original parent rock composition using immobile trace element composition of the samples.