Stratigraphy of two sandrocks at rainbow beach, Queensland, Australia, and a note on humate composition

Abstract Two intervals of coastal sandrock development are proved by the occurrence of sandrock boulders in a fossil beach that is itself preserved in sandrock. The fossil beach also contains driftwood, and carbon dates obtained by previous workers indicate that its age is greater than 40,000 years. The ancient beach rises 3 m above the present ocean beach and is the highest Pleistocene shoreline recognized on Queensland's southern coast. It is therefore likely to represent the warmest interval of the last two million years. Several lines of evidence indicate that the fossil beach is hundreds of thousands of years old. This is in agreement with tentative correlations with other Pleistocene events described in a previous paper where an age of 400,000 years was attributed to it. Large amounts of aluminium can be extracted with alkaline ammonium citrate from the two sandrocks, which are composed of quartz sands cemented with organic matter. Gibbsite is present in the older sandrock and in the boulders of older sandrock in the fossil beach. There is much less C relative to Al than in the younger sandrock. This is possibly due to decomposition of Al-humate by loss of organic matter, leaving Al in situ . If the rate of decomposition could be calibrated, by for example radiocarbon dating of the younger sandrock, it might be possible to use the C/Al ratios to date the older sandrock.