Abstract

The beach rock at Heron Island may be divided into three shore parallel zones, based on differing algal assemblages. The algae form laminated mats on the beach rock surface. No evidence has been discovered which indicates that older algal mats occur within the beach rock. Thus, algal mat formation has not played an important part in the actual growth of the beach rock. Thirteen bore holes were put down to a depth of 5 ft (1.5 m) through the beach rock on the south side of the Island. Petrological investigation of these indicates a marked increase of fibrous aragonite cement with depth, this effecting a porosity loss of between 20% to 50% in a vertical depth of only 3 ft (1 m). Most allochems are surrounded or partially surrounded by a micrite/algal mucus envelope. The chemistry of natural beach rock pools (high organic activity) and artificial sand pools (low organic activity) has been examined. The chemistry of the beach rock pools is being substantially controlled by factors other than those operative within the temperature aided photosynthetic cycles. Of special importance are the day time rise of calcium, the night time drop in calcium, and the night time rise in pH. This pattern is entirely opposite to the overall pattern expected in the vicinity of coral reefs.

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