Lithification of carbonate sediments in the Heron Island Reef, Great Barrier Reef

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Abstract
In this reef area lithification has resulted in the formation of beach rock at the surface and reef rock in the subsurface. The process of lithification which leads to the formation of beach rock is an unusual and interesting one, but does not play a major rôle in the consolidation of reef sediments. Beach rock results largely from the protective action of encrusting algae and to a less degree from carbonate precipitation from sea water. Because of the general lack of “fines” in reef flat and beach sands, matrix recrystallisation plays no part in beach rock formation. By contrast, the reef rock from the subsurface generally contains a higher percentage of fines and, below 100 feet, this material has been largely recrystallised to form a strong binding medium. The process of recrystallisation is apparently selective, those sediments with high matrix, high coral/algal ratio and low MgCO3 content showing the highest degree of recrystallisation. Depth of burial shows no direct relationship to this process. Thus, of the two lithifying processes operative in the reef province, the less spectacular one of compaction and recrystallisation is the more important. Beach rock formation is significant in its influence on the present distribution of carbonate sands, but it makes little or no contribution to the total rock mass underlying the present reef.