Guide to the geology of reefs of the Capricorn and Bunker Groups, Great Barrier Reef Province, with special reference to Heron Reef

Abstract

ABSTRACT. Published geological studies of the Capricorn and Bunker Groups of reefs together with preliminary results of our researches indicate that the reefal masses which comprise the southern end of the Great Barrier Reef Province had commenced growth by the early Pleistocene. Since then the reefs have been exposed during the. Pleistocene glaciations to subaerial weather-ing associated with the dissolving action of meteroic water. Subsequent sea level rise allowed coral growth to recolonize the pre-existing reefal bodies which in places may have exhibited a karst topography. The present stage of development of individual reefs can only be partly related to the relative heights of the sea and to the height and slope of the pre-existing reefal body, because a northwesterly trend in the development of reef types occurs irres-pective of the size of the reefs and irrespective of differences or similarities in the depth to the pre-Holocene disconformity. A possible explanation is that the rate of reef productivity might increase in the northwesterly direction. Further research is clearly warranted. Detailed analyses of the skeletal components of reef-top sediments from several reef types (e.g. closed ring, lagoonal platform, and platform) show that variations in composition result mainly from differences in the percentage contribution made by four dominant skeletal types: coral, coralline algae, Halimeda, and foraminiferans. As the reef progessively changes according to a recognisable sequence in the development of reefs, the component com-position of the bulk of the sediments also changes. This trend reflects the changing nature of the biota of the reef top. Recognition of this gradual evolution of reef types allows one to relate conclusions concerning the com-ponent and textural composition not only to individual depositional environ-ments on each reef, but also to a scheme of reef development, thereby pro-viding insight into the possible variations in the composition of sediment in time as well as in space.