In the southern Great Barrier Reef a relativel y thin (< 20 m) Holocene reef has developed on a foundation comprising a succession of older reefs. This reef foundation extends to a depth of up to 420 m in the subsurface and ranges from late Pleistocene to middle Miocene in age. The boundary between the Holocene and late Pleistocene reef is marked by a prominent solution unconformity that shows development of soils, calcrete, and extensive leaching in the underlying limestone. This has resulted from relative lowering of sea level during the period from the last interglacial to the post-glacial transgression. Late Pleistocene reef lithologies within individual drill holes are similar to their overlying Holocene counterparts, and there seems to have been a duplication of environments during both growth periods. However, Halimeda limestones, which are a prominent framework infill in the Pleistocene reef are not as abundant in the Holocene reef. Petrographic analysis of the late Pleistocene reef limestones shows that they have been subjected to near-surface subaerial diagenesis in both the vadose and phreatic freshwater zones. The effects of subaerial diagenesis increase progressively with depth below the solution unconformity, initially within the vadose zone and, ultimately, into the underlying phreatic zone. In the vadose zone, aragonite skeletons (e.g. corals. Halimeda) show two distinct processes of transformation to calcite: (1) total leaching and subsequent infill by sparry calcite cement: and (2) neomorphic replacement of aragonite by calcite, by a process of thin-film transformation. Variations in vadose and phreatic environments suggest that a more permanent freshwater lens was established around reef perimeters than beneath lagoons during the period of subaerial exposure.
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