Lithology and diagenesis of the carbonate foundations of modern reefs in the southern Great Barrier Reef

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.

在大堡礁（Great Barrier Reef）南部，一套厚度相对较薄（<20米）的全新世（Holocene）珊瑚礁发育于一套由多期古礁体组成的基底之上。该礁体基底在地下延伸深度可达420米，形成时代跨度为晚更新世（late Pleistocene）至中中新世（middle Miocene）。全新世与晚更新世珊瑚礁之间的界面以显著的溶蚀不整合面（solution unconformity）为标志，该界面显示下伏灰岩中发育土壤层、钙结层（calcrete）及广泛的淋滤作用。该现象源于末次间冰期至冰后海侵阶段的海平面相对下降。单个钻孔内的晚更新世珊瑚礁岩性与上覆的全新世礁体相似，两次造礁期的沉积环境似乎具有重现性。不过，作为晚更新世珊瑚礁主要骨架充填物的钙扇藻（Halimeda）灰岩，在全新世礁体中的丰度则显著偏低。对晚更新世珊瑚礁灰岩的岩石学分析表明，其在包气带（vadose zone）与潜水淡水带（phreatic freshwater zone）两类近地表大气成岩环境中均经历了成岩改造。大气成岩作用的影响随溶蚀不整合面之下的深度逐渐增强，最初作用于包气带，最终波及下伏的潜水带。在包气带中，文石（aragonite）骨骼（如珊瑚、钙扇藻）存在两种截然不同的向方解石（calcite）转化的过程：其一为完全淋滤后由亮晶方解石胶结物（sparry calcite cement）充填；其二为通过薄膜转化机制，由方解石完成对文石的新矿物交代。包气带与潜水带的环境差异表明，在大气暴露期，礁体周缘形成的永久性淡水透镜体（freshwater lens）比泻湖（lagoon）下方更为发育。