Cenozoic Evolution of the Nullarbor Plain Paleokarst, Southern Australia
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The Nullarbor Plain in southern Australia is an uplifted succession of Cenozoic marine carbonates whose surface has been exposed for ~14 m.y. This succession of limestones, particularly in the surfical middle Miocene Nullarbor Limestone, hosts a complex and prolonged record of meteoric diagenesis. Alteration took place through 3 broad phases of diagenesis encompassing 8 stages that are interpreted to have taken place over a dramatic regional climate change. Phase 1 diagenesis occurred under a humid middle Miocene climate and involved mineralogical equilibration with meteoric fluids, calcite cementation, widespread microkarst, and regional lacustrine and palustrine sedimentation producing copious amounts of ooids. These ooids are interpreted to have formed via microbial secretions and sediment aggradation over multiple seasons of changing rainfall and soil hydration states. Cortical laminations are proposed to represent microbial mucus envelopes during wet seasons alternating with dehydration during dry seasons and precipitation of fibrous clay minerals and CaCO3 that preserve the pre-existing microbial fabrics. Phase 2 alterations took place under a more temperate climate from the late Miocene to Pliocene with a later pronounced humid interlude. This phase encompassed ~8 m.y. and was dominated by karst process where deep cave dissolution occurred at depressed water tables related to globally low sea levels and later shallow caves developed during a Pliocene sea level highstand. Phase 3 has occurred since the late Pliocene and is indicative of the onset of modern semi-arid climatic conditions. This final phase involved the creation of subsoil hollows filled with blackened limestone lithoclasts, deep and shallow dolines, and indurated pedogenic calcrete that now forms much of the surface of the Nullarbor Plain. Blackened limestone clasts have been shown to form at the B-C boundary in soil profiles where roots have their cellular structures calcified and during this process incorporate trapped organics that ultimately produce the distinctive black colouration. The importance of this comprehensive diagenetic record is its direct applicability to the understanding of ancient subaerial exposure surfaces.