SHALLOW BURIAL DOLOMITIZATION AND DEDOLOMITIZATION OF MID-CENOZOIC, COOL-WATER, CALCITIC, DEEP-SHELF LIMESTONES, SOUTHERN AUSTRALIA

Oligocene to mid-Miocene, deep-shelf, bryozoan-rich limestones across southern Australia are variably altered to gray to orange, Ca-rich, me dium-crystalline, sucrosic dolomite. Degree of replacement ranges from scattered rhombs to complete dolostone units several tens of meters t hick and many kilometers in areal extent. The locale and timing of dol omitization are tightly constrained to shallow burial and mid- to late Miocene. Dolostone varies from friable to dense and well lithified. D olomitization is fabric specific; muddy sediments are preferentially r eplaced; calcite bryozoans and brachiopods form biomolds. Geochemistry suggests that dolomitization was predominantly by seawater-limestone interaction, but admixing of continental, possibly meteoric groundwate r is required by data on stable and radiogenic isotopes and trace elem ents. Sr isotopes confirm a mid-Miocene age for the dolomite, if preci pitated from seawater with no inherited limestone values. Dolomite cry stals have undergone variable degrees of dissolution, and meteoric cal cite cement has locally filled the resultant rhombic voids. Dissolutio n began in the crystal cores, implying some sort of metastability, and expanded outward until, in some instances, the whole dolomite crystal was dissolved, leaving dolomolds in otherwise unaffected limestone. T his dolomite occurrence, well constrained by regional geology, shows t hat widespread, porous and permeable, fabric-destructive, medium-cryst alline, sucrosic dolostone can form in the shallow subsurface soon aft er sediment deposition. Such dolomite, however, may be metastable in t he presence of variably mixed continental-marine waters, and newly for med crystals can undergo dedolomitization soon after precipitation.