ABSTRACT

Oligocene to mid-Miocene, deep-shelf, bryozoan-rich limestones across southern Australia are variably altered to gray to orange, Ca-rich, medium-crystalline, sucrosic dolomite. Degree of replacement ranges from scattered rhombs to complete dolostone units several tens of meters thick and many kilometers in areal extent. The locale and timing of dolomitization are tightly constrained to shallow burial and mid- to late Miocene.

Dolostone varies from friable to dense and well lithified. Dolomitization is fabric specific; muddy sediments are preferentially replaced; calcite bryozoans and brachiopods form biomolds. Geochemistry suggests that dolomitization was predominantly by seawater-limestone interaction, but admixing of continental possibly meteoric groundwater is required by data on stable and radiogenic isotopes and trace elements. Sr isotopes confirm a mid-Miocene age for the dolomite, if precipitated from seawater with no inherited limestone values. Dolomite crystals have undergone variable degrees of dissolution, and meteoric calcite cement has locally filled the resultant rhombic voids. Dissolution began in the crystal cores, implying some sort of metastability, and expanded outward until, in some ins ances, the whole dolomite crystal was dissolved, leaving dolomolds in otherwise unaffected limestone.

This dolomite occurrence, well constrained by regional geology, shows that widespread, porous and permeable, fabric-destructive, medium-crystalline, sucrosic dolostone can form in the shallow subsurface soon after sediment deposition. Such dolomite, however, may be metastable in the presence of variably mixed continental-marine waters, and newly formed crystals can undergo dedolomitization soon after precipitation.