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ABSTRACT

A 3 km long lateral facies transition from Oligo-Miocene skeletal grainstones into packstones and mudstones displays systematic variation in carbonate skeletal fragment alteration and secondary carbonate mineral composition.

Within the skeletal grainstone end-member all aragonitic grains have dissolved, all Mg-calcite skeletal fragments have altered to calcite, and calcite has been precipitated as both void-filling spar and as syntaxial overgrowths on echinoid fragments. Within the transitional grainstone-packstone lithologies some skeletal fragments show preservation of their original mineralogy, while calcite, ferroan calcite spar and dolomite have precipitated in void spaces. Within the packstone-mudstone end-member of the transition, aragonitic grains have been preserved and Mg-calcite skeletal fragments have retained most of their magnesium. Thin, originally porous grainstone layers within the pack-stone-mudstones contain sideroplesite and ferroan calcite spar in void spaces, and iron has been incorporated in skeletal fragments which were originally Mg-calcite. In these grains iron has been incorporated at the expense of magnesium until the grains approach the composition of the surrounding ferroan calcite cement.

The lateral variation in diagenetic alteration across this facies transition is a function of differing groundwater movement and composition, controlled ultimately by the composition, porosity and permeability of the host lithologies.