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Dolomite cement is a significant and widespread component of Phanerozoic sucrosic dolomites. Cements in dolomites that were never deeply buried are limpid, have planar faces (non-saddle forms), often distinct zonation in cathodoluminescence and form syntaxial overgrowths on crystals facing pores. Five samples of sucrosic dolomites, interpreted as having had mostly limemudstone or wackestone precursors in four carbonate aquifers, provide insights into the abundance of planar cements in sucrosic dolomites. Such cement comprises 11% to 45% (32% mean) of peritidal to sub-tidal dolomites on an outcrop in the Edwards aquifer (Early Cretaceous) of central Texas; 19% to 33% (25% mean) of ramp dolomites in the Hawthorn Group (Oligo-Miocene) and 50% to 70% in shelf dolomites of the Avon Park Formation (Eocene) in the Upper Floridan aquifer of sub-surface peninsular Florida; 18% to 45% (32+% mean) of sub-tidal shelf dolomites in quarry sections of the Burlington-Keokuk Formation (Early Mississippian) in south-eastern Iowa; and 18% to 76% (50% mean) in shallow cores and outcrops of outer-shelf dolomites from the Gambier Limestone (Oligo-Miocene) of South Australia. Backstripping the cement phases revealed by cathodoluminescence colour photomicrographs documents the effects of cements on textural coarsening, pore-space reduction, induration and general 'maturation' of these dolomites. Most pre-Holocene dolomites are multiphase crystalline rocks composed of: (i) seed crystals or 'cores'; (ii) crystal cortices that concentrically enlarged the cores; and (iii) free-space, syntaxial precipitates of limpid cement around the crystals. Remaining CaCO 3 grains and micrite can be replaced by dolomite, but typically they are dissolved between stages (ii) and (iii), creating systems of intercrystal and mouldic pores typical of sucrosic dolomites. Networks of cement overgrowths, aided by water-filled pore systems under hydrostatic to lithostatic pressure, are judged to slow or prevent compaction in sucrosic dolomites. It can be argued that cortex growth involves both replacement of CaCO 3 particles and microcementation of their interparticle pores. This interpretation, and the abundance of cements in so many dolomites, would obviate the controversy over the volumetrics of 'replacement dolomitization'. Limpid, planar and syntaxial dolomite cements of early diagenetic origin are interpreted to have precipitated from clear pore waters, at low temperatures (<30 to 35°C) and shallow burial depths (<100 m), in water-saturated networks of dolomite 'silt' and 'sand'. Cements in many dolomites in island and continental-aquifer systems appear to result from event-driven processes related to sea-level highstands. Cementation events can follow 'replacement 1

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Shallow marine lime-mud sediments of the Ste. Genevieve Formation (Mississippian), in part of the Illinois Basin, underwent at least three diagenetic changes: (i) local dolomitization in seawater or a brine, producing dolostone having average deltaC(13) of +2.5 per mille and deltaO(18) of +1.9 per mille (versus PDB-1); (ii) more usually cementation of unreplaced CaCO(3), in intrasediment seawater, yielding isotopically marine lime mudstone mainly composed of calcite, 4-micron or finer, with deltaO(18) of from -1 to +1 per mille; (iii) later partial alteration of CaCO(3), in permeable dolomitic rocks, by isotopically "lighter" waters, to calcite with an estimated deltaO(18) of -10 per mille or less. Isotope data appraised by petrographic analysis thus suggest "submarine" cementation of these carbonates in shallow marine conditions.