Marine Diagenesis of Shallow Marine Lime-Mud Sediments: Insights from 
O18 and 
C13 Data
Philip W. Choquette 1
1 Denver Research Center, Marathon Oil Company, Littleton, Colorado 80120
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 C13 of +2.5 per mille and O18 of +1.9 per mille (versus PDB-1); (ii) more usually cementation of unreplaced CaCO3, in intrasediment seawater, yielding isotopically marine lime mudstone mainly composed of calcite, 4-micron or finer, with O18 of from -1 to +1 per mille; (iii) later partial alteration of CaCO3, in permeable dolomitic rocks, by isotopically "lighter" waters, to calcite with an estimated O18 of -10 per mille or less. Isotope data appraised by petrographic analysis thus suggest "submarine" cementation of these carbonates in shallow marine conditions.
