Two postglacial carbonate deposits in Saskatchewan were studied to determine diagenetic changes occurring under different chemical and physical environments. The presence of aragonite and calcareous shells suggests that the dominant source of the carbonates was biologic precipitation. The well drained Sturgeon Lake site contains predominantly aragonite. Percolating rainwater is converting aragonite to calcite in the upper 69 cm. The poorly drained and saline Debden site contains a mixture of aragonite, calcite and dolomite. The dolomite content increased with depth, particularly in the fine silt (2–5 μm) and clay fractions (<2 μm). X-ray diffraction patterns indicate that the calcite contains approximately 5 mol percent MgCO3 and that the dolomite is enriched with Ca. High concentrations of magnesium and sulfates in the interstitial water is preventing the transformation of aragonite to calcite, instead the aragonite is being converted to dolomite.