Diagenesis of lime mud involves mineralogic stabilization, addition of large amounts of porosity-reducing cement and/or compaction, and grain shape changes, processes that may occur rapidly when fresh water invades the sediment pore systems. Scanning electron microscopy of lithified and semilithified Pleistocene lime mudstone from a subsurface location on Barbados (Borehole ) illustrate some of these early diagenetic processes. Mineralogically stable muds that have resided in fresh water phreatic lenses are well cemented with porosity less than 15%. Mud particles are 0.5 to 2 mu m in diameter Three to 8 mu m grains of cement occur in aggregates lining irregular coarse silt- and sand-sized cavities (molds?). In the modern zone of mixing of fresh and marine ground water. mineralogically metastable muds contain 20-30% calcite crystals, 3 to 5 mu m in size, partially filling micropores and cementing 0.25 to 2 mu m mud particles; porosity is 20-40%. These data indicate that early cementation of lime mud can occur in fresh and brackish phreatic environments. Mineralogic stabilization of the muds involves dissolution of 0.25 to 2 mu m sized metastable grains and precipitation of calcite crystals up to 8 mu m in size. Interpretation of the scanning electron micrographs indicates that aggrading neomorphism has not been an important process in the early evolution of the mud fabric.

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