Abstract

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.

You do not currently have access to this article.