Abstract

• The phacoidal (nodular) limestones of early Silurian age were studied in three selected areas on the Pindaya Range of Southern Shan State. In Yegyanzin area, the limestones are entirely crinoidal; the Linwe area are red crinoidal limestones, grey ostracoda limestones and lime mudstones; while the Thayetpya area consists of grey ostracoda limestones and lime mudstones. The limestones commonly show evidence of early lithification taken place near the sediment/water interface. Petrographic observation of both mud and grain-supported facies being rich in crinoids, brachiopods, trilobites, thin-shelled bivalves enabled the reconstruction of the digenetic evolution. Some layers, underwent a phase of early lithification, acquired a rigid framework that prevented deformation and rearrangement of grains during later burial. In the mud-supported facies, the effects of subsequent mechanical and chemical compaction are easily recognizable. Mechanical compaction includes overburden pressure, flattened burrows, fossil breakage, shrinkage pores and transformation from wackestone to packstone fabrics. The response to chemical compaction varied with texture; grain-supported sediments developed fitted fabrics whereas in mud-supported sediments stylolitic seams were generated. As a result of these diagenetic processes, laterally extensive stylolities would have developed in these phacoidal limestones.