 This occurs by dehydration of the clays during compaction, and heating due to blanketing of the sediments by continued deposition of sediments above.Zeolite facies is considered to start with temperatures of approximately 50 - 150 °C and some burial is required, usually 1 - 5 km.Zeolite facies tends to correlate in clay-rich sediments with the onset of a bedding plane foliation, parallel with the bedding of the rocks, caused by alignment of platy clay minerals in a horizontal orientation which reduces their free energy state.