Correlation between fragipans and permafrost with special reference to silty Weichselian deposits in Belgium and northern France

A comparison is made between the properties of permafrost and those of fragipans. The permafrost characteristics are summarized from published data on active permafrost and from personal observations on fossil permafrost. The study of the fragipans mainly concerns the silty sediments of Weichselian age which occur in Belgium and northern France.The distribution and the physical, macromorphological and micromorphological properties of fragipans can all satisfactorely be explained by the growth of segregated ice during permafrost aggradation. Particularly striking is the relationship of both permafrost and fragipans, to drainage conditions: the quantity of segregated ice is highest in moist or poorly drained soil and it is at such places that the best developed fragipans are observed.Certain arguments that can be stated against the hypothesis of fragipan genesis by permafrost are discussed: (1) the development of a platy structure by freeze-thaw or by wet-dry cycles, (2) the discontinuous distribution of the fragipan, (3) the development of the fragipan by pedogenetic weathering and migration processes. It is concluded that only the permafrost hypothesis can explain the complete set of properties of the fragipan.