Fragipan formation in argillic brown earths (Fragiudalfs) of the Milfield Plain, north‐east England. II. Post Devensian developmental processes and the origin of fragipan consistence

Abstract

SUMMARYPedogenetic processes following a permafrost stage of development in four argillic brown earths with fragipans (Glossic and Ochreptic Fragiudalfs) are investigated by soil microscopy, including SEM observations of unimpregnated soil material, and by determination of clay mineralogy. Micromorphology of the apparently massive fragipan confirms both the presence of subhorizontal fissures infilled with illuvial deposits separating dense lenticular structures formed by ice‐lens growth, and former vertical fissures formed by dessication on freezing with subsequent widening by ice‐vein development. The firmness and brittle failure of the fragipans are attributed to a closely‐packed, well‐graded matrix of skeleton grains bonded by clay bridges consisting of non‐swelling illite and chlorite and to weak interconnection of macrovoids. Weak cementation is rejected as an explanation of fragipan consistence. Sequential stages of particle translocation have been critical to fragipan formation. High bulk density is attributed not only to irregular compression and contraction of the soil matrix under permafrost conditions but also to the infilling of voids by illuvial silt and clay. The full development of the fragipans has depended on Flandrian clay migration which has contributed to clay‐bridge formation, void infilling and localized seasonal impedance of drainage resulting in eluviation of iron oxides, the development of grey polygonal patterning, and processes of degradation at the pan surface leading to glossic features.