The interpretation and environmental significance of a buried Middle Pleistocene soil near Ipswich Airport, Suffolk, England

Abstract

A buried Middle Pleistocene soil at Ipswich Airport, Suffolk, England, was studied by using macromorphological, textural, mineralogical, chemical and micromorphological techniques. This soil, developed on a low-level terrace surface in the Kesgrave Sands and Gravels, and buried beneath solifluction deposits and the Barham Sands and Gravels, is a composite of the Valley Farm and Barham Soils which have been recognized over wide areas of East Anglia. Clay illuviation, gleying, rubification (haematite formation) and periglacial disruption were the major pedogenic processes active during its formation; mineral weathering and temperate pedoturbation appear to have played only a minor role. After deposition of the Kesgrave Sands and Gravels and establishment of a stable land surface, clay was translocated from the eluvial horizons into the lower illuvial horizons. Initially, this process consisted solely of fine clay but as the environment deteriorated, coarser and more poorly sorted clay was translocated. Biotic, shrink—swell or frost turbation processes led to localized disruption of some limpid (fine) clay coatings before, or simultaneous with, commencement of this phase of coarser clay illuviation. However, most fragmentation of coatings occurred later when the environment had deteriorated to one characterized by seasonally frozen ground. At this stage, silt grains were translocated and small-scale contraction cracks or microscale cryogenic features (silty clay cappings and duplex textural lamellae features) formed. Further deterioration of climate led to formation of large-scale contraction cracks and soil (or incipient ice) wedges, truncation of the soil and deposition of two solifluction deposits. The older sediment contains components of the eroded eluvial horizons, whereas the other solifluction deposit and the overlying (glacifluvial) Barham Sands and Gravels contain minerals derived from the Anglian ice sheet. The soil at Ipswich Airport is developed in the Waldringfield Member of the Kesgrave Formation, which is assumed to be of Beestonian age. As the overlying sediments were apparently deposited during the Anglian Stage, it appears that the soil probably formed during the Cromerian and early parts of the Anglian. Such a chronology would not be in dispute with the proposed environmental reconstruction derived largely from pedological evidence, which suggests a simple environmental deterioration from a temperate optimum to that of periglacial conditions. However, much depends on the significance of the first disruption phase. If the fragmentation of limpid clay coatings represents a sharp climatic oscillation, the environmental reconstruction and stratigraphic implications of this soil may be more complex.