Loessial deposits in southern South Island, with a definition of Stewarts Claim Formation

Abstract

The loess deposits in the southern part of the South Island represent at least four major periods of accumulation. The deposits include the present soil and three well-defined paleosols separating individual loess members. A fourth poorly defined paleosol within the uppermost loess member is considered to represent a minor period of soil formation. The name Stewarts Claim Formation is proposed for the loess deposits of the region. The formation comprises four members: Yellow loess member (the youngest); Brown A loess member; Brown B loess member; and Brown C loess member (the oldest). The distribution pattern and morphology of the loess deposits is described. It is considered that in most localities, paleosols do not have a complete paleo-solum. The break between a paleosol and the overlying loess of the succeeding deposit is marked by a sharp boundary, in many places associated with a discontinuous stone line. The morphology of paleosols is comparable to that of the lower part of the solum or sub-solum horizons in present day soils developed on loess, particularly the yellow-grey earths. Thus it is considered that a period of “pedosphere stripping” has decapitated the soil prior to the deposition of the succeeding loess layer. Wood fragments at the base of the second youngest loess member immediately above the second major paleosol give a radiocarbon date (NZ1457) of 21 500 ± 1100 years B.P. The loess deposits are considered to span the period from the Waimean Glacial Stage to the present day and are correlated with advances and retreats of the ice during the Pleistocene.