Definition of two new members in the upper Kreftenheye and Twente Formations (Quaternary, the Netherlands): a final solution to persistent confusion?

Abstract

The upper part of the Late Saalian to Early Holocene Kreftenheye Formation in the Netherlands usually consists of a relatively fine-grained, clastic bed (frequently referred to as 'loam bed'), which is generally regarded as an overbank deposit of incipient meandering fluvial systems. It was formed essentially in two phases, during the Allerød and the Early Holocene. The 'loam bed' is sometimes covered by eolian river dune deposits which have also been considered as part of the Kreftenheye Formation. Both sedimentary units have a large areal extent, a number of characteristic lithologic properties and a distinct stratigraphic position. It is shown that the 'loam bed' in a lithologic sense rarely consists of loam. The currently existing terminological confusion concerning the 'loam bed' and (to a lesser extent) the river dune deposits is caused by the mixing of lithologic, genetic, lithostratigraphic and chronostratigraphic criteria. In order to avoid further confusion, two formal lithostratigraphic units are introduced and lithologic descriptions are given for the holostratotypes of these units. The Wijchen Member of the Kreftenheye Formation essentially contains the 'loam bed'. The definition of the areally variable Wijchen Member is supported by a hypostratotype. The river dune deposits are incorporated in the Delwijnen Member. The Delwijnen Member is included in the Twente Formation because its lithologic characteristics and stratigraphic position are more similar to the Younger cover sands (Wierden Member) of the Twente Formation than to the fluvial coarse-grained deposits of the Kreftenheye Formation.