The Diest Formation: a review of insights from the last decades

1. Introduction and aim of this review The Diest Formation is an important Neogene deposit in NE Belgium (Fig. 1a). It consists of greyish-green to brownish‐green, fine to coarse grained, locally clayey, glauconiferous sand. In outcrop in the Hageland and Zuiderkempen areas (Fig. 1a, b), the sand is locally iron‐cemented. The formation constitutes the interior of the longitudinal, parallel Hageland Hills around Leuven and Diest (Gullentops, 1957; Houthuys & Matthijs, 2018). More to the north, in the Kempen (Campine) subsurface, it is found as a tens of metres thick sand deposit that hosts an important aquifer. The main lithological composition and appearance in both regions are similar, though certain aspects of their lithology and style and time of deposition are different (Vandenberghe et al., 2014). Below the Hageland outcrop area and its NE subsurface extension, an important incised depression is present at the base of the Diest Formation. To the west, some outliers on the Flemish hills, a long row of hills near the Flanders‐Wallonia boundary, from Flobecq to Cassel in northern France (Fig. 1a), were traditionally assigned to the Diest Formation although doubt about this designation has always existed (review in Houthuys, 2014). Figure 1. Location and outcrop maps. a) Outcrop map of Northern Belgium (DOV, 2019). Thin Quaternary cover and locally thick Quaternary perimarine, estuarine and fluvial sediments have been stripped. The outcrop area of the Diest Formation (F.)