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Stratigraphic analysis of cored and geophysically logged boreholes in the Kasterlee-Geel-Retie-Mol-Dessel area of the Belgian Campine has established the presence of two lithostratigraphic units between the classical Diest and Mol Formations, geometrically related to the type Kasterlee Sand occurring west of the Kasterlee village and the study area. A lower ‘clayey Kasterlee’ unit, equivalent to the lithology occurring at the top of the Beerzel and Heist-op-den-Berg hills, systematically occurs to the east of the Kasterlee village. An overlying unit has a pale colour making it lithostratigraphically comparable to Mol Sand although its fine grain size, traces of glauconite and geometrical position have traditionally led stratigraphers to consider it as a lateral variety of the type Kasterlee Sand; it has been named the ‘lower Mol’ or ‘Kasterlee-sensu-Gulinck’ unit in this study. In the present analysis, the greenish glauconitic Kasterlee Sand in its hilly stratotype area evolves eastwards into the lower ‘clayey Kasterlee’ unit and possibly also into an overlying ‘lower Mol’ or ‘Kasterlee-sensu-Gulinck’ unit, although it is equally possible that the latter unit has an erosive contact and therefore is stratigraphically slightly younger than the type Kasterlee Sand west of the Kasterlee village. A lateral extension of this detailed stratigraphic succession into the faulted zone of east Limburg is proposed.

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Previous studies have shown that the Kasterlee Formation occurs as far south as the hilltops of Heist-op-den-Berg and Beerzel in the south of the province of Antwerp. In fragmentary outcrops on these hills, a clear threefold subdivision can be recognized. In the current study, a large temporary outcrop on the Heist-op-den-Berg hill was studied, providing a unique view into the small-scale vertical and lateral variations in the Kasterlee Formation. Based on field observations, grain size analyses and mineralogy, the characteristics of the three units could be clearly defined. Cone penetration test logs show that these units can also be traced further north in the basin. As the three units can be easily identified based on the parameters provided in this paper and as they occur over a wide area, it is proposed to introduce them as formal members of the clayey Kasterlee unit. The observations made in this outcrop also allowed to refine the paleogeographic model of the Kasterlee Formation, with deposition of sand-clay alternations in restricted lagoons, separated from the marine environment by a coastal barrier, during an overall regression. Coastal barrier deposits are found in the south and northwest, but in the northeast only deposition in restricted lagoons occurred.

During processing and interpretation of about 200 Cone Penetration Tests (CPT) with electrical cones in a limited area of 60 km² in NE Belgium, a close analogy was observed between the vertical trend and signature of two CPT derived parameters (normalized soil behavior index Ic and hydraulic conductivity kSBTn) and borehole gamma ray (GR) measurements. This close analogy could be repeatably observed on five locations with a CPT executed along a cored and logged borehole. Using this feature proves to be a considerably help for stratigraphic interpretation, especially for sandy units displaying low lithological contrast. Lithostratigraphic units and their boundaries are determined by the specific log response patterns of these CPT derived parameters (kSBTn and/or Ic). These log responses prove to be more consistent and better to correlate than for the classical parameters (cone resistance qc and friction ratio Rf). This paper describes the case of a Neogene setting in the NE of Belgium where this feature was first observed. An informal stratigraphy based on CPT log signal is proposed for the geological units for the research area. Some other cases are briefly described to illustrate the usability of this feature for other geological settings or issues.