Donk Member

The information on this page is a summary description.
The full formal description is available here: Donk Member

Abbreviation MlDo
Parent unit Mol Formation
Child units
Lithological description The Maat lignite Bed subdivides the Mol Formation into a Donk Member below the lignite and a Maatheide Member above the bed. Both sand units are pale grey to white quartz sand with an exceptionally high quartz content. The Donk Sand Member is finer grained than the Maatheide Sand Member; the latter has a modal size generally above 250 µm (Gullentops & Vandenberghe, 1995). However the Donk Sand Member close to the Mol Rauw Fault in the Witgoor-Dessel area also has a median grain size well above 250 µm, whilst further west in the Retie-Geel-Kasterlee area its median size is smaller than 200 µm (Vandenberghe et al., 2020). In the Poppel-Rauw fault zone and eastwards of it the Donk Sand Member contains a substantial 250-355 µm size fraction. Compared to the underlying Retie Member, the Donk Member is characterized by a fraction > 250 µm; in addition during drilling the borehole mud starts to take an olive-green hue at the top of the Retie Member. Some rare thin clay laminae are reported in boreholes, as are some quartz and quartzite gravels at the base (Gullentops and Vandenberghe, 1995). No macrofossils are present.
Age See LIS file Mol Formation for information on the age of this member.
Thickness In the Kasterlee-Mol-Dessel area west of the Poppel-Rauw Fault zone, the thickness of the Donk Member is between 5 and 10 m. The Donk (SIB-DON-02-03) and Pinken (SIB-PIN-03-03) sand pits closer to the Poppel- Rauw Fault zone show a marked increase in the Donk Member thickness up to 24 m. From this fault zone eastwards the Donk Member is about 25 m thick.
Area of occurrence The Maat lignite Bed, subdividing the Donk Member below from the Maatheide Member above, is outcropping along the Campine Canal in the Poppel-Rauw Fault zone west of Rauw 1 Fault ( Geological map 17 Mol, Gullentops & Vandenberghe (1995)). The faults are normal faults bordering the extent of the lignite and down dropping it at their eastern side. Therefore the Donk Member occurs near surface directly underlying the Quaternary only west of the Rauw 1 Fault. In the subsurface to the north in the Antwerp province the Mol Formation the distinction with the Brasschaat Formation is not obvious (Laga, 1976 –profile PGL76/106/3) and geometrically the formation seems to transition into the relatively coarse Merksplas Formation ( Laga, 1976 – profiles 76/106/2 and 75/104/1) [see also Lithostratigraphic Identification sheets of the Merksplas Formation (Note: in the Neogene Volume Louwye et al. (2020) and Vandenberghe and Louwye (2020) have named the Pliocene quartz sand in the west the Merksplas Formation and not Brasschaat Member as this name is reserved in the NCS for Pleistocene fine-sized sand.). East of the Rauw -1 Fault the Donk Member occurs in the subsurface below the Maatheide Member and Maat Lignite Bed. On the geological maps, the Mol Formation, including the Donk Member, is conventionally limited in the east by the major Roer Valley Graben (RVG) western boundary fault of Reppel to the east of which occurs the quartz sand of the Kieseloolite Formation.
Type locality The area west of the Poppel-Rauw Fault as figured in a series of boreholes in Vandenberghe et al. (2020, fig. 2) is a reference area for the Donk Member. In particular the boreholes in the Dessel area are considered as reference: ON-Dessel-2 (031W0338/kb17d31w-B299), ON-Dessel-3 ( 031W0354) & ON-Dessel-4 (031W0353/0376). These boreholes by ONDRAF-NIRAS have cores, sediment and mineral analyses, CPT logs and resistivity and gamma-ray logs (references in Vandenberghe et al., 2020 and Schiltz , 2020). In the area of the Poppel-Rauw Fault zone and eastward of it, the Donk Member is observed only in the subsurface below the Maat lignite Bed as figured in the boreholes in fig. 5 in Vandenberghe et al. (2020).
Alternative names
Authors Vandenberghe, N., Berwouts, I. & Vos, K.
Date 01/09/2023
Cite as Vandenberghe, N., Berwouts, I. & Vos, K., 2023. The Donk Member, 01/09/2023. National Commission for Stratigraphy Belgium.
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