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2.10. Merksplas Formation

Authors:Frieda Bogemans & Timothy Lanckacker.

 

Description:

Consists of grey medium to very coarse sand, less common medium fine (180 - 250µm). Can contain gravel, but not in large quantities and mostly concentrated, as well as intercalations of silt and/or clay layers, laminae and lenses. The layers may attain a thickness of 10cm. Also typical is the presence of glauconite, peaty material, vegetation remains and wood fragments. The dominant stratification is massive, horizontal and planar. The silt/clay lenses are often deposited as flasers.

Within the Merksplas Formation two depositional facies are recognized:

-          A sand facies with silt/clay intercalations. This facies is non-calcareous (example drilling 8W157 (DOV kb8d8w-B46), coordinates: x = 181.039, y = 231.282).

-          A sand facies with few fine siliciclastic intercalations. Within the dominant grain size distribution medium fine sand is observed more often. This facies contain shell remains, most often broken even reduced to fine grid. This facies is calcareous (example drilling 17E340 (DOV kb8d17e-B89), coordinates: x = 193.690, y = 227.365).

If both facies are present the first one lays on top of the second one. 

 

Stratigraphic position: The Merksplas Formation lies directly on the Neogene Lillo Formation in the west (north and northeast of the city of Antwerp), on the Neogene Poederlee Formation in the neighbourhood of Kasterlee and on the Neogene Mol Formation further eastwards. The Merksplas Formation is covered by the Pleistocene Malle Formation (Brasschaat Member) in the whole Campine area.

 

Stratotype:

Area:The northern Campine area. In the southwest, near Stabroek, Kapellen, Brasschaat, Schilde and Zoersel, the Merksplas Formation is absent under the Malle Formation (Brasschaat Member). The formation is at first observed north of those communities. An overview of the spatial distribution of those deposits is presented in Figure 1. Near Malle, the Merksplas Formation crops out directly under the Late-Pleistocene and Holocene deposits, continuing eastwards to Retie via Lille and Kasterlee. The outcrop area is thereby situated directly north of the Poederlee ridge. In the east, the Merksplas Formation is delimited by the Mol Formation – this formation is in that vicinity also underlying the Merksplas Formation. However, it is not yet completely clear where the exact boundary between those two units is situated due to the fact that the lithology of both formation is rather similar.

Figure 1. Geographical distribution of the Brasschaat Member and the Merksplas Formation (according to the G3D-model, available on http://dov.vlaanderen.be).

Thickness: The Merksplas Formation typically reaches a thickness of about 15m. However, in the north, thicknesses of more than 25m can be observed for this formation. On the contrary, in the southwest, the thickness of the Merksplas Formation is severely reduced to only a few meters near Brecht and Malle. In the neighbourhood of Brasschaat and Zoersel, the Merksplas Formation is even absent, here does the overlying Malle Formation rest directly upon the Neogene Lillo Formation.

Age: Pliocene (Pleistocene - pars)

Well logs: The Merksplas Formation is characterised by a low gamma-ray signal and a high resistivity signal. The spontaneous potential typically increases towards the top of the formation. The base of the Merksplas Formation can best be recognized by the sudden rise of the gamma-ray signal (towards the Lillo Formation), while the resistivity signal may be more difficult to interpret, since its trend may be disturbed by the underlying sediments (e.g. the Merksem Member of the Lillo Formation, which has a quasi equally high resistivity value as the Merksplas Formation). The top of the formation can deduced by the abrupt augmentation of the gamma signal, as well as by the noticeable drop of the resistivity signal (both towards the Weelde Formation). Examples of clarifying well logs can be found in e.g. 15E298 (DOV kb7d15e-B329, x = 160.120, y = 227.760), Merksplas Formation in between 15m and 25m below the surface) and 6E132 (DOV kb7d6e-B295, x = 154.338, y = 236.220), Merksplas Formation in between 38m and 63m below the surface). The example of well log 6E135 (DOV kb7d6e-B298, x = 154.850, y = 232.630) is shown in Figure 2.

Figure 2. The interpretation of the Kalmthout well log.

 

Remarks:

-          The term Merksplas (Merksplas sands) is used for the first time by Gulinck in 1962.

-          Concerning the archives of the Geological Survey of Belgium: in the areas where both the Merksplas Formation and the Brasschaat Member are present no distinction is made between the two stratigraphic units. The term Merksplas Formation is used.

-          In the publication of Buffel et al., (2001) Brasschaat (consisting of Malle, Hemeldonk and Schorvoort – see their figures 2 and 3) corresponds with/stands for the Merskplas Formation, as defined above.