Terhagen Member

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

Abbreviation BmTe
Parent unit Boom Formation
Child units
Lithological description The Terhagen Member is the part of the Boom Clay in between the Belsele-Waas Member and the Putte Member. If the former member is not present, the Terhagen Member occurs at the base of the Boom Clay Formation above the Bilzen Formation. The Terhagen Member is a banded pale grey clay consisting of a several-dm-scale alternation of silt-enriched and clay-enriched layers (Figure 12). This banded nature is in contrast with the thick silt layers of the underlying Belsele-Waas Member. The pale grey colour is in contrast with the overlying Putte Member which is darker grey to even black stained. The black staining is due to layers with a few percent silt-sized detrital plant particles and such layers in the Terhagen Member are well expressed only at two levels, although under particular conditions a few more very subtle ones can be observed (Figures 13, 14, 15, 16). In the middle of the Terhagen Member occurs a characteristic pinkish to brownish stained silty layer labelled R-horizon (Figures 12, 14, 15, 16). In its part below the R-horizon the clay is calcareous (Figure 15), above the R-horizon carbonate is only occasionally present (Vandenberghe et al., 2014 fig. 11). Septaria horizons S10, S20 (key layer for correlation), S30 and S40 (Figures 12, 13, 14 and photographs 2 & 7 in Vandenberghe (1978)) occur in the Terhagen Member. In the subsurface of the Mol-Dessel area a geophysical continuous carbonate content log has shown a periodicity in the carbonate content all along the complete Boom Clay section, also in the Terhagen Member (Vandenberghe et al., 2022).
Age The Terhagen Member contains the lowest part of nannoplankton biozone NP23 (Figure 5).
Thickness The thickness varies between 20 and 25 m.
Area of occurrence The Terhagen Member occurs in the same area as the Boom Clay Formation. The Boom Clay Formation outcrops in the Waasland area, north of the Rupel river, between the Grote Nete and the Dijle-Demer rivers, and in southern Limburg (Figure 1a). North of this outcrop area, the clay occurs in the subsurface except for some locations in the Hageland where the erosion surface underlying the late Miocene Diest Formation has locally removed the Boom Clay Formation.
Type locality The Terhagen Member type area is the Rupel-cuesta exploitation area where it general forms the lower part of the extracted clay. The active clay extraction pits have shifted in the past but are located around x= 154 000 , y= 197 500 and Z =+30. An almost complete section of the Terhagen Member is given by Vandenberghe (1978, photo 2 p. 25). In the field and on geophysical well logs, the Terhagen Member is fairly easily picked between the top of the silty Belsele-Waas Member (Figures 2 and 15) and the overlying Putte Member (Figures 13, 16). The boundary with the Belsele-Waas Member is identified by the sudden incursion of clay expressed by a low resistivity signal (lowest oblique black arrow 1 on logs in Vandenberghe et al., 2001 figs. 3, 5-7 /Figures 8a, 8b, 8c, 8d, 11). On geophysical GR logs the boundary with the overlying Putte Member is identified by a sudden increase in GR announcing the organic richer Putte Member (Figure 2).
Alternative names In a now obsolete twofold subdivision of the Boom Clay, Vandenberghe (1974, fig. 8.1. and p. 186; 1978) distinguished a lower grey clay and an upper black clay, respectively named Waasland clay and Putte clay. The Waasland clay included the present Belsele-Waas and Terhagen Members. In the section by Vandenberghe (1980) it was indicated as part of the ‘Argile grise’.
The Terhagen Member of the Boom Clay Formation is included pro parte in the unit labelled R2c on the geological map 1:40 000 (Anonymous, 1892,1896,1900,1909) and pro parte in the unit labelled R2b in the Stratigraphic Register (Anonymous, 1929, 1932). On the recent 1: 50 000 geological maps the Terhagen Member is coded BmTe.
Authors Vandenberghe, N. & Wouters, L.
Date 05/04/2024
Cite as Vandenberghe, N. & Wouters, L., 2024. The Terhagen Member, 05/04/2024. National Commission for Stratigraphy Belgium. http://ncs.naturalsciences.be/lithostratigraphy/Terhagen-Member


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