Boom Formation

The information on this page is a summary description.
The full formal description is available here: Boom Formation

Abbreviation Bm
Parent unit Rupel Group
Child units Belsele-Waas Member, Terhagen Member, Putte Member, Boeretang Member
Lithological description An extensive synthesis on the Boom Clay geology is given in Vandenberghe et al. (2014). The Boom Clay is a grey silty clay or clayey silt with fairly constant chemical and mineralogical properties. Macroscopic fossil content is limited and mostly single shells of the mollusc Portlandia deshayesiana (in the literature also reported as Nuculana and Leda) are observed.. The ecological analysis of all observed fauna and flora in the clay proves the marine character of the clay deposit. The Boom Clay has been deposited in an open shelf sea under warm climatic conditions as a life-supporting mud at an approximate depth of about 100 m for the most clay-enriched parts, although the water depth fluctuated with several tens of meters due to eustatic variations during the full history of the clay sedimentation. The most silty horizons contain glauconite pellets. The clay contains a few percent of very early diagenetic, mm to cm and even dm scale concretionary pyrite that formed clearly associated with the presence of organic matter in the clay. Black staining of the clay is due to the presence of an increased amount of fine particles of land-derived plant remains. Small quantities of marine organic matter adsorbed on the clay minerals are always present. Marl horizons occur and typically large septaria concretions have developed in the most calcareous horizons. The septaria are an iconic property of the Boom Clay Formation (Septarienton in Germany). Detailed clay mineralogy of the Boom Clay can be found in Zeelmaekers et al. (2015).
Age Based on available chronostratigraphic data (Figure 5) the Boom Clay Formation is older than about 29.3 Ma and younger than 32 Ma.
Thickness In the Campine subsurface, the maximal thickness of the Boom Clay Formation is about 100 to 120 m, but the formation thickness is reduced where its top is either eroded or where it is replaced by the Eigenbilzen Formation and/or the Bilzen Formation pro parte (Figure 5): e.g. a few meter thickness in the southern part of the Waasland area, 40 m in Rupel area, 40 m in the Hasselt area, 80 m in the Antwerp area and about 10 m in the Albert-Canal section near Eigenbilzen.
Area of occurrence 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 completely removed the Boom Clay Formation.
Type locality The area in which the Boom Clay Formation traditionally has been and still is exploited for brick making or other structural clay products, along the Rupel river between Rumst and Boom (Rupel Cuesta) and along the Scheldt between Temse and Antwerpen, is designated as the type area of the Boom Clay ( Figure 1a). Instead of the former numerous brick yards with their own clay extraction pits, nowadays only two actively exploited large pits are active: the Wienerberger clay pit at Rumst and the Argex clay pit at Kruibeke-Burcht. Both clay pits are therefore the practical type sections at present. Many sections of former clay pits in the type area are documented in Vandenberghe (1978) and the cored and geophysical borehole at Reet (water tower) documents the Rupel cuesta section (Van Echelpoel, 1991 fig. 4.5; Vandenberghe et al., 2001 fig. 3).
As the upper part of the Boom Clay Formation is considerably eroded in the designated type area (Figures 4 and 5), a group of additional boreholes around Mol-Dessel in the Antwerp Campine (Mol SCK 15, ON-Mol-1 and ON-Dessel-1), is selected as reference boreholes to demonstrate the complete Boom Clay Formation section (Figures 2 and 12). The geophysical well logs of the three boreholes presented in Figure 2 are interpreted in terms of the 4 members of the Boom Clay Formation and a few key horizons. The link of the geophysical logs to the lithology section of the boreholes is easy as the same interpretation terms are figured on the lithology section of the same Mol-Dessel area in Figure 12 which is representing the most complete information extracted from cores and geophysical logging including core imaging. A correlation between the Reet borehole section in the outcrop area and the equivalent part in the Mol-Dessel area is shown in Figure 11.
Alternative names Informally this unit has also been referred to as Rupelian clay or Rupel clay. In the description of the area around Bilzen, Van den Broeck (1883b) used the term “glaise schistoïde du Limbourg”. Coding on the geological maps and the stratigraphic legends has differed through time: R2c on the 1:40 000 map (Anonymous 1892,1896,1900,1909), R2b in the stratigraphic registers (Anonymous 1929,1932) and Bm on the most recent 1:50 000 geological maps in use today (
Authors Vandenberghe, N. & Wouters, L.
Date 05/04/2024
Cite as Vandenberghe, N. & Wouters, L., 2024. The Boom Formation, 05/04/2024. National Commission for Stratigraphy Belgium.


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