2.11.2. Boom Formation - Bm
Name
The name Boom Clay has been cited for the first time by De Koninck (1837). Boom is a town along the river Rupel (Antwerp province).
General characteristics
The Boom Clay is a grey silty clay or clayey silt with fairly constant chemical and mineralogical properties. This clay is rich in pyrite and contains glauconite in the most silty horizons. The clay is typically banded with layers of about 20 to 50 cm thickness, expressed by rhythmic variations in silt content, land-derived organic material and carbonates. Typically septaria have developed in the calcareous horizons. All these thin layers occur in a well-defined vertical succession, remaining constant in the outcrop area. In the subsurface the upper part of the clay becomes more sandy east of Mol but in the lower part of the clay the layer succession known from the outcrop area can be identified. Therefore the rhythms can be used as a microstratigraphic instrument (Vandenberghe, 1978; Vandenberghe et al., 2001).
Mollusc content is limited in numbers but several species have been described, dominated by a.o. Leda deshayesiana (Vincent, 1889; Glibert, 1957). The ecological analysis of the fauna and flora in the clay, leaves no doubt about the marine character of the deposit. The Boom Clay has been deposited in an open shelf sea under warm climatic conditions, as an aerobic mud at an approximate depth of about 50m or even a few tens of meter deeper.
Type area
The area in which the clay traditionally is exploited for a.o.brick making along the Rupel between Rumst and Boom and along the Scheldt between Temse and Antwerp is designated as the type area of the Boom Clay.
Occurrence
The Boom Clay Formation outcrops in the Waasland and north of the Rupel and Nete rivers. North of this outcrop area the clay occurs in the subsurface.
The Boom Clay Formation is underlain by the Bilzen Formation (Berg Member, Kleine-Spouwen Member, Kerniel Member) in Limburg and Flemish-Brabant and by the Zelzate Formation (Ruisbroek Member) in the provinces of East-Flanders and Antwerp. The Boom Clay Formation is overlain by Pliocene deposits (Waasland area), Miocene deposits (Antwerp and Brabant provinces), the Eigenbilzen Formation (east of the Antwerp province and Limburg), locally by the Voort Sand (north of Antwerp and in the north of the Antwerp province) or by Quaternary deposits in the outcrop area.
The thickness of the Boom Clay varies between a few meter in the southern part of the Waasland, 40 m in Rupel area, 80 m in the Antwerp area and till 150m in North Belgium; to the east its thickness diminishes.
Subdivision
In the outcrop area of the Waasland and the Rupel area, the Boom Clay Formation can be lithologically split up in three members:
Belsele-Waas Member;
Terhagen Member;
Putte Member;
In addition, a fourth member has been identified in the Campine subsurface, namely the Boeretang Member.
In all these members lithologically distinct layers systematically occur.
As these layers have a microstratigraphic significance some of the prominent layers can be considered as key horizons for correlation and therefore formally designated with a stratigraphic 'bed' status.
All layers in the outcrop area have been numbered by Vandenberghe (1978, p.39). Based on a detailed analysis of cores and geophysical borehole logs, related to the NIRAS/ONDRAF SCK/CEN research project for the deep storage of nuclear waste, a more complete and logically constructed numbering scheme has been developed (Mertens & Wouters, 2003) represented in figure 4 together with the litholog. This new numbering system keeps identical numbers as in the outcrop area for the layers 10 to 59. In his study of the Weelde borehole Van Simaeys (2004, fig. C2, p175) has numbered all the resistivity wiggles conformably the numbering in the outcrop area above the layer 10 and the additional younger section conformably the numbering system as elaborated by Mertens & Wouters (2003). In their study of the borehole ON-Dessel 1 Abels et al. (2007) have introduced another numbering system; their study of the periodicities in the resistivity signal needed to characterise all details in the signal.
A remarkable example is the layer 39 sensu Abels et al. (2007), easily recognisable on resistivity logs but less well outspoken in field logging (see fig. 139 in Vandenberghe, 1980) and therefore not individually numbered; nevertheless the original grain size data (Vandenberghe 1978) already suggested the presence of an additional silt horizon, later confirmed by detailed analyses (Van Boven, 1998).
Especially in the lower thick silt layers a consistent numbering between areas will be difficult as the thickness of this basal part of the clay increases to the northeast (com-pare e.g. the boreholes Reet and ON-Dessel-1 on fig.7 in Vandenberghe et al. (2001), with a numbering only appliquable to this specific figure). Abels et al. (2007) have numbered all the details of these basal silty layers and continued the numbering from this lower part upwards. This difficulty is also the reason that the numbering of Vandenberghe (1978) and Van Simaeys (2004) below layer 10 differs from the numbering in Mertens & Wouters (2003). The difference in numbering of the layers 60 to 68 in Vandenberghe (1978) and Mertens & Wouters (2003) can be explained by the practice in Vandenberghe (1978) to additionally number septaria layers occurring in the middle of a silt- and clay layer (see e.g. layer 9=S10 in Vandenberghe, 1978). The different numbering systems and their correspondence is shown in figure 4; for details of the numbering of the basal silty layers, the original references have to be consulted. Regarding the Boeretang Member in the Boom Clay Formation, the numbering of the silty resistivity layers is also indicated.
It is proposed to give a formal 'bed' status to (numbers between brackets refer to Vandenberghe, 1978):
- pink horizon R (nr 21)
- boundary surface between grey and black clay (between layers nr 31 and 32), also very well expressed in the natural gamma ray logs.
- the very silty to fine sandy double layer (layers nr 39-41)
- the septaria layer S20 characterised by large and numerous septaria (nr 14)
- the septaria layer S50 characterised by platy septaria containing multicoloured pyrite crystals in the septae (nr 49)
- the septaria layer S60, conatining siderite, rusty coloured, with visible bioturbation tracks (nr 56).
These layers are figured in Vandenberghe (1978, photographs 3, 4,5,6).
Previous designations
In the description of the area around Bilzen, Van Den Broeck (1883) uses the term "glaise schistoïde du Limbourg".
Geological Map 1/40.000: R2c
Stratigraphical Register (1929,1932): R2b
New Geological Map 1/50.00: Bm.