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Abstract

The Brabant Massif is the largest Lower Palaeozoic unit of Belgium, only exposed at its southern part along a few river incisions. The outcrops at Halle-Lembeek, located 20 km SW of Brussels, form the northern part of the Zenne outcrop area. Widening of the railway track Lille-Brussels offered a rare opportunity to study this area in more detail.

Temporary outcrops and cored drillings provided a discontinuous lithostratigraphic log of the middle part of the Lower Cambrian Tubize Formation. Lithology and analysis of turbidite sequences show the transition of the relatively well-known Rogissart Member, characterised by proximal and thick turbidites, to the poorly documented lowermost and currently unnamed member of the Tubize Formation, which is characterised by a more distal facies.

A magmatic body in one of the permanent outcrops crosscuts the host-rock and is therefore of intrusive nature. Amygdules and vesicles in nearby decimetric magmatic interstratifications indicate volcanic activity in the Lower Cambrian. The intrusive and volcanic rocks have a similar chemistry, suggesting a relation to one episode of, previously unrecognised, Lower Cambrian magmatism.

Logging with a magnetic pendulum shows that younger rocks in the southern outcrops are more magnetic than older rocks in the northern outcrops. This trend corresponds to an increase in aeromagnetic field values from north to south. Outcrops of the non-magnetic rocks of the Blanmont Formation, northeast of Halle-Lembeek, correspond to well delimited aeromagnetic trough anomalies.

Folds occur at two locations in Halle-Lembeek. Near Lembeek a fold train of three steeply plunging anticlines and three synclines with divergent cleavage fans, is found. North and south of this fold train, non-folded, steeply dipping beds are found that are younging towards the SW and WSW respectively, and show a counterclockwise cleavage to bedding relation. In the northern outcrops near Halle, tight, gently plunging folds and open, steeply plunging folds are found.

The relation between lithostratigraphy and aeromagnetism allows defining the distribution of lithostratigraphic units around Halle-Lembeek. However, several interpretations are possible of the nature of the contacts between different lithostratigraphic units and of aeromagnetic anomalies. Three models are therefore proposed, assuming respectively isoclinal folding, steep shear zones, and absence of folds or shear zones. Also combinations of the three models are possible.