Geophysics | EUROGIO – The Bree uplift at the southern boundary of the Rur valley graben

Presented at Meuse - Rhine Euregio Geologists Meeting, Krefeld 9.5.2003.

Abstract

Borehole Opitter 48E 294 (KB 225) was drilled in 2002 as a joint project of the Royal Observatory of Belgium and the Geological Survey of Belgium for the emplacement of a seismometer on the Campine Plateau as close as possible to the Bree Scarp, southern margin of the Rur Valley Graben. The borehole Opitter reached a depth of 571 m and traversed Tertiary, Cretaceous, Triassic and Carboniferous formations.

Borehole Opitter is the first deep well drilled on the ‘Bree Uplift’, a Cretaceous inversion structure parallel to the Rur Valley Graben, in between the converging terminations of the Heerlerheide and Grote Brogel faults, discovered by a coal seismic survey ‘Neeroeteren-Rotem’ in 1982. The areal extent of the Bree Uplift was defined by the coal seismic survey ‘Meeuwen – Bree’ in 1982; timing and effect of the inversion was fully described by Rossa in 1987. However, the new geological map 18-10 Maaseik-Beverbeek, published in 1999 displays the Bree Uplift as a late Tertiary horst with strongly dipping and thinning Tertiary strata. A high-resolution shallow reflection seismic survey carried out in 1999 over the southern boundary faults of the Rur Valley Graben confirmed the previous seismic interpretation and invalidated the structural interpretation shown on the geological map. Tertiary formations extend with rather constant thickness and almost undisturbed from borehole 146 (Neerglabbeek) in the Campine basin towards borehole Opitter near the edge of the Rur Valley Graben, meaning that the southern boundary of the Bree Uplift was largely inactive during the Tertiairy. Cretaceous sediments are reduced to half on the Bree Uplift, which thus takes an intermediate position between Campine basin and Rur Valley Graben. A reduced Buntsandstein sequence was found, covering the, probably youngest Carboniferous strata drilled in the Campine, which indicates that the Cimmeric downthrow of the southern boundary fault of the Bree Uplift was not in balance with subsequent Cretaceous inversion, as is generally the case. Faults marking the southern boundary of the Bree Uplift apparently were short-lived and not reactivated, resulting in a vague deformation zone.

The stratigraphic divergence between the Tertiary succession of borehole Opitter and outcrops in the former Solt sandpit, 1 km further to the SE, parallel to the Bree Scarp, needs to be explained. Either the special facies observed in the Solt sandpit are not local facies types of the Diest and Bolderberg formations as postulated by Gullentops & Huyghebaert (1999) but of the younger Kasterlee formation. Or the uplifted sediments of the Solt sandpit are deformed in the relay zone between Feldbiss and Heerlerheide faults. The 1999 seismic survey showed a similar structure along the Bichterweerd Scarp in the Meuse valley, the relay zone between the Feldbiss and Geleen faults. Here, the Kieseloolith formation displays an antiform structure with dips of 6° and vertical denivellation of 50 m below the Meuse river gravels.

Summarising, the Bree Uplift remains a Cretaceous inversion structure, not reactivated as a horst during the Tertiary. Local uplift of Tertiary strata is probably due to compressional events in fault relay zones along the Rur Valley Graben boundary.