Seismic stratigraphy of the Mesozoic and Cenozoic in northern Belgium: main results of a high-resolution reflection seismic survey along rivers and canals

This paper presents the results of high-resolution reflection seismic surve ys carried out between 1989 and 1996 along rivers and canals in northern Be lgium. The seismic data penetrate down to 900 m in the sedimentary cover or to the Paleozoic basement. The reflection response of the acoustic basemen t provides clear indications with regard to the top of the Paleozoic: cryst alline basement and Lower Paleozoic metasediments and volcanics of the Lond on-Brabant Massif and NE-dipping Devonian and Carboniferous strata. The sub horizontal Mesozoic and Cenozoic sedimentary cover comprises 20 unconformit y-bound seismic units: 5 in the Cretaceous and 15 in the Cenozoic. Based on borehole information, these units are correlated with lithostratigraphical ly defined formations or groups. Some of the unit-bounding unconformities a re of regional importance. They are attributed i) to eustatic sea-level cha nges causing regional flooding during the Late Cretaceous or incision of de ep valleys during the Late Oligocene and Late Miocene, ii) to regional tect onic tilting between Late Eocene and Early Oligocene, or iii) to a combinat ion of eustasy and tectonics causing valley incisions during the Lutetian. Faults of the Roer Valley Graben have offset different stratigraphic levels by sometimes considerable amounts (up to 230 m in the Oligocene to Quatern ary succession). Although the main tectonic phase took place during the Mio cene, the activity has varied considerably through time, and also from faul t to fault. Most faults seem to have a 10 to 30-m displacement since the La te Pliocene.