Seismic evidence for deep Palaeozoic sedimentary units in the Ringkøbing-Fyn High offshore Denmark

Interpretation of refracted and reflected arrivals in seismic data from the MONA LISA combined wide-angle and normal incidence deep seismic experiment in the south-eastern North Sea has revealed deep sediments in the basement high between the Central Graben and the Horn Graben.

Evidence for such deep sediments in the Ringkøbing-Fyn system of basement highs is most clearly found in refracted arrivals in wide-angle seismic data. P-waves refracted in the deep sedimentary units show velocities between ~4.5 km/s and ~5.1 km/s which are significantly below the observed velocities of about 6.0 km/s for crystalline basement. Outline of the shape and fine structure of sedimentary units are constrained by coincident normal incidence seismic reflection data.

A seismic velocity model and a reflectivity section for a 150 km section of the east-west trending MONA LISA line 3 are presented. The velocity model is consistent with both the wide-angle data and the normal incidence section. Pre-Zechstein sedimentary units locally up to 4 km in thickness are observed, and locally the top of the crystalline basement is found at depths of about 7 km to 10 km.
The boundary between the deepest sediments and the crystalline basement is characterized by several large block faults. On the flanks of the Horn Graben and the Central Graben a pronounced thinning of the pre-Zechstein sedimentary strata is observed indicating uplift and erosion of rift shoulders.
A correlation between free-air gravity anomalies, local basement highs and deep basins is observed, and the gravity data indicate that Pre-Zechstein sedimentary structures of similar thicknesses are present in the offshore part of the Ringkøbing-Fyn High both north and south of the analyzed seismic line.