Geological Evolution of the Chalk Group in the Northern Dutch North Sea (2015)

This MSc thesis has been written by former intern Eva van der Voet.

The Late Cretaceous to Early Tertiary chalk play in the Dutch North Sea area has received increased attention since many hydrocarbons fields were found in these sediments in the Norwegian and Danish North Sea. However, not much is known on the sedimentology and stratigraphy of the Chalk Group in the Dutch North Sea. A relatively new 3D seismic survey in the northern Dutch North Sea provided the opportunity to study the interior of the Chalk Group in detail.

An analysis of the tectono-sedimentary history of the Chalk Group in a northern Dutch North Sea area is performed, by integration of biostratigraphic well data and seismic data. Seven seismic units, based on the seven chronostratigraphic stages of the Late Cretaceous and Early Tertiary, are interpreted throughout the roughly five different structural elements of the study area. Reflection terminations have been identified and the tectonic evolution of the area was reconstructed, based on mostly onlap-structures and truncations. Periods of relative subsidence and erosional events can be distinguished.

The most important geological events that must have taken place during and after the deposition of the Chalk Group are described. The first period of chalk deposition was a phase of relative tectonic quiescence. However, the thickness map of the Turonian shows a SW-ward tilting, which has not been recognized before. Another exception to the regional subsidence is little or no deposition of Coniacian and Santonian sediments in the western part of the study area, which is likely due to subsidence of the eastern part during the Coniacian and Santonian. During the Campanian and Maastrichtian, a widespread inversion phase occurred in two pulses, visible as seismic truncations. It started in the Step Graben and western Dutch Central Graben at the end of the Campanian and the second pulse during the Maastrichtian affected the Elbow Spit Platform and eastern Dutch Central Graben. The uplift and subsequent erosion had a differential effect and was strongest in the Step Graben, where even the Turonian sequence was partly eroded. Subsequently, a period of regional subsidence resulted in the deposition of thick Maastrichtian sequences. The subsidence possibly continued during the Danian (early Tertiary) but this is uncertain since the Danian sequence is very thin in most of the study area as a result of a renewed uplift and erosion phase at the end of the Danian. This inversion phase ended the chalk deposition and from this time onward, siliciclastic sediments were deposited instead.