THE CONCEPT OF SEISMIC CYCLOSTRATIGRAPHY APPLIED ON THE LATE CRETACEOUS CHALK GROUP IN THE DUTCH OFF-SHORE

Cyclostratigraphy has been one of the go-to tools for the construction of high-resolution timescales. Age-models based on the identification of Milankovitch cyclicities have been constructed by visual identification from outcrops and cores or through spectral analysis of proxy records. Seismic reflection data from sedimentary basins remains a surprisingly undervalued proxy record for the construction of cyclostratigraphy calibrated timescales. However, having astronomically calibrated seismic data has two crucial implications: (1) the orbital signal from seismic reflectors can be used to assign ages to those seismic reflectors which can then be used to give basin wide age constraints helping us understand basinal dynamics where well-log-based age constraints are lacking. (2) it can also help us to optimize the drilling locations for paleoclimatic coring purposes. Seismic reflection data containing an orbital signal allows one to make a pre-drilling seismic cyclostratigraphy based age model. This age model will help us reduce pre-drilling uncertainties, leading to more successful coring/drilling projects. Here we present cyclostratigraphic results from a study on well-log and seismic reflection data from the Late Cretaceous Chalk Group in the Dutch offshore. The well-logs and seismic data were analysed using windowed spectral analysis (evolutive harmonic analysis and wavelet) and the evolutive Average Spectral Misfit analysis to identify the expression of the different orbital cycles in Chalk Group proxy records. We find that both the seismic and well-log contain a 40-60 m cycle, which we interpreted as the 1.2 Myr eccentricity cycle, as well as an 80-120 m cycle, interpreted as the 2.4 Myr orbital cycle and a 140-180 m cycle, interpreted as a 3.5 to 4.8 Myr orbital cycle. The identified orbital cycles were band-pass filtered from the well-log and seismic data plotted next to each other to show that there is a clear link between the orbital cycles observed in the seismic data and those observed in the well-logs. The 3.5 to 4.8 Myr orbital cycle manifests itself as a multiple high amplitude reflector which can be traced over a distance of 220km from the Dutch Central graben through the Schill Grund Platform to Terschelling Basin showing us the power of seismic cyclostratigraphy.