Towards a cyclostratigraphic framework of the Upper Cretaceous Chalk Group in the Dutch offshore

Abstract

The current geological timescale has already been astronomically calibrated down to the early Campanian. This study evaluated the Danian to Cenomanian Chalk Group in the Dutch offshore on the Schill Grund Platform as a possible extension for the astronomically calibrated timescale. The Schill Grund Platform in the Dutch offshore is one of the few areas where the Chalk Group is relatively undisturbed and virtually complete. Spectral analysis on well-logs by van der Molen (2004) already showed promising cyclostratigraphic results for the Schill Grund Platform. Van der Molen (2004) also proposed a non-tectonic origin for some of the seismic reflectors on the Schill Grund Platform. An integrated petrophysical, seismic, biostratigraphic and core material based approach was used to investigate the presence of Milankovitch cycles on the Schill Grund platform. The EHA, Blackman-Tukey transformation and Redfit spectral analyses of the well-logs in depth domain showed consistent peaks for the 4-6, ~8, 15-22 40-60, ~105, 125-160 and 300-400 m periods. Based on eASM results and the (bio)stratigraphic constraints the 15-22 m period is interpreted as the 405-kyr eccentricity cycle and was subsequently used for a depth-time conversion. Spectral analysis in the depth and time domain led to the identification of cycles with periods of 100, 200, 300, 405, 700-1000, 1200, 2400, 3500 and 9500-kyr. The 100, 405, 1200, 2400, 3500-kyr periods could be readily assigned to an orbital cycle, but the assignment of an orbital cycle to the 200, 300, 700-1000 and 9500-kyr cycles is more problematic. The enigmatic presence of the 1200 and 2400-kyr cycles could hint to an episode of chaotic resonance between the orbits of Earth and Mars. No definite orbital phase-lithology relationship exists for the Chalk Group in the North Sea Basin, which results in two tuning options; an eccentricity minimum and an eccentricity maximum tuning option. The 1200, 2400 and 3500-kyr (40-60, ~105 and 125-160 m periods) cycles are also registered in the spectral analysis of the seismic trace data. The filtering of the seismic trace and well data of the 1200 and 3500-kyr cycles (in the depth domain) display a good fit with the seismic record and an astronomical modulation of the reflector pattern is therefore likely. XRF, core observations, well-logs and literature indicate that the Chalk Group has two different depositional settings; a cool water detrital/anoxic setting with marl deposition and an oxic warm water chalk deposition. The two settings can be explained by an upwelling or a stagnation model. The upwelling model is linked to the eccentricity maximum tuning option and the stagnation model is linked to the eccentricity minimum tuning option. The upwelling model is preferred over the stagnation model due to more evidence in literature for the upwelling model, the presence of Prasinophyte algae in the core of well A12-02 and the phase relationship between the biostratigraphy and the filtered gamma ray log data of well A12-02. The cyclostratigraphic framework enabled a long-distance correlation of stage boundaries across the Schill Grund Platform, a re-evaluation of the ages assigned to the CK sequence stratigraphic boundaries of van der Molen (2004) and a re-evaluation of the ages assigned to the formation boundaries of the Chalk Group in the Dutch offshore.