The Rupel Clay Member in the Netherlands: towards a comprehensive understanding of its geometry and depositional environment

Abstract

This study presents the 3-D geometry of the Oligocene Rupel Clay Member and a review of its depositional environment based on new and published data. The Rupel Clay Member (RCM) is a clay layer in the Dutch subsurface which is informally known as the Boom Clay. New depth and thickness maps show that the member is present in nearly the whole subsurface of the onshore Netherlands to a depth of about 1500 m. The thickness of the member is variable but rarely exceeds 125 m. We identified three subunits: RCM-U, with a thickness of 15–40 m; RCM-M, with a thickness of 40–90 m; and RCM-L with a thickness of 25–50 m. The Rupel Clay Member is not a homogeneous clay; both vertical and lateral grain-size trends are present. These trends match with general palaeogeographic trends which foresee near-shore facies in the south and southeast of the onshore Netherlands and a more distal setting when moving northward. The three subunits are correlated with global climatic and regional tectonic events, which have mainly affected marginal facies. Faults are known to cut through the clay layer, but have not been mapped. A mismatch of lithostratigraphic nomenclature between the Netherlands and neighbouring countries is present. Part of the solution for this mismatch lies in the proper interpretation of well data in the Netherlands, both on- and offshore.

Characterisation of the geochemical heterogeneity of the Rupel Clay Member in the Netherlands

“…The difference in clay mineral content and heterogeneity between north and south is consistent with grain size distributions, which show consistently smaller grain sizes in the north and limited variability in distributions with depth (Vis et al, 2016). This is explained by the depositional environment, which was characterised by the Cenozoic Southern North Sea Basin, the Rupelian sea, with an approximately east-west trending coastline located in Belgium (Vis et al, 2016). At the southern margin of the basin, the deposition of the Rupel Clay was under the influence of sea level variation, resulting in the presence of quartz-rich intervals, and an increase in quartz content and grain size towards the bottom (towards the Tongeren and Dongen Formations in the east and southwest) and top (towards the sandy Steensel Member or Breda Formation) (Vis et al, 2016).…”

“…The variation in pyrite content is independent of both location and depth. The difference in clay mineral content and heterogeneity between north and south is consistent with grain size distributions, which show consistently smaller grain sizes in the north and limited variability in distributions with depth (Vis et al, 2016). This is explained by the depositional environment, which was characterised by the Cenozoic Southern North Sea Basin, the Rupelian sea, with an approximately east-west trending coastline located in Belgium (Vis et al, 2016).…”

“…This is explained by the depositional environment, which was characterised by the Cenozoic Southern North Sea Basin, the Rupelian sea, with an approximately east-west trending coastline located in Belgium (Vis et al, 2016). At the southern margin of the basin, the deposition of the Rupel Clay was under the influence of sea level variation, resulting in the presence of quartz-rich intervals, and an increase in quartz content and grain size towards the bottom (towards the Tongeren and Dongen Formations in the east and southwest) and top (towards the sandy Steensel Member or Breda Formation) (Vis et al, 2016). The Boom Clay in Belgium shows comparable geochemical characteristics to the Rupel Clay in the south of the Netherlands.…”

“…In this area, the Rupel Clay is overlain by the claydominated Veldhoven Member or Breda Formation (Vis et al, 2016).…”

“…Baeyens et al, 1985;Honty, 2008;Honty & De Craen, 2009). The Boom Clay in the investigated area in Mol is located at a depth of approximately 200-300 m. In the Netherlands, the Rupel Clay reaches much greater depths in the south eastern and northern parts of the country (see Vis et al, 2016). Much less is known about the physico-chemical properties of the Rupel Clay in the Netherlands and the effect of the current depth on its suitability for a potential geological disposal facility.…”