Rhaetian to Middle Jurassic

Abstract

The Altena Group consists mainly of dark argillaceous mudstones deposited between the Rhaetian and the Oxfordian in the Laurasian Sea, an epeiric sea on the northwestern margin of the Tethys Ocean. The group has been subdivided into the Sleen, Aalburg, Posidonia Shale, Werkendam, and Brabant formations. Calcareous intercalations and carbonaceous mudstones characterize the Rhaetian—Toarcian part of the group, whereas the Aalenian—Oxfordian part comprises an alternation of (sandy) mudstones, sandstones, marls, and limestones. The mudstones of the lower to middle Toarcian Posidonia Shale Formation have particularly elevated organic carbon contents and constitute the main source rock for oil in the Dutch subsurface. The present-day distribution of the group is largely confined to Cimmerian basins, where it attains locally a thickness of more than 1500 m. In the northern Dutch offshore, Jurassic rocks have also been preserved outside the Dutch Central Graben in smaller basins, where accommodation was generated by salt withdrawal or along strike-slip faults. The Altena Group was also deposited on the platforms and some highs, where the Lower Jurassic succession was condensed due to limited accommodation and Early Jurassic erosion. Condensed successions on platforms and highs were subsequently removed by erosion during the Middle and Late Cimmerian uplift (Middle to Late Jurassic), resulting in the present-day patchy distribution of the group. In the Late Cretaceous, the Altena Group was also locally eroded from the Cimmerian basins due to tectonic inversion.