Sequence stratigraphic framework of the Lower and Middle Jurassic in the North German Basin: Epicontinental sequences controlled by Boreal cycles

Abstract

In the Early Jurassic, the epicontinental Central European Basin (CEB), including the North German Basin (NGB), was covered with a semi-enclosed inland sea that was in the late Middle Jurassic transferred to a continental shelf sea connected with the Boreal and the Tethyan oceans. The pronounced facies architectures of Lower to Middle Jurassic coastal-deltaic and basinal deposits challenged many workers to apply sequence stratigraphic methods. However, resulting regional schemes are in places contradicting, which is caused by both common problems with precise dating and processes of regional to local scale obliterating the sea-level signal, such as tectonically enhanced subsidence, enhanced sediment supply, thermal doming, or salt diapirism.This paper proposes a Lower to Middle Jurassic sequence stratigraphic framework of the NGB. The application of a multi-method approach, including intensive macro- and micropalaeontological dating as well as mapping of biofacies and lithofacies architecture to an extensive data set of cored and logged wells resulted in a hierarchical framework of 2nd-, 3rd-, and 4th-order epicontinental sequences. This approach enabled the identification and exclusion of areas and locations where subordinated controls, such as salt diapirism, superimposed the signature of sea-level fluctuations within stratal pattern architectures. The proposed sequence stratigraphic framework correlates well with the Boreal Standard but to much lesser extent with the Tethyan Standard and, thus, demonstrates the principle control of Boreal cycles on epicontinental stratal pattern architectures of the NGB. This coincides with more significant faunal exchanges between the Boreal and Subboreal provinces. In particular, the opening of a marine strait via the Russian Platform in the late Middle Jurassic enabled the immigration of Boreal ammonites and ostracods into the NGB and the northern CEB and illustrates the change from a semi-enclosed inland sea to a continental shelf sea. Furthermore, the recognition of Lower and Middle Jurassic 4th-order sequences supports suggestions of glacio-eustatic sea-level changes in the Jurassic.