Stratigraphic revision of the upper Lower and Middle Cenomanian in the Lower Saxony Basin (northern Germany) with special reference to the Salzgitter area

Abstract

Re-logging and high-resolution integrated stratigraphic analysis of the Baddeckenstedt quarry section (Salzgitter area, northern Germany) suggests that the Lower/Middle Cenomanian boundary falls into a stratigraphic gap at the base of marl MIa. This interpretation is in contrast to that in previously published papers which indicate an unconformable contact of Lower and Middle Cenomanian rocks at this locality c. 5.3 m lower, at the top of the so-called ‘double limestone’. Based on interbasinal carbon stable isotope (δ 13C) correlation, the flat δ 13C signature of the beds below marl MIa, including the Turrilites scheuchzerianus Event and marls MII and MIb, indicates that they belong to the Early Cenomanian Mantelliceras dixoni ammonite Zone. Available biostratigraphic data are in accordance with this interpretation. Inoceramid bivalves from marl MII and the overlying scheuchzerianus Event, formerly thought to represent the Mid-Cenomanian Inoceramus schoendorfi Heinz, are better referred to I. ex gr. crippsi Mantell. The Middle Cenomanian ammonite basal marker taxon Cunningtoniceras inerme (Pervinquière) has never been found in Baddeckenstedt, partly because the lower part of its range is cut out at the stratigraphic gap at the base of marl MIa (‘ inerme hiatus’). The secondary marker taxon for the base of the Middle Cenomanian, the planktonic foraminiferan Rotalipora reicheli Mornod, is reported to enter c. 3 m below the chemostratigraphically unconformable boundary between Lower and Middle Cenomanian rocks defined herein. Its suitability as a proxy taxon for the definition of the base of the substage is, therefore, questioned. The basal Acanthoceras rhotomagense ammonite Zone, Praeactinocamax primus Event, is represented within the limestones immediately overlying marl MIa, probably in the horizon containing the ‘gastrolith lens’. The scheuchzerianus Event, formerly inferred to be the equivalent of the primus Event, is in fact an intra- M. dixoni Zone marker horizon and may correlate with the Turrilites scheuchzerianus Bed recognized at Southerham Grey Pit, southern England, in the Anglo-Paris Basin. In a sequence stratigraphical framework, the erosive base of marl MIa represents sequence boundary SB Ce III capping depositional sequence DS Ce III ( M. dixoni Zone), and coincides with the succeeding transgressive surface of DS Ce IV (Middle Cenomanian). Transgressive onlap is not coincident with the primus Event; it started earlier and marl MIa is probably broadly equivalent to the arlesiensis Bed of southern England. The interval between marl MIa and the Mid-Cenomanian Event (upper C. inerme to lower A. rhotomagense zones) is strongly condensed in the marginal Salzgitter area, probably in response to limited accommodation space during transgression. The capping sequence boundary SB Ce IV (lower Acanthoceras jukesbrownei ammonite Zone) occurs 1 m below the Pycnodonte Event at a conspicuous facies change. A detailed correlation of the Lower/Middle Cenomanian boundary interval between the proximal Baddeckenstedt and the distal Wunstorf quarry section near Hannover is proposed. The new stratigraphic data are extrapolated to the nearby Konrad 101 cored borehole, the most complete Cenomanian section in the Salzgitter area.