Ammonites and associated macrofauna from around the Middle/Upper Albian boundary of the Hannover-Lahe core, northern Germany

Abstract

The ammonite fauna from the Hannover-Lahe borehole, northern Germany, is described. The core comprises a monotonous claystone succession that is late Middle Albian Euhoplites nitidus ammonite Subzone at the base and early Late Albian C. auritus Subzone at the top. Furthermore, inoceramid taxa are discussed in relation to the ammonite biostratigraphy, establishing this core as a reference profile for the Middle/Upper Albian boundary interval of the Lower Saxony Basin. A continuous record of nektonic and benthic organisms, as well as strong bioturbation throughout the section, indicates well-oxygenated conditions during deposition. The uppermost Middle Albian sedimentary succession of the Hannover-Lahe core is expanded compared to sections in the Anglo-Paris Basin. The Middle Albian part of the Hannover-Lahe core yielded almost exclusively ammonites of the genus Euhoplites, typical for the Hoplitinid Faunal Province. There is a break in faunal assemblages at the Middle/Upper Albian boundary. Half of the Late Albian fauna of the core is characterised by Callihoplites spp., as members of the Hoplitinid Faunal Province, and half by mortoniceratids of the Brancoceratinid Faunal Province. This Middle/Late Albian faunal change to more cosmopolitan faunas coincides with a rapidly rising sea level. This is also indicated by a condensed interval in the lower Upper Albian enriched in debris of inoceramids, phosphoritic concretions, ammonites, and complete inoceramid specimens. This level contains ammonites of the Hysteroceras orbignyi and H. varicosum Subzones. There is no ammonite evidence for the earliest Late Albian Dipoloceras cristatum Subzone.