The stratigraphy of the Bathonian-Callovian boundary (Middle Jurassic) in Northern Germany

摘要

The aim of this work is a description of the Bathonian-Callovian boundary in Northern Germany, to gather more insights for those looking for a GSSP for the base of the Callovian Stage. The Upper Bathonian and the lowermost Callovian (Herveyi Zone) consists in Northern Germany of sandstones and shales. Some beds of ironoolitic concretions are situated at the boundary of the Hodsoni and Orbis zones and in the Herveyi Zone, indicating starving sedimentation in the vicinity of submarine swells. The correlation of all biohorizons of Central Europe and England makes a further chronozonal subdivision possible. For the Upper Bathonian and the Herveyi Zone 19 ammonite biohorizons have been determined, of which 16 were detected in Northern Germany, two more indirectly by ostracods. This succession has been puzzled from over 20 recorded sections and is described in detail. It allows a high-resolution biostratigraphy. The result is a chronostratigraphic chart. It shows that the sedimentary succession is developed very patchy. This concerns especially the lowermost Callovian (Herveyi Zone), which is very thin or missing. At the particular localities only two or three horizons are present, the basal keppleri horizon seems to be missing entirely. The detailed biostratigraphy should be the basis for further studies of sequence stratigraphy. First conclusions are that hiatuses and condensed layers are widespread at the Bathonian-Callovian boundary. At this time there was probably a sea-level highstand, so that sediments were deposited only in tidal areas or coastal plains, while in the inner parts of the basin was a somewhat starving sedimentation. With the subsequent rapid regression during the Herveyi Chron most of these sediments were reworked. On the other hand, the sea-level highstand allowed a good exchange of ammonites between different faunal provinces. Kepplerites, Cadoceras and Chamoussetia immigrated from the Boreal Realm, Macrocephalites and Cadomites from the Tethyan Realm.