Early Aptian bay deposits at the southern margin of the Lower Saxony Basin: Integrated Stratigraphy, palaeoenvironment and Oae 1a

Abstract

Lehmann, J., Friedrich, O., von Bargen, D. and Hemker, T. 2012. Early Aptian bay deposits at the southern margin of the lower Saxony Basin: Integrated stratigraphy, palaeoenvironment and OAE 1a. Acta Geologica Polonica, 62 (1), 35-62. Warszawa. The present study establishes a lithostratigraphic subdivision, a bio- and chemostratigraphy and describes the sedimentology of the Alstatte Bay at the southern margin of the Lower Saxony Basin in northern Germany. The socalled Fischschiefer is developed in the lower part of the section, a 2.1 m thick laminated interval that shows TOC values of up to 2 % and is interpreted as local expression of the global Oceanic Anoxic Event 1a (OAE 1a). The basal part of the Fischschiefer is fairly continuously laminated, whereas its upper part is characterized by an alternation of laminated and non-laminated beds. Stable carbon isotopes (δ13 C carb ) show a characteristic pattern for the Fischschiefer interval that can be correlated with sections from the Tethyan Realm. Immediately above the Fischschiefer, however, the studied section might reveal a diagenetic overprint as indicated by very negative carbon stable isotope values. The remaining part of the section is characterized by a rather monotonous claystone succession, terminated by a greenish layer rich in glauconite overlain by glacial till of Pleistocene age. The fauna is typically Boreal in the lower part of the section, but changes around 2 m above the Fischschiefer towards an assemblage indicating a distinct Tethyan influx. The faunal shift is considered to be due to the maximum flooding of the continuous, long-term sea level rise of the Early Aptian which led also to a supposed shift from a restricted to open bay environment.

Morphological variability in response to palaeoenvironmental change – a case study on Cretaceous ammonites

“…Full locality details are given by Lehmann et al . (). Additionally, Hoffmann & Mutterlose () and Bottini & Mutterlose () refer to different aspects of the same section.…”

“…Belemnite biostratigraphy is of limited value, but some supporting evidence for placing the boundary at this level comes from Halimedides itself, earlier referred to as Chondrites (Lehmann et al . ). In the assumed boundary bed, this trace fossil might indicate a hiatus, as it is associated with a firmground.…”

“…The rich faunas and their distribution in the Alstätte section permit a comparison of morphological change in ammonites to sea‐level, temperature and oxygen content changes that have been determined in this section (Lehmann et al . ).…”

The OAE1a in Cuchía (Early Aptian, Spain): C and O geochemistry and global correlation

“…Ammonites from the Alstätte section (Germany) date the OAE1a equivalent in the Deshayesites fissicostatus Zone, the first Zone of the Early Aptian (Lehmann et al 2012). On biostratigraphic grounds, this interval is correlatable with the earliest preOAE1a facies of the Cuchía section.…”

Ammonite Biostratigraphy of the Cretaceous—An Overview

“…Correlation of Early Aptian ammonite zones between England and North Germany is less complex than previously believed considering the different taxonomic approach. For example synonymy of the two traditionally used index species Deshayesites bodei (Germany) and Deshayesites fissicostatus (England) has been assumed for decades (Casey 1964in Casey 1960-1980Lehmann et al 2012), but no biostratigraphic consequences has been drawn resulting in a seemingly different zonal scheme that is still used (Kemper 1967(Kemper , 1995.…”