Integrated stratigraphy and palaeoenvironment of the Cenomanian-Lower Turonian (Upper Cretaceous) of northern Westphalia, North Germany

Abstract

The present study provides an integrated stratigraphy of the Lower Cenomanian-Lower Turonian of the northwestern Munsterland Basin, Westphalia. This is important to establish a standard section allowing an interregional correlation as well as an interpretation of single environmental conditions, their changes through time and their geographical extent. Numerous sections have been investigated in northern Westphalia, in addition to data from other profiles in North Germany. Macrofossils and thin-sections have been sampled, stable isotope and gamma ray data have been obtained from a part of the sections. Investigation of the sedimentary sequence is based on a analysis of events. Many events are diachronous, whereas others are difficult to define and do not show a wide geographic distribution. For ecological or sedimentological reasons, correlation is not possible. The discussion of events leads to a compound picture of the evolution of the depositional sequence, allowing the reconstruction of palaeo-environmental changes. Sea-level changes and their influence on the fauna is discussed. During maximal sea-level rising, macrofossils occur more frequently for ecological reasons, however, some macrofossil accumulations are lag deposits. Some biostratigraphical problems find their origin in a tectonic separation leading to different habitats. The local tectonics was caused by the intial phase of transpression of the Osning Zone, that can be traced down to the Lower Cenomanian. A correlation of the Cenomanian-Turonian Boundary Event (CTBE) in Westphalia (Lengerich), Colorado (USA) and England (Eastbourne), is possible due to very dense sampling of carbon-isotopes (δ13C). In Westphalia, definition of the stage boundary is possible by correlation of carbon isotope curves only. A sequence from the upper Middle Cenomanian, up to the lower Upper Cenomanian, is investigated concerning the controlling factors of biogenic sedimentation. The cyclicity of lithology is investigated by Fast Fourier Trans-formation. It can be shown that sedimentation is forced by orbital cycles, mainly by the precession cycle of the Milankovitch band (P1 and P2, 18 500 and 22 300 years, respectively). This confirms the primary origin of the marlstone-limestone couplets that are obvious in the field. Calculation of sedimentation rates is based on these data. There is a high variability of sedimentation rates, maybe due to a strong variation of productivity in this epicontinental environment.