FACIES 140
25-70
PI. 4-5
29 Figs. 2Tab. ERLANGEN 1999
Integrated Stratigraphy and Palaeoenvironment of the Cenomanian-Lower Turonian (Upper Cretaceous) of Northern Westphalia, North Germany Jens Lehmann, TLibingen and Davis Dedicated to the memory of Jost Wiedmann (1931-1993) KEYWORDS: LITHOSTRATIGRAPHY - BIOSTRATIGRAPHY - EVENT-STRATIGRAPHY - CYCLOSTRATIGRAPHY CHEMOSTRATIGRAPHY - PALAEOENVIRONMENT - NORTHERN GERMANY - UPPER CRETACEOUS (CENOMANIAN -TURON1AN) OFG-Schwerpunkt
CONTENTS Summary I Introduction 2 Methods 2.1 Samples 2.2 Sections 3 Ammonite and inoceramid zonation 3.1 Lower Cenomanian 3.2 Middle Cenomanian 3.3 Upper Cenomanian 3.4 Lower Turonian 4 Description of sections investigated in detail 4.1 Rheine-Waldhiigel (locality 1) 4.1.1 Lower Cenomanian (section 10, 11) 4.1.2 Middle Cenomanian (general) 4. !.3 Upper Cenomanian-Lower Turoniaa (section 1-5) 4.2 Brochterbeck near :Ibbenbtiren (locality 2) 4.2. I Middle Cenomanian 4.2.2 Upper Cenomanian 4.3 Lengerich, Dyckerhoff quarry (locality 3) 4.4 Lengerich, Galgenknapp quarry (locality 4) 4.4.1 Lower Cenomanian (section 4) 4.4.2 Middle Cenomanian (section 4) 5 The sedimentary sequence: Event Stratigraphy and environmental conditions 5.1 Lower Cenomanian 5.1.1 Ultimus/A ucellina Event 5.1.2 Crippsii Event 5.1.3 Hypoturrilites Event 5.1.4 Schloenbachia/virgatus Event 5. 1.5 Mantelliceras dixoni Event 5. 1.6 Orbirhynchia Event 5.1.7 Lower-Middle Cenomanian Boundary Event 5.2 Middle Cenomanian 5.2.1 Turrilites scheuchzerianus Event 5.2.2 Actinocamax primus Event 5.2.3 Austiniceras Event 5.2.4 Mid Cenomanian Event 5.2.5 Pycnodonte Event 5.3 Upper Cenomanian-Lower Turonian 5.3.1 Inoceramus pictus Event 5.3.2 Arnphidonte Event 5.3.3 Oxic-anoxic Event
Chondrites Events BIOGENE ~EDIMENTATION Actinocamax plenus Event Neocardioceras juddii Event Mytiloides Events 5.3.7 Stable isotopes of the CTBE 6 Description of sections 6.1 Discussion and Conclusions 6.2 6.2.1 Carbon isotopes Oxygen .isotopes 6.2.2 Gamma ray stratigraphy 7 7.1 Previous work Description of logs 7.2 Comparison and conclusions 7.3 Orbital forcing of sedimentation 8 Sedimentary couplets and investigated units 8.1 8.2 Results 9 Conclusions PalAeoenvironment, litho-, bio- and event stratigraphy 9.1 9.2 Chemostratigraphy 9.3 Cyclostratigraphy References 5.3.4 5.3.5 5.3.6
S U M M A R Y
The present study provides an integrated stratigraphy of the Lower Cenomanian-Lower Turonian of the northwestern M0nsterland 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 geo-
Address: Dr. J. Lehmann, i nstitut und Museum fiir Geologie u. Pal~iontologie, Universit~it Ttibingen, Sigwartstr. 10, D-72076 Ttibingen. Fax: ++49-(0)7071-295727, e-maih jens.lehmann @uni-tuebingen.de, present address: Department of Geology, University of California, Davis, CA 95616-8605, USA. Fax: ++1-530-752-0951, e-mail:
[email protected]
26
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.
1 INTRODUCTION
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Fig. 1. Map of Upper Cretaceous outcrops of North Germany (stippled) with main tectonic blocks, the Osning overthrust and localities focused here. graphic distribution. For ecologial 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 ofpalaeoenvironmental 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 initial 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 (813C). 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 oflithology is investigated by Fast Fourier Transformation, It can be shown that sedimentation is forced by orbital cycles, mainly by the precession cycle of the Milankovitch band (PI 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.
The Cretaceous is characterized by extreme climatic conditions and several dramatic environmental changes within comparatively short periods of time. Among these environmental changes are the five Oceanic Anoxic Events (OAEs), Aptian-Lower Turonian in age (see ERSACHZR& THUROW 1997: Fig. 7). The lower Late Cretaceous is especially interesting, because it represents one of the main turning points of the earth's history. In the Upper Albian, sea-level started to rise (e.g. HAQet al. 1988), reaching the maximum highstand of the whole Phanerozoic at the Cenomanian-Turonian Boundary (CTB). This drastic environmental change goes hand in hand with a drawdown of CO 2, initiating a cooling of the climate (inverse greenhouse effect), stepwise extinction events and continuous radiation of organisms after the CTB, as well as ocean-wide anoxic conditions (HARRIES 1993, JENKYNSet al. 1994, KAUFFMAN• HART 1996). Evolutionary, ecologic, and/or biogeographic changes are an expression of rapid environmental changes. In the Cretaceous, a lot of those short-term changes occurred; their environmental backround is one of the most discussed topics recently. In the literature, they are usually called 'events' (e.g. WALelSER 1996), lasting between days to less than 100.000 years (KAUFFMAN& HART 1996). TO analyze the reasons for these events, factors controlling deposition of sedimentary sequences have to be investigated and compared in great detail and a global scale. This topical target of research was focused also by the priority program 'Global and Regional Controls on Biogenic Sedimentation', founded by the Deutsche Forschungsgemeinschaft (DFG). The present publication is a part of it (REITNERet al. 1996). Within the last decade, the Event Stratigraphy was established, referring to the Cenomanian and Turonian of North Germany (e.g. ERNST et al. 1983; KAPLAN 1986, 1992a-c; TROCF.R1995); providing a high resolution stratigraphy. However, stratigraphical research was hitherto limited mostly to traditional methods of field-work. Furthermore, these results have not been improved yet and local tectonics was hitherto believed to be rather insignificant in hemipelagic sequences of most of the Cenomanian. It was only clear from different facies that regional tectonics have masked global conditions at the CTB in the northern Mtinsterland. The present study first provides an integrated stratigraphy for the working area in the northern Miinsterland Basin (Fig. 1) and is based on previous work, especially by KAPLANet al. (1984), KAPLAN(1986), ERNSTet al. (1984), WIEDMANN et al. (1989), MEYER (1990), GALE (1995), K A P L A N (1995), L E H M A N N & W I E D M A N N (1996) and O W E N (1996).
27
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