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Integrated stratigraphy of the upper Lower - lower Middle Cenomanian of northern Germany and southern England

Wilmsen M.

Acta Geologica Polonica

2007

Vol. 57, no. 3

263-279

A high-resolution stratigraphic calibration of the upper Lower (upper Mantelliceras dixoni Zone) and lower Middle Cenomanian (Cunningtoniceras inerme Zone and lower Acanthoceras rhotomagense Zone) based on an integrated analysis of macrofossil biostratigraphy, event, cyclo-, stable-isotope and sequence stratigraphy of northern German and southern England key sections is presented. Classic event stratigraphy has a good potential in refining biostratigraphic correlations as most of the classic bioevents are isochronous within the integrated stratigraphy. Many lithological event beds such as marker marls can be incorporated into the cyclo- and sequence stratigraphic framework, explaining their significance in interregional correlation. The best stratigraphic resolution provides the cyclostratigraphy based on the typical Cenomanian marl-limestone couplets and their stacking pattern, inferred to reflect orbital forcing of the Milankovitch frequency band: detailed bed-by-bed correlation of couplets (precession cycle, ca. 20 kyr) allows a stratigraphic calibration within [similar to]10 kyr time slices. Conspicuous marker marl beds embrace bundles of [similar to]five couplets and are related to the short eccentricity (100 kyr) cycle. However, for the upper Lower Cenomanian (dixoni Zone) it appears that the existing couplet scale is incomplete. Sequence stratigraphic analysis demonstrates that the investigated interval comprises the maximum flooding and highstand interval of an Early Cenomanian sequence, capped by a significant late dixoni Zone sequence boundary, followed by uppermost Lower to Middle Cenomanian lowstand and transgressive deposits grading into a Middle Cenomanian maximum flooding zone ("calcimetry break"). Carbon stable-isotope values are stable around 2[per mil] vs. V-PDB within the mid- and late dixoni Zone,related to equilibrium conditions during maximum flooding and highstand conditions of sea-level. The latest Early to earliest Middle Cenomanian sea-level fall and lowstand was accompanied by a negative [delta] [^13]C excursion of ca. 0.4[per mil] in couplets B34-B40 (Lower-Middle Cenomanian boundary isotope Event, LMCE, new name) followed by a rise of 0.4.0.6[per mil] [delta] [^13]C in couplets B41-C2 during the early transgressive systems tract (Middle Cenomanian [delta] [^13]C excursion MCE 1). These observations support the interpretation that the [delta] [^13]C signal is a good proxy for (eustatic) sea-level changes. The LMCE is suggested as a proxy marker for the base of the Middle Cenomanian Substage.