The middle–upper Cenomanian of Zilly (Sachsen- Anhalt, northern Germany) with remarks on the Pycnodonte Event

Markus Wilmsen, Thomas Voigt

Abstract


A detailed stratigraphic log of the 28 m thick Cenomanian succession at Zilly (Sachsen-Anhalt) is presented. The succession is composed of 11 m of middle Cenomanian hemi-pelagic marl–limestone alternations (‘Pläner Limestones’) grading into 15 m of upper Cenomanian calcareous pelagites (‘Poor rhotomagense Limestones’) unconformably overlain by 1.5 m of red-coloured marly clays and limestones (‘Rotpläner’). The proof of the interregional marker beds of the Pycnodonte Event at the 11 m level, the Facies Change at 26 m, and the base of the plenus Bed at 26.9 m allow a bio-/chronostratigraphic correlation of these levels with the middle/upper Acanthoceras jukesbrownei Zone (upper middle Cenomanian), the Calycoceras (Proeucalycoceras) guerangeri/Metoicoceras geslinianum Zone transition, and the lower Metoicoceras geslinianum Zone, respectively (middle upper Cenomanian). Litho-/microfacies and sequence stratigraphic analyses indicate an overall increase of pelagic influence up to the Facies Change. This retrogradational trend was shortly interrupted by the Pycnodonte Event, the base of which correlates with the late middle Cenomanian sequence boundary SB Ce IV and the succeeding transgressive surface. The Facies Change indicates a significant mid-late Cenomanian sea-level fall (sequence boundary SB Ce V), followed by more shallow water Rotpläner deposition. The Pycnodonte Event is very thick and proximal in character at Zilly. Its monospecific oyster fauna consists of small pycnodonteines assigned to Pycnodonte (Phygraeavesicularis (LAMARCK) vesiculosa (J. SOWERBY), a secondarily free-lying oyster which lived as a ‘cup-shaped recliner’. The patchy occurrence of the oysters, the sorting and partial damage of valves prior to final burial along with significant supply of terrigenous materials suggest episodically elevated water energy and strong environmental stress during deposition of the Pycnodonte Event. This situation promoted colonization of the sea-floor by, and reproductive success of the inferred eurytopic oyster. The Pycnodonte Event is a classic example of an ‘onlapping bioevent’, the formation of which was controlled by different factors such as sea-level rise, terrigenous influx, environmental stress, and preferential preservation.


Keywords


Cretaceous, Cenomanian, Subhercynian Basin, Event stratigraphy, Microfacies, Pycnodonte Event, Correlation

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