Sequence stratigraphy and palaeoceanography of the Cenomanian Stage in northern Germany

Abstract

The sequence stratigraphic and sedimentologic analysis of a 180-km nearshore (SSE) to outer shelf (NNW) transect shows the pulsatory nature of the ‘Cenomanian transgression’ in northern Germany. Five complete and the lower part of a sixth third-order depositional sequence can be recognized: DS Ce I and DS Ce II ( Mantelliceras mantelli Zone); DS Ce III ( Mantelliceras dixoni Zone; DS Ce IV ( Cunningtoniceras inerme to lower Acanthoceras jukesbrownei Zone); DS Ce V [ Acanthoceras jukesbrownei Zone to Calycoceras ( P.) guerangeri Zone]; DS Ce VI ( Metoicoceras geslinianum Zone to lower Turonian). In inner and mid-shelf settings, sequence boundaries are mostly represented by stratigraphic gaps. A major sea-level fall occurred at the Early/Middle Cenomanian transition (sequence boundary SB Ce III). Maximum transgression during the Cenomanian is indicated by the deposition of homogeneous pelagic sediments of the ‘Poor rhotomagense Limestones’ (Late Cenomanian sequence DS Ce V). Graphic correlation of the sections investigated with a composite standard suggests that the inner and middle shelf sections were strongly controlled by available accommodation space during the Early and Middle Cenomanian. From the late Middle Cenomanian, non-accommodation controlled deposition of the ‘Poor rhotomagense Limestones’ occurred across the whole of the transect. Apart from the third-order depositional sequences, a hierarchy of stacked higher frequency (fourth- to sixth-order) cycles can be recognized. The basal elements are sixth-order cycles, represented by dm- to metre-scale marl-limestone ‘precession couplets’. Fifth-order cycles are represented by bundles of four to six couplets separated by dark marker marls, inferred to record the ca. 100 ky short eccentricity cycle. Fourth-order cycles are sets of four (three to five) fifth-order cycles. Their boundaries are significant erosion surfaces associated with subordinate sea-level fall, occasionally coinciding with third-order sequence boundaries. Most of the ‘classic’ Cenomanian bioevents can be accommodated into the sequence stratigraphic framework. Three main types of bioevents are recognized: ‘onlapping bioevents’ at the transgressive surface or in the early transgressive systems tract, ‘maximum flooding bioevents’, and ‘late highstand bioevents’. Onlapping and late highstand bioevents developed mainly either in response to sedimentologic and taphonomic processes such as winnowing and condensation, or owing to faunal migration during transgression. Maximum flooding bioevents are related to blooms of specialist taxa adapted to low food supplies and low-energy conditions. Two contrasting sedimentary systems can be recognized based on the nature of the dominant planktic carbonate component: an Early to early Middle Cenomanian ‘calcisphere system’ (‘Pläner’), characterized by high CaCO 3accumulation rates (up to 80 m/my), a relatively high input of terrigenous material, and dense macroinvertebrate faunal communities; and a late Middle and Late Cenomanian ‘coccolith system’ (‘Poor rhotomagense Limestones’), characterized by low CaCO 3accumulation rates (13–18 m/my), low terrigenous input, and impoverished benthic communities. The ‘calcisphere system’ is interpreted to reflect eu-/mesotrophic middle shelf ‘green’ water masses with high primary productivity and high food supplies supporting a rich suspension- and deposit-feeding benthos. The ‘Poor rhotomagense Limestones’ document deposition from a low-productivity, oligotrophic outer shelf water mass (oceanic ‘blue water’). The change between the two systems was caused by the Middle Cenomanian transgression, associated with the breakdown of shelf-front systems and the onlap of stratified, ‘oceanic’ water masses. The strongly reduced surface water productivity of the oligotrophic ‘coccolith system’ resulted in the disappearance of dense benthic communities during the late Middle and Late Cenomanian.