Abstract
Calcareous nannofossils are described from Lower Cretaceous strata of four borings in the Central Trough of the Danish North Sea sector, and from three Aptian units on Helgoland and at Sarstedt, northern Germany. The assemblages range in age from Upper Hauterivian to Albian-?Cenomanian. Abundances of calcareous nannofossils varied considerably throughout the investigated sequences. They were rock-forming in several Lower Barremian - Aptian sediments, but rare or lacking in many of the Hauterivian and Albian strata. The Middle Barremian - Lower Aptian assemblages were often dominated by nannoconids. Preservation was generally bad in chalk and good in marl deposits. Some finely laminated black sediments yielded extremely well-preserved assemblages. One hundred and sixteen species were recognized. Eighteen species were selected as biostratigraphically particularly useful and their chronostratigraphic ranges are shown and discussed. The ages of the investigated sequences were determined on the basis of the selected nannofossil events. Some distinct sedimentary units in the North Sea could be correlated with time-equivalent formations in eastern England and northern Germany.
Highlights
During the Early Cretaceous up to 1.200 m of sediments were deposited in the Central Trough of the Danish North Sea
Last occurrences are usually less reliable due to possible reworking. Using coccoliths this preference is accentuated as coccoliths are reworked more than any other fossil group used in stratigraphy
The calcareous nannofossils indicate that the shale unit belongs to the Upper Barremian and the lower most Aptian. This conclusion is based on the presence of Chiastozygus platyrhethus, Nannoconus abundans, and Octocyclus reinhardtii and on the absence of Eprolithus anterior, typical Rhagodiscus angustus, and Flabellites biforaminis
Summary
During the Early Cretaceous up to 1.200 m of sediments were deposited in the Central Trough of the Danish North Sea. The Ryazanian to mid Hauterivian sequence consists mainly of homogeneous clay deposits. Jensen et al (1987) subdivided the Cromer Knoll Group into five formations (Fig. 1) Four of these - namely the Valhall, Tuxen, Sola, and R0dby Formations - are widespread and can be found in both the Danish, Norwegian, British, and German sectors. The bed was described as the Munk Marl Bed by Jensen et al (1987) It produces a characteristic high gamma ray peak, which has been recognized in many wells in the central North Sea. The lower part of the Sola Formation consists of a dark grey, finely laminated, organic rich calcareous shale, while the upper part is dominated by reddish and light grey claystones and marlstones. Dinoflagellates were absent in some of the calcareous units in the upper part of the Lower Cretaceous, where calcareous nannofossils were common
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