Ommelanden Formation CKGR


Premise The definition of the Ommelanden Formation in NAM and RGD (1980) has been amended to exclude intervals of a Paleocene age. These have now been placed in separate units, the Ekofisk Formation and the Houthem Formation. The lithological affix is redundant and not always appropriate, and is therefore omitted.
Derivatio nominis Named after the surroundings of the town of Groningen, commonly known as the Ommelanden, where the type section is situated.
Type section Location map See figure (pdf)
  Well De Paauwen-l (pdf)
  Location N 53°16'30.4
E 06°45'15.5
  Depth 822 to 1654 m
  Length 832 m along hole
  Reference NAM and RGD (1980)
Definition Succession of white, yellowish-white or light-grey, fine grained limestones, in places argillaceous. Layers of chert nodules can be very common over thick intervals. Along the basin edge coarse, bioclastic limestones and tongues of sandstone occur. In southern Limburg, where the Upper Cretaceous is exposed, these are classified as separate formations (see Subdivision in Chapter 2). The formation comprises mainly hard, dense limestones (as result of compaction and cementation), but in its upper reaches it tends to be softer and more chalky in texture. Locally the total thickness may exceed 1800 m. Biostratigraphical and log correlation studies indicate in places several unconformities within the succession. Their regional extent and correlation still remains uncertain.
Upper Boundary Over most of its range the formation is overlain by Tertiary sands or clays of the Lower or Middle North Sea Groups. In this case the upper boundary can be recognised easily by the lower gamma-ray response and higher acoustic velocity of the chalks on wire-line logs, coinciding with a marked change in lithology and colour. Where the Ommelanden Formation is overlain by the Ekofisk Formation the boundary may be somewhat difficult to pinpoint. However, the gamma-ray curve of the Ommelanden Formation tends to show a slightly lower response and smoother pattern of the gamma-ray curve and to have a somewhat higher porosity in its uppermost reaches compared to the basal beds of the Ekofisk Formation, as can be observed on the appropriate wireline logs. Very rarely a high peak is seen on the gamma-ray log is seen, representing a Cretaceous/Tertiary boundary clay layer that equates the Fish Clay in Denmark Sorgenfrei (1957) .
Lower Boundary The lower boundary, with the Texel Formation, is usually well marked by a wire-line log kick, indicating the presence of the Plenus Marl Member. Where this member is absent the Texel Formation is recognised by the slightly more serrate, higher gamma-ray readings. In those parts of the Central Graben and Broad Fourteens Basin, where inversion occurred during the Subhercynian phase (Santonian-Campanian), a thin cover of Ommelanden Formation may rest unconformably on older deposits (Schieland Group, Scruff Group, Altena Group, or Upper Germanic Trias Group), which represents onlap of the post-inversion development of the formation. In the far southwest of the country, on the London-Brabant Massif, the Ommelanden Formation can rest unconformably on deposits of the Limburg , Carboniferous Limestone Group or Banjaard Group (or possibly even older deposits in Belgium). On the crest of the Mid-North Sea High Elbow Spit High the unit can onlap onto the Farne Group or Old Red Group.
Distribution The Ommelanden Formation is found over most of the Netherlands and the Netherlands offshore. As a result of Subhercynian (Santonian-Campanian) and Laramid (earliest Paleocene) inversion of Late Jurassic-Early Cretaceous basins, it is absent in large parts of the Dutch Central Graben, Broad Fourteens Basin, Central Netherlands Basin, West Netherlands Basin and Roer Valley Graben (see maps in van Wijhe (1987) ; Burgers and Mulder (1991) ). The formation can be equated with the Hod and Tor Formations in the Norwegian and Danish offshore. Towards the southeastern onshore Netherlands it grades into the formations that have been identified in southern Limburg Felder (1975) .
Age Turonian to Maastrichtian. Rich microfossil assemblages permit a division into separate chronostratigraphic units. Characteristic microfossils (Ostracoda to), planktonic Foraminifera (p) and benthic Foraminifera (b)) are, in the Upper Maastrichtian: Bolivinoides giganteus (b), Orbignyna rimosa (b), Parvacythereis postparva to), Pseudotextularia elegans (p), Globigerina biforaminata (p) and Tumidoleberis bonnemai (o). In the Lower Maastrichtian Heterohelix glabrans (p), Bolivinoides australis (b), Pyramidina minute (b) and Bolivinoides draco miliaris (b) (not exclusively Lower Maastrichtian, however) are useful age-indicators. The Upper Campanian is identified by e.g. Bolivinoides decoratus (b), Bolivinoides draco miliaris (b), Gavelinella clementiana clementiana (b), Gavelinella clementiana laevigata (b), Gavelinella monterelensis (b), Globorotalites hiltermanni (b), Globotruncana rugosa (p), Krithe bonnemai (o) and Neoflabellina rugosa leptodisca (b). Lower Campanian markers are Bolivinoides strigillatus (b), Stensioeina exsculpta gracilis (b), Stensioeina granulata incondita (b), Gavelinella cristata brotzeni (b), Gavelinella cristata cristata (b), Curfsina senior (o), Loxostomum eleyi (b), Gavelinella thalmanni (b), Reussella cushmani (b) (the last four also Santonian), and a few others. The Santonian also contains several diagnostic forms, such as Dicarinella asymetrica (p), Dicarinella concavata (p), Golcocythere calkeri to), Neoflabellina suturalis praecursor (b), Praebulimina pusilla (b), Stensioeina granulata perfecta (b), Stensioeina granulate polonica (b) and Curfsina senior(o) (not exclusively Santonian). The Coniacian is identified in many cases by the presence of Golcocythere calkeri (o), Dicarinella primitiva (p), Eouvigerina robusta (b), Eouvigerina stormi (b), Marginotruncana sinuosa (p) and Stensioeina granulata granulata (b), and the first occurrence of Dicarinella imbricata (p). Characteristic microfossils found in the Turonian are Gavelinopsis tourainensis (b) (also Coniacian), Globorotalites minuta (b), Praeglobotruncana gibba (p), Praeglobotruncana stephani (p), Spiroplectinata jaekeli (b), Stensioeina cf. pokornyi (b), Stensioeina granulata humilis (b), Stensioeina granulata kelleri (b) and Stensioeina granulata levis (b). Representatives of the peridinoid, especially deflandroid, dinoflagellates, such as Chatangiella and Isabelidinium and gymnodinoids are rare in the lower part (Turonian) of the Ommelanden Formation. Species first occurring in the Turonian include Acanthaulax wilsonii, while Stephodinium coronatum occasionally ranges into the lowermost Coniacian. Dinoflagellates with distinct ranges which could be confirmed by other (micro)fossil groups, are uncommon in the Coniacian and Santonian. Psaligonyaulax deflandrei has its top at the Coniacian-Santonian boundary. A possible marker species for the top Santonian is Cassiculosphaeridia reticulata. In marked contrast, the Campanian and Maastrichtian part of the Ommelanden Formation shows numerous well-defined first appearances and last occurrences. These include, to mention only some, Areoligera spp., Cailaiosphaeridiurn asymmetricum, Cerodinium diebelii, Dinogymnium spp., Gillinia hymenophora, Leberidocysta chlamydata, Odontochitina spp., Palaeocystodinium, Palaeohystrichophora infusorioides, Palynodinium grallator, Phelodinium tricuspe, Samlandia solida, Senoniasphaera rotundata, Triblastula utinensis, Xenascus ceratioides, etc. An overview of the biomarkers is available in biomarker table I (see pdf) .
Depositional Setting Deposited under relatively stable, low-energy conditions in carbonate-shelf and upper bathyal environments. The rocks consist essentially of pelagic, biogenic remains, which settled from suspension. A variety of subfacies has been recognised, reflecting variation in sedimentation rate and water depth. In many localities Campanian-Maastrichtian fossil associations indicate considerably shallower water conditions than Turonian-Santonian associations (e.g, lower plankton/benthos ratio).
Subdivision The monotonous lithological development of the Ommelanden Formation does not permit a regionally recognisable formal subdivision. Various attempts at a subdivision on the basis of wire-line log characteristics have proved useful for local purposes, but not reliable over larger areas.
References See References Upper Cretaceous

Van Adrichem Boogaert, H.A. & Kouwe, W.F.P., 1993-1997. [Stratigraphic unit]. In: Stratigraphic Nomenclature of the Netherlands.
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