|In Northwest Europe the Upper Cretaceous is developed as a uniform cover of predominantly friable, white or cream-coloured, micritic limestones. This type of limestones which has an earthy texture is commonly known as 'chalk'. In the Netherlands chalky sediments were deposited from the Cenomanian (or locally latest Albian) to the Maastrichtian (or locally earliest Paleocene: Danian). The Harlingen gas field is the only proven economic hydrocarbon accumulation in the Dutch chalk deposits van den Bosch (1983) . Figure H.1 (see pdf) presents a number of reliable palynological and micropalaeontologicaI markers for the Late Cretaceous, as used by the RGD. For additional data on the Late Cretaceous biostratigraphy of the North Sea area and the Netherlands onshore see for instance Crittenden et al (1991) , Herngreen et al (in press) and Sissingh (1977) .|
|During the latest Albian the influx of fine-grained, clastic material into the marine environment, which covered most of the Netherlands, suddenly diminished. This appears to be associated with a short-term regression that preceeded the large-scale transgression, which terminated deposition of the Holland Formation. The decrease inelastic influx allowed carbonates to be deposited, mainly in the form of carbonatic remains of planktonic plants and animals, which settled from suspension. In the Netherlands the widespread carbonate deposition started at the beginning of the Cenomanian, and was tolast till the end of the Maastrichtian, and locally even into the earliest Paleocene (Danian). The deposits have been placed in the Chalk , a unit defined in NAM and RGD (1980) , modified for local use after Rhys (1974) .Typical chalk deposits consist mainly of coccoliths, usually rounded calcite platelets of 0.5 till 20 pm in diametre. They are produced by various taxa of the phylum Haptophyta (golden brown algae), which thrive in a pelagic, open marine habitat Hancock (1990) . Other characteristic fossil groups are: planktonic and benthic Foraminifera, Ostracoda, Bryozoa, Echinodermata, calcareous dinoflagellates, sponges and some bivalves (e.g. Inoceramus spp.). The ratio of sessile, benthic versus planktonic/nektonic fossil taxa can be used as palaeo-waterdepth indicator. For instance, the bioclastic limestones found in the Maastrichtian in the south of Limburg are characterised by bryozoans, echinids, ostracods and benthic foraminifers, whereas planktonic foraminifers are rare. This association reflects deposition in a shallow marine environment (inner to middle neritic), well within the photic zone. On the other hand, the Cenomanian-Turonian chalk deposits of the Central Graben, which consist only of coccoliths and planktonic foraminifers, were probably deposited in an outer neritic to bathyal setting. Apart from direct settling from suspension, redeposition as mass flows played a large role in areas with sufficient syndepositional topographic relief, e.g. in areas with active faulting and halokinesis.|
|During the Early Cenomanian the palaeo-coastline ran from the south of the province of Zuid Holland to southern Gelderland (Nijmegen). Close to this coastline shallow marine conditions existed, which are reflected in the deposition of greensands and thick, marry successions in the West Netherlands Basin and the Broad Fourteens Basin. To the north these marls grade into pure chalk. All these deposits have been placed in the TexelFormation. The basal greensands found close to the paleo-coastline have been assigned to the Texel Greensand Member. At the transition of the Cenomanian to the Turonian an anoxic event resulted in the deposition of a thin, dark coloured marl bed, the Plenus Marl Member. This remarkable marker interval can be traced throughout Northwest Europe, and represents a period of stagnation of deep basinal water circulation.|
|From the Turonian to the Maastrichtian, carbonate deposition in the Netherlands continued to prevail, forming the bulk of the Chalk Group. In this period, the marine realm gradually expanded southward, and during the Campanian/Maastrichtian even the crest of the thus far exposed London Brabant Massif (south of Limburg, S quadrant offshore) was flooded. The formations that have been recognised in the south of the province of Limburg Felder (1975) reflect the Santonian-Maastrichtian stages of the Late Cretaceous transgression of the northern flank of the London-Brabant Massif. Deposition started with a succession of lagoonal clays and coastal sandstones (Aken Formation) which show a strong continental influence (e.g. abundant silicified wood fragments), followed by a sequence of locally silty glauconitic sandstones, reflecting deeper marine elastic deposition without much coastal influence (Vaals Formation). In the Late Campanian the influx of siliciclastic material decreased, and inner and middle neritic carbonates became the prevailing sediment. These limestones consist of fine to coarse-grained bioclastic grainstones (calcarenites/calcirudites), composed of a hash of calcareous fossils, amongst others bryozoans and echinids.|
|This succession has been subdivided into two formations: the Gulpen Formation (Late Campanian-early Late Maastrichtian) and the Houthem Formation (Late Maastrichtian) and several members. Chert nodules are commonly observed in layers in the upper part of the Gulpen Formation and lower part of the Houthem Formation. In the rest of the succession a few isolated nodules are present. Repeated influx of terrigenous material locally resulted in the deposition of marly chalk. The distribution of these marly intercalations has not yet been mapped in detail, but appears to be related to inversion tectonism. Most basins with a Late Jurassic and Early Cretaceous fill suffered inversion during the Late Cretaceous and/or earliest Paleocene. Although the actual movement involved many small pulses, they can be attributed to two major tectonic phases: the Subhercynian phase, which climaxed in the Santonian-Campanian, and the Laramide phase, during the earliest Paleocene. The depocentres of the basins were uplifted, and consequently deeply truncated. The mechanism behind this inversion is still a matter of debate (Heybroek (1974) ; Ziegler (1987) ; van Wijhe (1987) ; Dronkers and Mrozek (1991) ; Baldschuhn et al (1991) ),but it appears to be more or less inherent to the life cycle of an extensionally formed basin in a subsequent transpressional tectonic setting. In areas that were not inverted, such as the Late Jurassic-Early Cretaceous highs and terraces (see Fig. A.6 (see pdf) ), very thick successions of chalk could develop. Fossil associations from these successions tend to demonstrate a gradual shoaling of the North Sea Basin, with sudden shallowing during inversion events.|
|Danian (Early Paleocene)|
|Chalk deposition persisted during the Early Paleocene. At the close of the Danian a regression occurred. Patches of Danian chalk escaped the ensuing mid-Paleocene erosion, i.e. in and around the Central Graben, and in the southern Netherlands (West Netherlands Basin, Roer Valley Graben, Central Netherlands Basin and southern Limburg). Where present, the Paleocene chalk has the same lithofacies as the underlying Cretaceous chalk.|
|The Chalk Group is usually characterised by a thick interval of a more or less transparent seismic appearance, dissected by a number of high amplitude, continuous reflectors. The transition from the Chalk Group to the overlying Lower North Sea Group is marked by a continuous, very high-amplitude reflector. The basal part of the Chalk Group shows a gradual downward increase in reflectivity, which continues into the underlying Holland Formation. Furthermore, especially in the highest intervals of the Chalk Group, a number of parallel, very high-amplitude, low-frequency reflectors can be encountered. The origin of the intra-Chalk Group reflections is not yet fully understood, but some of them have been interpreted as associated with hardgrounds and/or chert-rich horizons. Particularly in areas of intense halokinesis of Zechstein salts, substantial angular unconformities occur at some of these intra Chalk reflectors. The Chalk Group is notorious for its erratic seismic velocity anomalies.|
|Regional correlation||Regional lithostratigraphic correlation chart of the Upper Cretaceous for the Netherlands and neighbouring countries
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|Chrono-stratigraphy||Late Cretaceous litho-chronostratigraphic chart
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|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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