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updated to follow: Stratigraphic Guide to the Cromer Knoll, Shetland and Chalk Groups of the North Sea and Norwegian Sea. Felix M. Gradstein & Colin C. Waters (editors), Mike Charnock, Dirk Munsterman, Michelle Hollerbach, Harald Brunstad, Øyvind Hammer & Luis Vergara (contributors). Newsletter on Stratigraphy, vol 49/1 pp71-280, 2016
Chalk Group, Southern North Sea
Lott & Knox (1994, fig.7/4) working in the Southern North Sea introduced the term Rowe Chalk Formation to cover the very highest Chalk concealed beneath Quaternary deposits in the Hornsea area and immediately offshore.
After the eminent geologist A.W. Rowe who carried out extensive studies of the English Chalk successions in the early 1900s.
The Rowe Formation ranges up to 380 m in thickness (e.g. well 49/25-1, Lott & Knox, 1994). However, it may vary significantly in thickness due to intra-basinal tectonic controls and post-Cretaceous erosion notably around the Sole Pit Inversion structure.
Sumbler (1999) described these beds, based on geophysical logs from boreholes, as 70m or so of flint-bearing chalks characterised by the inferred presence of Belemnitella mucronata and also indicated that still younger beds were present offshore.
The Rowe Formation is widely distributed throughout the Southern North Sea Basin. In eastern England the formation is known only in the subsurface where it overlies the Flamborough Chalk Formation of Wood & Smith (1978; refer also to Whitham, 1993). In the Trunch borehole, on the north Norfolk coast, the base of the formation probably equates with the hardground development marking the base of the Basal Mucronata Chalk (base Belemnitella mucronata Zone) (McArthur et al., 1993).
44/28-1: 1651.5В1918.5 m (5418В6294 ft) |
Lat. 54° 03’ 42.0”N Lat. 53° 52’ 05.0”NВ В В В Lat. 53° 43’ 51.6”N Lat. 52° 52' 50.0"N |
Long. 02° 26’ 52.6”E Long. 02° 49’ 04.0”E Long. 00° 48’ 17.1”E Long. 02° 47' 16.0"E |
The top of the Rowe Formation is overlain by Danian chalky limestones (Ekofisk Formation), with the boundary usually being marked by a downward into chalk with a lower clay content. Where the Ekofisk Formation is absent, the Rowe Formation is overlain by Paleocene mudstones (Lott & Knox, 1994).
The character of the log responses for the top of the Rowe Formation is variable, depending on whether the unit is overlain by Paleogene or younger Cenozoic or Pleistocene sequences. In a few wells the formation is overlain by Danian chalky limestones (Ekofisk Formation), with the boundary usually being marked by a downward decrease in gamma-ray values, reflecting a lower clay content. The sonic log signature is, however, variable, with some sections showing a downward decrease in velocity (e.g.В 44/28-1) and others a downward increase (e.g. 39/7-1, Panel 1 Lott & Knox, 1994). Where the Ekofisk Formation is absent, the Rowe Formation is overlain by Paleocene mudstones and there is a corresponding sharp downward decrease in gamma ray values and increase in velocity (e.g. 44/28-1:В Panel 2, Lott & Knox, 1994).
The base of the Rowe Formation is defined by a marked downward increase in velocity but only shows a slight downward decrease in gamma-ray values. The sharp increase in velocity is probably related to a significant basinwide regressive hardground surface (Lott & Knox, 1994).The top of the Rowe Formation is marked by the Pseudotextularia elegans foraminiferal biomarker and the Nephrolithus elegans calcareous nannofossil biomarker (Lott & Knox, 1994). In the middle part of the formation, Reussella szajnochae has its FDO (First Downhole Occurrence), but it is rare and patchily distributed in the upper part of its range. However, within the formation the Reussella szajnochae acme biomarker, which may be accompanied by Tritaxia capitosa, is biostratigraphically important. In the lower part of the formation, the FDO of a number of species of Gavelinella and Bolivinoides miliaris forms a characteristic biomarker. At approximately the same horizon as the FDO of R. szajnochae, the FDO of Reinhardtites levis forms a key nannofossil biomarker at the early/late Maastrichtian boundary. The FDO of Reinhardtites anthophorus is within the early Maastrichtian. Palynomorph recovery in the Rowe Formation is generally relatively low
The Rowe Formation passes northwards into the upper part of the Mackerel Formation and the Tor Formation of the Central North Sea (Table 1 and Table 2).
The chalky limestones of the Rowe Formation were deposited in an open marine setting as pelagic carbonates and consist primarily of fine bioclastic skeletal debris (dominated by coccolith plates). Microfaunas are dominated by planktonic foraminifera (Lott & Knox, 1994).
None in UK waters.