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
The term Jukes Formation was introduced for a unit of hard, comparatively chert-free chalky limestones which underlie the Rowe Formation and overlie the more argillaceous chalks of the Lamplugh Formation (Lott & Knox, 1994; Table 1 and Table 2).
After the eminent survey geologist A. J. Jukes-Browne (1851В1914) who carried out extensive studies of the Cretaceous rocks of Britain.
The Jukes Formation typically consists of moderately hard, white, occasionally greyish white, variably argillaceous, chalky limestones. Occasional harder limestone bands occur together with comparatively few nodular chert developments (Lott & Knox, 1994).
The Jukes Formation ranges up to 500 m (Panel 2, Lott & Knox, 1994), but may vary in thickness due to intra-basinal tectonic controls and post-Cretaceous erosion.
The Jukes Formation is widely distributed throughout the Southern North Sea Basin.
38/24-1: 1448В1585.5 m (4751В5202 ft)
|Lat. 55° 12’ 35.3”N
Lat. 54° 45’11.3”NВ В В В
Lat. 53° 52’ 05.0”N
|Long. 02° 37’ 33.1”E
Long. 1° 33’07.8”E
Long. 02° 49’ 04.0”E
The upper boundary of the Jukes Formation is characterized by a downward change from relatively soft chert-bearing argillaceous chalks to harder white chalks with fewer cherts. The top of the unit is often marked by a thin, hard chalk unit (e.g. well 49/5-1 Lott & Knox, 1994).
The FDOs (First Downhole Occurrence) of a number of taxa form good biomarkers within the formation: Stensioeina granulata incondita and Bolivinoides strigillatus in the mid Campanian; the radiolarian Cenosphaera sp. at the top of the Santonian (where Stensioeina exsculpta exsculpta is also found); and Stensioeina granulata polonica. The key calcareous nannofossil biomarkers within the formation are the FDOs of Broinsonia enormis, at the top of the early Santonian; and Watznaueria barnesae acme, which marks the mid/early Santonian boundary. Palynomorph recovery in the Jukes Formation is generally relatively low.
The Jukes Formation passes northwards into the middle part of the Mackerel Formation of the Central North Sea (Table 2). In the UK onshore area the formation equates largely with the Flamborough Chalk Formation (Table 1) of Wood & Smith (1978; refer also to Whitham, 1993).
The chalky limestones of the Jukes Formation were deposited in an open marine setting as pelagic carbonates and consist primarily of fine bioclastic skeletal debris (dominated by coccolith plates). Thin more argillaceous beds present represent increased terrigenous input into the basin (Lott & Knox, 1994).
None in UK waters.