Jukes Formation

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

Introduction

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).

Name

After the eminent survey geologist A. J. Jukes-Browne (1851В­1914) who carried out extensive studies of the Cretaceous rocks of Britain.

Lithology

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).

Thickness

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.

Geographical distribution

The Jukes Formation is widely distributed throughout the Southern North Sea Basin.

 

Type well

Well name: 49/24-1

WGS84 coordinates: Lat. 53° 16’ 49.5”NВ В В В В  Long. 02° 41’ 30.4”E
UTM coordinates:
UTM zone: 31
Drilling operator name: Shell UK Exploration and Production Ltd
Completion date: 30.04.1972
Status: Suspended
Interval of type section & thickness in type well: 840В­1060 m (2756В­3478 ft)

UK Reference Wells

38/24-1: 1448В­1585.5 m (4751В­5202 ft)
43/8a-2: 810В­948 m (2657В­3110 ft)
49/5-1: 1265.5В­1679.5 m (4152В­5510 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

 

Upper and lower boundaries

Upper Boundary

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).

Lower Boundary

The base of the Jukes Formation is defined by a downward change from moderately hard, relatively chert-free chalks, to darker, consistently harder, argillaceous, cherty chalks of the Lamplugh Formation.

Well log characteristics

The top of the Jukes Formation is marked by a slight downward decrease in gamma-ray values and a more marked increase in sonic velocity. There is considerable variability in the sonic response over the formation as a whole as a result of the cyclic interbedding of thin argillaceous units, hard chalks and occasional hard chert-rich bands. The lower boundary of the formation corresponds to a consistent increase in sonic velocity, sometimes accompanied by a slight downward increase in gamma values (Lott & Knox, 1994).

Biostratigraphy

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.

Age

Santonian to early Campanian

Correlation

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).

Depositional environment

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).

Subdivisions

None in UK waters.