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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, Central North Sea
The Mackerel Formation was introduced by Johnson & Lott (1993) for a unit of limestones and argillaceous chalky limestones with interbedded mudstones that lies between the Herring Formation and the Tor Formation (Table 2). The strata were included in the Hod Formation (Table 1) by Deegan and Scull (1977) and Gatliff et al. (1994), which was abandoned again by Johnson & Lott (1993).
After the salt-water fish.
The Mackerel Formation consists of white, pale grey, and occasionally pink or red, fine-grained, and commonly argillaceous chalky limestones (Johnson & Lott, 1993). Sections with relatively high amounts of detrital clay are reflected in higher gamma values and lower velocity. The formation becomes more argillaceous towards the South Viking Graben, where it grades laterally into the Flounder Formation (Shetland Group). The northward passage into the mudstone-dominated Flounder Formation is reflected in an increasing proportion of argillaceous limestone intercalations in the Mackerel Formation of the Outer Moray Firth.
In depocentres the Mackerel Formation is between 200 and 500 m thick (Panels 4 and 13W, Johnson & Lott, 1993).
The Mackerel Formation is present over much of the Central North Sea. It onlaps and covers the intrabasinal ridges, such as the Jaeren High, but is absent from the higher parts of the Forties-Montrose High and the Western Platform, where it borders the Central Graben (Johnson & Lott, 1993).
13/28-2: 1212.5-1405.5 m (3978-4611 ft) |
Lat. 58° 06’ 59.3”N Lat. 58° 28’ 11.2”NВ В В В Lat. 56° 50’ 37.0”N |
Long. 01° 25’ 27.0”W Long. 00° 03’ 07.8”W Long. 01° 45’ 14.4”E |
The top of the Mackerel Formation is normally marked by a relatively abrupt downward change from the hard chalky limestones of the Tor Formation to the interbedded argillaceous chalky limestones, which are commonly stained pink or red (Johnson & Lott, 1993).
The base of the Mackerel Formation is taken at a downward change from the argillaceous chalky limestones to cleaner, higher velocity limestones of the Herring Formation (Johnson & Lott, 1993).
On wireline logs, the upper boundary of the Mackerel Formation corresponds to a downward increase in gamma values and decrease in velocity. Commonly, the basal part of the Mackerel Formation displays a downward decrease in gamma values and increase in velocity immediately above the Herring Formation (e.g. 14/20-6A, Panel 4; 21/4-1, Panel 6, Johnson & Lott, 1993).
The top of the Mackerel Formation is marked in the Reussella szajnochae acme foraminiferal biomarker, which in the Central North Sea is accompanied by the FDO (First Downhole Occurrence) of Tritaxia capitosa. In some areas this biomarker occurs at or just below the top of the formation (King et al., 1989, fig. 8.4). Long-ranging planktonic foraminfera, particularly species of Rugoglobigerina, are common (Johnson & Lott, 1993).В The Cenosphaera sp. radiolarian biomarker is present in the middle part of the formation. The Stensioeina granulata polonica and S. granulata granulata biomarkers also occur within the formation.
Three key nannofossil biomarkers are present (Johnson & Lott, 1993): the Reinhardtites anthophorus biomarker occurs at the top of the formation, the Broinsonia enormis biomarker, at the top of the early Santonian, and the Watznaueria barnesae acme biomarker, which marks the mid/early Santonian boundary, in the middle of the formation. The Helicolithus trabeculatus biomarker and H. valhallensis acme biomarker are also present.
The Mackerel Formation becomes more argillaceous towards the South Viking Graben, where it grades laterally into the Flounder Formation (Table 1 and Table ).
The Mackerel Formation consists of dominantly pelagic chalky, coccolith-rich limestones that accumulated in marine, low-energy shelf to upper bathyal conditions (Johnson & Lott, 1993). Phases of more open marine circulation are indicated by a higher proportion of planktonic foraminifera in the Coniacian and Upper Campanian (King et al., 1989).
Two informal subdivision schemes have been applied to beds assigned to the Mackerel Formation. King et al. (1989) recognized a sub-unit of pink-stained chalks at the top of the formation, and in the Moray Firth, Andrews et al. (1990) recognized four chalk-marl cycles (designated units F1 to F4), each up to about 100 m thick. The Mackerel Formation was not subdivided by Johnson & Lott (1993).