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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, Northern North Sea
The term Herring Formation was introduced by Deegan & Scull (1977), in the Central North Sea area (Table 1 ), to describe a unit of hard, chalky limestones and interbedded calcareous mudstones lying between a thin mudstone unit, the Plenus Marl Formation, and the overlying chalks of the Flounder Formation. These terms were subsequently revised by Johnston & Lott (1993) for the Central North Sea and as the unit is recognizable into the Southern North Sea Basin the definition was also followed by Lott & Knox (1994). The Herring Formation now includes the thin mudstone unit at the base of the formation, the Black Band (refer to Johnson & Lott, 1993; p.95), formerly known as the Plenus Marl Formation Table 1 ), and the lower sequence of high-velocity harder, thinly bedded Turonian limestones and chalks, formerly included within the Hod Formation (Gatliff et al., 1994).
The Herring Formation is named after the fish (Deegan & Scull, 1977).
The Herring Formation consists of hard to very hard, dense, chalky limestones with interbedded argillaceous chalks and mudstones (Johnson & Lott, 1993; Lott & Knox, 1994). Chert is abundant in the unit. The limestones are white to pale grey, occasionally medium grey and buff, glauconitic and occasionally pyritic. The interbedded mudstones, including the Black Band, are soft to hard, dark grey to black or variegated red-brown, carbonaceous, micaceous non-calcareous to calcareous and pyritic. In the South Viking Graben, thin, glauconitic, quartzose sandstones are present in the lower part of the formation (e.g. 16/28-7, Panel 3, Johnson & Lott, 1993).
In the Central North Sea the formation ranges from 25 to 75 m, but locally reaches over 120 m (e.g., 22/3a-1, Panel 7, Johnson & Lott, 1993) and over 140 m in the Fisher Bank Basin (e.g., 22/1-2A, Gatliff et al., 1994, fig. 48). In the Peterhead sub-basins and south-east part of the Witch Ground Graben of the Outer Moray Firth, the formation, including the ‘Plenus Marls’, is up to 285 m thick, but is virtually absent in the Inner Moray Firth (Andrews et al., 1990). The formation is up to 60 m thick in the Southern North Sea Basin (Lott & Knox, 1994).
The Herring Formation can be recognized throughout the Central North Sea and extends into the South Viking Graben. It is absent, however, over some contemporary structural highs (Johnson & Lott, 1993).
The Herring Formation can be recognized over most of the Southern North Sea Basin (Lott & Knox, 1994). The unit is absent over the Sole Pit Basin because of subsequent inversion and erosion, and is also missing over some contemporary tectonic highs and salt diapirs (e.g. 48/10-1, 52/5-3).WGS84 coordinates: Lat. 57Вє 56’ 12.2”NВ В В В В Long. 01Вє 02’ 55.8”E
UTM coordinates:
UTM zone: 31
Drilling operator name: BP Exploration Operating Company Limited
Completion date: 13.04.1974
Status: P & A
Interval of type section & thickness in type well: 3605В3738.5 m (11827В12265 ft) below KB (Deegan & Scull, 1977) (revised depths).
Central and Northern North Sea (Johnson & Lott, 1993) |
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13/28-2: 1405.5-1468.5 m (4611-4818 ft) |
Lat. 58° 06’ 59.3”N Lat. 58° 39’ 17.7”NВ В В В Lat. 56° 50’ 18.6”N |
Long. 01Вє 25’ 27.0”W Long. 01° 23’ 59.0”E Long. 01° 48’ 52.5”E |
Southern North Sea (Lott & Knox, 1994) |
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| 49/20-1: 1433В1495 m (4701В4905 ft) 53/4-2: 1515В1573 m (4970В5161 ft) |
Lat. 53° 24’ 07.0”N Lat. 52° 52’ 50.0”N |
Long. 02° 51’ 52.0”E Long. 02° 47’ 16.0”E |
In the Central North Sea the top of the Herring Formation is normally marked by a downward change from argillaceous chalky limestones of the Mackerel Formation, or calcareous mudstones of the Flounder Formation, to cleaner, harder chalky limestones (Johnson & Lott 1993). In the South Viking Graben, the Herring Formation is locally overlain by the Kyrre Formation (e.g. 16/3-1).
In the Southern North Sea the top of the Herring Formation is normally marked by a downward change from softer, more argillaceous chalks of the Lamplugh Formation to cleaner, harder chalky limestones (Lott & Knox, 1994).The base of the Herring Formation is normally marked by a sharp downward change from dark coloured mudstones (Black Band) to chalky limestones of the Hidra Formation (Johnson & Lott, 1993; Lott & Knox, 1994). The Black Band forms a key log marker throughout much of the basin (refer to Panel 1, Lott & Knox, 1994). Where the Black Band mudstones are poorly developed the boundary is often more difficult to identify without biostratigraphic control. Where the Black Band is absent, limestones of the Herring Formation disconformably overlie the Hidra Formation (e.g. 21/1-7ST, Burnhill & Ramsay, 1981 and 14/26-1). On intrabasinal highs, the Herring Formation locally rests unconformably on Lower Cretaceous or older rocks (e.g. 14/13-1, 16/28-4 and 16/23-4, Johnson & Lott, 1993).
The Plenus Marl Formation and/or Black Band at the base of the Herring Formation marks the base of the onshore White Chalk Subgroup (Hopson, 2005). However, this author noted that the use of the two terms Plenus Marl Formation and/or Black Band were not necessarily mutually exclusive offshore.In the Central North Sea, the top of the formation is marked on the downhole log responses by a downward increase in velocity accompanied by a decrease in the gamma ray values (e.g.В 16/28-7 and 15/28a-3, Johnson & Lott, 1993). В In the Southern North Sea, this upper boundary is marked on the downhole log responses by an increase in velocity sometimes accompanied by a slight decrease in the gamma-ray values (e.g. 49/24-1, Lott & Knox, 1994). The lower boundary of the formation is marked by a sharp downward decrease in gamma-ray values and increase in velocity. The Black Band, commonly present at the base of the formation, usually has characteristic high gamma-ray and low velocity spikes. Gamma-ray values may be expressed as a single peak (e.g. 13/17-1, Panel 4 Johnson & Lott, 1993) or two peaks (e.g. 29/1b-1, 29/3-1, Panel 10 Johnson & Lott, 1993), although, occasionally the gamma spike is not well developed (e.g. 49/24-1, Panel 2 Lott & Knox, 1994). The low velocity Black Band generates a high-amplitude seismic reflector present across much of the Central North Sea (Gatliff et al., 1994).
The Praeglobotruncana stephani biomarker can be recognized towards the middle of the formation, together with other common planktonic foraminifera such as Dicarinella hagni, Praeglobotruncana gibba, Dicarinella imbricata and Marginotruncana renzi (Johnson & Lott, 1993; Lott & Knox, 1994). Calcareous benthonic foraminifera are rare, but Lingulogavelinella globosa and Stensioeina granulata humilis are occasionally found within the formation. The Eprolithus spp. nannofossil biomarker is situated in the middle part of the formation. Palynological recovery in the Herring Formation is generally rather low, but by analogy with the Central North Sea and onshore areas, the Litosphaeridium siphoniphorum dinoflagellate cysts biomarker should be present immediately above the top of the Black Bed (Johnson & Lott, 1993). The dinoflagellate cyst Heterosphaeridium difficile is typically common to abundant (Costa & Davey, 1992).
Early to mid Turonian
The Herring Formation is the lateral equivalent, in part, of the Welton Chalk Formation of Eastern England which includes the Black Band bed (Wood & Smith, 1978; Table 4.3). The Black Band bed is widely recognized in Eastern England (Hart & Bigg, 1981). It is also the lateral equivalent of the more argillaceous Macbeth Formation in the Viking Graben and East Shetland Basin.
The micritic, chalky limestones of the Herring Formation are pelagic sediments deposited as nannofossil-rich oozes in a well-oxygenated sea of moderate depth (Johnson & Lott, 1993; Lott & Knox, 1994). The Black Band mudstones, in contrast, were deposited during a phase of bottom-water stagnation and is dominated by planktonic foraminifera and impoverished benthonic faunas (Gatliff et al., 1994). It represents a widely documented global marine transgression and phase of anoxia close to the CenomanianВTuronian boundary (e.g. Hart & Leary, 1989).
The four informal sub-units of the Herring Formation (G1-G4) were recognized by Burnhill & Ramsay (1981) in the Outer Moray Firth based upon wireline-log responses. G1 and G3/4 comprise high velocity, thin-bedded, often hard chalk commonly topped by a ‘hot shale’ band, with G2 dominated by calcareous mudstone and thin interbedded chalk. Johnson & Lott (1993) extended recognition of these units into the Central Graben and South Viking Graben, with an additional G0 unit to substitute the Plenus Marl Formation of Deegan & Scull (1977). These informal sub-units are not, with the exception of the Black Band, readily apparent in the wells south of the Mid-North Sea High (Lott & Knox, 1994).
The Black Band (of bed status, formerly the lower part of the Plenus Marl Formation of Deegan & Scull, 1977 and Plenus Marl Formation unit A of Crittenden et al., 1991) consists of up to 40 m of dark grey to black, fissile, carbonaceous and non-calcareous mudstones (Johnson & Lott, 1993; Lott & Knox, 1994). The base of the Black Band is defined by a sharp downward change from black mudstones to argillaceous, white, chalky limestones of the Hidra Formation.