Flounder 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

Shetland Group

Introduction

The term Flounder Formation was introduced by Deegan & Scull (1977) for a unit of mudstones and interbedded limestones that lay between the Herring Formation and the Tor Formation (Table 1). This definition was adopted by Johnson & Lott (1993) except that, in contrast to Deegan & Scull (1977), the Flounder Formation is included in the Shetland Group.

Name

From the salt-water flat-fish.

Lithology

The Flounder Formation consists of pale to dark grey, occasionally pink to red, very calcareous mudstones, grading into pale to dark grey argillaceous chalky limestones. Thin beds of white brittle cryptocrystalline limestone are also present (Johnson & Lott, 1993).

Thickness

The Flounder Formation is up to 500 m in thickness in the Fisher Bank Basin, but is absent over the crest of the Forties-Montrose High (Gatliff et al., 1994). Seismic evidence suggests that it broadly onlaps, and becomes very thin over, contemporary structural highs (Gatliff et al., 1994). Andrews et al. (1990) interpreted a significant unconformity or hiatus within the Flounder Formation.

Geographical distribution

The Flounder Formation is confined to the southern part of the South Viking Graben and the extreme north of the Central North Sea.

 

Type well

Well name: 22/l-2A

WGS84 coordinates: Lat. 57° 56’ 12.2”N; Long. 01° 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: 3198-3605 m (10492-11827 ft) below KB (Deegan & Scull, 1977, p.27, fig.28) (revised depths by Johnson & Lott, 1993). Highly argillaceous chalky limestones and mudstones that were placed within the Tor Formation by Deegan & Scull were included within the Flounder Formation by Johnson & Lott (1993).

UK Reference well

16/17-6: 3249-3590.5 m(10660-11779 ft)

 Lat. 58°25’ 55.6”N Long. 01°18’ 26.0”E

 

Upper and lower boundaries

Upper Boundary

The top of the Flounder Formation is normally marked by a relatively abrupt downward change from the hard chalky limestones of the Tor Formation to calcareous mudstones with interbedded argillaceous chalky limestones, which are commonly stained pink or red.

Lower Boundary

The base of the Flounder Formation is usually taken at a downward change from mudstones with interbedded argillaceous chalky limestones to cleaner, higher velocity limestones of the Herring Formation. On intrabasinal highs, the Flounder Formation locally rests unconformably on Jurassic or older rocks (e.g. Pentland Formation in 15/26a-2).

Well log characteristics

The Flounder Formation has a similar wireline-logВ  signature to the more argillaceous developments of the Mackerel Formation, but is distinguished on the basis of its higher overall gamma values (generally over 30-35 API units, Johnson & Lott, 1993). The top of the Flounder Formation corresponds to a downward increase in gamma values and decrease in velocity on wireline logs. The base of the Flounder Formation corresponds to a downward decrease in gamma values and an increase in velocity.

Biostratigraphy

The top of the Flounder Formation is marked by the Reussella szajnochae acme foraminiferal biomarker, which in the Central North Sea is accompanied by the FDO of Tritaxia capitosa. In some areas, this biomarker occurs at the top of the formation; in others it is just below the top (this is also implied in King et al., 1989, fig.8.4). Long-ranging planktonic foraminifera, particularly species of Rugoglobigerina, are common. 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 calcareous nannofossil biomarkers occur: the Reinhardtites anthophorus biomarker, at the top of the formation, the Broinsonia enormis biomarker, at the top of the early Santonian, and Watznaueria barnesae acme biomarker, which marks the mid/early Santonian boundary, in the middle part of the formation. The Helicolithus trabeculatus biomarker and H. valhallensis acme biomarker also occur within the formation.

The Senoniasphaera protrusa, Callaiosphaeridium assymetricum, Cassiculosphaeridia reticulata, Heterosphaeridium difficile, Endoscrinium campanula, Florentinia deanei and Stephodinium coronatum dinoflagellate cysts biomarkers all occur within the formation (Johnson & Lott, 1993). Spiniferites spp. (especially S. ramosus) dominates and Chatangiella spp. are locally common in the Coniacian.

Age

Mid Turonian to early Maastrichtian

Correlation

In the Central North Sea, the Flounder Formation grades laterally into the Mackerel Formation. In the South Viking Graben, the Flounder Formation passes northwards into the Kyrre Formation and the lower part of the Jorsalfare Formation (J1 and the basal part of J2) (see Panel 3 of Johnson & Lott, 1993). Red coloration in the unit of interbedded argillaceous chalky limestone and mudstone near the top of the formation in well 22/l -2A supports a wireline-log correlation with the characteristically red, calcareous unit (J1) at the base of the Jorsalfare Formation.

Depositional environment

The Flounder Formation comprises marine hemipelagic mudstones and pelagic limestones that accumulated in low-energy shelf to upper bathyal conditions. 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).

Subdivisions

King et al. (1989) informally divided the Flounder Formation into three units of pale grey mudstone with argillaceous limestone, and three intervening units of red-stained, and generally more argillaceous mudstone. On the basis of wireline-log signatures, Andrews et al. (1991) recognized four chalk-marl cycles, each up to about 100 m thick in the Flounder Formation of the Moray Firth and subdivided it into four informal units (designated F1 to F4). The Flounder Formation was not formally subdivided by Johnson & Lott (1993), although informal subdivisions are recognized and correlated locally.