Valhall 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

Cromer Knoll Group

 

Introduction

The term Valhall Formation was  introduced by Deegan & Scull (1977, p.24) in the Norwegian Central North Sea. The usage of the term Valhall Formation was redefined by Johnson & Lott (1993) for the Central and Northern North Sea areas, and extended into the southern North Sea area by Lott & Knox (1994), emphasising the continuity of the sequence over the North Sea as a whole.

The Valhall Formation (together with the Carrack Formation) replaces the term Speeton Clay Formation, introduced by Rhys (1974), for the unit of grey to black, calcareous mudstone and chalky mudstone of early Cretaceous age. The Valhall Formation therefore forms only the lower part of the Speeton Clay Formation as originally defined by Rhys (1974).

 

Name

From the Valhall Field in the Norwegian Central North Sea (Deegan & Scull, 1977, p.24).

Lithology

The Valhall Formation consists of olive-grey to dark grey to black calcareous mudstones, with occasional off-white chalky mudstones and thin limestones (Lott & Knox, 1994). Local sandstones and conglomerates also occur in the Central and Northern North Sea (Johnson & Lott, 1993).  The mudstones and chalky mudstones are soft to firm or firm to hard, blocky, locally sandy and silty, micromicaceous, glauconitic, pyritic and carbonaceous. They are generally pale to dark grey but locally black, grey-green and red-brown. Thin, but widespread units of black, laminated, non-calcareous mudstone are developed at two levels ('Munk Marl Bed' and 'Fischschiefer').  Limestones are hard to soft, microcrystalline or argillaceous and locally microlaminated or sandy; white to pale grey, but locally tan and yellow-orange or red-brown. Locally, particularly on palaeohighs, where condensation occurs, lithologies are predominantly limestones, argillaceous limestones and chalky mudstone.  Sandstones are fine- to coarse-grained, calcareous and locally pass into conglomerates (some sandstones have been given formal status: Devil's Hole Sandstone, Scapa Sandstone, Sloop Sandstone, Yawl Sandstone members). In some areas of the Southern North Sea, bentonite horizons occur (Lott & Knox, 1994).

Thickness

The maximum thickness of about 1000m is attained in the Central North Sea, although thicknesses of up to about 1800m are indicated by seismic data (Gatliff et al., 1994).  In general the formation is <100m in thickness over most of its subcrop area of the Southern North Sea (Lott & Knox, 1994). However, in rim synclines marginal to major salt diapirs and along the Dowsing Fault Zone thicknesses in the order of c.500m have been proved e.g. 53/2-5;  Cameron et al., 1992). Up to 400m of strata, bounded at base and top by unconformities, is present in the Unst Basin of the Northern North Sea (Johns & Andrews, 1985) and up to 800m in the Outer Moray Firth (Andrews et al., 1990).

Geographical distribution

The Valhall Formation is widely distributed across the UK Sector of the Central and Southern North Sea Basin  but thins, both to the north and south, onto the Mid-North Sea and Anglo-Brabant highs respectively (Lott & Knox, 1994). Eastwards the formation thickens into the Cleaver Bank Basin. Comparable strata of Valanginian to mid Barremian age are recognized in the Unst Basin of the Northern North Sea (Johns & Andrews, 1985).

Type well

Well name: N2/11-1 (Norwegian Sector)
WGS84 coordinates: Lat. 56º 14’ 17”N    Long. 03º 27’ 07”E
UTM coordinates: 6232804.37 N 528015.41 E
UTM zone: 31
Drilling operator name: Amoco Norway Oil Company
Completion date: 03.10.1969
Status: P & A
Interval of type section & thickness in type well: c. 2954 to 3539 m (9691-11610 ft) below KB (revised depths; Johnson & Lott 1993). Deegan & Scull (1977, p.24, fig. 29) placed the formation boundaries at 2910 m (9548 ft) and 3540 m (11610 ft) below KB.

UK Reference wells

Central and Northern North Sea (Johnson & Lott, 1993)

14/4-1: 1329-1754.5 m  (4360-5756ft)
14/20-8: 2771.5-3058 m (9092-10032ft)
16/12b-6: 4126.5-4367.5 m (13538-14329ft)
29/3-1: 3401-3859 m (11158-12660ft)

 Lat. 58º 59’ 52”N
Lat. 58º 24’ 41.3”N
Lat. 58º 37’ 23.076”N
Lat. 56º 50’ 22.195”N		 Long. 00º 23’ 44”W
Long. 00º 06’ 20.9”W
Long. 01º 20’ 57.813”E
Long.01º 33’ 59.090”E
Southern North Sea (Lott & Knox, 1994)
42/13-1: 839-1020 m (2752-3346ft)
48/17-1: 402-559 m (1318-1835ft)
48/22-3: 430-553.5 m (1411-1816ft)
49/25a-5: 1853-1957 m (6080-6420ft)		 Lat. 54º 36’ 54.6”E
Lat. 53º 22’ 09.7N
Lat. 53º17’ 31.3”N
Lat. 53º 13’ 34.6”N		 Long. 00º 35’ 13.8”E
Long. 01º 14’ 49.6”E
Long. 01º 15’ 58.9”E
Long. 02º 49’ 39.7”E

Upper and lower boundaries

Upper Boundary

In the Central and Northern North Sea, the upper boundary is a downward change from dark grey, non-calcareous mudstones (Carrack Formation) to chestnut brown, brick-red, greyish red and pale grey calcareous mudstones, chalky mudstones and limestones.  Red stained planktonic foraminifera are also a characteristic element of the upper Valhall Formation. 

In the Southern North Sea the upper boundary is defined by a downward change from dark grey to brown, non-calcareous mudstones (Carrack Formation) to pale to dark grey chalky mudstones (Valhall Formation).

In the Outer Moray Firth and South Viking Graben there is a downward change from sandstone (Britannia Sandstone Formation) to calcareous and chalky mudstones (Valhall Formation) (e.g. Well 15/28a-3) and in the Inner Moray Firth there is also a downward change from sandstone (Wick Sandstone Formation) to the mudstones of the Valhall Formation (e.g. wells 13/14-1, 13/17-1 and 13/24-1, Johnson & Lott, 1993).

Lower Boundary

In the Central and Northern North Sea, the base of the Valhall Formation is a sharp downward change from pale grey and grey interbedded calcareous mudstones, chalky mudstones and limestones (Valhall Formation) to dark brown-grey, olive-grey or black, variably calcareous to non-calcareous, organic-rich mudstones (Kimmeridge Clay Formation). Locally sandstones of the Valhall Formation rest directly on the Kimmeridge Clay Formation (e.g. Scapa Sandstone Member at 14/19-9 and 14/29a-2, Johnson & Lott, 1993).  On some structural highs the Valhall rests on older strata. Locally, on the margins of the Central Graben (e.g. 29/24-1), calcareous sandstones (Devil's Hole Sandstone Member) rests on less calcareous sandstone of the Fulmar Formation.

In the Southern North Sea, the lower boundary of the Valhall Formation is marked by a downward change from dark calcareous mudstones to either sandstones of the Spilsby Sandstone Formation or to harder more variably calcareous olive-grey to black mudstones of the Kimmeridge Clay Formation (Lott & Knox, 1994).

Well log characteristics

On wireline logs the upper boundary of the Valhall Formation Central and Northern North Sea is marked by a sharp downward increase in velocity and density, together with a local small downward decrease in gamma-ray values (e.g. wells 15/23-6A and 15/23a-8 Johnson & Lott, 1993). In the Southern North Sea, there is a corresponding downward decrease in gamma response and increase in velocity (Lott & Knox, 1994).

The base of the Valhall Formation corresponds with a marked downward increase in gamma-ray values and a decrease in velocity on wireline logs. In the Southern North Sea, where the formation is underlain by sandstones there is a marked downward decrease in gamma values and increase in velocity. Where the formation is underlain by mudstones of the Kimmeridge Clay Formation (e.g.  42/13-1) there is a downward increase in gamma and decrease in velocity.

Biostratigraphy

Eleven calcareous nannoplankton biomarkers have been recognized in the Valhall Formation: Nannoconus abundans, N. borealis, Stradnerlithus comptus, Tegulalithus septentrionalis, Tegulalithus septentrionalis (acme), Cruciellipsis cuvillier, Corolithion silvaradion, Eprolithus antiquus, Micrantholithus speetonensis, Sollasites arcuatus and Nannoconus sp. (discs) (Johnson & Lott, 1993; Lott & Knox, 1994). Of the foraminifera, the following biomarkers occur: Hedbergella infracretacea, Gavelinella barremiana, small Hedbergella sp. (acme), Epistomina caracolla, Gavelinella sigmoicostata, Falsogaudryinella moesiana, Ammovertella cellensis and Haplophragmoides spp. (at the base of the formation; Lott & Knox, 1994). Of the ostracods, the FDOs of the following form useful biomarkers within the formation: Protocythere intermedia, Protocythere triplicata, Paranotacythere inversa costata, Paranotacythere diglypta diglypta, Mandocythere frankei frankei, Protocythere hannoverana and Galliaecytheridea teres (Lott & Knox, 1994). Valhall Formation dinoflagellate cyst floras are abundant, well preserved and diverse. The following biomarkers are present in the formation: Ctenidodinium elegantulum, Batioladinium longicornutum, Kleithriasphaeridium corrugatum, Nematosphaeropsis scala, Lagenorhytis delicatula, Endoscrinium pharo and Dingodinium spinosum (Lott & Knox, 1994).

Age

Late Berriasian to intra Late Aptian.

Correlation

The Valhall Formation as defined by Lott & Knox (1994), equates with part of the Speeton Clay Formation of the Cleveland Basin and the lithologically more varied marine Lower Cretaceous successions of the East Midlands and north Norfolk (Rawson, 1992).

Depositional environment

The calcareous mudstones of the Valhall Formation were deposited as hemipelagic muds in an offshore, low-energy, oxygenated marine setting (Lott & Knox, 1994). The mudstones are calcareous with abundant and varied macro- and microfaunal assemblages. Occasional off-white coccolith-rich, pelagic chalks are developed at some levels. A thin basal unit of anoxic mudstones is present in a cored sequence from the Southern North Sea (Lott et al. 1989) and particularly during deposition of the Munk Marl and Fischschiefer (unit V5) (e.g. Crittenden et al. 1991; Riley et al. 1992). In the Witch Ground Graben, sandstone members, e.g. Scapa Sandstone Member mark active rifting, when submarine fan deposits accumulated in half-grabens (Harker & Chermak 1992). The Devil’s Hole Sandstone Member has been interpreted as a possible shallow marine deposit (Gatliff et al. 1994). Sporadic volcanic activity around the basin margins is apparent from the presence of thin bentonite horizons throughout the succession (Lott et al. 1985, 1986).

Distribution and thickness of the Cromer Knoll Group and main component sandstones (derived from Cameron et al., 1992, fig. 79; Gatliff et al., fig. 39; Andrews et al., 1990, fig. 34; Johnson et al., 1993, fig.47).

Subdivisions

Harker et al. (1987) and Riley et al. (1992) defined formal members within the Valhall Formation. The Scapa Sandstone Member was defined Harker et al. (1987) in the Scapa Field of the Witch Ground Graben, with the definition revised and expanded to include more extensive and coeval mass-flow sandstones and laterally equivalent conglomerates in the Witch Ground Graben (Johnson & Lott, 1993). Riley et al. (1992) correlated thinly laminated, organic-rich mudstones of intra-early Aptian age in the North Sea Graben with similar, possibly coeval deposits of the ‘Fisch-Schiefer’ in the Lower Saxony Basin and gave it formal status as the Fischschiefer Member, used informally by Johnson & Lott (1993).
In the Central and Northern North Sea the formation has been divided into seven informal units (V1 to V7) based upon wireline-log responses that can be correlated across a wide area of the Central North Sea and South Viking Graben (e.g. Bisewski 1990, Crittenden et al. 1991) and fully described by Johnson & Lott (1993). Thicker, more widespread sandstone bodies have been defined as four formal members (Johnson & Lott, 1993): Devil’s Hole Sandstone, Scapa Sandstone, Sloop Sandstone and Yawl Sandstone members. Gatliff et al. (1994) recognized the Devil’s Hole Sandstone as a formation.

In the Southern North Sea, no formal subdivision of the Valhall Formation was presented by Lott & Knox (1994). However, the presence towards the top of the formation, of a prominent, thin, low gamma/high velocity chalky unit, beneath which a marked gamma spike may sometimes be developed (e.g.  49/25a-5; 44/24-1) suggests a possible correlation with the V6 and V5 ('Fischschiefer') units respectively, recognized more consistently in the Valhall Formation sequences of the Central North Sea area. Other chalky developments also occur, notably the thin unit at the base of the formation (?V1 equivalent), but are not formally named.