lithostratigraphy using wireline logs and lithology of the early to middle Cretacoue of the Central North Sea. calibrated with biostratigraphy

1. THE EARLYTO “MIDDLE” CRETACEOUS
LITHOSTRATIGRAPHYOF THE CENTRAL NORTH SEA
(UK SECTOR)
S. Crittenden*, J. Cole* and C. Harlow*
A revised lilhostratigraphical framework of the Early to “Middle” Cretaceous strata of the
Central North Sea (UK sector) derived from the rigorous examination of more than 100 commercial
boreholes is presented. Previously published frameworks have tended toward oversimplification of
the Early to “Middle” Cretaceous as the interval was considered initially to be of minor importance
with regard to prospectivity. However, there is a general feeling in the industry that the Early to
“Middle” Cretaceous is prospective, particularly with regard to subtle stratigraphical traps like the
Kilda/Bosun and Scapa fields. It is inevitable that as more attention, due to hydrocarbon exploration,
is paid to an interval that the stratigraphical models used progress from the simplistic to the complex.
The framework presented here is the result of a pragmatic approach in that it is based upon the
integration of previously published schemes, with the erection of new units derived from new analyses
and interpretation. The complete lithostratigraphical framework is defined, explained and discussed
with reference to borehole examples in the Central North Sea.
INTRODUCTION
The lithostratigraphical model described in this paper is based upon the interpretation of more
than 100 released wells from the Inner and Outer Moray Firth (Fig.1) of the Central North Sea
(UK Quadrants 12, 13, 14, 15, 16, 20, 21, 22). A detailed lithostratigraphical and biostratigraphical
interpretation of these wells (Early to “Middle” Cretaceous) is included in two non-exclusive oil
industry reports (Crittenden et al., 1989, 1990).
Despite the acknowledged prospectivity of the interval, to date there is no detailed complete
lithostratigraphy published for the Early to “Middle” Cretaceous (Ryazanian to Turonian) of the
Central North Sea (UK sector). Previous subdivisions have dealt with small areas within the region,
such as the Witch Ground Graben (Harker et al., 1987) and the Buchan Horst (Burnhill and Ramsay,
1981), where the Early and “Middle” Cretaceous stratigraphy has been discussed in the context of
basin or area evolution, but have not covered the whole interval, although some units have been
formally defined.
Journal of Petroleum Geology, vol. 14 (4), October 1991, pp. 387-416 387
* Geolink UK Ltd, Bridge of Don, Aberdeen AB2 8EW

2. The exploration for subtle stratigraphical hydrocarbon traps in the Early to “Middle” Cretaceous
of the Inner and Outer Moray Firth area of the Central North Sea requires a detailed stratigraphical
model. Such a model compiled from an abundance of well data can be integrated with seismic
sequence stratigraphy in order to define prospective areas of sand-prone facies development. The
lithostratigraphical scheme proposed here (Fig. 2) is a first but important step toward a detailed
model. Seismic data is available in the literature which supports the proposed subdivision of the
sequence (e.g. O’Driscoll et al., 1991).
The basic nomenclature used in this paper is based upon that established for the southern North
Sea (Rhys, 1974) and Central North Sea (Deegan and Scull, 1977). Other more recent revisions of
that scheme have been referred to, and in most instances are applicable to the Central North Sea area
(e.g. Burnhill and Ramsay, 1981; Hesjedal, 1981; Crittenden, 1982a, 1987a,b; Hesjedal and Hamar,
1983; Hamar et al., 1983; Vollset and Dore, 1984; Jensen et al., 1986). Where appropriate, such
revisions are discussed in this paper. Other units used in this study are at present informal
lithostratigraphical units recognised and used by the present Authors, which have good correlation
potential in the Central North Sea area.
The lithostratigraphical framework proposed in this paper has been integrated with a
biostratigraphical zonation scheme formulated by the present Authors. The biostratigraphical zonation
scheme (foraminifera and palynomorphs) will be the subject of a subsequent paper (Harlow et al., in
prep.). The age range assigned to the lithostratigraphical units discussed in the following sections of
this paper has been derived from that work. For this reason, it is considered unnecessary to include
here extensive details of the biozonation schemes and faunas and floras recovered from the
lithostratigraphical units, and it thus follows the format used in other lithostratigraphical papers (e.g.
NAM and RGD, 1980). However, for comparisons to be made with published schemes, the
palynofloral data is related to the biozones mentioned in Rawson and Riley ( 1982).
The lithostratigraphical subdivision of the Early to “Middle” Cretaceous strata of the Central
North Sea area studied can be substantiated by other workers by applying the defined unit tops to
other wells in the region. The age relationships can then be confirmed by subsequent
biostratigraphical analysis.
For example, published/released biostratigraphical and lithological well data has been re-
evaluated in the light of our stratigraphical models, to provide a consistent interpretation of a whole
suite of wells useful for exploration purposes. This is illustrated by the re-evaluation of the data of
Burnhill and Ramsay (1981) and is displayed as Fig. 3.
The Early to “Middle” Cretaceous (Ryazanian to Turonian) was, in NW Europe, a time of gradual
and sustained eustatic sea-level rise, the effects of which were, in the Moray Firth region, strongly
overprinted by the effects of relative rise and fall of sea-level due to localised tectonic activity. On
this basis, the studied interval can be divided broadly as follows:
1. Latest Ryazanian to early Aptian: Valhall Formation, generally a transgressive sequence of
limestones, marls and claystones and interbedded coarse clastics. Thick clastic units such as the
Scapa Sand Member and equivalents are a result of turbidite/ submarine fan deposition derived
from structurally-higher areas, and were emplaced as a result of tectonic activity (localised
relative falls in sea level).
2. Aptian to early Albian: Sola Formation and the Bosun Sand Member (and equivalents), generally
a transgressive sequence of dark claystones and marls and interbedded coarse clastics. The sands
generally replace the lower part of the Sola Formation and represent turbidite/submarine sand fans
emplaced as a result of tectonic activity (Austrian) and relative regression.
Early-”Middle” Cretaceous lithostratigraphy, UK North Sea388

4. 3. Middle Albian to Turonian: Rodby, Hidra, Plenus Marl and Herring Formations. Generally, an
increase in the relative rate of transgression, associated with the waning tectonic activity and the
continuing eustatic sea-level rise, resulted in an upsection decrease in supply of clastic material
and an increase in pelagic carbonate deposition. The inundation and peneplanation of the intra-
basin highs and basin margins resulted in the establishment of an extensive shelf sea with good
open-ocean connections to the Tethys in the south, the Boreal ocean in the north, and to the proto-
North Atlantic.
LITHOSTRATIGRAPHICAL FRAMEWORK
Chalk Group (Pars)
Author: Rhys, 1974.
Regional Reference: Deegan and Scull, 1977.
The Herring, Plenus Marl and Hidra Formations, which comprise the lower part of the Chalk
Group, have been formally recognised in the literature. Each are discussed in turn:
Herring Formation
Lithology: Limestone, white to pale-grey, dense with interbeds of claystone, varicoloured, sub-fissile,
calcareous, tuffaceous in part.
Age: Regional data suggests that this formation is intra-Turonian in age.
Author: Deegan and Scull, 1977.
Regional Reference: Burnhill and Ramsay, 1981. Type UK well-section 22/1-2a (Fig. 4).
Comments: The upper boundary of this unit (base Flounder Formation) is marked by a good gamma-
ray and sonic log break. The lower boundary is often difficult to place with precision, as the lower G1
unit may grade into the upper, “B” unit of the Plenus Marl Formation. Based on wireline log character
(gamma-ray and sonic logs), Burnhill and Ramsay (op. cit.) subdivided this formation into four
informal units (G4 - G1). These units are correlatable across the region. However, unit G3 is thin and
often difficult to recognise.
1. G4 - G3: The top is defined by a downhole reduction in the gamma and sonic log values. The unit
is characterised by relatively clean limestones displaying a relatively low and constant gamma and
sonic log profile. Tuffaceous claystones have been recorded in this interval in some wells in the
region. This unit is late to early Turonian in age.
2. G2: The top of this unit is defined by a downhole increase in gamma and sonic log response. The
log profile through the unit is a pronounced saw-tooth pattern, which is a reflection of the
alternation of pure limestones and thin, argillaceous interbeds. Tuffaceous material has been
recorded in this interval in some of the wells in the region. This unit is early Turonian in age.
3. G1: The top of this unit is defined by a downhole decrease in gamma and sonic log response. The
unit as a whole depicts a steady downhole increase in both gamma and sonic log values. A saw-
tooth pattern is still evident, although not so pronounced as in the overlying G2 unit. This unit is
early Turonian in age.
The magnitude of the stratigraphical hiatus associated with the mid-Cretaceous disconformity of
Burnhill and Ramsay (1981) varies with structural location. It is associated with structural movements
and continued marine transgression, and in basinal locations a small hiatus is postulated at the base of
the G2 unit.
The thin, argillaceous interbeds noted in this formation are in some instances laterally quite
persistent, and can be correlated over a large area. For example, these interbeds in the G2
unit (Fig. 4) are important for correlation purposes. It may well be that these interbeds are
a product of the degeneration of volcaniclastic material (Leary and Wray,
Early-”Middle” Cretaceous lithostratigraphy, UK North Sea390

6. 1989). Postulating a volcanic source for these claystone interbeds would explain their lateral
persistence, as they are discernible in wells over the whole of the Central North Sea region. Indeed,
Deegan and Scull (1977) postulate the same origin for the dark claystones of the Plenus Marl
Formation.
Foraminiferal faunas recovered are sparse (the effects of diagenetic destruction, hard lithology and
diamond-bit drilling) but include specimens of the benthonic taxon Stensioeina. For example
S. granulata humilis ranges no higher than earliest middle Turonian. Planktonic foraminifera are more
common, and include Dicarinella hagni, Praeglobotruncana stephani, P. gibba, Whiteinella spp. and
Marginotruncana pseudolinneiana, which confirm the Turonian age.
Plenus Marl Formation
Lithology: Claystone, dark-grey to light-grey, red and dark-green mottling is occasionally present,
subfissile to fissile, hard, often pyritic, glauconitic, non-calcareous, tuffaceous, grading up-hole to
light-grey argillaceous limestone.
Age: It is common practice for stratigraphers working on North Sea sections to take a pragmatic
stance and to assign an early Turonian age to the Plenus Marl Formation, rather than late Cenomanian
to Turonian as proposed by Deegan and Scull (1977).
Author: Deegan and Scull (1977).
Regional Reference: Burnhill and Ramsay, 1981; type UK well section 22/1- 2a (see Fig. 4).
Comments: The top boundary of this formation is placed at a downhole increase in both gamma and
sonic log values. The values of both traces increase gradually downhole through the formation, and
form a characteristic triangular profile reflecting a downhole transition from argillaceous limestones
to dark, non-calcareous claystones. The upper boundary may be difficult to place in some instances,
because of the gradational contact with the overlying G1 unit of the overlying Herring Formation.
However, in some wells, it is an unconformable contact. The lower boundary of the Plenus Marl
Formation is in most cases easy to place, as it is represented by a sharp reduction in gamma-ray and
sonic log response which is easily recognised.
In the Central North Sea, the Plenus Marl Formation is often divisible into two facies-types as
follows.
1. “B” unit: This unit comprises calcareous claystones usually with abundant planktonic
foraminifera. The wireline log profile displays a saw-tooth pattern, but with an overall increase in
gamma-ray and sonic log response.
2. “A” unit: This unit comprises dark-grey to black, essentially non-calcareous, pyritic claystones
which are usually barren of calcareous microfauna, although rare agglutinating foraminifera and
radiolaria are often recorded. The wireline log response for this unit displays a spike of very high
gamma-ray and sonic log values.
The Plenus Marl Formation is an easily-recognised formation and because of its distinctive
wireline log trace, particularly for the dark, basal claystone horizon (very high gamma and interval
transit times), provides a marker horizon over a wide area. It is commonly used as a correlation datum
throughout the North Sea area (Carter and Hart, 1977; Crittenden, 1982a; Hart and Ball, 1986).
In southern Britain, according to recent work by Hart and colleagues (pers. comm.), the
Turonian/Cenomanian boundary occurs above the Actinocamax plenus Marl (Jeffries, 1962, 1963).
Onshore UK, the base of the A. plenus Marl is marked by a well-developed erosion surface known in
the literature as the sub-Plenus erosion surface (Robinson, 1986). The A. plenus Marl as defined
onshore southern Britain comprises eight distinct beds (numbered 1 - 8 in ascending order). Above
this, in the overlying very marly chalks, there are a number of argillaceous beds.
The exact relationship between the Plenus Marl Formation as defined in the North Sea by
Deegan and Scull (1977) and the A. plenus Marl as defined in southern Britain is very
uncertain. This has important implications for any discussion attempting comparisons
Early-”Middle” Cretaceous lithostratigraphy, UK North Sea392

8. between the two areas in terms of formational boundaries and age boundaries. In fact, Robinson
(1986) should not have formally redefined the A. plenus Marl in southern Britain, as the Plenus Marl
Formation, without clarification of the precise age-relationships of the two units.
The exact position of the Turonian/Cenomanian boundary and its relationship to the Plenus Marl
Formation/Hidra Formation contact in the Moray Firth region of the North Sea is also uncertain.
However, a pragmatic approach is taken as the available evidence from the examination of ditch
cuttings and side-wall cores indicates an earliest Turonian age for the unit offshore. The top
occurrence in the study region of the genus Rotalipora, which world-wide is within the late
Cenomanian, occurs in the underlying Hidra Formation and not in the Plenus Marl Formation.
Foraminiferal faunas recovered include non age-diagnostic agglutinating taxa and, from the more
calcareous beds, planktonic foraminiferal taxa similar to those encountered in the Herring Formation.
For a fuller discussion of the Turonian/Cenomanian boundary, reference should be made to the
publications of Hart et al. (1989), Leary et al. (1989) and Hart and Ball (1986). Tuffaceous material
has been recorded in wells in the region.
Hidra Formation
Lithology: Limestone, white to light-grey with occasional pink coloration, firm to hard, argillaceous
grading to calcareous claystone. Tuffaceous material has been recorded from this interval in wells in
the region.
Age: Cenomanian.
Author: Deegan and Scull (1977).
Regional Reference: Burnhill and Ramsay (1981). Reference well-section: UK well 22/1-2a
(see Fig. 4).
Comments: This formation is regionally of Cenomanian age, and is subdivisible into informal
wireline log units based upon the fluctuations of the argillaceous content of an otherwise pelagic
carbonate lithology. The absence in some areas of the basal units of the formation is related to non-
deposition and/or condensation on topographically high areas, and may be a function of the “mid”-
Cenomanian discontinuity (Carter and Hart, 1977). The upper boundary is marked by a sharp and
distinctive decrease in gamma response. The lower boundary is the contact with the underlying
Rodby Formation of the Cromer Knoll Group. Based on wireline log character (gamma and sonic),
this unit was subdivided by Burnhill and Ramsay (1981) into four informal units, H4 - H1. Their
subdivision is followed in this paper and is as follows:
1. H4: The top of this very thin unit (late Cenomanian in age) is placed at a sharp decrease in gamma
and sonic log response. The log profile through this unit shows a very marked decreasing trend in
the sonic and gamma log readings. This unit is usually thin, and comprises dense, pure limestones.
2. H3: The unit (late to middle Cenomanian in age) displays as a whole a gentle downhole increase in
gamma-ray and sonic log values. The top of this unit is placed at the top of this trend. The overall
saw-tooth pattern reflects the interbedded nature of the argillaceous limestones and marls which
comprise this unit.
3. H2: The top of this unit (middle Cenomanian in age) is picked at a marked decrease in gamma and
sonic log values. The unit as a whole represents an interval of cleaner limestones, and is
characterised by relatively uniform gamma and sonic log profiles.
4. H1: The top of this unit (early Cenomanian in age) is placed at a downhole increase in gamma and
sonic log values. The unit in general displays a distinct downhole increasing trend of values for the
gamma and sonic log response. At the base of this unit, a distinctive, thin limestone bed, or beds,
is usually present. This is shown by distinct gamma and sonic log lows. In some wells/areas, the
H1 unit is thinly-developed or condensed, while in others it contains a high percentage of
glauconite.
Early-”Middle” Cretaceous lithostratigraphy, UK North Sea394

10. Foraminiferal faunas recovered from the upper part of the Hidra Formation are dominated by
planktonic taxa, while the lower part is dominated by benthonic taxa. The faunal turnover boundary is
difficult to place, but may be equatable with the “mid”Cenomanian event of Carter and Hart (1977). It
is placed at the proximity of the H3/H2 unit boundary. The faunas include a number of age-diagnostic
species, which enables a biozonal subdivision (Fig. 3) to be made (see Harlow et al., in prep.). In the
upper part of the formation, deeper-water, keeled planktonic foraminifera are also recorded,
indicating good open-ocean connections as a result of relative high sea-levels. The benthonic
foraminiferal faunas display a gradual increase in abundance and diversity downhole, and include
Gavelinella ex. gr. intermedia, G. cenomanica, Arenobulimina advena, Eggerellina mariae and
Textularia sp.1, all of which top in the Cenomanian. The small benthonic foraminifera Stensioeina
pokornyi usually ranges no higher than the top of the Hidra Formation, and its downhole
disappearance coincides with the topmost occurrence of the benthonic foraminifera Lingulogavellina
ciryi inflata in the middle Cenomanian.
Cromer Knoll Group
Author: Rhys, 1974.
Regional Reference: Deegan and Scull, 1977. The type area is the southern North Sea Basin, and
Rhys (1974) designated UK well 8/22-2 as a typical section. The Cromer Knoll Group is,
pragmatically, entirely Early Cretaceous in age. For further discussion, reference should be made-to
Crittenden (1982a).
In the Inner and Outer Moray Firth area, a number of formations and members are formally
recognised. These are:
the Rodby Formation;
the Sola Formation;
the Valhall Formation;
the Fischschiefer Member (Riley et al., in press);
the Scapa Sand Member;
the Devil’s Hole Formation.
Other lithostratigraphical schemes and units are in use elsewhere in the North Sea region, and the
nomenclature has been applied by some operators to what are regarded as coeval units in the Central
North Sea (UK sector). For example, reference has been made on Department of Energy released well
composite logs to the Kopervik Formation (Hesjedal and Hamar, 1983) in describing Aptian-age
sands in the area. This term should be restricted to the type area in the Norwegian sector of the North
Sea. In the southern Witch Ground Graben, the present Authors (Crittenden et al., 1989, 1990) refer
coeval sands to the informally-named Bosun “Formation”, although the nomenclature Bosun Sand
Member of the Sola Formation is more accurate. A further example is the use of the term Devil’s
Hole Formation for sands in the Inner and Outer Moray Firth area of Neocomian to ?Late Jurassic age
at the base of the Valhall Formation. This term should be restricted to the type area of block 29/25.
There is a proliferation of informally-designated lithostratigraphical nomenclature applied by various
operators to various subunits recognised in the Cromer Knoll Group of the Inner and Outer Moray
Firth (e.g. Shirley Sands, Asgard Sands, Glenn Sands, etc.). Other lithostratigraphical terms which are
perfectly valid, in that they have been formally designated, for the area outside the Central North Sea
(UK sector) such as the Utvik, Floro and Klepp Formations of Hesjedal and Hamar (1983), have not
been referred to in this paper.
Early-”Middle” Cretaceous lithostratigraphy, UK North Sea396

12. Rodby Formation
Lithology: Claystone, varicoloured red, light-grey, dark-grey and brown, calcareous, silty,
tuffaceous, with interbeds of argillaceous limestone and sand. There is a progressive upwards increase
in the overall calcareous content of this formation.
Age: Late to middle Albian.
Author: Larsen, 1966.
Regional Reference: Deegan and Scull, 1977.
Comments: This formation is probably equivalent in part to the Red Chalk Formation (Rhys, 1974)
and Upper Holland Marl Member of the Holland Formation (NAM and RGD, 1980) of the southern
North Sea (Crittenden, 1982a), although the base of the formation when considered over the whole of
the North Sea is undoubtedly diachronous. In complete and thick sequences in the Central North Sea,
the Rodby Formation is divisible into three, broad, informal units on the basis of wireline log
response character and lithological variation (Figs. 4 and 5). The three informal units
(see well 15/30-2) proposed are as follows:
1. R3: The top of this upper calcareous unit of late Albian age is picked on wireline logs at a well-
defined shoulder, where the gamma and sonic log response values increase. This reflects the
change from the Hidra Formation to Rodby Formation claystones. The unit is characterised
overall by a variable wireline log profile, which in general portrays a downhole increase in gamma
and sonic log values intermediate between the H1 unit above and the R2 unit below. At the base
of this unit, there is usually a thin limestone bed which is distinguishable on the wireline log
profile by a reduced gamma-ray and sonic log value. The R3 unit has to be identified with care, as
although the gamma and sonic log values are higher as a whole than in the overlying Hidra
Formation, the log profile can be confused with the H1 unit, especially if there is a limestone bed
at the base of the R3 unit. Correct identification should be confirmed by biostratigraphical data, as
is seen in Fig. 3 (in addition, see Crittenden, 1988; and Harlow et al., in prep.).
2. R2: The top of this middle argillaceous unit of earliest late to middle Albian age is picked at a
marked downhole increase in both gamma and sonic log values. This unit is more argillaceous
than both the R1 and R3 units, and displays generally higher gamma and sonic log values. The
usually abrupt wireline and lithological break comprising the R3/R2 boundary interpreted with
biostratigraphical data (see Crittenden, 1988; Harlow et al. in prep.), while not indicating a hiatus,
does suggest a strong transgressive pulse at this time (Crittenden, 1982a), perhaps equatable with
the intra-late Albian (dispar Biozone) transgressive pulse recognised onshore NW Europe.
Throughout this unit, the log values initially increase and then decrease, giving rise to a gentle
“waist” pattern profile.
3. R1: The top of this more calcareous unit of earliest middle Albian age is picked at a pronounced
downhole decrease in gamma and sonic log values. The log profile for this interval shows an
increase in the degree of fluctuation of the gamma and sonic log values as compared to the
overlying unit. This saw-tooth response is due to the repetition of calcareous claystones and
argillaceous limestone interbeds. In some areas of the North Sea, this transgressive basal unit of
the Rodby Formation is referred to as the basal limestone unit. It is recognised by Deegan and
Scull (1977), and indeed a similar unit can be identified at the base of the Red Chalk Formation in
wells in the southern North Sea (Crittenden, 1982a and 1988).
This formation contains rich and diverse palynofloras (mainly dinocysts) and planktonic and
benthonic foraminiferal faunas, which, by comparison with sequences onshore, provide a reliable
means of dating the sequence.
The planktonic foraminiferal fauna recovered from the Rodby Formation is dominated
by the Hedbergella brittonensis—H. delrioensis—H. infracretacea plexus. Other species
Early-”Middle” Cretaceous lithostratigraphy, UK North Sea398

13. present include H. planispira, H. simplex and Globigerinelloides bentonensis. Influxes of abundance
of the planktonic fauna are of local correlative use. For example, a minor influx of H. planispira is
frequently seen towards the top of the formation. A major influx of the same species defines an
interval within the late Albian (R3/R2 unit boundary event), and is associated with an increase in the
abundance of G. bentonensis. By comparison with studied sections onshore the UK (Harris, 1982)
and elsewhere in NW Europe (Price, 1977; Crittenden, 1988), this is an intra-late Albian event.
The diversity and abundance of the planktonic foraminiferal faunas increase “up-hole” through the
Rodby Formation, reflecting the continued (eustatic) marine transgression, the increase in carbonate
content of the formation, and the increase in open-oceanic influences.
Within the middle part of the Rodby Formation (R2 unit), specimens of planktonic foraminifera
are frequently red- or green-stained. Red staining is commonly encountered in the genus Gubkinella,
a planktonic genus which characterises the middle Albian (top occurrence is intra-R2 unit).
The benthonic foraminiferal fauna recovered from the Rodby Formation increases in diversity and
abundance downhole. The uppermost part of the formation (R3 unit) contains a number of
taxa, which range no higher than the Albian. These include Osangularia schloenbachi
(usually small, smooth variants) and the calcareous cement agglutinant Arenobulimina
chapmani. Quinqueloculina antiqua occurs commonly in this formation, particularly
towards the top, but it is also recorded (rarely) in the lower part of the overlying Hidra
Formation. The R3 unit is usually dominated by the planktonic foraminiferal fauna. The R3 unit
foraminiferal fauna is a typical outer-shelf biofacies.
The top of the argillaceous R2 unit is associated with a downhole increase in benthonic
foraminiferal faunal abundance and diversity. This lithological and faunal boundary can be very
pronounced, and is characterised by a distinctive dark-red staining of the fauna, although the fauna
within this unit is commonly stained green.
The top of the middle Albian (intra-R2 unit) is taken at the topmost occurrence of
Uvigerinammina sp.1 (Burnhill and Ramsay, 1981) (U. bulimoides of other Authors), and is a
valuable correlatable datum. Most commercial stratigraphical laboratories refer to the genus
Uvigerinammina, but recent research has shown that the assignation of this foraminifera should be to
the genus Falsogaudryinella (see Crittenden, 1988 for discussion). In addition, many stratigraphers
working on material from the North Sea regard the R3/R2 contact as defined in this study as the
Rodby/Sola contact (for example King et al., 1989).
Large, robust and very ornate specimens of the benthonic foraminifera Osangularia schloenbachi
usually occur in the middle part of the Rodby Formation (R2 unit) accompanied by green-stained
agglutinating foraminifera (Crittenden, 1983), including Recurvoides spp., Haplophragmoides spp.,
Trochammina spp. and Glomospira spp.
The calcareous benthonic fauna increases in relative abundance downhole through the R3 unit and
upper part of the R2 unit, particularly the genera Gavelinella and Valvulineria.
The top occurrence of the distinctive foraminifera Lingulogavelinella gyroidinaeformis in the
basal part of the R1 unit of the Rodby Formation is an important regional marker recognised in the
Central (Burnhill and Ramsay, 1981) and Southern North Sea (Crittenden, 1988). Previous
Authors have related this occurrence to the middle/early Albian boundary which was
thought to coincide with the Rodby/Sola Formation contact. This view was prevalent for
Central and Southern North Sea sections (Crittenden, 1988). Regional data and further
research now suggests that the species “tops” in the basal part of the Rodby Formation
(intra-R1 unit), and the range of L. gyroidinaeformis spans the Rodby/Sola Formation
contact. Where this species is recorded in the basal part of the R1 unit (limestone bed), it
is invariably stained a light-red/tan colour. Elsewhere in Europe, the range of this species has been
documented as middle to early Albian, with the highest occurrence at the top of the Hoplites dentatus
ammonite Biozone, the initial phase of the middle Albian transgression,
S. Crittenden et al. 399

14. The Rodby Formation broadly coincides with the dinoflagellate Subzones OIIC-OIIA. The
downhole appearance of the dinocyst Ovoidinium (Ascodinium) scabrosum marks the top of the
Albian, and the top of the Ovoidinium verrucosum subzone (OIIA) of latest late Albian age, at the top
of the R3 unit. Dinocyst assemblages within this Subzone are typically dominated by Spiniferites
spp., Oligosphaeridium complex, Hexagonifera chlamydata, and Odontochitina operculata. Other
important taxa that may be locally common are Pseudoceratium turneri, Ovoidinium (Ascodinium)
verrucosum and Litosphaeridium siphonophorum. Generally corresponding to the basal part of the R3
unit is the Apteodinium grande Subzone (OIIB) of late Albian age, defined by the downhole
appearance of A. grande. Assemblages are generally dominated by Oligosphaeridium complex,
Spiniferites spp., Hystrichodinium pulchrum, Cyclonephelium hystrix and Cribroperidinium
edwardsii, with locally-common Cleistosphaeridium ancoriferum and Xiphophoridium alatum.
Important index taxa that appear downhole within the zone include Litosphaeridium arundum and
L. conispinum.
Units R2 and R3 are generally encompassed by the Systematophora cretacea (OIIC) Subzone of
middle to late Albian age, marked by the downhole appearance of “ratty”Cribroperidinium(large robust taxon
with cones). The dinocyst assemblage otherwise remains dominated by taxa similar to the zone above, with the
addition of locally common Ellipsoidictyum imperfectum (circular forms) and Stephodinium
“fenestratum”. Dinocyst taxa having their first downhole appearance within the zone include
Kiokansium prolatum, Leptodinium cancellatum, Muderongia pariata and Systematophora cretacea.
Sola Formation
Lithology: Claystone, dark-grey to black, green-grey, occasional red/brown, carbonaceous in part,
non- to slightly calcareous, tuffaceous, with occasional thin, argillaceous limestone interbeds. A
distinctive siderite horizon is frequently present within the middle part of the Sola Formation (S2 unit
herein), and has been used by many stratigraphers working in the North Sea as a pragmatic early
Albian/late Aptian marker.
Age: Early Albian to late Aptian.
Author: Hesjedal (1981), Hesjedal and Hamar (1983).
Regional Reference: UK well-section 15/23-1 (per Hesjedal and Hamar, 1981).
Comments: The top of this formation is marked by a downhole increase in both gamma and sonic
log response. The boundary between the Rodby and Sola formations is a major lithological and
microfaunal event (see Harlow et al., in prep.).
In complete and thickly-developed sequences, the Sola Formation is subdivided informally into
three wireline log units designated S3, S2 and S1 (well 14/19-5, Fig. 6). The S1 and S2 units are
replaced in some areas by a well-developed sand interval—the Bosun Sand Member and regional
equivalents.
1. S3: The top of this unit is picked at a sharp increase in gamma-ray and sonic log values. The trace
through the unit is generally stable.
2. S2: This is a unit of consistently higher gamma-ray and sonic log values. The wireline log profile
through this unit is characterised by pronounced sonic and resistivity spikes. These are thought to
represent tuffaceous horizons.
3. S1: This unit is characterised by overall slower gamma-ray and lower sonic log responses. The
wireline log profile displays a saw-tooth pattern through the unit.
The Sola Formation was defined, with reference to a type-section, by Hesjedal in an oral
presentation (1981) but was not formally published.
Published reference was subsequently made by Hesjedal and Hamar (1983) to the oral
presentation of Hesjedal (1981), which was cited as the type reference. This is an improper procedure
and, strictly speaking, is an invalid citation for the Sola Formation type designation, even though the
oral presentation of Hesjedal is available as a Statoil Internal Report.
Early-”Middle” Cretaceous lithostratigraphy, UK North Sea400

16. Jensen et al. (1986) later published a formal description of the Sola Formation and defined a type-
section in the Danish sector of the North Sea. That reference, strictly speaking, should take
priority, although Jensen et al. (1986) did refer to the oral presentation of Hesjedal (1981).
To further complicate matters, the Jensen et al. (1986) concept of the formation differs
from that originally proposed by Hesjedal (1981) and Hesjedal and Hamar (1983). Jensen
et al. (1986) regard it as middle/late Barremian to Albian in age rather than mid-Aptian to
mid- Albian and included the upper part of the Valhall Formation (as defined by Hesjedal and
Hamar) within it.
The original concept of the Sola Formation as defined by Hesjedal and Hamar ( 1983) is used in
this study, and Jensen et al.’s concept is not accepted. The reasons for this are three-fold:
1. to prevent unnecessary proliferation of terminology;
2. the concept as defined orally is entrenched in subsequent literature;
3. the formation as originally defined (however improperly) is geologically useful and accepted by
many explorationists working in the Central North Sea (e.g. Harker et al., 1987).
Within the middle part of the Sola Formation (S2 unit), tuffaceous horizons have been recorded
which probably correspond to marked sonic and gamma-log spikes. These volcanic horizons may be a
result of:
1. the intra late Aptian nutfieldensis/nolani Biozone volcanic activity referred to by Zimmerle (1979),
and therefore related to the tuffs/bentonite horizons of the Sandgate Beds in southern England;
2. the basal Albian or Aptian/Albian boundary tuff as identified by Zimmerle (e.g. 1979).
The lateral persistence of these tuffs needs to be investigated in detail, as they may provide good
correlatable horizons in the region. Initial research by the present Authors is encouraging. The
boundary between the Rodby and Sola formations is a major lithological and microfaunal
event. The top of the Sola Formation is characterised by the occurrence of an abundant,
agglutinated foraminiferal assemblage including Recurvoides spp., Bathysiphon spp.,
Glomospirella/Glomospira spp., Ammodiscus spp., Haplophragmoides spp., Pelosina spp.,
Reophax minuta and rare Textularia bettenstaedti The fauna is usually stained green, but
may be often stained dark-red where the formation contact is unconformable. This faunal
assemblage is similar to that recorded in the R2 unit of the overlying Rodby Formation,
but differs in being more diverse. It is a restricted-basin biofacies, reflecting both water
depth and oxygen depletion in the marine environment.
Green-stained, and occasional red-stained, benthonic foraminiferal assemblages, dominated by
agglutinating species, characterise the Sola Formation.
Calcareous benthonic and planktonic foraminifera in the Sola Formation are rare or
absent, but when present are usually green-stained (Gavelinella ex. gr. intermedia,
Valvulineria gracillima and Hedbergella spp.). These assemblages are obviously facies-
controlled, and have been observed in early Albian and late Aptian sequences throughout
NW Europe (Crittenden, 1988).
The foraminifera L. gyroidinaeformis is recorded from the upper part of the Sola
Formation, and is usually stained green.
Sideritic/dolomitic limestone has been recorded on the composite logs in the upper part
of the Sola Formation, and is often associated with the first downhole occurrence of the
agglutinated foraminifera Verneuilinoides ex. gr. neocomiensis-subfiliformis (V. chapmani
of various Authors) and with an increase in the proportion of coarser clastic material. The
significance of the presence of siderite is not clear. In this area, it may be related to the
transgressive early Albian (tardefurcata Biozone) event.
In complete sequences, the Albian/Aptian boundary occurs within the upper part of
the Sola Formation, but the top of the Aptian is difficult to determine with precision by
micropalaeontology (Crittenden, 1987, 1988). In previous reports, stratigraphers have
Early-”Middle” Cretaceous lithostratigraphy, UK North Sea402

17. regarded the top occurrence of siderite (when recorded) and the top occurrence of V. neocomiensis-
subfiliformis as a pragmatic top-Aptian marker. The lowest occurrence of L. gyroidinaeformis is
regarded as coincident with the Albian/Aptian boundary (van Hinte, 1977), but exact placement is
difficult with ditch-cuttings material.
In the lower part of a complete Sola Formation, horizons of calcareous benthonic (including
Gavelinella cf: brielensis and G. rudis) and planktonic foraminifera (including Hedbergella
infracretacea and Globigerinelloides spp.) occur at sporadic intervals, are often green-stained, and in
some cases provide intra-late Aptian correlatable horizons. The influxes of calcareous benthonic and planktonic
foraminifera in a sequence dominated by agglutinating foraminifera suggests that periodic influxes of well-aerated
surface and bottom waters affected the region. Such open-ocean influences and faunal turnover may be related to
the onset of a regional, marine transgression at this time (nutfeldensis Biozone) associated with regional tectonic
activity. The presence of volcanic-derived sediments attests to this tectonic activity.
The Sola Formation broadly coincides with dinoflagellate Subzones OIID-OIB (pars) of general
late Aptian to early Albian age. The top of the OIID Subzone is marked by the downhole appearance
of the important dinocyst marker Cauca parva, together with impersistent Kleithrisphaeridium
simplicispinum. Dinoflagellate assemblages are generally rich and diverse, with a dominance of
Oligosphaeridium complex, Spiniferites spp., Hystrichodinium pulchrum, Cyclonephelium hystrix and
Odontochitina operculata. Significant influxes of Ellipsoidictyum imperfectum (circular form,
followed lower-down by the oblong cigar-shaped form) and Leptodinium cancellatum, as well as the
marker Cauca parva, occur within the Subzone. The top-Aptian Dingodinium albertii Zone (OI) is
indicated by the appearance of the marker Dingodinium albertii, together with Cerbia tabulata and
increased numbers of Kleithriasphaeridium simplicispinum. An upper Aptea polymorpha Subzone
(OIA) may be distinguished by a large influx of Ellipsoidictyum imperfectum (cigar-shaped), from a
lower Cyclonephelium tabulatum (OIB) Subzone.
Bosun “Formation”
Lithology: Sandstone, white-to-grey, fine- to medium-grained, poorly-sorted, rarely coarse-grained,
angular to subrounded, glauconitic, hard-to-friable, calcareous cement in part. Interbedded claystone,
dark-grey to light-grey, occasionally red/brown, non- to slightly calcareous, silty, occasionally
tuffaceous.
Age: Early?-late Aptian to ?early Albian.
Author: The name Bosun “Formation” has not been formally published.
Regional Reference: There is no formal regional reference to the Bosun “Formation” although the
type area (quadrants 15/29, 15/30, 16/26, 16/27 and 16/28) is in the southern Witch Ground Graben
(Crittenden et al., 1989; Bisewski, 1991).
Comments: The sands of the Bosun “Formation” in the type area have been informally referred by
various Authors to the Kopervik Formation, as formally defined by Hesjedal (1981) and Hesjedal and Hamar
(1983), and as the Shirley Sands (not formally defined). In the 13/30 area, sands of a similar age have been referred
to, informally, as the Asgard Sands, while to the south of the Renee Ridge in the 21/2 area (Fig. 7), similar-age
sands have been referred to as the Glenn Sands. Like the Kopervik Formation, the Bosun ‘’Formation” is a sand
sequence restricted in areal extent and of variable thickness. It is thought to be the lateral time-equivalent of the
lower and middle part of the Sola Formation (S1 and S2 units), as defined in this paper. Indeed, the sand body in the
type area is best referred to as the Bosun Sand Member of the Sola Formation, and it is hoped that in
future such a terminology is formalised.
The Bosun “Formation” and equivalents in other areas (op. cit.) are of a mass-flow/ turbidite
origin, and comprise a number of superimposed sandbodies. In contrast, however, Hesjedal proposed
that the Kopervik Formation was probably deposited under low-energy shallow-marine conditions
(sub-littoral).
S. Crittenden et al. 403

18. The whole Bosun “Formation” as informally defined here is difficult, but not impossible, to
subdivide lithostratigraphically into units which have areal extent and which are thus mappable.
Bisewski (1991) using detailed wireline-log correlations has mapped the formation across the
Southern Witch Ground Graben, and his proposed subdivision is in general accord with that proposed
by the present Authors (1989) although there are differences.
In a number of wells in the southern Witch Ground Graben, a simple bipartite subdivision can be
made into an upper “ratty” sand unit and a lower “blocky” sand unit. The “ratty” sand unit comprises
sandstones and claystone interbeds, and is portrayed on the wireline log response. The sand unit
beneath has few claystone horizons, and has a block-like wireline log response. In the east of the
southern Witch Ground Graben area, a third, lower unit can be identified, comprising a massive
sandstone interval which appears to grade laterally into a lower “ratty” unit.
Elsewhere in the region, the Bosun “Formation” equivalents display a striking similarity across
the region in that they are dissected by pronounced gamma-ray and sonic log spikes, representing
claystone deposition and a cessation of sand deposition at that time. The lateral persistence of these
claystone horizons is probably related to normal marine deposition, associated with a regional
decrease in tectonic activity at that time, with a concomitant decrease in turbidite sand emplacement,
rather than a pulse(s) of eustatic rise in sea-level. These clays are associated in some wells with an
increase in the planktonic foraminifera content (green-stained), again reflecting more normal marine,
clear-water, open-oceanic influenced sedimentation.
Previous Authors have dated the Bosun “Formation” in some wells as Barremian to Aptian in age
(Banner and Desai, 1988), but the Barremian date has been interpreted from a reworked palynoflora.
The presence of tuffaceous material in some wells is important, as it may represent horizons
suitable for correlation. The tuffaceous material is present in all subunits, and may be related to the
tuff horizons seen in the Sola Formation. However, care has to be taken as these sands are texturally
immature, and the volcanic material in the sands may in fact be reworked from the Jurassic Rattray
Formation. In addition, from examination of the Department of Energy composite logs and ditch-
cuttings samples, it is apparent that turbo-drilled formation cuttings have in the past been
misidentified. However, some of the volcanic material within the claystone interbeds may well
represent indigenous volcanic activity at this time.
Valhall Formation
Lithology: Claystone, dark-grey to light-grey, brown, reddish horizons are present, fissile,
calcareous, pyritic, glauconitic. Limestone, grey-to-tan, hard, argillaceous. Tuffs have been recorded
in some wells in the region and in the BGS borehole 81/40 in the Forth Approaches Embayment (Lott
et al., 1985).
Age: Early Aptian to Ryazanian.
Author: Deegan and Scull (1977) (pars), Hesjedal (1981), Hesjedal and Hamar (1983).
Regional Reference: Deegan and Scull (1977).
Comments: The Valhall Formation was originally erected to include all mudstones and shales above
the Kimmeridge Clay Formation but below the Rodby Formation, Deegan and Scull (1977)
distinguished the Devil’s Hole Formation from the Valhall Formation, but cited data restrictions for
their reason in not describing any other formations from the Early Cretaceous in the Central and
Northern North Sea. Hesjedal ( 1981) later described and defined the Sola Formation from what was
originally the upper part of the Valhall Formation in the type well-section (Norway, 2/11-1). Rawson
and Riley (1982) in their discussion of Early Cretaceous events used the original Deegan and Scull
(1977) concept of the Valhall Formation, but divided it into “A” and “B” intervals without defining
their units. They did not recognise the Sola Formation.
Early-”Middle” Cretaceous lithostratigraphy, UK North Sea404

20. In this paper, the Valhall Formation, as defined by Hesjedal and Hamar (1983) in basinal and non-
condensed sequences, is divided into six informal units. No formal status is given to any of them,
apart from the V5 unit, but further research and study may make it necessary and desirable in the
future to accord the other units formation or member status. In condensed and incomplete sequences
on highs and basin margins, the Valhall Formation is developed as a carbonate facies. Sand units are
also recognised within the Valhall Formation in the region, and are related to tectonics and basin fill.
Age-diagnostic foraminiferal taxa and assemblages are recovered from the V6 and V5 units of the
Valhall Formation, and provide a means of age-dating these lithostratigraphical subdivisions.
The foraminiferal assemblages recovered from the V4 to Vl units of the Valhall Formation often
comprise long-ranging and/or facies-controlled taxa, which make it difficult to subdivide and
correlate the sequences. A more precise biostratigraphical subdivision of the Valhall Formation is
provided by palynological analysis.
The six units, in descending order (Fig. 8), are as follows (well 16/26-4):
1. V6: In complete, argillaceous Sola/Valhall sequences, the top of this unit is picked at a well-
defined shoulder, where the gamma and sonic log responses decrease, reflecting the change from
non-calcareous to calcareous claystones. In Bosun (or equivalents)/ Valhall sequences, the top of
this unit is picked at an increase in gamma and sonic log response, reflecting the change from
sandstones to calcareous claystones. However, according to Bisewski, 1991, sand deposition is
considered to have commenced in this unit and to comprise, in some areas, the basal part of the
Bosun Formation, and is developed as either a “ratty” sand/claystone unit or a massive sand unit.
This unit comprises light- to dark-grey marls and limestones, usually with a red coloration. It is
correlatable with the ewaldi Marls of NW Germany, Heligoland and eastern England (Crittenden,
1982a, 1983b, 1984a,b, 1987a,b, 1988), which are of the same age: latest early Aptian to earliest
late Aptian. This unit is characterised by horizons of abundant red-stained planktonic foraminifera
of the genus Hedbergella, which are referable to Hedbergella D11 of Hecht (1938)—see
Crittenden (op. cit.) for discussion.
It may be appropriate formally to name this unit as the ewaldi Marl Member of the Valhall
Formation, with the type well being either the 14/19-5 or 14/19-15 well or the 16/26-4 well.
Calcareous benthonic foraminifera occur in flood-abundance at the same horizons (e.g.
Valvulineria gracillima). This horizon is equatable to the H. D11 horizon of Hecht (1938)
described from the latest early Aptian of Germany (ewaldi Kreide). The sample interval and
complications associated with caving preclude confirmation of the possibility that there is more
than one flood occurrence of red/orange-stained planktonic foraminifera. The V6 unit falls within
the Cerbia tabulata (OIB) Subzone discussed for the lower part of the Sola Formation. Taxa that
appear downhole within this zone, that broadly coincides with the top of the Valhall Formation, V6
unit, are Kleithriasphaeridium cristulatum, Sirmiodinium grossii and Pseudoceratium pelliferum,
together with an increase in numbers of Aptea polymorpha, and other Apteoid cysts, Florentinia
spp. and Cribroperidinium spp.
2. V5: This unit, with a higher gamma and sonic log response than the overlying unit,
comprises dark-grey to black, occasional brown, bituminous claystones, with a thin but
prominent limestone horizon. This unit is readily identifiable on the wireline logs by
high gamma and sonic log spikes. It is equatable with the Tock Facies of Heligoland
and the Fischschiefer of NW Germany (Crittenden, op. cit,), of intra-early Aptian age
(forbesi-deshayesi ammonite Biozones). This unit is quite distinctive, and can be used
as a regional correlation datum (e.g. Harker et al., 1987). It is characterised
Early-”Middle” Cretaceous lithostratigraphy, UK North Sea406

22. by the occurrence of flattened, brown-stained, often pyritised, planktonic foraminifera of the
genus Hedbergella (= H. D9 of Hecht, 1938, = Hedbergella aptiana Bartenstein).
The VS unit contains a dinocyst assemblage closely similar to the V6 unit, lying as it does
within the same Cerbia tabulata (OIB) Subzone. However, it is distinguished by the presence of
common Tasmanites and AOM associated with the prevalent anoxic conditions. An acme of
Subtilisphaera perlucida may occur within the VS unit.
While this paper was in press, Riley et al. (in press) have formally established this unit as the
Fischschiefer Member, and refer to the type well-section as 14/19-15 (UK), 8,100-8,112 ft
(RKB).
The equivalent of the V6 and V5 units has been recognised in the southern North Sea, where it has been
referred to as the “Aptian Belly”, because of the wireline log shape, by Crittenden (1982a, 1987a,b).
The V6 and V5 units are equatable with the basal part of the Lower Holland Marl Member of the
Holland Formation in the Dutch sector of the southern North Sea (Crittenden, 1987a,b.). Publications
by various other Authors (e.g. NAM and RGD, 1980) agree that the V5/V6 equivalent interval in the
North Sea is the equivalent of the “Tock’’ and ewaldi marls of Heligoland and Germany (see also
Jensen et al., 1986; King et al., 1989).
A stratigraphical break is postulated between the V5 unit and the underlying V4 unit, as part or all of the late
Barremian is considered to be missing or highly condensed in many areas of the North Sea (Crittenden, 1982a) and
NW Europe (e.g. Heligoland: Bartenstein and Kaever, 1973). This hiatus is apparent in many wells
studied for this paper (see Crittenden, et al., 1989, 1990; Harlow et al., in press).
The literature dealing with the Central North Sea consistently has the V5 unit, whether recognised
as such by various Authors or not, as within the Valhall Formation (Harker et al., 1987;
O’Driscoll et al., 1991).
3. V4: The base of the gamma and sonic log spikes of the overlying V5 unit define the top of this
unit. The unit is a sequence of calcareous claystones and limestone interbeds. They are dominantly
grey, but occasional reddish/orange horizons may be present. Regional biostratigraphical evidence
suggests that this unit is entirely of ?late to mid-Barremian age (see Harlow et al., in prep.).
Foraminiferal assemblages recorded in the V4 unit comprise calcareous benthonics (e.g.
Gavelinella barremiana, Conorotalites aptiensis/bartensteini, Lenticulina heiermanni, and
L. ouachensis), and predominantly non-calcareous agglutinants. Small planktonic foraminiferids
are also present (Hedbergella spp.). Specimens of Uvigerinammina spp. (Falsogaudryinella spp.
of various Authors) are often recorded from this interval. The assemblage as a whole is
attributable to an outer-shelf biofacies. The foraminiferal fauna is characteristic of late and middle
Barremian age sequences in NW Europe.
Palynology has identified an hiatus offshore between the V5 and V4 units, broadly
encompassing the “Astrocysta” cretacea (IA) Subzone (bidentatum and upper rudefissicostatum
zones) of late Barremian age, that is rarely if ever seen offshore. Penetration of the Barremian is
normally first seen with the downhole appearance of Hystrichodinium ramoides, Batioladinium
longicornutum and Hystrichosphaeridium arborispinum, together with Aptea anaphrissa,
Spiniferites dentatus, Hystrichogonyaulax cladophora and Hystrichodinium ornatum/phoenix.
This assemblage is of mid-Barremian aspect, corresponding to the Doidyx anaphrissa (IB)
Subzone. Dinocyst assemblages tend to be dominated by Aptea anaphrissa, Spiniferites spp.,
Dingodinium albertii, Cassiculosphaeridia magna and Diphasiosphaera stolidata.
4. V3: This unit comprising calcareous claystones and limestones is the time-equivalent of
the Tuxen Formation of Jensen et al. (1986), of part of the Valhall 3 unit of Hansen and
Buch (1982), and part of the Utvik Formation, in some areas, of Hesjedal and Hamar
(1983). The original designation of the Tuxen Formation defined it as a calcareous
claystone unit overlain by the Sola Formation (sensu Jensen et al., 1986) and underlain
by the Valhall Formation (ibid., 1986). The unit is characterised overall
Early-”Middle” Cretaceous lithostratigraphy, UK North Sea408

23. by slightly lower gamma and sonic log values than the overlying unit. However, the log profile
shows a gentle waist pattern, as the gamma and sonic log values increase in value downhole.
Within this unit, Jensen et al. (1986) recognised a thin bed of dark, laminated AOMrich
claystone, which they called the Munk Marl Bed. It is equatable with the Blatterton facies of the
early to mid-Barremian of NW Germany and Heligoland. This dark claystone, deposited during a
phase of anoxia associated with the “Middle” Barremian transgressive pulse, is recognised in a
number of the studied well-sections in the Central North sea by a spike log response of increased
gamma and sonic values. It is a valuable stratigraphical marker, if identified correctly.
Confrmation of the identity of the claystone horizon (the Munk Marl Bed of Jensen et al.
1986) should be made whenever possible by biostratigraphical analysis (palynology), as it can be
confused with similar log spikes seen elsewhere in the section.
Regional biostratigraphical data indicates that the whole unit is intra-late Hauterivian to intra-
mid Barremian in age (see Harlow et al., in prep.).
The V3 unit is regionally middle Barremian to late Hauterivian in age. Microfaunal
assemblages comprise calcareous benthonic and agglutinating foraminiferal faunas and
ostracoda. The taxa recorded are in the main long-ranging and environmentally
controlled in their distribution (e.g. Glomospirella/Glomospira spp.). Better
stratigraphical subdivision is provided by palynology. In the lower part of the V3 unit,
there are often recorded rare specimens of the calcareous benthonic foraminifera
Gavelinella sigmoicosta, which indicates an age no younger than earliest early
Barremian. The downhole disappearance of Gavelinella barremiana, if not complicated
by caving, is usually good evidence for an age no younger than early Barremian. Other
age-diagnostic taxa commonly recorded from this unit include Falsogaudryinella spp.
(identified variously as U. moesiana moesiana and Falsogaudryinella sp.x and U.
moesiana subsp. A).
Palynomorphs below the Munk Marl Bed within the V3 unit are essentially early Barremian,
corresponding to the Kleithriasphaeridium corrugatum (IC) Subzone, marked by the downhole
appearance of the dinocyst K corrugatum that can occur as an influx, together with
Hystrichodinium furcatum. The remainder of the assemblage is typically dominated by
Cassiculosphaeridia magna, Spiniferites spp., Cyclonephelium hystrix, Hystrichodinium
pulchrum, Gardodinium trabeculosum and Kleithriasphaeridium simplicispinum. Other
important markers first appearing downhole in the IC Subzone include
Adnatosphaeridium vetusculum, Gonyaulacysta teicha, Muderongia crucis/ tetracantha
and Trichodinium ciliatum. Below this, but still within the early Barremian, a narrow
interval — the Adnatosphaeridium vetusculum (IIA) Subzone — can sometimes be
recognised, based on the downhole appearance of Gonyaulacysta confossa and G.
kostromiensis. The general palynomorph assemblage remains similar to the zone above,
with the addition of Pseudoceratium pelliferum and Phoberocysta neocomica, that can
be common. The lower part of the V3 unit is mainly defined by the Canningia cf
reticulata (IIB) Subzone extending down into the latest Hauterivian. This Subzone is
thin and not always recognisable. Dinocyst taxa that first appear downhole within this
Subzone are Canningia cf reticulata and Ophiobolus sp. A. An acme of
Oligosphaeridium abaculum occurs within this subzone, this taxon being quite rare
elsewhere. The palynomorph assemblage is generally dominated by Cribroperidinium
sepimentum, Cyclonephelium hystrix, Oligosphaeridium complex, Hystrichodinium pulchrum,
Trichodinium ciliatum, Spiniferites spp. and Cassiculosphaeridia magna.
5. V2: The top of this unit is not clearly defined, but is picked at a very small “neck”
profile on the wireline logs. The unit as a whole is characterised by higher gamma and
sonic log values than the overlying and underlying units. Lithologically, the u nit
S. Crittenden et al. 409

24. comprises dark-grey to light-grey calcareous claystones, with thin limestone interbeds.
The whole unit, from regional biostratigraphical evidence, is considered to be late Valanginian
to intra-late Hauterivian in age.
Rawson and Riley (1982) have documented a number of”events” through the lower part of the
Early Cretaceous. They are tentatively recognised in this unit as follows:
1. The top of the V2 unit is indicated by a log break considered to be equatable with the gottschei
Biozone transgression (intra-late Hauterivian).
2. Within the V2 unit, a stratigraphical hiatus, usually associated with gamma and sonic log
spikes, is thought to correspond to the “mid” Hauterivian transgressive pulse of the inversus
Biozone.
3. The amblygonium Biozone transgressive event (base Hauterivian) occurs in the basal part of
the V2 unit and is probably reflected by a slight decrease in the sonic log response.
4. The base of the V2 unit/top of the Vl unit is a log break associated with the Dichotomites
Biozone transgressive event (basal late Valanginian).
The microfaunal recovery from the V2 and Vl units is sparse and sporadic and faunas
comprise long-ranging taxa. In addition, over “high” areas, condensation of the V3, V2 and Vl
units is characterised by a distinctive carbonate biofacies (?Urgonian facies). The top of this
carbonate biofacies is diachronous and could be as old as ?late Ryazanian where it is developed
as Unit Vl (Utvik Formation equivalent), or as young as early Barremian.
The top of the V2 unit very generally corresponds to the top of the late - “mid” Hauterivian
Chlamydophorella trabeculosa (IIIA) Subzone, marked by the first appearance downhole of the
important marker Muderongia simplex and Oligosphaeridium amplexum. Assemblages are
typically rich and diverse, dominated by Gardodinium trabeculosum, Hystrichodinium pulchrum,
Cyclonephelium hystrix and Spiniferites dentatus. Influxes of Phoberocysta neocomica and
Ctenidodinium elegantulum may occur. Below this, a Kleithriasphaeridium simplicispinum (IIIB)
Subzone can be recognised, based on an influx of Hystrichogonyaulax cladophora (sensu
Duxbury), Stephanelytron cretacea and Kleithriasphaeridium simplicispinum, together with the
first downhole appearance of Oligosphaeridium “quadrum”. Muderongia extensiva,
Aprobolocysta varigranulata, Gochteodinia villosa and Cymososphaeridium validum may be
common near the base of the Subzone. Assemblages are typically dominated by Hystrichodinium
pulchrum, Spiniferites spp., Chlamydophorella sp. A (Davey, 1973), Oligosphaeridium complex
and O. amplexum. Acmes of Dingodinium albertii, Hystrichogonyaulax cladophora,
Gardodinium trabeculosum and Phoberocysta neocomica occur within the Subzone. The V2 unit
is generally recognised to extend into, and include most of, the late Valanginian, which is
encompassed palynologically by the Muderongia extensiva (IVA) Subzone. This Subzone is
defined by the downhole influx of ornate forms of Hystrichogonyaulax cladophora and the
appearance of Apteodinium spongiosum. Also appearing within the zone is the taxon Speetonia
delicatula and consistent Gochteodinia villosa. Common within the zone can be the taxa
Phoberocysta tabulata and tabulate varieties of Muderongia simplex. Palynological assemblages
within the IVA Subzone are typically dominated by Cassiculosphaeridia magna, C. reticulata,
Gonyaulacysta helicoidea, Trichodinium ciliatum, Hystrichodinium pulchrum, H. voigtii,
Oligosphaeridium complex and Pseudoceratium pelliferum.
6. Vl: This unit, which is present in both basinal and high areas, is characterised by low gamma and
sonic log values, but with an overall fluctuating profile reflecting the calcareous claystone with limestone
interbed lithologies. This unit is equivalent to the “basal Valhall Limestone” of Rawson and Riley (1982), to
the Valhall I unit of Hansen and Buch (1982), to the basal part of the Utvik Formation of Hesjedal and
Hamar (1983), and to the Leek Member of Jensen et al. (1986).
The unit is regarded as late Ryazanian to early Valanginian in age, although locally it may
extend through to the Barremian.
Early-”Middle” Cretaceous lithostratigraphy, UK North Sea410

26. The Vl unit, of broadly late Ryazanian to early Valanginian age, is subdivided by at least
three dinoflagellate Subzones. The uppermost of these, the Tubotuberella apatela (IVB) Subzone
within the early Valanginian is defined by the downhole appearance of Tubotuberella apatela
and Senoniasphaera frisia, commensurate with acmes of Surculosphaeridium sp. II and
Hystrichosphaeridium scoriaceum. The first downhole appearance and influx of
Occisucysta cf. evittii occurs within this Subzone. The general palynological assemblage is
otherwise similar to the zone above. At the base of the Valanginian, the Endoscrinium pharo
(IVC) Subzone can be recognised on the downhole appearance of Endoscrinium pharo. General
palynological assemblages are otherwise similar to the two Subzones above. Into the Ryazanian
Dingodinium spinosum (V) Zone, palynofloras show a profound change with Prasinophyte algae
(Pterospermella spp. and Tasmanites spp.) attaining local numerical significance, reflecting the
waning effect of anoxia during the Stenomphalus transgressive event, just above the top of the
Kimmeridge Clay Formation. The Prolixosphaeridium torynum (VA) Subzone within the latest
Ryazanian is generally dominated by large, ornate Millioudodinium spp. and
Cribroperidinium spp., and is marked by the downhole appearance of Egmontodinium torynum,
Dingodinium spinosum and Gonyaulacysta sp. A/B (Davey).
The base of this unit is easily identified when it rests upon the Kimmeridge Clay Formation.
SAND DEVELOPMENT
Sand bodies are developed at various localities and stratigraphical horizons within the Early
Cretaceous of the Central North Sea. The only formally defined sand bodies in the area so far not
discussed here are as follows:
The Scapa Sand Member (Harker et al., 1987) is a turbidite sand accumulation developed in the
NW part of the Witch Ground Graben (Fig. 9). It is, according to Harker et al. (1987), in the type well
section 14/19-15, early Valanginian to early Hauterivian in age. Its temporal and spatial distribution
and origin is discussed elegantly in detail by O’Driscoll et al. (1991) and Riley et al. (in press). It is
interpreted as regionally equivalent to part of the Devil’s Hole Formation.
The Devil’s Hole Formation is a sand body which is Late Jurassic to Early Cretaceous in age, and
is restricted areally to Quadrant 29 of the UK Sector of the Central North Sea. In the type-section well
29/25-1 (Fig. 10), an incomplete development of the Devil’s Hole Formation is present in comparison
with other wells in the vicinity. It was originally erected by Deegan and Scull (1977), and the name
should ideally only be used in Quadrant 29. According to Holland et al. (1978), a “Formation” is a
primary local unit which is mappable, and unless physical continuity or its former presence can be
reasonably inferred, should not be used in other areas. However, it is common practice to use the
name Devil’s Hole Formation for similar-aged sand bodies elsewhere in the region, but with the
addition of “equivalent”.
Other sand bodies are developed in the area, and at present have not been formally defined.
Examples of sand accumulations which are the time-equivalent of the Kopervik Formation have been
discussed in the section on the Bosun “Formation”, and, strictly speaking, should be regarded as un-
named sand bodies until they are formally defined. Examples of sand accumulations which are time-
equivalent to the Devil’s Hole Formation occur in quadrants 13/30 and 14/26, and in the Inner Moray
Firth adjacent to the Wick Fault.
Sand bodies undoubtedly occur in other areas of the region throughout the Early Cretaceous, and
are awaiting discovery by the exploration drill.
Early-”Middle” Cretaceous lithostratigraphy, UK North Sea412

27. CONCLUSIONS
It is a valid statement that different areas of the North Sea region during the Early Cretaceous
were characterised by broadly-similar patterns of sedimentation. These patterns (the effects) are the
results of similar genetic processes (the causes) and environments of deposition. The various Early
Cretaceous depositional basins and subbasins within the North Sea region underwent the same, or
very similar, stratigraphical development, related both to eustatic sea-level changes, and regional and
local tectonics. A thorough lithostratigraphical subdivision, coupled with detailed biostratigraphical
S. Crittenden et al. 413

28. modelling, is of the essence in seismic sequence stratigraphy. It is not surprising, therefore, that a
thorough understanding of the Early to “Middle” Cretaceous lithostratigraphy and biostratigraphy
achieved from a detailed evaluation of a large number of wells drilled in the Central North Sea is a
powerful tool for assisting in the prediction of likely areas where sands were generated and where
sand deposition occurred.
The lithostratigraphic scheme presented in this paper defines and subdivides the Herring and
Hidra Formations in the manner of Burnhill and Ramsay (1981). The Rodby, Sola and Valhall
Formations have been newly subdivided into defined units which are regionally correlatable and
mappable.
As a whole, the lithostratigraphy presented is a pragmatic scheme, which, with use, will
undoubtedly be refined as shortcomings become apparent. We have refrained from the formal
erection of new units as we appreciate the need for oil companies and consequently groups as a whole
to agree on an accepted terminology and subdivision.
ACKNOWLEDGEMENTS
This paper was made possible by the release to the Authors of material from the Department of
Energy Core Store in Edinburgh. In particular, we are grateful to Dick Sutherland, Chief Curator at
the Core Store, for his invaluable assistance and advice. We would like to acknowledge fruitful
discussion with a number of geologists working in the North Sea exploration industry, including Hans
Bisewski, Richard Benmore, Barbara Dickinson, Brian Moseley, Stuart Harker, Martin Kirk, Bill
Braham and Colin R. Harris. An anonymous reviewer provided useful comment. This paper is
published with the permission of the Directors and Management of Geolink (UK) Ltd, Aberdeen.
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