Jurassic stratigraphy of the Western Barents sea region: A review

J U R A S S I C S T R A T I G R A P H Y OF THE WESTERN BARENTS SEA REGION : A

MORTEN SMELROR SMELROR M. 1994. Jurassic stratigraphy of the Western Barents sea region : a review. [Aper~u stratigraphique du Jurassique de la partie occidentale de la Mer de Barents.] GEOBIOS, M.S. 17 : 441-451.

ABSTRACT The Jurassic succession of the western Barents Sea Region comprises Hettangian to Bathonian marginal to open marine sandy and shaly sequences, and Callovian to Volgian open marine dominantly shale and claystone deposits. In overall character and development the Jurassic sequences on the Barents Shelf are related to those found on the Svalbard archipelago. However, since Svalbard emerged in Late Bajocian - Bathonian time, strata of Hettangian - Aalenian age on Spitsbergen occur mainly as thin erosional remnants, while Bajocian to Middle Bathonian deposits apparently are missing. Contemporaneously, thick clastic sequences were deposited in depressions in the present offshore Troms area. On Svalbard the Hettangian - Aalenian sequences are assigned to the Wilhelmcya Formation (Kap Toscana Group), while the Bathonian to Volgian sequences comprise the Brentskardhaugen Beds and the Agardh0ellet Formation (Janusfjellet Subgroup). In the offshore Troms area, contemporaneous deposits to the Wilhelmya Formation are represented by the Tubaen, Nordmela and lower St¢ Formations (Realgrunnen Group). The Late Middle and Late Jurassic Fuglen and Hekkingen Formations (Teistgrunnen Group) of offshore Troms are time-equivalen t to the Janusi~ellet Subgroup on Svalbard. In these Jurassic to lowermost Cretaceous successions, important TR cycles boundaries are recognized at the base of the Pliensbachian and in the early Toarcian, Late Toarcian]?.Aalenian, Late Bathonian and Early Oxfordian ; also at the base of the Valanginian. The Jurassic ammonite record of the western Barents Sea Region is still incomplete, but faunas which allow a subdivision at zonal level are found in the Toarcian - Aalenian, Upper Bathonian - Callovian, Oxfordian, Kimmeridgian and Middle Volgian. Microfossils are well documented and provide a good mean for correlations of the Middle and Upper Jurassic sequences. Formal biostratigraphic zonations have been proposed for the Callovian to lowermost Cretaceous based on foraminifera and for the Toarcian to lower Oxfordian based on dinoflagellates. KEY-WORDS : BARENTS SHELF, SVALBARD,JURASSIC, STRATIGRAPHY.

RI~SUMI~ La succession s6dimentaire jurassique de la Mer de Barents occidentale est compos~e de d6p6ts sableux et argileux de milieu matin ouvert. D'apr~s leur caract~re et leur d6veloppement, les s6quences jurassiques du plateau continental de la Mer de Barents sont proches de celles des iles de Svalbard. Svalbard 6mergeant au Bajocien tardif - Bathonien, les couches de l'Hettangien - Aal6nien y sont repr6sent6es par des minces t6moins d'6rosion, tandis que les d~p6ts du Bajocien au Bathonien moyen manquent selon toute apparence. Dans le m6me temps, des 6paisses couches clastiques se sont d6pos~es dans les r6gions qui sont actuellement au large de Troms. A Svatbard, la s6quence de rHettangien h l'Aal6nien est repr6sent6e par la Formation de Wilhelm0ya (Groupe de Kap Toscana), tandis que la s6quence du Bathonien au Volgien est "epr6sent~e dans les Couches de Brentskardhaugen et la Formation d'Agardijellet (Sous-groupe de Janusflellet). Au large de Troms, des d6p6ts contemporains de la Formation de Wilhelm0ya sont repr6sent6s par les Formations Tubaen et Nordmela et la partie inf6rieure de la Formation St~ (Groupe de Realgrunnen). Les Formations Fuglen et ttekkingen (Groupe de Teistgrunnen) de la m6me r6gion, sont contemporaines du sous-groupe Janusi~ellet de Svalbard (Jurassique moyen tardif et Jurassique tardif). Dans ces couches du Jurassique au Cr6tac6 Inf6rieur, de remarquables limites de s6quence existent la base du Pliensbachien, dans le Toarcien prO:oce ~ la limite Toarcien tardif ~ ?Aal6nien, dans le Bathonien tardif, l'Oxfordien pr6coce, et finalement ~ la base du Valanginien. Bien que la biostratigraphie des ammonites jurassiques de la Mer de Barents occidentale ne soit pas compl6tement connue, les faunes du Toarcien-Aal6nien, du Bathonien sup6rieur - Callovien, de l'Oxfordien, du Kimm6ridgien et du Volgien Moyen permettent une subdivision jusqu'au niveau de la zone. Les microfossiles sont bien documents dans le Jurassique moyen et SUl~rieur, permettant des correlations excellentes. Une zonation formelle ~ l'aide des foraminif'eres a 6t6 propos6e du Callovien au Cr6tac6 basal, et une autre sur les dinoflagell6s allant du Toarcien ~ l'Oxfordien Inf6rieur. MOTS-CL]~S : PLATEAU CONTINENTAL DE BARENTS, SVALBARD,JURASSIQUE, STRATIGRAPHIE.

442 0,

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INTRODUCTION The Barents Sea is situated on the Norwegian and Soviet continental shelfs ; it is bounded by the Norwegian-Greenland Sea to the West, the Svalbard and Franz Josef Land archipelagos to the North, Novaya Zemlya to the East and the Norwegian and Russian mainland to the South (Fig. 1). The Barents Sea region has an intracratonic setting ; it has been affected by several phases of tectonism since the Caledonian orogenic movements terminated in Early Devonian times (Gabrielsen et al. 1990). Structurally, the Barents Sea continental shelf is dominated by ENE-WSW to NE-SW and N N E - S S W to NNW-SSE trends

with local influence of NWN-ESE striking elements (Fig. 1). Since the search for exploitable hydrocarbons reserves on the Norwegian shelf spread northwards in the early 1980's, much attention has been paid to studies of the Jurassic succession of the western Barents shelf, where Lower-Middle Jurassic reservoirs and Upper Jurassic source rocks are among the main geological exploration targets. So far several gas discoveries have been made, but only a few good oil shows have been encountered. The most prolific gas fields are the Sn0hvit, Albatross and Askeladd discoveries, where the gas is trapped in LowerMiddle Jurassic deltaic and shallow marine sandstones.

443 ==

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Figure 2 - Lithostratigraphic correlation of the western Barents Sea area, northern Noi~vay and northeastern Greenland. Seq u e n c e b o u n d a r i e s s h o w n to t h e r i g h t a r e : B P - b a s e P l i e n s b a c h i a n , E T - E a r l y T o a r c i a n , L T / A - L a t e T o a r c i a n / ? A a l e n i a n , L B l a t e B a t h o n i a n , E O - E a r l y O x f o r d i a n . Corrdlation lithostratigrapbiquc (l trauers la rdgion de Ia M e r de Barents Occidentate le

N o r d de la Noru~ge et le N o r d - E s t du GrfY2nland. A droite, les limites de sgquence sont indiquges : B P - Base du Pliensbachien de base, E T - Toarcien prdcoce, L T I A - Toarcien t a r d i f / ?Aaldnien, L B - B a t h o n i e n tardif, E O - Oxfordien prdcoce.

In overall character and development, the Jurassic sequences on the Barents Shelf are closely related to those found on Svalbard. The Jurassic exposures on Svalbard have been intensively studied by geologists from several nations for almost a century. The first commercial well seeking oil or gas on the Svalbard archipelago was spudded already in 1963, but the exploration activity has been low and no commercial discoveries have been made so far. However, during the last few decades the scientific parties studying the Jurassic successions have provided a large amount of new information ; the interest of exploration companies has improved our knowledge of the development of these successions significantly. The aim of this paper is to review the Jurassic stratigraphy of the western Barents shelf and Svalbard and to provide a correlation of the Jurassic sequences (i.e. T-R cycles) in the western Barents sea region and adjacent onshore areas (i.e. northern Norway and northeastern Greenland). JURASSIC LITHOSTRATIGRAPHY OF SVALBARD A number of lithostratigraphic units (Fig. 2) were proposed by early geologists studying the Jurassic succession on Svalbard, but the first modern lithostratigraphic scheme was proposed by Par-

ker (1967), who assigned the Lower and Middle Jurassic sequences to the Kapp Toscana Subgroup (originally described by Buchan et al. (1965) ; and which also comprised the Upper Triassic beds) and the Upper Jurassic sequences to the Agardhi~ellet Formation of the Janusi~ellet Subgroup. The Lower-Middle Jurassic lithostratigraphy was revised by Worsley (1973), who defined the Wilhelmoya Formation to embrace the ?Norian/Rhaetian to Bathonian succession of Svalbard. On Spitsbergen the Withelmcya Formation is divided into a lower Knorringfjellet Member, which comprises mainly shales, sandstones and carbonates, and an upper Smalegga Member, which is dominated by quartzitic sandstones (M0rk et al. 1982). In the eastern Svalbard archipelago, the Wilhelmya For~lation has been separated into a lower, mainly sandy Sjcgreni~ellet Member and an upper, domirantly silty and clayey Passet Member (L0faldli & Nagy 1980). On Spitsbergen the base of the formation is marked by a phosphatic conglomerate overlying a typical coarsening-upwards sequence. The upper boundary of the Wilhelmya Formation is here clearly marked by the (transition to) phosphatic conglomerate of the Brentskardshaugen Bed. It should be mentioned that a local lithostratigraphic nomenclature for the Kong Karls Land in

444 AMMONITE ZONES

Remarks

I

D. maximus D. panderi

Dorsoplanites zones as recognized in Siberia and on the Russian platform

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?A. autissidorensis A. eudoxus A. rnutabilis

Correlation to the subboreal zones is based on recognized boreal Amoeboceras horizons

AGE

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Cardioceras spp. P. athleta E. coronatum

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Boreal Amoeboceras zones as defined in East Greenland

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C. apertum C, calyx A. eranocephaloide A. ishmae

Boreal zones as recognized in East Greenland

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P. macklintoki

L. opalinum P. rosenkrantzi

Reworked Porpoceras Pseudolioceras faunas recognized in the Brentskardhaugen Bed related to zones defined in Arctic Canada, East Greenland and Siberia

P. polare m'-

3 I.-

Figure 3 - Ammonite faunas described fi'om the Jurassic successions of the Barents Shelf and Svalbard. Faunes d'ammonites des sddiments .]urassiques du plateau contineutal de Barents et de Svalbard.

eastern Svalbard was proposed by Smith et al. (1976). However, following the arguments presented by Edwards et al. (1979), this is here omitted. The Upper Jurassic - Lower Cretaceous lithostratigraphy of central Spitsbergen has recently been revised by Dypvik e t al. (1991) ; they retained the shale-dominated Janust]ellet Subgroup, as originally defined by Parker (1967) to comprise the Bathonian to Volgian/Lower Berriasian Agardhfjellet Formation and the Lower Cretaceous Rurikfjellet Formation. The boundary between the Agardhfjellet Formation and the overlying Rurikfjellet Formation is marked by a thin interval of yellowish-weathering clays (Dybvik 1985 ; Dypvik et al. 1991). The transition between the Wilhelm0ya Formation and the base of the Agardh~ellet Formation is marked by the Brentskardhaugen Bed (BfickstrSm & Nagy 1985) ; its position with regard to higher categories, is not well established. The bed was first claimed to be part of the Janust]ellet Subgroup (Buchan et al. 1965). Parker (1967), however, prefelTed to place the Brentskardhaugen Bed at the top of the Kapp Toscana Group, owing to lithological similarities between the mat~ix of the conglomerate and the sandstones forming the uppermost part of the group. These sandstones compose the Wilhehn0ya Formation as introduced by Worsley (1973). In more recent literature the Brentskardhaugen Bed is sometimes regarded as the uppermost unit of the Wilhehnoya Formation (Worsley 1973 ; Harland et al. 1974 ; Bjmrke & Dypvik 1976) and sometimes as part of the Janusfjellet Subgroup (Flood et al. 1971 ; Major & Nagy 1972 ; Birkenmajer 1975 ; BfickstrSm & Nagy 1985 ; Birkenmajer et al. 1982). A number of workers have ranked the Janust~ellet Subgq'oup merely as a formation, and given the Agardh0ellet Formation the of member (Major & Nagy 1972 ; Birkenmajer et al. 1982 ; L0faldli & Nagy 1983). In eastern Svalbard (i.e. Kong Karls Land), Edwards et al. (1979), Lofaldli & Nagy (1980) and Sinelror (1988) assigned the Upper Jurassic succession to the Retzius~ellet Member of the Janust]ellet Formation, while Smith et al. (1976) had, as mentioned already, proposed several local lithostratigraphic units to cover the Upper Jurassic succession at this eastern islands in the Svalbard archipelago. The formal lithostratigraphy of this eastern area of Svalbard is presently not well established.

445

JURASSIC LITHOSTRATIGRAPHY O F F S H O R E T R O M S ( N O R T H E R N NORWAY) A formal lithostratigraphic scheme for the Mesozoic and Cenozoic succession offshore (Fig. 2) from northern Norway has been proposed by Worsley et al. (1988), replacing the informal units used in Berglund et al. (1986), Gjelberg et al. (1987) and Olaussen et al. (1984). The Jurassic succession of offshore Northern Norw a y is partly assigned to the Realgrunnen Group and partly to the Teistgruppen Group. The Realgrunnen Group, which is of Early Norian to Bajoclan age, is generally time-equivalent and shows a similar development to the Wilhelmoya Formation of eastern Svalbard. The Realgrunnen Group comprises the Fruholmen Formation of Norian Rhaetian age (the top of the formation COlTeSponds in general to the Triassic/Jurassic transition), the Tuben Formation of Rhaetian to Hettangian/Sinemurian age, the Nordmela Formation of Sinemurian to Late Pliensbachian/Toarcian age and the Sto Formation of Late Pliensbachian to Bajocian/Bathonian age. These formations represent a variety of coastal marine environments and are generally dominated by sandstones, but with occasionally shale intervals and with thin coals. The transition between the Realgrunnen Group and the Teistgrunnen Group is marked in the type wells by sharp increase in gamma ray and density responses and by an accompanying decrease in intelwal transit time (Worsley et al. 1988). In contrast to the sandy Realgrunnen Group, the overlying Teistgrunnen Group contains dominantly shales and claystones, with thin interbeds of limestone and only rare siltstones or sandstones. This group represents relatively deep and quiet marine environments, occasionally becoming anoxic. It is of Middle Bathonian to Volgian/Early Boreal Berriasian age, thus being comparable to the Agardhi~ellet Formation of the Janusi~ellet Subgroup on Svalbard. The upper boundary of the Hekkingen Formation in the type wells is defined by changes in gamma ray, sonic and density log response.

BIOSTRATIGRAPHY Paleontological and biostratigraphic studies on Svalbard commenced more than a century age ; a number of fossil groups has been well documented from the archipelago. However, very little in-

tbrmation have been published from the offshore western Barents Sea area. In the following section the status of and the recent advances in, ammonoid and microfossil biostratigraphy of these areas are briefly summarized. In North-West Europe, ammonite zones provide the orthochronolo. gy for the biostratigraphic subdivision of Jurassic System ; the ammonite record would be a major key in correlating the Jurassic sequences in the Barents Sea region with contemporaneous strata elsewhere. However, as microfossils (i.e. foraminifera and palynomorphs) are the most widely used groups in dating and correlating sequences penetrated by explorations wells, they provide an effective parachronology which will become increasingly important in interregional correlations. AMMONITES To my knowledge, no ammonites have (Fig. 3) been described from the regressive earliest Jurassic deposits, i.e. the lower Wilhetmoya Formation oll Svalbard. Reworked ammonite faunas of Toarcian and Aalenian have been frequently recovered from the Brendskardhaugen Bed (Wierzbowski et al. 1981 ; BtickstrSm & Nagy 1985) ; important species include Dactylioceras toxophorum, Porpoceras polare, P. spinatum, Pseudoliceras gradatum, P. compactile, P. p u m i l u m , P. macklintocki, P. rosenkrantzi and Leioceras opalinium. The Porpoceras - Pseudolioceras fauna is known from Arctic Canada, East Greenland and northern Siberia and, although its exact stratigraphic zonal position has been disputed (BSckstrSm & Nagy 1985), it provides a good mean for" correlation in the Arctic region. Late middle Jurassic (Upper Bathonian and Callovian) ammonite faunas from Svalbard have been descr'ibed by Sokolov & Bodylersky (1931), Blthgen (1936), Pchelina (1967), Smith et al. (1976), Lofaldli & Nagy (1980), Rawson (1982), and Kopik & Wierzbowski (1988). The Upper Bathonian fauna, including Costacadoceras bluethgeni, Arcticoceras harlandi, A. cf. ishmae and A. kochi, is related to the Ishmae Zone and Cranocephaloide Zone faunas described from East Greenland and Northern Siberia. On Svalbard these faunas are succeeded by a Kepplerites - Cadoceras (Paracadoceras) fauna, including Kepplerites svalbardensis, K. "tychonis", K. birkelundae, Radoceras (Paracadoceras) cf. victol, C. (P.) cir. multiforme and C. cf. aperture, which offers a correlation with the uppermost Bathonian and lowermost Callovian Cadoceras aperture Zone and C. calyx Zone faunas in East Greenland. The youngest Middle Jurassic fauna, which includes Stelzocadoceras multicostatum, Pseudocadoceras

446 1967) and the D. m axi m us Zone of Siberia (Nagy 1990). In addition to these two zones, Yershova (1983) also recognized faunas assigned to the Laugeites groenlandicus Zone in the Middle Volgian of Spitsbergen. The Jurassic Strata overlying the Middle Volgian is usually poor in macrofossils ; no age-diagnostic Upper Volgian ammonite faunas have been described from Svalbard.

et al. ~u

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FORAMINIFERAL ZONES

STAGES

VALANGINIAN

Abundant Glomospira and Glomospirella

- 128

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RYAZANIAN

Gaudryina aff. milleri

131

Recurvoides obskiensis U Trocharnrnina aft. abrupta VOLGIAN

M

Ammodiscus zaspelovae

L

Trochammina rosacea

U KIMMERIDGIAN . o..

g

.

L

Haplophragmoides canuiformis

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OXFORDIAN

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Recurvoides disputabilis

u CALLOVIAN

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Middle and Upper Jurassic ammonite faunas have also been encountered in other IKU shallow cores drilled elsewhere on the western Barents Shelf. Information on these is still confidential, but will be released in coming years. FORAMINIFERA

152

LU~ . j t.,O E3~ D~

Jurassic ammonite faunas from the western Barents Shelf have been described only from the IKU Shallow core 7227/08-U-03 in the Nordkapp Basin (Wierzbowski & ,£_rhus 1990). This core revealed a fairly complete succession of species of the ammonite genus Amoeboceras, enabling the identification of the Standard Boreal ammonite zones and some informal ammonite horizons of the Upper Oxfordian and Kimmeridgian, recognized in East Greenland•

Trochammina rostovzevi

F i g u r e 4 - F o r a m i n i f e r a t zones recognized in the Callovian to R y a z a n i a n succession of central S p i t s b e r g e n Ifrom Nag?." e t al. 1990). Les zones de [bramini/?res d u Callovien au Ryc~zanien d u S p i t z b e r g central.

nanseni, P. chitinense, Longaeviceras stenolobum and L. cf. pomeroyense, is related to the Upper Callovian E. coronatum Zone and P. athleta Zone

(Kopik & Wierzbowksi 1988). Lower Oxfordian Cardioceras ammonite faunas have been recorded from southern Spitsbergen (Pchelina 1967 ; Parker 1967) and Kong Karls Land (Lofaldli & Nagy 1980). Upper Oxfordian and Kimmeridgian Amoeboceras - Rasenia faunas have been documented from several areas by Frebold (1930), Sokolov & Bodylevsky (1931), Yershova (1983) and Kopik & Wierzbowski (1988). In the upper parts of the Agardh0ellet Formation, Dorsoplanites faunas have been recovered from several localities (Pchelina 1967 ; Parker 1967 ; Nagy et al. 1990), they include species which offer a correlation with the middle Volgian D. panderi Zone of the Volga Basin (Pchelina

Investigation of the foraminiferal stratigraphy of the Jurassic succession on Svalbard is of relatively recent date. The earliest published information consisted of short notes on species occurrences fl'om Wilhehn0ya (Klubov 1965) and Agardhbukta (Pchelina 1967). The distribution of Jurassic foraminifera in Kong Karls Land was discussed by Lofaldli and Nagy (1980). To my knowledge this is the only paper which deals with Lower as well as Upper, Jurassic assemblages from the Wilhelmoya Formation. Two characteristic foraminiferal assemblages were recognized, i.e. the Hettangian to ?Pliensbachian A m m odi scus rugosus assemblage and the ?Pliensbachian to ?Toarcian Ammodiscus asper assemblage. The regional distribution pattern of foraminifera in the Janus~ellet Subgroup was discussed in preliminar fashion by Nagy and L0faldli (1981). A subsequent paper by Lofaldli and Nagy (1983) recorded the foraminiferal stratigraphy of the Keilhaui~ellet section in southern Spitsbergen. Changing depositional conditions up through the Janusi]ellet Subgroup were discussed by Nagy et al. (1988), on the basis of major faunal parameters (species diversity and frequency of genera) combined with lithological features (mainly the organic carbon content).

447 Figure 5 Oxfordian

Geologic age

Dinoflagellate cyst zonation

ua

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Last apperanee datums

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Crussolia deflandrei Wanaea fimbriata z

Crussolia deflandrei I Wanaea fimbriata

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Liesbergia scarburghensis Wanaea thysanota

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Liesbergia scarburghel'=sis Wanaea thysanota

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:

tion of the Western Barents Sea area ffrom Smelror & Bel o w 1 9 9 1 ) . Zonation de dinofla-

gellds du Toarcien & l'Ox/brdien prdcc~'e dans la rdgion de la Mer de Barents.

Cassiculosphaeridia dictydia Arnbonosphaera calloviana

Meiourogonyaulax planoseptata Chlamydophorella ectotabulata

- Toarcian to early dinoflagellate zona-

Meiourogonyaulax planoseptata

Ambonosphaera calloviana I Meiourogonyaulax plenoseptata

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Sirmiodiniurn grossii

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I Mikrocysta erugata M i k rocysta spp.

M ikrocysta erugata

An important synthesis of the agglutinated foraminiferal stratigraphy of Middle Jurassic to lowermost Cretaceous of Central Spitsbergen was published recently by Nagy et al. (1990) ; they demonstrated t h a t Spitsbergen and other areas of the Boreal Realm (i.e. western Siberia, the northern North Sea, Sverdrup Basin and western Canada) show an increased development of agglutinated assemblages in Middle Jurassic to Early Cretaceous time. Nagy et al. (1990) used foraminiferal distribution data for stratigraphic zonation of the Agardhfjellet and basal Rurikfjellet Formations, proposing the following foraminiferal zones (Fig. 4) :1) Trochammina rostovzevi Zone - Caltovian ; 2)Recurvoides disputabilis Zone - Oxfordian ; 3) Haplophragmoides canuiformis Zone - Lower and Upper Kimmeridgian ; 4) Trochamnzina rosacea Zone uppermost Kimmeridgian and Lower Volgian ; 5) Ammodiscus zaspelovae Zone - Middle Volgian ; 6) Trochanzmina aft. abrupta Zone Middle and Upper Volgian ; 7) Recurvoides obs-

kiensis Zone - Upper Volgian and lowermost Ry:tzanian ; 8) Gaudryina aff. milleri Zone - Ryazanian.

PALYNOMORPHS Although plant fossils were described from Svalbard by Sch/3rter (1880) and Upper Triassic pollen and spores reported by Pchelina (1967), the record of Jurassic palynomorphs first commenced in the mid 1970's, with the papers of Smith et al. (1976), Bjmrke & Dypvik (1976) and Bjmrke (1977). While the two former papers mainly concerned the Lower Jurassic pollen and spore assemblages of the Wilhelm0ya Formation, the third also gave the first description of dinoflagellates recovered from the Middle to Upper Jurassic Agardhfjellet Formation. A more comprehensive record of the dinoflagellate assemblages was provided by Thusu (1978), in his paper on Toarcian to Aptian dinoflagellate cysts from Arctic Norway. Subsequently, more detailed descriptions of the Jurassic marine microfloras from the

448 Agardhfjellet Formation in Spitsbergen and from the Wilhelmoya and Janust~ellet Formations on

Pareodinia capillosa, Horologinella spinosigibberosa) may be more useful.

Kong Karls Land have been provided respectively by Bjmrke (1980) and Smelror (1988). Additional taxonomic papers on Jurassic dinoflagellate from Svalbard by Bjmrke (1980a),o Below (1987a,b), Smelror (1989), Smelror & Arhus (1989) and Smelror (1991) have been published.

SEQUENCE STRATIGRAPHY

In his pioneering account of palynomorphs from the Mesozoic succession of Kong Karls Land, Bjmrke (1977) mapped the stratigraphic distribution of 116 species of spores, pollen, dinoflagellates and acritarchs, recognizing six associations of palynological assemblages within the Upper Triassic to Lower Cretaceous sequences. A more detailed, but informal, palynological zonation for the Upper Triassic to Lower Cretaceous succession was later proposed by Bjmrke, but this has not been published. More recently, Smelror & Below (1993) evaluated the stratigraphic distribution of ninety Toarcian to Lower Oxfordian dinoflagellate species from outcrop localities on Svalbard and Franz J o s e f Land and from shallow and deep wells in the Nordkapp and Hammerfest Basins ; they used the range of selected species to propose seven dinoflagellate zones, as follows (Fig. 5) : Mikrocysta erugata Zone, of Middle to Upper Toarcian age ; Dodekovia bullula - Nannoceratopsis senex Zone, of Aalenian and Lower Bajocian age ; Nannoceratopsis gracilis Zone, of Upper Bajocian to Middle Bathonian age ; Sirmiodinium grossii Zone, of Upper Bathonian age ; Meiourogonyaulax planoseptata - Chlamydophorella ectotabulata Zone, of Lower and Middle Callovian age ; Liesbergia scarburghensis - Wanaea thysanota Zone, of Upper Callovian age ; and the Crussolia deflandrei - Wanaea fimbriata Zone, of Lower Oxfordian age. This zonation scheme was correlated with previously published dinoflagellate zonations of northwest Europe and the Arctic. The only published record of later upper Jurassic palynomorphs is the previously mentioned paper by Wierzbowski and Arhus (1990), describing the ammonite and dinoflagellate cyst succession of the upper Oxfordian - Kimmeridgian strata in IKU core 7227/08-U-03 in the Nordkapp Basin. Owing to a general low diversity, a detailed upper Jurassic dinoflagellate cyst stratigraphy has not been established for the Barents Sea region. Most of the recorded taxa are long-ranging species known also from sub-boreal areas. According to Wierzbowski and ,~krhus (1990), the few endemic Boreal elements (include Pareodinia borealis,

Descriptions and correlations of Jurassic sequences of the western Barents Sea region, following "modern" principles of sequence stratigraphy, were first published by Embry (1989) and Johannessen and Embry (1989). In his paper on correlation of Upper Paleozoic and Mesozoic sequences between Svalbard, the Canadian Arctic Archipelago and Alaska, E m b r y (1989) recognized three main correlatable sequences within the Jurassic successions. Throughout the study area, major sequence boundaries occur at the b a s e of the Jurassic, at the base of the Bajocian, at the base of the Oxfordian and at the base of the Valanginian. A more detailed sequence correlation of the Upper Triassic to Lower Jurassic succession of Arctic Canada and the western Barents Sea area was given by Johannessen and E m b r y (1989). Their approach differs from the frequently used depositional sequence model of Vail et al. (1987) in the sense that they are using T-R cycles as the genetic stratigraphic unit. The sequences discussed further in this present paper are also T-R cycles and the sequences boundaries are recognized by Transgressive Surfaces rather t h a n Subaerial Unconformities and Correlative Conformities, as is done in the Exxon model. At some studied locations on the basin margins, however, the Transgressive Surfaces coincide with Sequence Boundaries as defined in that model. Johanessen and Embry (1988) recognized three correlative sequences within the Norian - Pliensbachian time interval, with important boundaries at the base of the Norian, near the Triassic - Jurassic boundary (i.e. Late Norian) and at the base of the Pliensbachian, respectively. They placed the top of the Norian sequence on the western Barents Shelf tentatively at the base of the Tubaen Formation. The basal Pliensbachian sequence boundary (BP in Fig. 2) is placed either high in the Nordmela Formation (southern Hammerfest Basin) or at the base of the St~ Formation (eastern Hammerfest and BjornOya Basins). This boundary is also recognized on Kong Karls Land, at the base of the Passet Member of the Wilhelmoya Formation (Johannessen & Embry 1989). The next sequence which can be recognized in the western Barents Sea - Svalbard region spans the Early Pliensbachian to Early Toarcian time interval. It is very important in the Hammerfest

449 Basin, since it represents the main potential reservoir unit in the area (Gjelberg et al. 1987). The upper sequence boundary, of Early Toarcian age, can be recognized in the lower St~ Formation in the Hammerfest and Troms~ Basins, where the transition represents a rapid sea-level rise. Following the interpretation of BfickstrSm and Nagy (1985), the sequence is represented by the upper Wilhelm~ya Formation on Central Spitsbergen. In the Hammerfest Basin, the Early Pliensbachian - Early Toarcian sequence is succeeded by a unit comprising very fine/fine-grained sandstone, followed by a strongly cyclic alternation between offshore mudstones and lower/upper shoreface sandstones. The upper boundary of this sequence is of Late Toarcian/?Aa!enian age and corresponds to the transition between units II and I of the St~ Formation (LT/A on Fig. 2). The sequence represents m a x i m u m transgression in the area. During the Late Bajocian - Bathonian, the Svalbard area became emergent. Strata of Hettangian - Aalenian age occur on Spitsbergen mainly as thin erosional r e n m a n t s of shelf shales and sandstones (Steel & Worsley 1984). This, combined with r a t h e r poor biostratigraphic control of some units, makes it difficult to correlate the deposits with sequences in the Barents Shelf. In eastern Svalbard, i.e. east of the Billet]orden lineament, a more complete Lower Jurassic succession is developed. Tentatively it appears t h a t the early Pliensbachian - Early Toarcian and the Early Toarcian - ?Aalenian sequences in the Hammerfest Basin m a y be correlative to the Passet Member of the Wilhelmoya Formation on Kong Karls Land. The ensuing ?Aalenian to Bathonian shallow marine sequence in the Hammerfest Basin, comprising the S t , Formation Unit I, has not been recognized on Svalbard which, as mentioned above, was emergent during this time. The unconformity surface of this upper sequence boundary (LB on Fig. 2) transgresses from Bajocian to Bathonian in the Hammerfest and Troms Basins and from Late Bathonian on Svalbard ; much of the Bajocian and Bathonian is represented by very thin condensed deposits, i.e. the Brendskardhaugen Bed on Svalbard and the Sto I unit in the eastern Hammerfest Basin, which contain several erosional surfaces or hiatuses. On Spitsbergen the overlying Upper Bathonian Lower Oxfordian succession generally comprises a fining upwards sequence, starting with the basal B r e n t s k a r d h a u g e n conglomerate and followed by shales of the lower Agardht~ellet Formation. Eastwards on Kong Karls Land, the basal conglo-

meratic unit is not recorded, but the lowermost siltstones of the Retziusi~ellet Member contain common reworked ?Toarcian palynomorphs, comparable to those found elsewhere in the Brentskardhaugen Beds. The Upper boundary of this Upper Bathonian - Lower Oxfordian sequence is at the Hfirfagrehaugen section on Kong Karls Land, marked by the abrupt change from claystone to the overlying sandstones of Lower Cretaceous Helvetiafjellet Formation (Smelror 1988). Elsewhere in the Svalbard area, the appear to be no profound depositional breaks within the Lower Oxfordian ; however, the transition between the zones ("sequences") 1 and 2 defined by Dypvik (1985) in the Agardh§ellet Formation m a y correspond to this lower Oxfordian sequence boundary (EO on Fig. 2). In the deep wells drilled in the H a m m e r f e s t and Troms Basins, the lower boundary of the Upper Bathonian - Lower Oxfordian sequence is marked by sharp increases in gamma ray and density respons and by an accompanying decrease in interval transit time (Worsley et al. 1988). The sequence boundary corresponds to the transition between the Sto and the overlying Fuglen Formation ; in this area, the whole sequence can be assigned to the Fuglen Formation. In the Hammerfest Basin the upper boundary of the Upper Bathonian - Lower Oxfordian sequence is defined by the transition from carbonate-cemented and pyritic mudstones to poorly consolidated shales, producing a sudden increase in interval transit time and an abrupt decrease in bulk density values (Worsley et al. 1988). At IKU corehole 7230/05-U02 in the Nordkapp Basin, the correlative sequence boundary is marked by a erosional surface and hiatus spanning the ?Early/Middle Callovian to Late Oxfordian. The last sequence boundary considered here is the basal Valanginian, which represents a major regional sequence boundary in the Arctic region (Embry 1989). In the Barents Sea area, regression started about Middle Volgian time when there were often minor stratigraphic breaks in the successions (Kelly 1988). On Svalbard, this regressive episode is expressed by the transition between the Agardhfjellet Formation and the Rurikfjellet Formation, the Ruriki~ellet Formation being locally unconformable on the Agardhfjellet Formation and older strata or on intruded dolerites (Parker 1967). In the Hammerfest and Troms0 Basins the Middle Oxfordian - Boreal Berriasian sequence corresponds to the Hekkingen Formation, which locally shows breaks in deposition at base and top, probably most fully developed near the basin axis. In the BjcrnCya Basin, an

450 Upper Jurassic dark shale sequence is overlain by condensed dolomitic limestone of Valanginian a n d H a u t e r i v i a n a g e ( / ~ r h u s et al. 1990) ; t h i s c o r r e s p o n d s to w h a t is o b s e r v e d o n K o n g K a r l s Land, where a thin sandy bioclastic limestone of Valanginian age assigned to the Tordenskjold Member overlies the Upper Jurassic shales of the R e t z i u s f j e l l e t M e m b e r ( S m i t h et al. 1976).

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M. S M E L R O R Continental Shelf and Petroleum Technology Research Institute N-7034 Trondheim, Norway