405

Eastern Barents Sea Late Palaeozoic setting and potential source rocks

P.C. Alsgaard

The Late Palaeozoic basins or the Eastern Barents Sea arc in general controlled by three orogenic lineaments, the Baikalian,

the Calcdonian and the eastern plate boundary. The Baikalian in the south includes the onshore Timan-Kanin Ridge which turns

from a northerly to a northwest direction seaward o[ the Kola Peninsula. The trend appears to be cut by the Vyalozero and the

Tuloma fracture zones. Similar northwesterly lineaments including the Tiddlybankcn Basin occur on the west side of these fracture

zones towards northern Nor.vay. The Calcdonides extend from the Norwegian mainland to Svalbard and probably eastward to

Franz Josef Land. A northeasterly Ca lcdonian trend is shown by a number or structures like the Nordkapp Basins, Scntralbank

High, a nameless basin cast of Svalbard and to some degree the Olga Basins. During Devonian and Early Carboniferous the

eastern plate boundary was characterized by subduction of oceanic crust along volcanic arcs which were located parallel with the

present-day Urals and Novaya Zemlya.

The distribution o[ potential source rock [acies in the Eastern Barents Sea during Late Palaeozoic can be represented by

palaeogeographic maps [or the Late Devonian (Fransnian-Famennian), Early Carboniferous (Late Visean-Serpukhovian) and

Early Permian (Sakmarian) periods. The Late Devonian basin in the Eastern Barents Sea was probably a northward extension or

the onshore Timan-Pcchora, Volga-Ura l and North Caspian basins. The palacogeographic map shows the restricted Domanik

facies and the bordering open shelf carbonate facies and their continuation from the Pechora area and into the Barents Sea.

During the Late Viscan to Scrpukovian a wide low relief shcl[ existed between the continental setting of the western Barents

Sea and the open basinal facics at the eastern plate boundary. On this platform the only basins present were reactivations of

Caledonian structures. An equivalent to the Devonian basin was not presen t at this time. An open shelf facies existed in the cast.

Towards the west marine conditions may have been more restricted and favou rable for source rock deposition. The Sakmari an period coincided with the early stage or the Uralian continent collision. Much of the Barents Sea was in an

open shelf setting. I !owever, areas of restricted ci rculation and source rock potential occurred locally as a function of topography.

Introduction

The Eastern Barents Sea is bordered by the is­lands of eastern Svalbard and Franz Josef Land in the north, Novaya Zcmlya in the cast and the Timan- Pcchora, the Kola Peninsula and the Norwe­gian mainland in the south (Fig. 1 ). The geologic literature on the sur rounding onshore areas is quite extensive and key references arc listed below for the different locali ties. In ofTshorc areas most of the interpretations arc of recent date. D ibncr, 1978 give an outline mainly based on scabottom samples and onshore geology. Extensive use of refraction and rcnection seismic have given more structu ra l details (R0nncvik and Jacobsen, 1984; Yunov et al., 1985; Bogolepov et al. , 1986; Vinogradov, 1987; Scnin and Shipilov, 1988). Palaeogcographic maps arc shown by Gramberg (1988) and Ziegler ( 1988).

The present article is largely based on published data, particularly from the Soviet Union. Addi tional mapping and eva luations have been performed by

this author and the sta fT at Esso Norge. Throughout the article the name Fennoscandian is used for the plate which by other authors also has been called the Laurussian, Ba ltic, Fannosarmatian or Euro­pean. Lineaments arc given with only one geograph­ical direction like northeast instead of northeast­southwcst.

Setting

The Late Palaeozoic basins of the Eastern Bar­en ts Sea were mainly controlled by three orogenic lineaments, the Baikalian, the Caledonian and the eastern plate boundary (Fig. 2). These tectonic belts arc believed to have been important factors for the facics development.

Baikalian T he Baikalian orogcny is of latest Proterozoic

age (Sultanaycv and Danilcvskiy, 1979; Khain, 1985; Ulmishck, 1985). The most prominent Baikalian

Arctic Geology and Pc1rolc11111 Po1c111ial edited by T.O. Vorren, E. Bergsagcr, 0.A. Da hl-S tamnes, E. I l oller, B. Johansen, E. Lie and TB. Lund. NPF Special Publication 2. pp. 405-418, Elsevier. Amsterdam.© Nor.vegian Petroleum Society (NPF), 1992

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Eastern Barents Sea Late Palaeozoic selling and potential so11rce rocks 409

element present day is the Timan- Kanin Ridge. The onshore Timan Ridge extends into the Kanin Ridge and turns from a northerly to a northwest direction. Offshore the Kola Peninsula the Kanin Ridge trend continues based on a gravity high from a SEASAT map (Fig. 3) and GEOSAT profiles. It is problematic whether the gravity high is due to a structural high or a basement monocline with an internal high-density lineament, however seismic data seem to support a monocline interpretation (Shvarts, 1977; Bogolepov et al., 1986). The northwest trending gravity high is cut by both the Vyalozcro and Tuloma fracture zones. The onshore parts of the fracture zones on the Kola Peninsula are documented by Kirichenko (1970); Makiyevskiy and Pctersil 'yc (1971) and Kirichcnko (1978) . The Vyalozero fracture zone was active in the Late Devonian to Early Carboniferous and the Tuloma fracture zone was likely to be active at the same time. West of the Tuloma zone the gravity high is not present, but GEOSAT profiles show that a northwesterly lineament probably continues towards northern Norway. A Baikalian age has not been documented on the Norwegian side, but some ra­diometric datings in Finnmark arc quite close and possibly overlap with the Baikalian. A dolerite dyke near Bcrlcvag has given a K/Ar age of 542 ± 17 M a, Middle Cambrian (Beckinsale et al., 1975).

In addition to the Baikalian belt described above, a larger area is controlled by Baikalian tectonic clements and their reactivations. This include the Pechora Basin, Pay-Khoy and parts of the South Barents Basin (Bclyakov and Dcmbovskiy, 1984; Bo­golepov ct al. , 1986). The Tiddlybanken Basin also seems to have a Baikalian axis based on maps by R0nnevik and Jacobsen (1984); Bogolepov ct al. (1986); Vinogradov (1987) and the SEASAT data.

Caledonian The age of the main phase of the Ca lcdonian

orogeny is Silurian to Early Devonian. The existence of an earlier Finnmarkian phase is disputed (Krill ct al., 1988; Roberts, 1988b; Andersen, 1989). In the western part of the Barents Sea the Ca lcdonian orogeny probably extends from onshore Norway to the eastern Svalbard. The western part of Svalbard has a different development including a Late Devo­nian Svalbardian phase (Harland, 1985; Ohta et al., 1989) .

The intersect ion where the possible Baikalian trend is overprinted by the younger Ca lcdonidcs is seismically defined by a northeast Calcdonian fault trend about 50 km cast of the Nordkapp Basin South (Figs. 2 and 4). There is an additional 85 km shift eastward to match up with the easternmost onshore Ca ledonian thrust front at Berlevflg (Gee ct al., 1985;

Roberts, 1988a) . If the thrust at Bcrlcvag continues offshore, without this offset, it must be so thin that it docs not control the Late Palaeozoic fa ult reactiva ­tions.

The eastern extent of the Caledonian front north of the Nordkapp Basin is difficult to determine. The in general northeasterly structural trends west of the front suggest that the extent is limited by a line between the Nordkapp Basin North and the Tid­dlybanken Basin, along the west side of the North Barents Basin and northward to the Franz-Victoria Trench. The northeasterly trend is assumed to be inherited from Calcdonian tcctonism (R0nnevik and Jacobsen, 1984), and include the Nordkapp Basins, Norsel High, Vads0 High, Mercurius High, Gardar­banken High (Fig. 4), Sentra lbank High, a nameless bas in and to some degree the Olga Basins South and North (Fig. 2). East of the boundary proposed as the eastern extent of the Calcdonidcs, in the South and North Barents Basins, there is a general lack of documented Caledonian trends. They could have been overprinted by the Uralian event in the Per­mian or obscured by the very thick Triassic section. The outline of the structural elements east of 32° or north of 76° arc taken from R0nnevik and Jacobsen (1984); Bogolepov ct al. (1986); 11.nakhovskiy et al. (1987); Vinogradov (1987); Gramberg (1988); Senin and Shipilov (1988) and BGR and NPD seismic lines. The postulated eastern position of the Ca ledonian front will make the orogeny on the Fennoscandian plate about as wide as in the southern Norway/ northern North Sea.

To the north, from Nordaustlandc t on Svalbard to Franz Josef Land the Ca ledonian type tccton­ism appears to be relatively similar to further south. D espite this similarity the northern area is differ­entiated by colour to indica te the possibility of a difTerent development (Fig. 2). If the New Siberian Islands on the Chukotska block were located north of the Fcnnoscandian plate this could facilitate an explanation for the Caledonian like event from Franz Josef Land and further east to the islands of Sever­naya Zemlya. The plate reconstruction of this area has been discussed by Scotcse ct al. (1979), Ziegler (1988) and Rowley and Lattes (1988).

The Caledonian like descriptions from the Nor­daustlandet area and Franz Josef Land are based on isotopic ages, stratigraphic gaps and fold strike. On Nordaustlandc t isotopic ages are generally in the range 435-340 Ma., Early Silurian- Early Car­boniferous (Flood et al. , 1969). On Franz Josef Land there is a radiometric age of 360 M a., onset of the Carboniferous, for the basement in a well (Ulmishck, 1985) . The gap in the stratigraphy and the assumed palaco-highs seem to be about the

410 P.C. Alsgaard

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same on the northeastern Sva lbard and Franz Josef Land. In the Nordaustlandet area the pre Ordovi­cian Hecla Hock "basement" in genera l is overlain by no older than Early Carboniferous (Flood et al.,

1969; Lauritzen, 1981; Harland, 1985; ElverhOi et al., 1988). T he drill ing on Franz Josef Land show folded basement unconfo rmably overlain by a thin Carboniferous sect ion (Gramberg et al., 1985). This

Eastern Barents Sea Late Palaeozoic selling and potential source rocks 411

245 Ma

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hia tus is similar to that observed on the structural highs in the southwestern Barents Sea (Bj0rn0ya: Gjelberg, 1981; Well 7120/12-2 in the Hammcrfcst Basin: Oljedirektoratet, 1987). On Nordaustlan<let and Kvi10ya the strike of the major basement folds arc northerly while on Franz Josef Land there is no data (Flood et al., 1969; Hjelle ct al., 1978; Ohta, 1978, 1982).

Both the Baikalian and the ca tcdonian orogcnies preda te the Late Palaeozoic. The eastern Fennoscan­dian plate boundary is different in that it was ac­tive during parts of the Late Palaeozoic. This plate boundary will be described toge ther with each of the palaeogeographic maps and in particular for the La te Devonian interval (Fig. 5).

Palaeogeography and source rocks

Late Devonian (Frasnian-Famennian) To evaluate the extent of the Late Devonian Do­

manik source rock it is necessary to describe the setting at the eastern plate boundary including the present-day Ural Mountains and Novaya Zemlya.

Zonenshain c t al. (1984) outline the development of the Southern U rals, and in genera l the same events can probably be extended to the Northern Urals. East of the Timan- Pechora area there was a back arc basin, a volcanic arc and a subduction zone (Fig. 6). In the Frasnian the crust that was subducted represented the remnants of the Ura lian ocean. In

the Famennian the Mugodjarian micro continent was partly subductcd and later collided with the arc. Subsequently elastics were shed westward across the narrow back arc and onto the Fcnnoscandian platform. A departure from this model is that in the north elastics are seen coming from the east as ea rly as Eifelian, Middle Devonian (Nalivldn c t al. , 1973). Remnants of the arc described above can present day be found in the U ral Mountains.

On Pay-K.hoy and Vaygach between the Urals and Novaya Zemlya the Late Devonian consists mainly of shallow marine elastics and carbonates (Sergiyevsldy and Komarov, 1963; Nalivkin ct al., 1973; Cherkesova, 1973a, b) . An exception is a trend towards deeper water shales and a lack of dolomiti­zation of the ca rbonates to the northeast. Scarcity of observed volcanism on Pay-K.hoy and Vaygach con­trasts with the Urals and Novaya Z emlya and could be due to a possible transform along Pay-Khoy and Vaygach active in the Palaeozoic or only during parts of the main U ralian collision.

For the Late Devonian palaeogeographic mapping along Novaya Zcmlya it has been assumed that the tectonic setting in general is the same as for the Urals. This is in the best case an over-simplifica tion, and the plate reconstruction of Fcnnoscandia versus Siberia and Kazakhstan is uncertain (Scotcsc and McKerrow, 1990). Along the axis of Novaya Zemlya there are major fa ults separating different facics zones. Early Frasnian mafic volca nism with minor units of intermediate composition are found within these zones, but it is not sure 10 which degree they arc allochthon (Markov, 1975; Bonda rev et al., 1977; Andreyeva ct al., 1979; Sobolev and Shckoldin, 1982; Timofeyeva, 1982; Sobolcv et al., 1985). T he amount of Palaeozoic oceanic crust observed on Novaya Zcmlya is minor to what is found in the Urals. In addition to the volca nics a ca rbonate platform was the dominant in the western part of the island during the Late Devonian. In the very eastern part a deep wa ter facics probably prevailed. At about Early Frasnian time there were also conglomerates in the northwestern Novaya Zcmlya indica ting the existence of an uplift nearby.

The Domanik source rock is described by Grachevskiy and Solomatin (1977) ; Aliyev et al. (1978); Gurari (1981); U lmishck (1982); U lmishck (1985); Artyushkov and Baer (1986) and U lmishek (1988). The source rock is the result of deposition during an eustatic high and basin subsidence devel­opment. A major general transgress ion in the Late Devonian is described by Bondarcv c t al. (1977) and Johnson ct al. (1985). Within the same period, Fras­nian comparatively deep wa ter basins were formed and a restricted basinal source rock was deposited.

412 P.C. Alsgaard

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Shallow water carbonates fl anked the basins. The time span of the source rock generally va ries between Late Frasnian-Famennian and Early Frasnian- Early Visean.

Present day onshore the Domanik fades is proba­bly present from the Caspian Sea and up along the west side of the Urals to the Barents Sea. In the Timan- Pechora basins west of the Northern Urals and in the Volga- Ural basins west of the Southern Urals the source rock is well documented. Further south in the North caspian Basin it is assumed to

exist. There seems to be an empirica l relationship between the basins containing the Domanik fades and their proximity to the plate boundary in the east, but the cause of this relationship is not well understood. In addition to the proximity to the plate boundary, reactivations of the Baikalian trend were important for the development of the basins in the Timan-Pechora and at least some of the ones in the Volga-Ura l area.

Mainly based on the simila rities between the east­ern plate boundary along the Urals and Novaya Zemlya and the proximity of the Domanik facies basins, it is postulated that the Domanik facies prob-

ably extends from the T iman-Pechora and north­wards into the Barents Sea. Another similarity is a major unconformity at the base of the Late De­vonian found both in the Timan-Pechora and in the South and North Barents Basins (Onshore: Ul­mishek, 1982; Offshore: Murzin et al., 1984). This indicate an equivalent basinal development in the two areas. A difference north of 72° is the lack of well documented Baikalian trends. Ulmishek (1985) does not believe in the presence of Domanik rocks far north of Timan-Pechora presumably because of mainly shallow water facies seen on the western Novaya Zemlya.

The total organ ic carbon (TOC) in the Late Devon ian shales documented onshore range 1-20% whereas 5-10% is a more common spread (Katchenkov, 1975; Uspenskiy et al., 1979; U lmishek, 1982). Thicknesses are commonly a few tens of me­ters with up to 200-300 m in places. The organic matter has a high algal content and it is a very good source for oil (Grachevskiy ct al., 1976; Podel'ko et al., 1985). In the Volga-Ural area maximum palaeo­watcr depths are interpreted to have been 400 m (Artyushkov and Baer, 1986).

Eastern Barents Sea Late Palaeozoic serring and poremia/ source rocks 413

North of the Timan- Pechora the only data on a good Frasnian- Tournaisian source rock is from the southwestern part of Novaya Zemlya (Nepomiluyev, 1982; U lmishek, 1982). It is not certain that the source rock here represent the same setting as further south. Neither is it clear if it is from an autochthon part of the island.

The extent of the marine facies westward in the Barents Sea was mainly controlled by the Caledonian orogcny. This event in general terminated in the Early Devonian. In the Late Devonian the area of calcdonian influx is assumed to have had an erosion and interior basin facies. The origin of the Central Barents High is uncertain, but it has a thin Palaeozoic sequence which is likely to be younger than the Late Devonian. This thin Palaeozoic interpretation is de­rived from a depth to basement map by Shipilov and Scnin (1988) and a Triassic isopach map by Alekhin (1988). Below is a list of references which have been used for the Late Devonian palaeogcographic map and were not discussed above.

Northern Urals: Kolguyev Island: Kanin Peninsula:

West of Timan Ridge: Kola Peninsula:

Bogdanov ct al. (1979) Bro et al. (1988) Getsen (1975a, b); Gctsen and Naumov (1973) Yinogradov (1969) Maslyayev (1989)

Upper Early Carboniferous (Late Visean­Serpukhovian) For the Late V isean- Scrpukhovian period there is

no documented source rock; however if organic rich rocks occur in the Late Palaeozoic this unit is a good candidate. Below follows a general description of the facies distribution.

The Late Yisean-Serpukhovian was probably on­set by a major transgression and followed by severa l sea level fluctuations (Ramsbottom, 1979; Yeevers and Powell, 1987; Ross and Ross, 1988). Onshore south of the Eastern Barents Sea the Late Yisean­Serpukhovian is described by Yinogradov (1969); Chcrnov (1972); Dedeyev (1982) and U lmishek (1982). The l ithology is relatively uniform throughout the area, with alterna ting limestones, dolomites and in places evaporites. The Late Devonian basins were not present at this time. They had been filled with Tuurnaisian-Middle Visean mainly elastics. Except for a few exposed highs there was little topography. One trend was towards slightly more sha llow and restricted facies in the west with more dolomites and evaporites. In the east there was generally a higher content of shaly limestones (Fig. 7). The variation in dolomitization could be due to secondary diagenesis instead of deposi tional facies.

In the Barents Sea there is a lack of available data for the Late Viscan- Scrpukhovian. The area is in general assumed to correlate with the onshore to the south, that is a low relief carbonate pla tform with slightly more open marine conditions in the east. Based on seismic interpretation by the author in the Nordkapp Basin and by Turakhovskiy et al. (1987) and Gramberg (1988) in the Olga Basin North, there were probably marine basins present in these places. Particularly in the Nordkapp Basin area the seismic reflections arc laterally continuous and med ium high amplitude which could refl ect shales and carbonates with potential restricted facics intervals. Other basins further west were closer to the center of the Caledonide orogeny and more likely to be non-marine at this time. On Spitsbergen a broad coastal pla in is shown by Steel and Worsley (1984) and the same facies is possibly present in the Raddcda len 1 well on Edge0ya, eastern Svalbard (Svare, 1985).

On Novaya Zemlya the western part of the island was dominated by shallow water ca rbonates with a distinct transition to bathyal deposits in the east (Platunov and Chernyak, 1982; Sobolev et al., 1985). There is a lack of observed major igneous activity in the whole period of ca rboniferous and Early Permian. Only minor dykes are seen (Markov, 1975; Korago an Chukon in, 1984). On the " Fennoscandian side" of the Urals there is also a lack of volcanism from the Late Viscan to the Bashkirian (Zonenshain et al., 1984).

The relative facies changes mapped in Fig. 7 arc believed to have persisted through a larger period of time than just the Late Visean-Serpukhovian. The general change from the Late Visean to the Early Permian was that each transgression reached further west across the eroding and cooling Caledonides. The middle part of the carboniferous is poorly doc­umented in the Barents Sea, but it cou ld contain organic r ich rocks. The Late Visean- Serpukhovian is shown as a representative horizon. Rocks with source potential would primarily be expected in the possible restricted shelfa l area and in the loca l deeper basins which presumably also had a restr icted facies.

Early Permian (Sakmarian) During the Sakmarian there was probably a gen­

eral transgression. Much of the Barents Sea was dominated by open marine conditions. T here were shallow wa ter carbonates along the margins and on the local highs. A deeper shelf or basinal carbonate facies was found elsewhere (Fig. 8). Seismic interpre­tation of the western Barents Sea shows mounded fa­cies along many of the highs and shelf edges towards

414

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Eastern Barents Sea Late Palaeozoic selling and potential so11rce rocks 415

the deeper areas. On the northwest side of Novaya Zcmlya Vinogradov (1987) also shows buildups and shelf edges of possible Early Permian age.

The source rock potential is best illustrated on Svalbard (Skaug et al., 1982; Worsley and Hcllcm, 1982; Steel and Worsley, 1984; Hcllcm, pcrs. com ­mun., 1989). The Tyrcllfjcllet Member of Assclian­Sakmarian age contains restricted facics with carbon­ate biohcrms along the margins. The bioherms were probably important as a barrier to the open marine conditions. The water depth in the restricted areas could range from lagoonal to shallow basinal, but probab ly a plat form ra ther than a basin setting. The depositional texture is wackcstone to packstone with mainly fusulinid grains. The TOC is up to 25%, but more commonly around l % TOC in 1-4 m beds. T he kcrogen type is II - III. It is not unlikely that similar restricted lagoonal or shclfal sett ings existed elsewhere in the Barents Sea, but it can not be ex­cluded that any such occurrences have a difTercnt age. Further cast on Sva lbard the Raddcdalen J well probably shows a relatively open basinal ca rbonate facies (Svare, 1985).

In the Sakmarian the main continent collision started to afTect the Eastern Barents Sea. In the southern part of Novaya Zcmlya there were fi ne elastics (Ustritskiy, 1981) which arc likely to have come from the Siberian continent in the cast. To the south along the present -day Ura ls the Fcnnoscandian plate collided with the Kazakhstan plate at about the same time (Scotesc ct al., 1979; U lmishck, 1982; Zoncnshain ct al. , 1984; Scotese and McKerrow, 1990). Clastic input began to afTcct the ca rbona te regime in the T iman- Pcchora area. Some of the troughs had a deeper wa ter facics with carbonates and fine elastics. On the west side of the Timan " r idge" there was a restricted shallow marine facics (Yinogradov, 1969). Potential source rock is not observed as on Sva lbard.

Conclusions

The Ba ikalian and Calcdonian orogcnies played an important role for the facies development in the Eastern Barents Sea in the Late Palaeozoic. Tectonic activity along the eastern plate boundary including the present-day Urals and Novaya Zcmlya was also important, but we do not have sumcicnt knowledge to understand it fully.

The La te Devonian Domanik source rock is pos­sibly present in the Barents Sea along the west side of the Novaya Zemlya in Soviet territory. The Early Carboniferous and Early Permian only contain po­tential source rocks and they arc move likely to be found further west in Norwegian and Soviet areas.

Acknowledgements

This work would not have been possible without the full support by Esso Norge a.s and extensive help from the Esso librarians. The author is also grateful for the assistance and contributions from other colleagues including D. Bergslien, T Nardin and K G. R0ssland.

References

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PER Cl IRISTIAN ALSGAARD Essa Norge a.s., I~O. Box 60, N -./033 Forus, Nonvay

I' resent addre5s: Amoco Nonvay Oil Co111pa11); P.O. Box 388, N-./001 Stal'a11ger, Nonvay