Palaeocene and Eocene development of the Tromsø Basin sedimentary response to rifting and early sea-floor spreading in the Barents Sea area
STIG-MORTEN KNUTSEN, LARS-JOHAN SKJOLD & PÅL HERMANN SKOTT
Knutsen, S.-M., Skjold, L-J. & Skott, P. H.: Palaeocene and Eocene development of the Tromsø Basin - sedimentary response to tifting and early sea-floor spreading in the Barents Sea area. Norsk Geologisk Tidsskrift, Vol. 72, pp. 191-207. Oslo 1992. ISSN 0029-196X.
Multichannel seismic data combined with weU-log information of Palaeocene and Eocene (lower Palaeogene) strata reveal the stratigraphy and lower Palaeogene evolution of the Tromsø Basin and adjacent areas. Four sequences are recognized. A Palaeocene sequence TtAI, a late Palaeocene to early Eocene sequence TtA2, and an early-mid-Eocene sequence TtBI show progradation into the basin. Progradation ceased due to a relative rise of sea-leve! as sequence TtB2 of mid-?late Eocene times was deposited. The his tory of the Tromsø Basin mirrors the la test phase of tifting and early opening his tory of the Norwegian-Greenland Sea. Transpression caused uplift of highs adjacent to the basin area and led to progradation at the basin margins in late Palaeocene to early-mid-Eocene times, as represented by sequences TtAI, TtA2 and TtBI. Sequence TtB2 reflects increased subsidence rates and relative rise of sea-leve! in the ?late Eocene to ?early Oligocene as spreading of the Norwegian-Greenland Sea propagated northwards. Post ?early Oligocene times were characterized by several phases of erosion, leaving only Palaeocene to mid-?late Eocene marine facies preserved.
S.-M. Knutsen, JBG, University of Tromsø. Present address: Norsk Hydro a. s. P. O. Box 31, N-9401 Harstad, Norway; L.-J. Skjold, Norsk Hydro a. s. Bergen, Norway; P. H. Skott, Norsk Hydro a. s. P. O. Box 31, N-9401 Harstad, Norway.
The Tertiary tectonic evolution of the western Barents fest Basin and the deeper Tromsø Basin. Towards the Sea margin (Fig. la), and the tectonic movements during north of the Tromsø Basin, the Ringvassøy-Loppa Fault the Tertiary opening of the northeast Atlantic, have been Complex interfingers with the Bjørnøyrenna Fault Com much studied recently (Talwani & Eldholm 1977; Myhre plex (BFC) (Fig. la, b). The Senja Ridge and the et al. 1982; Spencer et al. 1984; Faleide et al. 1988; Veslemøy High make up the western boundary of the Myhre & Eldholm 1988; Nøttvedt et al. 1988; Ziegler basin. The elongated highs west of the Tromsø Basin are 1988). More recently, Vorren et al. (1991) have presented bounded by NNE-SSW oriented faults along the Eastern a regional study concerning the Cenozoic sedimentary Senja Ridge Fault System (ESRES) (Gabrielsen 1984). and erosional history of the Barents Shelf, and Richard Salt diapirs, located along the NNE-SSW basin axis sen et al. (in press) have discussed the Tertiary develop (Fig. lb) have probably originated from upper Carbonif ment in areas south and west of the Stappen High. erous and lower Permian evaporite deposits; they pene Knutsen & Vorren (in press) discussed the early Ceno trate both Mesozoic and lower Cenozoic sediments (e.g. zoic sedimentary history in the Hammerfest Basin. Here Faleide et al. 1984). we concentrate on the Tromsø Basin, located west of the The fault zones bounding the basin have a dominantly Hammerfest Basin (Fig. la). By using seismic and well N-S orientation, the same main orientation as the west data we present a stratigraphic framework for the ern continental margin. The Eastern Senja Ridge Fault Palaeocene and Eocene strata, discuss their sedimentary System shows evidence of having been active in both environments and related the sedimentary response in the Mesozoic and Cenozoic times, and.similar activity is seen Tromsø Basin to the rifting and early sea-floor spreading along the Ringvassøy-Loppa Fault Complex east of the of the Norwegian-Greenland Sea. Tromsø Basin. According to Hanisch (1984), this fault activity mirrors a regional Cretaceous rift system which covered the Tromsø Basin and its sourthern extension. Geographic and geo/ogic setting Such Cretaceous rifting and failed ocean-opening ex The Tromsø Basin has a N-S orientation, and its geo plains the N-S intersection of the E-W oriented Ham graphical limits are approximately 7l-72°30'N and 18- merfest Basin, resulting in the faulted transition zone 200E (Fig. la, b). To the east the basin is limited by three between the Tromsø and Hammerfest Basins (Øvrebø & structural elements: The Finnmark Platform, the Ham Talleraas 1977; Hanish 1984). High subsidence rates in merfest Basin and the Loppa High. The Ringvassøy the Tromsø Basin during the Cretaceous are reflected by Loppa Fault Complex (RLFC) delineates a transition the accumulation of an approximately 5 s TWT Creta zone between the shallower NE-SW oriented Hammer- ceous sediment thickness ( Gudlaugsson & Faleide 1987). 192 S.-M. Knutsen et al. NORSK GEOLOGISK TIDSSKRIFT 72 (1992)
1roo·
17" 18" 19" 20"
30km
Sorveatanaget Baaln
Asten.. Fwt AFC: Cotrøttc e.,.rneyr.,.. FIUtCon1liP BFC : KFZ: Kne1egg1iF•IAt Zone Airlgvaney ·L� F.Ut �· ALfC Tlomt·F�F·!Jt� iliiTf'FC :ttrnllllntofocHI'IC:cr�t a b
Fig. l. (a) Structural elements of the western Barents Sea (compiled from Talwani & Eldholm 1977; Faleide et al. 1984; Berglund et al. 1986; Bergsager 1986; Gabrielsen et al. 1990). Magnetic anomalies indicated on the oceanic crust. Framed area enlarged in (b). (b) Simplified structural map showing main elements in the Tromsø Basin area (compiled from Gabrielsen 1984; Gabrielsen et al. 1990). Salt is indicated in the Tromsø Basin. Black circles denote well positions. ESRES: Eastern Senja Ridge Fault System.
The late Cretaceous and Tertiary spreading history of the Barents Shelf and the western margin are covered the Northeast Atlantic and the Norwegian-Greenland with Quaternary glacial sediments deposited above the Sea had a stepwise evo1ution (Hanisch 1984; E1dho1m et URU during the last 0.8 Ma (Vorren et al. 1989). al. 1987; Myhre & Eldholm 1988). Prior to magnetic anomaly 25 (i.e. 56 Ma), sea-floor spreading occurred south of the Norwegian-Green1and Sea, which possib1y Database led to transpresion and transtension along rift zones on During the last decade the Tromsø Basin has been drilled the western Barents Shelf (Hanisch 1984; Knutsen & by three commercia1 wells (71 19/7-1, 71 19/9-1 and 72 19 f Vorren in press). From the onset of sea-floor spreading Fig. lb). We have had access to composite logs from west of the Greenland-Senja Fracture Zone in late 9-1, these wells, in addition to biostratigraphy reports. Sev Palaeocene to early Eocene times (magnetic anomalies eral seismic surveys have been shot in the Tromsø Basin 25-24), and until early Oligocene times, the fracture zone area both by the Norwegian Petroleum Directorate acted as an oblique transform between the new ocean and (NPD) and by the oil companies. We have interpreted the Barents Shelf. As sea floor spreading took place west several of these surveys. Emphasis has been placed on the of the Greenland-Senja Fracture Zone, the spreading NPD surveys from 1982-84, the Troms North Group gradually propagated northwards (represented. by mag Shot survey from 1983 (TNGS-83), and two surveys shot netic anomalies 23-13). At anomaly 13 (early Oligocene) by Norsk Hydro in 1984 (NH840 l and NH8403). Gener the spreading changed from a dominantly northwards ally, the data quality of the surveys is good, and the direction to a more west-northwest orientation. seismic lines have an average spacing of 2-3 km. Tertiary sediments on the Barents Sea Shelf are found mainly in the western intra-cratonic basins (Fig. la): the Hammerfest (Knutsen Vorren in press), Bjørnøya and & Cenozoic stratigraphy of the Tromsø Basin Tromsø (Vorren et al. 1991) basins. In exploration wells east of the Senja Ridge, Palaeocene and Eocene strata The Palaeogene succession in the Tromsø Basin is repre are separated from Pleistocene sediments by a hiatus (the sented by one Palaeocene sequence, TtA1 (T =Tertiary, upper regional unconformity (URU) ; Vorren & Kristoff t =Tromsø Basin, Al =Sequence number) and three ersen 1986; Eidvin & Riis 1989). Neogene sediments are Eocene sequences TtA2, TtB1 and TtB3 (Figs. 2, 3). The present only at the western margin of the shelf. Most of nomenclature of the seismic sequences and units used in NORSK GEOLOGISK TIDSSKRIFT 72 (1992) Palaeocene and Eocene deve/opment, Tromsø Basin 193
Knutsen and Vorren Vorren et al. This (Fig. 7). On the Senja Ridge the sequence is eroded, (1991) work (in press) (e =Egga Basin) (t = Tromsø Basin) h = Hammerfest Basin) and west of the ridge it onlaps older positive structural
;;; elements just east of the oceanic crust. ·;;; � ii: TeO Sequence TtBI is also thickest in the Veslemøy High area ( 1000 ms TWT, Fig. 4e). The maximum thickness j > !!! a.. w TeC l u coincides with an area of southwest directed, low-angle o � !!! � progradation (Fig. 8). Progradation towards the east is .2 -+ observed along the eastern fl.ank of the Senja Ridge (Fig. c: > .. TeB 9). In the Veslemøy High area and the northern part of a: "o < ;::: � the Tromsø Basin, a conspicuous high-amplitude refl.ec a: ___Q__ w � ... w tor is seen within sequence TtBl (Fig. 8). The refl.ectors Cl c:.. o .. T w "o TtB1 beneath seem to be parallel to the high-amplitude refl.ec < w TtA2 ..J ThA2 tor; younger refl.ectors above have a different dip and � � i TeA ThA1 "o TtA1 downlap on to the high-amplitude horizon. .; rTTT1hmmmmnlnrm Sequence TtB2 is thickest in the southern central part æ of the Tromsø Basin (Fig. 4f). The sequence probably Fig. 2. Simplified Cenozoic stratigraphy of the Tromsø Basin showing the age was more widespread during deposition, but owing to and relationships between the sequences discussed in the text. erosion only the basinal parts are preserved. They are represented by parallel, continuous and medium- to this work mainly follows the published regional studies high-amplitude refl.ectors. from the southwestern BarentsSea (Vorren et al. 1991) and from the Hammerfest Basin (Knutsen & Vorren in press). Post-Eocene In the western part of the Tromsø Basin, the Palaeocene Palaeocene and Eocene sequences are truncated by the base of unit Towards the Loppa High, the Finnmark Platform, the TeC (Fig. 6), which has a regional westerly dip (Fig. Senja Ridge and the Veslemøy High, the base of the 4g). Erosion at the base of unit TeC seems to have been Tertiary succession is an erosional unconformity truncat more extensive in the areas west of the Tromsø Basin ing underlying strata. The base Tertiary refl.ector (Fig. 4a) where several of the Palaeogene sequences (TtA-TtB2) shows a relief varying from 2500 ms TWT in the western are very thin or are absent (Faleide et al. 1988). The part of the Tromsø Basin to 600 ms TWT on the Loppa mainly progradational sedimentary wedge above the High, where the refl.ector is truncated by the URU. To the base of unit TeC pinches out east of the Senja Ridge, north, along the basin axis, the refl.ector shallows. and has been divided into units TeC, TeD and TeE The lowermost seismic sequence TtA1 has a relatively (Vorren et al. 1991; Figs. 2, 9). These three units have a uniform thickness (Fig. 4b), which reaches a maximum sigmoidal internat reflector configuration with a regional at the eastern margin of the Tromsø Basin. In the basin, N-S strike and westward dip, and were defined as the sequence is characterized by parallel bedded, laterally Units Illa, Illb and IV, respectively, by Spencer et al. extensive refl.ectors. They onlap the basin margins in the ( 1984). The base of sequence TeC and the base of lower part, and show low-angle westward progradation sequence TeD locally show large relief in the western in the upper part (Fig. 3). The Palaeocene sequence Tromsø Basin (Fig. 9). Wells on the Senja Ridge show along the Ringvassøy-Loppa Fault Complex and the that the Neogene is represented by late Pliocene and Eastern Senja Ridge Fault System is infl.uenced by fault Pleistocene sediments (Eidvin & Riis 1989), which
ing (Figs. 3, 5, 6). West of the Senja Ridge, in the south mainly correlated to sequences TeD and TeE of Vorren em Sørvestsnaget Basin, sequence TtAI drapes and et al. (1991). Sequence TeC is eroded by the base of onlaps older faulted rocks. sequence TeD in the Senja Ridge wells. It has been suggested that sequences TeC and TeD may represent upper Miocene to Pleistocene deposits in areas where Eocene the sequences are hetter preserved (Vorren et al. 1991). Sequences TtAl-TtB2 and units TeC and TeD are all The base Eocene unconformity shows a regional dip truncated by the URU (Figs. 3, 8, 9), which defines the towards the west (Fig. 4c). This refl.ector marks the base of the uppermost late Pleistocene glacially infl.u lower boundary of three Eocene sequences. The refl.ector enced sequence TeE on the shelf. is eroded by the base of unit TeC on the Senja Ridge (Fig. 6), but is preserved on the Veslemøy High. The lowermost Eocent sequence (TtA2) thickens in We/1 data the northern part of the Tromsø Basin, reaching a max imum of 800 ms TWT in the Veslemøy High area (Fig. The wells in the Tromsø Basin (Fig. lb) are located west 4d) ; progradation from the Loppa High is seen here of the Loppa High in the Ringvassøy-Loppa Fault 194 S.-M. Knutsen et al. NORSK GEOLOGISK TIDSSKRIFT 72 (1992)
Comp1ex (72 19/9-1), in the eastern (71 19/9-1) and cen tral (71 19/7-l ) parts of the bas in el ose to a salt dia pir. The lower Palaeogene is dominated by claystone and siltstone with thin stringers of dolomite and dolomitized carbonate (Fig. 10). Common minerals are glauconite, pyrite and mica. In wells 71 19/7-l and 72 19/9-1, the upper parts are interpreted as glacigenic diamictons. o o Figure 10 illustrates the re1ative1y stable gamma-ray N M UJ o � intensity in the Palaeogene strata in all three wells. It is �- � worth noting, however, that the gamma intensity is high ..c in the lower part of wells 7119/7-1 and 71 19/9-l ; in well
71 19/9-1 the gamma intensity is high at about 900- o 1000 m; and from about 1000-700 m in well 7219/9-l there is an increase in gamma intensity.
c: (J) Ages of the seismic sequences in the Tromsø Basin o o C! ·oas �M� o o .,.... C! ·oas �M� ei c-i ei .,.... c-i w o o o:t N g C') N o o N N o o .,.... N o o o N o o Ol .,.... o o co .,.... 196 S.-M. Knutsen et al. NORSK GEOLOGISK TIDSSKRIFT 72 (1992) 25km 25km a Base Tertiary isochrons b TtA 1 isopach (Palaeocene) 2Skm 25km c Base Eocene, isochrons d TtA2 isopach (Late Palaeocene-Early Eocene) NORSK GEOLOGISK TIDSSKRIFT 72 (1992) Palaeocene and Eocene development, Tromsø Basin 197 0 1r7-' ____ 1-rs·----� 19_ · -----.2 _· _____,21' 7 •3 . 2 0 7219/9-1 o 25km 25km f e TtB 1 isopach ( early-? mid. Eocene) TtB2 isopach (mid.-? late Eocene) Knutsen & Vorren (in press) in the Hammerfest Basin. This correlation is based on detailed seismic mapping 7219/9-1 o of the base of the Eocene, which is defined in well 72 19/9-1, and which represents the base of TtA2. The base of ThA2, however, is neither a chronostratigraphic nor a sequence boundary, but marks the onset of conspicuous westerly and southwesterly progradation in the western part of the Hammerfest Basin (Knutsen & Vorren in press) and eastern part of the Tromsø Basin. Early Palaeogene sedimentary environments Palaeocene 7119/9-1 Subsidence of the Tromsø Basin during this period is o indicated by normal faulting along the Ringvassøy Loppa Fault Complex and the Eastern Senja Ridge Fault System. This faulting was probably not extensive, as the isopach map of the Palaeocene sequence (Fig. 4b) shows 25km a uniform thickness. This indicates that the palaeorelief 9 Base Unit TeC isochrons of the basin at the time of deposition was more gentle than that seen today. Both the Tromsø and Hammerfest Fig. 4. (a-g) Isochron and isopach sketch maps of the Palaeocene and Eocene sediments. Contours are in ms TWT. Basins were transgressed during Palaeocene times, and 198 S.-M. Knutsen et al. NORSK GEOLOGISK TIDSSKRIFT 72 (1992) marine sediments accumu1ated both in the basins and on adjacent structura1 highs (Nøttvedt et al. 1988; Knutsen & Vorren in press). Draping and thinning of the Pa1aeocene sequence over the Ves1emøy High, the Loppa High, and towards the Finnmark Platform, show that these elements bad a positive relief relative to the basin areas. The general decrease in gamma-ray activity from the base of the Tertiary upwards into the Palaeocene (Fig. 10) may indicate an increase of clastic material. Increased sediment supply from the basin margins is supported by the development of a prograding margin migrating from east to west in the eastern part of the Tromsø Basin. This is shown by a slight increase in thickness of the Palaeocene sequence on the eastern flank of the basin (Fig. 4b), which coincides with the low-angle prograda tional pattern seen in the upper part of the Palaeocene sequence on the seismic data (Fig. 3c). The Gardar banken High (the Svalbard Platform) to the north pro b ably acted as a source area during this period (Dowling 1988; Knutsen & Vorren in press). Eocene In the north of the Tromsø Basin, westward prograda tion in early Eocene time (Fig. 8) suggests that the Loppa High acted as a source area. Progradation from ø the southwest flank of the Loppa High probably started ca in the late Palaeocene (Knutsen & Vorren in press, their O) fig. 13). Sediment input from the Loppa High area is o - suggested also by the considerable thickness of the lower Cl) most Eocene sequence (TtA2) in the north of the Tromsø :::J Basin (Fig. 4d). In the east, slumps and progradational 'C ø areas indicate sedimentation close to a basin margin Cl) (Fig. 3c) ; progradation started in the late Palaeocene and (.) c continued to early Eocene times. ca A similar pattern of evolution occurred in the early .o Lo mid-Eocene (TtBl). Progradation was no longer merely :::J - to the west, but also has a southward and an eastward C/) component from the Veslemøy High and the Senja o Ridge, respectively (Figs. 8, 9). This indicates that areas to the west and north were uplifted and probably eroded. Further south, Eocene progradation from the Stappen High area has been described also, by Rønnevik & Jacobsen (1984), Vorren et al. (1991, unit TeB) and Richardsen et al. (in press, unit R). A northerly prove nance also is indicated by the northward thickening of sequence TtBl (Fig. 4e). Contemporaneous with the accumulation of the mid ?late Eocene sequence TtB2, subsidence of the Tromsø Basin continued and possibly accelereted. The isopach map and the erosional boundary of the sequence (Fig. Fig. 5. N-S seismic profile from the Ringvassøy-Loppa Fault Complex (loca tion on Fig. lb) showing disturbances due to gas, which has migrated along deep-seated faults. z o " C/l ;>< Cl 8 § c;; ;>< -i C/l6 o Ø) C/l w E ;>< o ,... " co co 2500 2600 2700 N N 2900 � i::l � - u Q) en - UR � Base Eocene Ei Sequence TtA 1 � 2 � <":) !:l ;::s Base Tertiary � � <":) � � ,....,.,...._..,.:;s-:;;�m..z:··:i!'·"'�·,;;oc;;?�....,il!l 3 � Palaeocene, sequence TtA 1 Base Tertiary � l 2km S e�a. R"d ge � t �- � � � � s· - \0 Fig. 6. E- W seismic profile from the Senja Ridge (location on Fig. lb) showing erosion of the Palaeocene and Eocene strata by the base of sequence TeC. \0 200 S.-M. Knutsen et al. NORSK GEOLOGISK TIDSSKRIFT 72 (I992) w E 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 3000 0.0 1.0� � 2.0 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 3000 1.0 � � 2.0 Fig. 7. E-W seismic protile showing progradation from the Loppa High to the northern part of the Tromsø Basin. Position of Well 7219/9-1 is shown approximately by the arrow (the well is located abo ut 6-7 km to the nortlj). 4f) may indicate that greatest subsidence took place in Eocene volcanism? the central and southern parts of the basin. No basin The high amplitude reftector in sequence TtBl (at about margin coeval with sequence TtB2 has been observed. l s TWT on Fig. 8) has been interpreted as a tuff/ ash layer We suggest that this is the result of a rise of relative sea (Faleide et al. 1988). However, there are indications that level during TtB2 times. This led to general aggradation other origins are more likely. and possibly back-stepping of the basin margins. The margins later became eroded, and only the deep basin l. According to Faleide et al. ( 1988), the reftector termi deposits are represented today by the parallel continuous nates against the Loppa High and the Senja Ridge reftectors in the Tromsø Basin. owing to erosion. It should be stressed that the high The relative rise of sea level that started in the amplitude reftector also has a very abrupt termination Palaeocene, after the generation of the base Tertiary towards the south (Fig. 8, at sp. 465) where the unconformity, probably continued during deposition of reftector siopes towards the deeper parts of the the Eocene sequences. During deposition of sequences Tromsø Basin. This southward termination of the TtA I-TtB I, sediment supply kept up with the rise thus high-amplitude reftector cannot be explained by uplift generating the late Palaeocene and early Eocene basin and erosion. margin progradation. During deposition of sequence 2. The only Palaeogene tuffaceousmaterial reported from TtB2 relative rise in sea level increases and sediment wells in the Tromsø and Hammerfest Basins area supply could not keep up with the added accommoda (Westre I984; Knutsen & Vorren in press) does not tion. show the same seismic high-amplitude response as the The presence of early Eocene neritic diatoms in north reftector in question. em Finland and Sweden (Fenner 1988) indicates that at least the southern part of the Barents Shelf was trans gressed in the early Eocene (Vorren et al. 1991). The sediments that were transported into the basin areas were Fig. 8. E-W seismic profile from the northern central parts of the Tromsø Bas in probably at least partly derived from earlier fine-grained (location on Fig. lb) showing low-angle southward progradation in sequence transgressive sediments deposited during the Palaeocene TtB l. A conspicuous high-amplitude event is seen in TtB l betweensp. 480 and 930. The chaotic seismic pattern at sp. 950- l 050 is proba bly due to gas in the sediments. and Cretaceous. Some of the Mesozoic rocks on the highs surrounding the basins ( such as the Loppa High and the Finnmark Platform) could have contributed to an input Fig. 9. E-W profile from the Senja Ridge to the Tromsø Basin (location on Fig. lb) illustrating eastward progradation along the eastern flank of the ridge within of sand, but the timing of erosion of these rocks is difficult sequence TtB l. In the upper part of the profile, large erosional relief correlated to to determine (e.g. Knutsen & Vorren in press). the regional sequences TeC and TeD is indicated. s N ,. 300 400 500 600 700 800 900 1000 1100 1200 V> z o o "' :><: 8Cl o r Vi6 V> :><:V> -1 1 ,. � :><: "'.... :g � 2 - u !!...... : 3:Gl .... 2km Fig. 8 � iS" w E �<'1:> Cl � 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 <'1:>!:l. 0.0 1::1 �;:s � ! <'1:> 1.0... ;:s� ... <'1:> ;l: .[ <'1:>� 2.0 � �.... � � 3.0 �Cl � s· o 3km Fig. 9 N o 202 S.-M. Knutsen et al. NORSK GEOLOGISK TIDSSKRIFT 72 (1992) 7119/7- 1 7119/9- 1 7219/9- 1 _1 & _1 o o o 90 90 90 GR GR GR Sea bottom 201 \238 Sea bottom LLJ : · z " " . ••• LLJ {:0.- LLJ .... z (.) o· LLJ z � (.) o o :::.�0. � 1- �.....,k4 .. .,., 1- �. rJ) rJ) " " LLJ Sea bottom r�s. jjj � i) i)'. � 0.. 1- ·Ø'� ' rJ) ·�. o.. os2·s;�+�r 1::4 ·'C» o LLJ '($4 415 � F==' � ."4,: � 0.. 0.6 " " 0.. �� 458 � % � � o " " 441 sec. TWT 500 l� i ; c * 20"� * M Il:. " " $- ."r-- � = V LLJ = " " LLJ z * � z " " " } LLJ o LLJ (.) .... (.) o o o LLJ � � c LLJ s LLJ l� < M " " � 20 � LLJ " � c z � c � ... 20" :z:z: ! LLJ � � = 718 (.) 11 ' � o 756 � " " = � = LLJ M a: LLJ � 0.96 = et " " 851 ! LLJ c � = sec. TWT c � LLJ * � � z = M LLJ ' * LLJ z 1r-r-.065 LLJ � == (.) ?- (.) a: o " " b et It o sec.TWT" " 1000 LLJ -= LLJ LLJ1 08 et 1023 1 :::E:E l � " =- � ... sec. TWT ! � � a: l LLJ � - et :=. z c 11 LLJ l LLJ = } LLJ (.) M =o o " " LLJ = LLJ" M � = � = (.) et z � o -= ··=M P=t= LLJ � * � � M et o 13 3 8" " " LLJ � * � = ... � 25m3o5s!. U5 LLJ� � = = * c=* Maastrichtian- � -= L-U. Campanian sec. TWT � �� * .::i! � :::r<:::::IL: 1.45 � 1.40 � L-1'1440 1443- U. Cretaceous Albian sec. TWT s . TWT Casing Glauconite M Mica Macrofossils frag. Silty Carbonaceous mat. Sand y Strings of dolomite (not to scale) Pyrite Strings of limestone (not to scale) Rock frag. Strings of dolomitic limestone (not to scale) o Unconformity Fig. JO. Schematic lithology and age of the Cenozoic strata in the three wells drilled in the Tromsø Basin (location on Fig. lb). On the left of each well column the gamma-ray log is shown. Depths are in meters below sea leve!. NORSK GEOLOGISK TIDSSKRIFf 72 (1992) Palaeocene and Eocene development, Tromsø Basin 203 (·o as) .lM.i w o ...... N Cl) .. Ø) w N Øl Øl c( c( Q.CD l ... Ø) - - - r- Q...... � r ....; 1- 1- 1- 1- �o - -..... 3: ...... 204 S.-M. Knutsen et al. NORSK GEOLOGISK TIDSSKRIFT 72 (1992) 3. The closest known Tertiary volcanic activity occurred fest Basin and is of mid-Palaeocene age (Knutsen & in the Vestbakken volcanic province west of the Stap Vorren, in press). pen High (Faleide et al. 1988; Richardsen et al. in press), 150 km from the high-amplitude refiector in the Tromsø Basin. If the high-amplitude reflector in the Tromsø Basin represents volcanic material from Late rifting and early opening of the this source, these sediments must have been deposited Norwegian -Greenland Sea as a long-transported, widespread, ash layer on the The detailed history revealed by the Palaeocene and Tromsø Basin sea floor. The thickness of marine ash Eocene sediments in the Tromsø Basin can be correlated layers seldom exceeds lO cm (Fisher & Schminke with the early opening of the Norwegian-Greenland Sea 1984, pp. 168-173; Cas & Wright 1987, p. 287), which as proposed by Eldholm et al. (1987) and Myhre & is far too thin to produce a high-amplitude reflection Eldholm ( 1988): on seismic data. The high-amplitude reflector could represent several stacked ash layers, like the lower l. During the latest rifting, and as the sea-floor spread Tertiary Balder Formation in the North Sea (Malm et ing was initiated during the latest Palaeocene at mag al. 1984). netic anomaly 25 (i.e. approximately 56 Ma) south of the Greenland-Senja Fracture Zone (Eldholm et al. If the reflector in question does not represent a tuff/ash 198 7; Myhre & Eldholm 1988; Nøttvedt et al. 1988), layer, other possible explanations may be as follows. older fault zones along the Bjørnøyrenna Fault Com A. A diagenetic transition, i.e. opal CT to quartz, which plex possibly may have been reactivated (Knutsen & has been found in the Palaeogene strata on the Senja Vorren in press). This first period of Tertiary erosion Ridge wells (Ramberg Moe et al. 1988). Løvø et al. was accompanied by compression along the south ( 1990) have suggested that the high-amplitude reftec west of the Loppa High and the Senja Ridge, and tor shown on Fig. 8 originates from an opal CT to subsidence in the Tromsø Basin and the west of the quartz transition. Hammerfest Basin (e.g. Gabrielsen & Færseth 1988; B. The presence of gas along a lithostratigraphic Knutsen & Vorren in press, their fig. 12). The onset boundary also is possible. This area of the southwest of this first erosional Tertiary phase accompanied Barents Sea contains shallow gas accumulations (An westward margin progradation into the Tromsø dreassen et al. 1990; Løvø et al. 1990). Basin area in latest Palaeocene time (the upper C. A third interpretation is that the reflector originates most, prograding part of sequence TtAl) (Figs. 12a, from a condensed section representing a period of 13b). starved sedimentation in the basin (Vail et al. in 2. At the time sea-floor spreading started east of the press) downlapped by younger strata as the margin Greenland-Senja Fracture Zone (tentatively starting prograded basinward. at magnetic anomaly 23, i.e. 54 Ma according to Eldholm et al. 1987), the infilling of the Tromsø We tentatively suggest a combination of alternatives A Basin areas continued (Figs. 11b, 12b). The creation and C; the high-amplitude reftector results from a fine of an oblique shear margin and gradual development grained condensed marine horizon formed during a pe of oceanic crust east of the Greenland-Senja Frac riod of low sediment supply. Our seismic data do not ture Zone was followed by stepwise sea-ftoor spread show any clear truncation of the stratigraphy by the ing northwards. As areas both north and west of the high-amplitude reflector (Løvø et al. 1990). This does not Tromsø Basin became uplifted due to local and re exclude a diagenetic transition along the condensed sur gional transpression (e.g. Spencer et al. 1984; Nøt face. We believe that high biogenic activity and low tvedt et al. 1988), sediments from these areas, re clastic input of sediments led to high concentrations of presented by sequence TtBl, continued to infill the silica-rich sediments a1ong the condensed surface. Tromsø Basin (Figs. 12c, 13c). The age of the high-amplitude reflector has been as 3. By mid-?late Eocene, represented by sequence TtB2, signed to the end of the Palaeocene (Faleide et al. 1988). a change occurred; the established evolutionary pat Seismic correlation in the northern part of the Tromsø tern, in which the Tromsø Basin was receiving sedi Basin to well 7219/9-1 indicates a younger age; the ments from adjacent areas, ceased. We suggest that high-amplitude reflector is eroded by the URU about by this time the spreading bad passed by the margin lO km west of the well location and the Palaeocene west of the Tromsø Basin and Senja Ridge area. As Eocene boundary is identified by biostratigraphic analy the result of a general post-opening subsidence of ses of well cores. Thus, the high-amplitude reflector has both the Tromsø Basin and adjacent areas, more an Eocene, probably early/mid-Eocene age. It is impor , widespread marine sedimentation on the southwest tant to stress that no volcanic material Q.as been found in Barents Shelf took place (Fig. 13d). This period is the thick Eocene strata in the Tromsø Basin wells. The represented by sequence TtB2, deposited during a thickest and most conspicuous Tertiary tuff layer that relative rise and highstand of sea level. Vorren et al. has been observed in the wells is found in the Hammer- ( 1991) suggest that the period represented by their NORSK GEOLOGISK TIDSSKRIFT 72 (1992) Palaeocene and Eocene development, Tromsø Basin 205 a unit TeB represents the time of maximum sediment TtA 1 (Palaeocene) thickness on the southwestern shelf areas. Richardsen et al. (in press) also show that from intra-Eocene, and probably to early Oligocene times, thick sedi ment accumulations formed in areas west of the Stappen High ( their unit R). This timing of the max imum sediment thickness is reinforced by the fact that our sequence TtB2 reflects a relative rise of sea leve! and probably more accommodation on the shelf areas. The erosion of the Palaeocene and Eocene sediments at the base of the TeC surface (Fig. 6) marks the end of the infilling seismic pattern of the Tromsø Basin area and areas west of the Senja Ridge. Erosion was less in the Hammerfest Basin and the eastern part of the Tromsø Basin than to the west of the Senja Ridge, suggesting that the latter area was more elevated (e.g. Eldholm et al. 1987; Faleide et al. 1988) and intensively eroded com pared with the Tromsø Basin area. The timing of the b25km shift from widespread marine conditions on the shelf to TtA2 (Late Palaeocene-Early Eocene) intensive erosion of the western parts of the Tromsø Basin, and areas west of the Senja Ridge represented by base TeC, is not dated. We tentatively suggest an early Oligocene age (Fig. 13e) for renewed uplift of the west ern margin. This is based on the proposed maximum age for the base of TeC as suggested by Vorren et al. ( 1991), who have dated the base of TeC by its downlap on oceanic crust. Uplift could have occurred in response to the change of sea-floor spreading from a mainly north ward to a more west-northwest orientation in early Oligocene times (Spencer et al. 1984; Eldholm et al. 198 7; Myhre & Eldholm 1988; Richardsen et al. in press). The oldest preserved evidence of a change in evolution c TtB1 (early-?mid. Eocene) 17" 18' 19' 20' Tr Sequence truncated � Sequence en?ded � Marine deposits æ Basin margin deposits 'C� Slumping along basin margin 71'30' Fig. 12. Seismic facies maps for (a) sequence TtAI (Palaeocene), (b) sequence TtA2 (early Eocene) and (c) sequence TtBI (early-(?)mid-Eocene). No facies map is presented for the uppermost Palaeogene sequence TtB2 owing to the fact that large parts of the sequence have probably been eroded. See text for discussion of evolution. 206 S.-M. Knutsen et al. NORSK GEOLOGISK TIDSSKRIFT 72 (1992) west of the Senja Ridge is the base of TeC (Fig. 13f) which defines the base of the middle Miocene (Vorren et w E al. 1991), or Plio-Pleistocene (Eidvin & Riis 1989) sedi ments. Correlating this erosional surface to areas west and northwest of the Tromsø Basin shows that older erosional surfaces are truncated by the base of TeC. Hence, the erosion represented by the base of TeC is the youngest prominent erosional phase in the area west of 9 and on the Senja Ridge. Vorren et al. ( 1991) relate this remodelling to a eustatic fall of sea level enhanced by Miocene uplift of the shelf areas, which led to a major relative fall of sea level on the Barents Shelf. Alterna tively the remodelling is younger, of Plio-Pleistocene age, and inftuenced by glacial processes (Eidvin & Riis 1989). 6 4 L----L------�------L------L------�----�Conclusions 12 �!!!f��tiimiiiii:��FF�S::::sl l. In mid-Palaeocene times marine conditions existed in 10 the Tromsø Basin. No tuffjash deposits, as found in the Hammerfest Basin, have been observed in the Tromsø Basin. 6 2. In late Palaeocene times, during the latest rifting and earliest spreding phase of the Norwegian-Greenland ? Oligocene e Sea, transpression led to evaluation of the highs sur 12 rounding the Tromsø Basin to the west, north and 10 east. This gave rise to progradational input to the basin area. Progradation kept pace with a relative rise in sea levet in early-mid Eocene times, and took place along the margins of the Tromsø Basin. In the basin itself, deep marine conditions and sedimenta ns? (m.-?1. Eocene) d tion mainly by turbidity currents prevailed. 3. After the spreading of the Norwegian-Greenland Sea propagated northwards, the Tromsø Basin and adja 10 cent areas experienced general post-opening subsi dence. Progradation could no longer keep up with the 6 relative rise of sea level. During this period, tenta tively dated as mid-Eocene to Oligocene, widespread c marine conditions occurred on the southwest Barents Shelf and adjacent areas. 4. A major shift is thought to have occurred during early 10 12 Oligocene times. Uplift and erosion of the margin and ------;/11! shelf areas took place owing to a shift of the spread l ing pole in the Norwegian-Greenland Sea. This ero TtA1-2 (Palaeocene-e. Eocene) b sional phase is today represented by a younger phase 12 of erosion that reworked older erosional surfaces in 10 mid-late Miocent or Plio-Pleistocene times. Sea watc• Acknowledgements. - The authors would like to thank Norsk Hydro a.s. for Senja Ridge Tromso Basin HammerfestBase Terllary Basin TtA1 (e.-m. Palaeocene) a giving perrnission to use and publish their data. Norsk Hydro also supported the first author through his PhD stipend. The figures were drawn by Hilkka Falkseth 11· 19° 20° 21° at the University of Tromsø, and G. Dietel and E. M. Olsen at Norsk Hydro. 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