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Foraminiferal, Palynomorph, and Diatom Biostratigraphy And Foraminiferal, palynomorph, and diatom biostratigraphy and paleoenvironments of the Torsk Formation: A reference section for the Paleocene-Eocene transition in the western Barents Sea JENONAGY1, MICHAEL A. KAMINSKIZ, KARI JOHNSEN3and ALEXANDER G. MITLEHNER4 1. Department of Geology, University of Oslo, P.O. Box 1047 Blindern, N-0316 Oslo, Norway 2. Research School of Geological and Geophysical Sciences, Birkbeck College and University College London, Gower Street, London, WCIE 6BT, U.K. 3. Saga Petroleum as., Kjorboveien 16, P.O. Box 496, N-1301, Norway 4. Department of Geography, Edinburgh University, Drumrnond Street, Edinburgh, EH8 9XP, U.K. ABSTRACT A microfossil-based biostratigraphic reference section is selected for the Paleogene Torsk Formation in the 7119/9-1 well, yocited in the Tromser Basin of the southwestern ~arentsSea. Benthic foraminifera, dinoflagellate cysts, and diatom infillings indicate an early late Paleocene to early Eocene age for the Torsk Formation on the eastern flank of the basin. A comparison of benthic foraminiferal last occurrences in the reference section with that established in neigh- bouring wells illustrates the potential use of these events for high-resolution stratigraphic cor- relation. Though differing in detail, the record of microfossil last occurrences in the western Barents Sea compares well with the biostratigraphy of the Central North Sea area. A morpho- group analysis of the Paleogene foraminiferal succession of the Tromser Basin reveals a batl~y- metric deepening of the depositional area culminating near the Paleocene-Eocene transition, followed by a shallowing. INTRODUCTION region is of early Eocene age, the early Paleogene The Barents Sea covers a continental shelf area that biotic record can only be obtained from continental is of a comparable size to the entire North Sea, but margin wells. Fortunately, Paleocene to Eocene the subsurface geology of the area is constrained sediments have been recovered from a number of by fewer than 53 wells, mostly concentrated in the exploratory wells drilled in the Tromser and Ham- Hamrnerfest and Tromser basins (Fig. I). This area merfest basins, off the northern coast of Norway. has been subjected to intense Mesozoic and Ceno- These basins are situated just east of the oceanic zoic tectonic movements that have affected the areas of the Norwegian-Greenland Sea, and consti- configuration of seaways linking the North Atlan- tute the northernmost offshore basins of the North tic with the Arctic Ocean. Therefore, studies of the Sea / Norwegian Sea rift system explored by con- sedimentary and biotic record preserved in the ventional drilling. The main objective of this study Barents Sea basins are important for reconstructing is to assess the stratigraphic and paleoenviron- the history of faunal connections between the mental significance of early Paleogene benthic for- North Atlantic and Arctic oceans, and constraining aminifera, dinoflagellate cysts, and diatoms from a paleobiogeographic patterns in the Boreal Realm. key section in the western Barents Sea. This study Reconstructing the development and history is intended to complement the assessment of of marine connections through the Norwegian- microfossil assemblages from younger Cenozoic Greenland Sea is the prime objective of the ,,Arctic material carried out under the auspices of the ODP Gateways" legs of the Ocean Drilling Program. As ,,Arctic Gateways" initiative. the oldest oceanic crust in the Norwegian Sea Among 12 wells that we have examined from In: Hass, H.C. & Kaminski, M.A. (eds.) 1997. Contributions to the Micropaleontology and Paleoceanogra- phy of the Northern North Atlantic. Grzybowski Foundation Specinl Publication, no. 5, pp. 15-38 Nagy, Kaminski, Johnsen and Mitlehner Figure 1. Location map of the western Barents Sea, with main struct~~ralfeatures and locations of 6 of the studied wells. the western Barents Sea area, well 7119/9-1 was Greenland Sea, but the structural basins on the chosen as a reference section because of the pre- Barents Sea platform were formed as a result of sence of a thick Paleogene succession (1030 m earlier (Cretaceous to Paleocene) rifting phases. according to electric logs), rich microfossil assemb- The geology and structural development of lages, and the availability of continuous sampling. the Barents Sea has been the subject of numerous In addition, Knutsen et 01. (1992) published a seis- studies published mainly after drilling in the area mic line that crosses the well site and defined seis- commenced in the early 1980's. The subsurface mic units in this well, so it is now possible to more geology of the area based on seismic evidence has accurately assess the age and significance of regio- been discussed by R~nnevik(1981), RPrmevik & nal seismic reflectors. Altogether, 129 ditch cutting Jacobsen (1984), Roufosse (1987), and Faleide et 01. samples were analysed from this well, with few (1993a,b). Recently, Riis (1992), Cloetingh et nl. exceptions spaced at 10 m intervals. The biostrati- (1992), Walderhaug (1992), and Richardsen ct nl. graphy from well 7119/9-1 is compared with data (1993) have provided estimates of the amount of from nearby wells in the Hammerfest and Tromser Cenozoic upiift and erosion in the western Barents basins. Sea based bn geophysical and petrophysical evi- dence. The early Cenozoic sedimentary history of STUDY AREA the area has been discussed by Knutsen & Vorren Geological Setting (1991). The Barents Sea overlies an intracratonic area of The area of this study is located in the south- structural basins, platforms, and highs. In the west, western part of the ~arentsSea close to the conti- the area borders the oceanic Norwegian-Greenland nental margin. The region is divided into four main Sea with a passive shear margin marked by the basins: Bjarnerya, Troms~r,Hammerfest, and Nord- Senja Fracture Zone in the south and the Hornsund kapp, with intervening structural highs such as the Fault Zone in the north. The Barents Sea margin Senja Ridge, Loppa High, and Veslemery High (Fig. started to develop in the early Paleogene in 1). The Tromser Basin is connected with the Ham- connection with the opening of the Norwegian- merfest Basin over the Ringvassoy-Loppa Fault Foraminiferal, palynomorph, and diatom biostratigraphy of the Paleogene Torsk Formation 17 Complex. In an easterly direction, the Hammerfest 1993a,b). The top of the Torsk Formation is cut by basin is linked to the Nordkapp Basin over a shal- an erosional unconformity, and it is overlain by low threshold on the southernmost extension of Plio-Pleistocene glacial and glacial-marine sedi- the Bjarmeland Platform. The reference well cho- ments. sen was drilled on the eastern flank of the Tromser Large areas of the Barents Shelf east of the Basin. Senja Ridge were uplifted and subjected to mul- tiple phases of erosion, lasting to the early Plio- Stratigraphical Framework cene. As a result, the upper parts of the Torsk For- The Mesozoic and Cenozoic lithostratigraphy of mation (as much as 1500 m) have been removed, the western Barents Sea was formally outlined by particularly in marginal parts of the basins where Worsley ct 01. (1988). The Paleogene Torsk Forma- the formation thickness is strongly reduced. tion has a rather lnonotonous lithology consisting of light grey to greenish grey, essentially non- MATERIAL & METHODS calcareous claystones. Thin beds of siltstone or Well 7119/9-1 was a wildcat well drilled in the limestone occur rarely throughout the formation, summer of 1984 at 71°24'53.19"N, and and tuffaceous horizons are commonly reported in 19'49'43.26"E in 201 m of water, with Elf Aquitaine the lower part. The thickness of the formation Norge a.s. acting as the operator. Samples for this varies from 200-300 m in wells drilled in marginal micropaleontological study were provided by the areas of the Hamrnerfest Basin, to over 1 krn in the Norwegian Petroleum Directorate in Stavanger. central parts of the basin. The formation thickens to The material analysed consists wholly of washed over 2 km in the undrilled deepest parts of the ditch cutting samples ranging in dry weight from Tromser Basin. 25 to 45 grams. The micropaleontological samples were disintegrated by boiling in sodium carbonate, and washed over a 63pm sieve. Foraminifera and diatoms were picked from the >125pm fraction and mounted on cardboard slides. Palynological sam- ples were analysed in accordance with standard techniques. All depths reported in this paper are quoted below rotary kelly bushing (therefore, a value of 226 m must be subtracted from the repor- ted depth to convert to depth below sea floor). RESULTS The succession of palynomorphs, diatoms, and benthic foraminifera in the 7119/9-1 well can each be subdivided into distinct assemblages based on the distribution of donlinant taxa, the occurrences of stratigraphically important species, and fluctua- tions in diversity and abundance (Fig. 3). The chro- nostratigraphy of the well is based primarily 011 palynomorph assemblages, which can be correla- ted with the zonations of the northern North Sea published by Schroder (1992) and Mudge & B~~jak (1994, 1996). a. Palynomorph assemblages Figure 2 Lithostratigraphic scheme of Upper Cretaceous Altogether, 70 samples have been analysed for and Cenozoic deposits of the southwestern Barents Sea palynomorpl~swith a spacing of 10 m except in the (modified after Worsley tt a/. (1988) and Faleide et al. upper 400 m of the well, where the sample spacing (1 993)). is mostly 30 m. The studied interval contains 90 palynomorph taxa, and their stratigraphical distri- The basal contact of the Torsk Formation is bution is presented in Table 1. The p alynomoph unconformable, and represents an important succession is subdivided into six stratigraphically depositional hiatus encompassing the latest Creta- significant assemblages (Fig. 3). ceous and early Paleocene (Fig. 2). This hiatus can From the uppermost part of the formation (at be traced on seismic lines throughout the south- 330 m), a single sample was analysed which con- western Barents Sea (Rernnevik, 1981; Faleide et nl., tains a poor palynomorph assemblage.
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