Miocene to Quaternary paleoenvironments and uplift history on the mid Norwegian shelf

Marine Geology, 115 (1993) 173-205

173

Elsevier Science Publishers B.V., Amsterdam

Miocene to Quaternary paleoenvironments and uplift history on the mid Norwegian shelf David A.R. Poole z and Tore O. Vorren

Department of Geology, University of Tromso, 9000 Tromso, Norway (Received April 6, 1993; revision accepted September 7, 1993)

ABSTRACT Poole, D.A.R. and Vorren, T.O., 1993. Miocene to Quaternary paleoenvironments and uplift history on the mid Norwegian shelf. Mar. Geol., 115: 173-205. Based on benthic and planktic foraminifera, Bolboforma, oxygen isotope measurements and seismic data, major changes in Miocene, Pliocene and Pleistocene paleoenvironments on the mid Norwegian shelf are discussed and a possible scenario of the late Cenozoic uplift history is given. The dating of the Neogene sequence has been done using foraminifera and Bolboforma. Four main assemblage zones have been identified with nine distinct subzones. Most of the Miocene sequence is preserved. The lower Miocene sediments contain only siliceous microfossils. A period of high fertility and upwelling in the study area prevailed. The early Miocene-early mid Miocene (15 Ma?) change from a siliceous to a calcareous rich microfauna, dominated by Nonion barleeanum, can be related to increased surface-water circulation due to overflow across the Iceland-Faeroe ridge. During the Miocene the temperature decreased in the study area. Evidence of increased amounts of coarser sediments may suggest that an uplift of the mainland areas occurred during the mid-late Miocene. Lower Pliocene sediments contain a foraminiferal fauna that seems to occur in slightly colder conditions than the late Miocene fauna suggesting a further cooling. Possibly, Arctic waters entered the study area in the early Pliocene. A very marked change in lithology (from compacted claystone to unconsolidated diamicton), fauna (from deep dwelling to shallow dwelling species) and seismic signature (from flat lying reflectors to prograding clinoforms) occurs during the mid?-late Pliocene. A two step cooling trend is indicated by the microfauna of these prograding wedges. (1) The first wedge buildups might have been associated with an uplift of the mainland during the early late Pliocene (mid Pliocene, ca. 4 Ma). However, the age determination is somewhat uncertain and may very well be of late Pliocene age. (2) The second step of wedge buildup is associated with a glacial phase where the dominating microfauna exists of arctic species. Large continental ice sheets might have occurred at this time reaching coastal areas and that possibly many of the geomorphological features such as the strandflat were made during this episode. The Pleistocene epoch is represented by an increased percentage of boreal foraminifera intermingled with high arctic species which indicates that interglacial-glacialcycles prevailed and the dynamics of the glacier system changed.

Introduction Very few studies on g l o b a l e n v i r o n m e n t a l c h a n g e s in the N e o g e n e have been d o n e on m a t e r i a l f r o m c o n t i n e n t a l shelves. This is p a r t l y due to oil c o m p a n y c o n f i d e n t i a l i t y o f the m a t e r i a l , p a r t l y to the cost o f o b t a i n i n g m a t e r i a l a n d , last b u t n o t least, the q u a l i t y o f the m a t e r i a l . O n the m i d 1Present address: Esso Norge AS, P.O. Box 60, 4033 Forus, Norway 0025-3227/93/$06.00

N o r w e g i a n shelf, a thick sequence o f T e r t i a r y sediments c a n be f o u n d (Fig. 1). T h e a i m o f this w o r k is to s t u d y N e o g e n e p a l e o e n v i r o n m e n t s o n the m i d N o r w e g i a n shelf, d a t e the N e o g e n e sediments a n d d e v e l o p a sedim e n t a t i o n a n d tectonics h i s t o r y o f the s t u d y a r e a a n d a d j a c e n t l a n d areas. T h e C e n o z o i c d e v e l o p m e n t o n the N o r w e g i a n c o n t i n e n t a l shelf has h a d i m p o r t a n t consequences for the m a t u r a t i o n a n d m i g r a t i o n o f h y d r o c a r b o n s ( B e r g l u n d et al., 1986; N o t t v e d t et al., 1988; Riis

© 1993 - - Elsevier Science Publishers B.V. All rights reserved.

SSDI OO25-3227( 93 )EO112-.1

174

D.A.R. P O O L E A N D T.O. V O R R E N

35ow

25oW

80 °

15ow

5oW

5OE

15OE

25°E

35°E

45°E

8(3°

55°E

25°W

~"-I C

/

/

.;:~

~i:i:i:::~ ¸¸ ~60o

5°E

15°E

25°E

Fig. I. Map showing the elevation of the mainland areas and thickness of tertiary sediments. The framed area represents the study area.The elevation contours are for every 200 m and the sediment thickness contours are for every 500 m.

175

PALEOENVIRONMENTS ON THE MID NORWEGIAN SHELF

et al., 1989; Vorren et al., 1990; Jackson and Hastings, 1986). The various parts of the continental margin have shown different styles of sedimentation and diastrophism. A key element of the Cenozoic geological history of Fennoscandia is its uplift. A wide variety of hypotheses as to the timing and cause of the Cenozoic uplift have been proposed. Theories relate it to the Alpine orogeny, to fault systems along the coast or to the opening of the North Atlantic (Reusch, 1901; Strom, 1948; Holtedahl, 1953; Gjessing, 1967; Torske, 1972, 1975). Recent studies from the Barents Sea have shown that much of the uplift occurred in the Neogene (Riis et al., 1989; Stuevold, 1989; Eidvin and Riis, 1991; Riis and Fjeldskaar, 1993; Eidvin et al., 1993) and that it was possibly initiated by glacial erosion (Riis and Fjeldskaar, 1993). However, Vorren et al. (1991), amongst others, suggested that about 1000 m of sediment seems to have been eroded from the uplifted Barents Sea, partly by fluvial and partly by glacial erosion. Calculations done by Riis and Fjeldskaar (1993) of the late Pliocene supposed glacial sediments on the mid-Norwegian Shelf showed that the amount of sediment deposited was less than the amount eroded from the hinterland. They suggested that a certain relief must have existed prior to the glaciations, and consequently that there must have been an uplift prior to the glaciations. From this it is clear that a key problem in understanding this phenomenon lies in dating the sediments produced during the uplift. It is also important to evaluate the paleoenvironments during the Neogene on the shelf areas so as to compare with the deep-sea areas, where large important global climate events are documented during the Neogene. The most important events include: the growth of the Antarctic ice cap (Shackleton and Kennett, 1975; Miller et al., 1987; Woodruff and Savin, 1989; among many others), the creation of deep bottom water in the Atlantic (Schnitker, 1980; Woodruff and Savin, 1989) and large scale northern hemisphere glaciations (Shackleton et al., 1984; Jansen et al., 1990). Most modern foraminiferal species, both planktic and benthic, are considered to have evolved during the course of the Miocene (Boersma, 1978). An increased understanding of the habitats of modern

continental shelf species has occurred in arctic regions in recent years (Hald and Steinsund, 1993; Schroeder-Adams et al., 1990), and detailed paleoenvironmental reconstructions of the Neogene and Pleistocene on the continental shelf can be performed with a certain degree of confidence. Material and methods

Foraminiferaland Bolboforma analysis Biostratigraphic analysis was completed on three wells 6407/1-2, 6507/7-1 and 6610/7-1 (Figs. 1 and 2; Table 1) to identify both chronostratigraphic bioevents and depositional environments. Drill cuttings from each well were collected (195 samples) at 10 m drilling intervals after they were transported to the surface by the drilling mud. Approximately the same amount of material was used for each sample, the samples were split and analyzed with respect to their benthic and planktic foraminiferal content. In addition to foraminifera, Bolboforma were analyzed in the Miocene section. All the samples were treated with paraffin and NaOH, except the Pliocene and Quaternary of well 6507/7-1, which were wet sieved. The fraction larger than 100 ~tm was analyzed for microfossils according to the method used by Phleger (1965). The foraminifera were divided into benthic assemblage zones and subzones according to Hedberg (1976). Due to the low abundance of planktic foraminifera, no planktic zonation has been done. Species occurrence and relative abundance were recorded. We counted between 200 and 300 specimens from each sample. There were very few planktic specimens throughout most of the stratigraphy, however, the planktic species are valuable

TABLE 1 Well locationand depth belowsea level Site

6407/I-2

6507/10-1

6610/7-1

Latitude(°N) 64°47'50.61" 65°13'10.75'' 66°17'32.82" Longitude(°E) 07°02'23.76'' 07°14'00.47" 10°16'52.96'' Water depth (m) 273 298 265

176

D.A.R. POOLE AND T.O. VORREN 2 °

4°

6 °

8 °

10 °

12°

14 °

I

~

© Negi

i i i-i i i;iii 66*

\ 65

o

Fig. 2. Bathymetric m a p over the study area. The wells used in this study are represented by black hatched circles, whereas the open circles represent the O D P drillings and the wells studied by Eidvin and Riis (1991).

in constraining ages and have thus been included in this work. As the samples are taken from ditch cuttings, several problems occur (King, 1989). The most important of these is that a certain amount of caving (material from levels higher up in the stratigraphy mix with material from lower levels) occurs. This will have a marked effect on the zonation and must be kept in mind. Stable isotope measurements

A total of 80 samples were measured for their stable oxygen- and carbon isotopic content at The National Geological Mass Spectrometry Laboratory at the University of Bergen, Norway

(Tables 2 and 3). The stable isotope measurements were done on a Finnigan Mat 251 mass spectrometer after reaction with ortho-phosphoric acid at 50°C. Before reaction, the foraminiferal shells were ultrasonically cleaned in methanol and roasted in vacuo at 380°C for 30 minutes. The automatic preparation line is described by Jansen et al. (1988). Analytical precision is 0.07%0 for 6180 and 0.069/00 for 613C based on repetitive measurements of carbonate standards (Jansen et al., 1988). Three species were measured. Respectively, two Cibicides grossa, five Cibicides lobatulus, and twenty Nonion barleeanum were needed to obtain an accurate measurement. The Cibicides species were chosen as they had a good continuation throughout the Plio-Pleistocene and only a few

177

PALEOENVIRONMENTS ON THE MID NORWEGIAN SHELF

TABLE 2

TABLE 3

Oxygen isotope data for well 6407/1-2

Oxygen isotope data for well 6507/10-1

Sample 6180g~o 613Cg~o

Sample 61801ob O13Clob

Depth

61s0

613C

520 540 560 580 580 590 600 610 630 640 650 660 680 690 710 740 760 780 810 830 850 880 930 950 960 990 1010 1030 1050 1070 1090 1110 1150 1160 1170 1180 1220 1260 1280 1290 1330 1330 1360 1390 1430 1450 1480

420 450 470 480 490 500 520 600 960 1010 1220 1260 1280 1290 1330 1430 1470 1510 1520 1540 1550 1560 1580 1590 1600 1620

670 700 730 760 830 900 930 970 1000 1070 1100 1220 1260 1295 1325 1350 1400 1430 1490

3.63 4.1 4.02 3.41 3.8 3.1 3.56 3.45 2.49 3.12 3.48 3 2.48 2.68 2.62 2.58 3.01 3.06 3.3

0.96 0.36 0.68 -0.37 0.8 1.03 1.29 1.1 0.51 - 1.18 0 -0.12 - 0.47 0.38 0.13 -0.08 0.07 1.37 0.19

61801ob

613C1ob

2.31 3.31 3.63

0.85 -0.31 -0.03

6180bar

613Cba r

3.54 4.15 3.83

- 1.52 -2.17 - 1.91

3.552 3.722 1.814 1.851 1.33 2.954 3.083 3.112 2.293 4.19 3.8 2.87 3.802 3.339 3.26 3.46 2.8 2.41 2.735 3.182 3.25 2.691 3.346 3.072 2.486 3.354 3.398 3.482 3.504 3.06 3.203 3.853 3.28 2.5 3.47 2.11 2.947 2.41 2.623 2.66 2.332 3.25 2.827 2.727 3.082 3.237 2.802

0.525 1.342 -0.75 -0.934 - 1.6 -0.656 0.316 1.169 - 1.086 0.157 0.41 -0.66 -0.71 1.03 0.26 0.64 -0.06 0.385 0.101 0.469 0.67 0.627 0.032 1.17 0.533 -0.157 -0.157 0.29 -0.184 0.48 0.094 -0.089 -0.55 -0.69 0.91 - 1.912 1.538 -0.521 0.4 -0.57 -0.669 -0.221 -0.023 - 1.349 0.242 -0.35 -0.035

2.9 2.195 0.817 1.817 2.63 1.652 1.732 2.363 1.511 1.631 1.932 1.428 1.491 2.3 1.788 2.316 3.53 3.26 3.06 2.45 2.96 2.78 2.78 2.56 2.78 2.53

0.422 1.155 - 1.615 0.817 0.673 -0.656 -0.074 -0.386 1.108 -2.174 1.096 - 1.743 0.683 0.451 0.27 0.81 1.04 -0.2 0.98 0.49 0.68 0.7 0.99 0.52 0.48 1.11

Sample 6180~r 613Cbar 1430 1440 1460 1470 1490 1500 1510 1550 1570 1620

3.262 3.38 3.493 3.57 4.07 3.706 3.97 3.706 3.359 3.164

0.81 - 1.93 -2.232 -2.27 - 1.78 -2.061 -2.03 -2.061 - 1.911 - 1.839

550 1635 1670

1490 1555 1575

C a l i b r a t i o n t o e q u i l i b r i u m w i t h t h e a m b i e n t seaw a t e r 6 1 8 0 w a s d o n e b y a d d i n g 0.58%0 t o C.

lobatulus a n d 0.16%o t o N. barleeanum ( P o o l e e t al., in prep.). Ten parallel measurements were done on C. grossa a n d C. lobatulus. C. grossa's 6 1 8 0 v a l u e s were

higher

than

C. lobatulus's v a l u e s b y a n

a v e r a g e o f 1.04%o a n d w e t h u s c o r r e c t e d C. grossa b y s u b t r a c t i n g 0.46%o f r o m t h e a n a l y t i c a l r e s u l t s . The

paleotemperature

was

derived

using

Shackleton's (1974) equation: specimens were needed

to make

a measurement

thus reducing the possibility of "pollution" by c a v e d s p e c i m e n s . Nonion barleeanum a n d Cibicides

lobatulus w e r e m e a s u r e d i n t h e M i o c e n e a n d e a r l y Pliocene because of their high abundance.

T = 16.9 - 4 . 3 8 ( ~ c - ~w) + 0.1 ( 6 c - ~w) 2 w h e r e 6c is t h e c o r r e c t e d c a l c i u m c a r b o n a t e i s o t o p e v a l u e a n d 6~, is t h e i s o t o p e v a l u e o f t h e a m b i e n t water, both are relative to PDB.

178

D.A.R.POOLEANDT.O.VORREN

Lithology and seismic analysis No attempt to construct a lithostratigraphy has been made. Due to the fact that the material comes from ditch cuttings, an uncertain amount of drilling mud is incorporated into the samples. This makes such an analysis difficult, especially for the Pliocene and Pleistocene sediments which are unconsolidated. Information of the lithology has, however, been obtained from porosity logs and onsite drilling reports. Based on these results we have constructed a schematic general lithology for the three wells (Fig. 14). A seismic grid was used to correlate the wells (Fig. 3). A total of 23 seismic lines were used for interwell correlation. Three different surveys were used, "Nopec 1986", "Geco 1984" and "Norwegian Petroleum Directorate 1977". Generally, the data quality of the surveys is good. Isopach maps of Miocene and Pliocene sediments were made for the study area on the basis of work done by Stuevold (1989).

Tectonic setting The region between Norway and Greenland has been a depositional area since the Carboniferous

67 °

ORING//-~\~r___ ~ ~ ' , . @ " ~

-1

"

-66 °

SUBBASIN ~ - . ~ Z /I~'~VHELGELAND ~50 7 / BASIN -

6 4~ 0 7 / ~

T~ AONDg~AG

-65 °

I 5°

10 °

Fig. 3. The map shows the structural setting in the study area amd the seismic grid used in the correlation of the wells. The location of the seismic lines in Fig. 15 are represented as thick lines.

(Eldholm et al., 1984; Hamar and Hjelle, 1984; Jorgensen and Navrestad, 1981). A number of tectonic phases (late Paleozoic, Kimmerian, early and late Cenozoic) and quiescent periods (early Mesozoic, Paleocene) have occurred on the Norwegian shelf (Gjessing, 1977; Eldholm et al., 1984; Bukoviks et al., 1984). The opening of the Norwegian-Greenland Sea started around 57.5 Ma (Eldholm et al., 1984) and was probably accompanied by an uplift of the surrounding areas (Torske, 1972; Riis and Fjeldskaar, 1993). Up until anomaly 13 (36 Ma), Greenland moved northwestward relative to Eurasia causing a transpressional regime with Svalbard. At anomaly 13 time, the plate geometry changed and Greenland's movement has since been westward relative to Eurasia (Eldholm et al., 1987). During the early phase of rifting, the Voring Plateau Escarpment (VPE) (Fig. 3) became an exposed high (Skogseid and Eldholm, 1989). The marginal high (VPE) sank throughout the Oligocene and became completely submerged during the Miocene (Skogseid and Eldholm, 1989). Subsidence of the whole mid Norwegian margin was governed by progressive lithospheric cooling and contraction and crustal loading by water and sediments (Bukovics et al., 1984; Skogseid and Eldholm, 1987). Locally it was also affected by compressional deformations that can be related to the Oligocene rearrangement of the sea-floor spreading axes in the Norwegian Greenland Sea. The entire Tr~enabanken-Haltenbanken region (Fig. 2) underwent rapid subsidence during late Pliocene-Pleistocene times according to Jackson and Hastings (1986). A late Cenozoic oblique uplift of the mainland is thought to have occurred (Holtedaht, 1953; Gjessing, 1967; Peulvast, 1985; Riis and Fjeldskaar, 1993; among others). According to Peulvast (1985) a flexure hinge line, related to the oblique uplift, is located off the outer edge of the strandflat. Stuevold (1989) suggests that this hinge line runs parallel with the Nordland Ridge (Fig. 3). The timing and magnitude of this possible phase of uplift is a debated theme and different views are held. Stuevold (1989) suggests that the uplift started already during the Oligocene and terminated during the early Pliocene. Riis and Fjedskaar

PALEOENVIRONMENTS

ON

THE

MID

NORWEGIAN

179

SHELF

the Naust Formation. The Kai Formation (early Miocene-late Pliocene?) consists mostly of claystone, however, silstone and sandstone do occur especially in the upper parts of the wells. In many of the samples analysed in this work, an abundance of crystalline fragments were found. This might be fallout from higher up in the stratigraphy, but could also be input of ice rafted debris.

(1993) suggest that a late Pliocene uplift phase occurred depositing large prograding wedges on the whole of the mid Norwegian shelf. The uplift was supposedly mainly due to isostatic reequilibration after glacial erosion.

Seismo- and lithostratigraphy Miocene-lower Pliocene sediments

Middle~upper Pliocene sediments (Top Hordaland- Base Pleistocene reflector)

According to Stuevold (1989), over most of the mid Norwegian margin the thickness of Miocene sediments varies between 100 and 400 ms (ca. 100-400 m). However, the sediments are very thin over the northern part of Nordland Ridge and the northern part of the Tronderlag Platform. The thickness increases southward (Fig. 4). Dalland et al. (1988) have developed a formal lithostratigraphy of the Cenozoic sediments from the mid Norwegian margin. Miocene, Pliocene and Pleistocene sediments from mid Norway belong to two formations, the Kai Formation (older) and

•

~o i

6° I

Middle/upper Pliocene sediments are characterised by wedge buildups represented as clinoforms on the seismic. Stuevold (1989) mapped a total of 11 major Pliocene wedges. According to Stuevold (1989) only the five oldest wedges are found. The P1 11 (oldest) to P1 7 reflectors mark the top of each of these wedges. The younger six are found over V~ring Plateau, however, wedge 3 does drape over some of the study area (Fig. 5). The Naust Formation (late Pliocene-

7° I

8°

9o

10°

11°

_

66 °

g _ 85°

Fig. 4. Isopach map of Miocene sediments in the study area, compiled from Stuevold (1989). The ODP wells are represented as open circles. The wells studied in this work are shown with black hatched circles.

180

D . A . R . P O O L E A N D T.O. V O R R E N

4°

5*

¢

7°

¢

I

i.

9°

10 °

11°

,r---- - x ~

1

_

67 °

_

66 °

ag YI

Isopach P] 0

ODP

•

Coral wells this

.-.

_

65 °

Shell

Contour int. I

Y

F i g . 5. I s o p a c h m a p o f P l i o c e n e s e d i m e n t s i n t h e s t u d y a r e a , c o m p i l e d f r o m S t u e v o l d (1989).

Pleistocene) is mostly unconsolidated clay, silt and sand. The general colour of the sediment is grayish. Throughout most of the formation, coarse clastics can be found. These increase upward throughout the formation. The coarse fraction includes a large percentage of crystalline rock fragments such as gneisses and granites. In certain samples, mica can be found indicating mineralogically immature sediments, however, these might be part of the drilling mud. Foraminiferal assemblage zones

Micropaleontological zonation When using ditch cuttings for dating purposes, the most important parameter is the tops/exits or the Last Appearance Datum (LAD) of the individual fossils. The LAD defines the last time the fossil appears in the material. Emphasis in this work is placed on the paleoenvironmental changes taking place and we therefore decided to define the biozones into assemblage

zones that mark out dominant species (Hedberg, 1976). However, we have also defined subzones within the assemblage zones where the LAD of certain important stratigraphical species and the influx of important secondary taxa define these subzones. Only the most important taxa are represented on Figs. 6-8. Neogene foraminiferal zones

Assemblage Zone I This zone (Figs. 6-8) is characterised by a sparse abundance of benthic and planktic foraminifera (semi-barren). The zone is, however, rich in siliceous microfossils (both radiolarians and diatoms). Assemblage Zone H Nonion barleeanum/affine Assemblage Zone: This assemblage zone is characterised by a dominance of Nonion barleeanum/affine. The zone is subdivided into four subzones based on benthic foraminifera (Figs. 6 and 7).

PALEOENVIRONMENTS ON THE MID NORWEGIAN SHELF

181

WELL 6 4 0 7 / 1 - 2

Fig. 6. Detailed foraminiferal and Bolboforma diagram of the most important taxa in well 6407/I-2. Statistical analyses has only been done on the benthic foraminifera. The circles represent values less than 5%. Only the presence of the planktic foraminifera and the Bolboformawas noted, this is represented by the squares.

Subzone IIa Nonion barleeanum/affine, Cibicides dutemplei a n d Trifarina gracil& d o m i n a t e this assemblage. T h e b e n t h i c f o r a m i n i f e r a a c c o u n t for a b o u t 8 5 % o f the fauna. O t h e r species t h a t a r e f o u n d include a m o n g o t h e r s Epistominella nippon-

ica, Pullenia bulloides, Ehrenbergina

variabilis,

Cibicides lobatulus, Uvigerina venusta a n d Uvigerina semiornata. Bolboforma are f o u n d in large a m o u n t s in this zone (B. badenensis, B. subfragori, B. laevis a n d B. metzmacheri d o m i n a t e ) . This zone is o n l y f o u n d in 6407/1-2 (1680-1530 m, Fig. 6).

182

D.A.R. POOLE AND T.O. VORREN

WELL 6507/10-1

Fig. 7. F o r a m i n i f e r a l d i a g r a m over the m o s t i m p o r t a n t t a x a for well 6507/10-1.

PALEOENVIRONMENTS ON THE MID NORWEGIAN SHELF

Neogloboquadrina acos,,ensis, Neogloboquadrina mayeri and many juvenile species dominate the planktic assemblage. N. mayeri dominates the lower part of the zone, whereas N. acostaensis dominates the upper part of the zone. Other species include sinistral and dextral forms of Neogloboquadrina pachyderma, N. atlantica, Globigerina bulloides, Sphaeroidinellopsis subdehiscens and others. There are a number of probably resedimented species in this zone such as Globigerina angulisuturalis which occurs sporadically. The zone was not found by Eidvin and Riis (1991), in wells on the "outer continental margin" (Fig. 2). Subzone lib Very few samples comprise this zone. It is only found in well 6507/10-1 (1705-1620 m, Fig. 7). The zone is very special both with regards to planktic and benthic foraminifera. Nonion barleeanum/affine, Pullenia bulloides, Cassidulina teretis and Cibicides dutemplei are the dominant benthic species. There is also a large percentage of agglutinating foraminifera where Trochammina spp. and Haplophragmoides spp. dominate. In some samples the ratio of planktic to benthic foraminifera is as much as 50%. The zone is similar to Eidvin and Riis's (1991) Ehrenbergina variabilis-G, subglobosa Zone. The abundant planktic foraminifera in this zone include G. bulloides, N. acostaensis, N. atlantica (sin. and dex.), N. altispira globosa, N. dehiscens and G. scitula. The planktic fauna is dominated by G. bulloides. Subzone IIc Nonion barleeanum/affine, Cassidul&a teretis and Epistominella nipponica dominate. The percent of planktic foraminifera decreases relative to both subzones IIa and IIb. The zone is characterized by an increase in E. nipponica and a decrease in C. dutemplei, although the latter species is still present in small amounts. Bulimina marginata seems to increase and P. bulloides decreases. There is a fairly large percentage of C. grossa. Other species that can be found include C. reniforme, C. lobatulus, C. laevigata, E. excavatum f. clavata. The latter species is probably caved from above, as is E. groenlandicum. This zone is found in 6407/1-2 (1530-1480 m, Fig. 6) and in 6507/101 (1620-1575 m, Fig. 7). N. acostaensis and N.

183

atlantica (dextralis) represent the dominant planktic species. The zone can be correlated to zone A2 from the Voring Plateau of Osterman and Qvale (1989). Subzone lid N. barleeanum/affine and T. fluens are the species that dominate in this zone. Many of the species typical to the Miocene, become extinct or migrate southwards. There does not seem to be any kind of hiatus between this zone and zone IIc. Other foraminifera that are present include C. lobatulus, B. marginata and E. nipponica. There is a very low planktic to benthic ratio, with N. pachyderma (sin.) and N. atlantica (sin.) dominating the planktic assemblage. Zone A2 from the Vering Plateau may also comprise this zone. Eidvin and Riis (1991) also find a similar zone in their material. Assemblage Zone III Cibicides lobatulus, Elphidium groenlandicum Assemblage Zone: This zone is found in wells 6407/1-2 (1430-1180 m, Fig. 6) and 6610/7-1 (480-580 m, Fig. 7). Many of the dominant Miocene-early Pliocene species have given way to species that enjoy a different environment. The dominating species in this zone include C. lobatulus, B. marginata, E. groenlandicum, C. grossa and C. teretis. Auxiliary taxa include E. hannai, C. pseudoungerianus, A. beccarii, P. orbiculare and B. aculeata. There are a few Miocene resedimented species such as A. staeschei. Assemblage Zone I V Elphidium excavatum, Cassidulina teretis Assemblage Zone: The species in this zone are similar to the C. lobatulus Zone, however, Elphidium exeavatum f. elavata comes to dominate. C. teretis is still very common. This assemblage zone can be divided into five distinct subzones (a-e), where subzone IVa is the oldest and subzone IVe is the youngest. Subzone IVa: E. excavatum f. elavata, C. teretis, B. marginata. Subzone IVb: E. excavatum f. clavata, C. teretis, C. grossa. Subzone IVc: E. excavatum f. clavata, N. niveum.

184

Subzone IVd: E. excavatum f. clavata, C. teretis. Subzone IVe: E. excavatum f. clavata, C. reniforme, C. teretis. The important difference between subzone IVe and the others is the extinction of C. grossa and the increase in percentage of C. reniforme. There is also a large increase in number of planktic foraminifera, increasing from 1% to 20%.

Chronostratigraphy For dating the stratigraphy, benthic and planktic foraminifera were used throughout the whole of the Neogene. Additionally in the Miocene section, Bolboforma spp. were used, these "problematica" were first described from the Oligocene and Miocene of north Germany (Von Daniels and Spiegler, 1974). They have since been found in the Miocene of Belgium, Holland, the north Atlantic, the North Sea and around Antarctica (see King, 1983, 1989; Qvale and Spiegler, 1989; Spiegler and Von Daniels, 1991). Bolboforma spp. have restricted vertical ranges and are valuable indexfossils.

Age of Assemblage Zone I The age of this zone is difficult to determine due to the lack of foraminifera. However, diatom sp. 4 (King, 1983) was found in wells 6507/10-1 and 6407/1-2, indicating a minimum age of early Miocene (King, 1989, minimum 18 Ma). Globigerina praebulloides leroyi was found in well 6407/1-2 (1730 m) also indicating possible early Miocene sediments (Blow, 1969). G. praebulloides leroyi may also indicate late Oligocene sediments. Almanea osnabrugensis was found in well 6507/101 (1765 m) which according to King (1989) is found in late Oligocene-early Miocene sediments of the North Sea. Neither of the two above named species were found in well 6610/7-1, however, in a very thin interval Globorotalia praescitula was found which might indicate a late early Miocene-early mid Miocene age (minimum 16 Ma). Some individuals of both Cibicides dutemplei and Nonion barleeanum/affine were also observed pointing to a Miocene age. This zone overlies a completely

D.A.R. POOLE AND T.O. VORREN

barren interval (Barren zone 1 on Fig. 8) of unknown age. In the interval below this appeared an abundance of Subbotina linaperta. King (1989) finds this species to die out just prior to the Eocene/Oligocene boundary. Goll and Hansen (1992) in their study on the mid Norwegian shelf find a similar zone whose base lies between 24 and 30 Ma, and its uppermost boundary lies between 20 and 21 Ma. To summarise, Assemblage Zone I probably represents both upper Oligocene and late Miocene sediments (its base has an age of at least approximately 30 Ma and the uppermost boundary is about 16-18 Ma).

Age of subzone IIa Trifarina gracil& is found in Miocene and Oligocene sediments from the northwest European margin (Batjes, 1958; Spiegler, 1974; Doppert, 1980; King, 1983) and Ehrenbergina variabilis was found by Eidvin and Riis (1991) from the Miocene from the mid Norwegian continental margin and the northern part of the North Sea (Eidvin and Riis, 1991). In the lowermost samples of well 6407/1-2, 1720-1680 m (Fig. 6), Sphaeroidinellopsis subdehiscens and G. quadriloba trilobus appeared. Sphaeroidinellopsis subdehiscens and Globigerinoides quadrilobatus trilobus suggest a possible middle Miocene age for the sediments. King (1989) found Sphaeroidinellopsis subdehiscens (renamed Sphaeroidinellopsis disjuncta) up to the middle Miocene. Blow (1969), however, found the species to range into the late Miocene. King (1983) found Globigerinoides quadrilobatus trilobus to have its LAD in the middle Miocene. In these lower samples Uvigerina serniornata was found, this species also occurs in middle Miocene sediments in the North Sea (King, 1989). The presence of Neogloboquadrina acostaensis and Neogloboquadrina mayeri in the upper part of the zone suggests a middle-late Miocene age for these sediments. Spiegler and Jansen (1989) and Bolli and Saunders (1985) found Neogloboquadrina mayeri in middle Miocene sediments. According

185

P A L E O E N V I R O N M E N T S ON T H E MID N O R W E G I A N SHELF

WELL

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186

D.A.R. POOLE AND T.O. VORREN

Age of subzone IIb E. variabilis is considered to have lived on the Norwegian continental shelf from Oligocene to early Pliocene (Skarbo and Verdenius, 1986). The planktic assemblage indicates a late Miocene age, with several Neogloboquadrina species present such as N. acostaens& and N. atlantica. Age of subzone IIc

Fig. 9. Table showingthe age distribution of the sedimentsin the three wells.The hatched areas represent hiatuses.

to Kennett and Srinivasan (1983), N. acostaensis is found from the late Miocene and through the early Pliocene. King (1989) recorded the species in late Miocene sediments from the North Sea. Spiegler and Jansen (1989) recorded the species in upper Miocene sediments as did Weaver and Clement (1987). Bolli and Saunders (1985) also reported the species in upper Miocene-Pliocene sediments. Three zones of Bolboforma were observed. The zonation was very similar to the V~ring Plateau drillings (Qvale and Spiegler, 1989). Using new dates by D. Spiegler (pers. commun., 1992), our zones can be dated as follows: the oldest group corresponds to Bolboforma rotunda, Bolboforma badenensis (13.70-13.15 Ma), Bolboforma subfragori (13.00-9.85 Ma), Bolboforma laevis (9.85-9.2 Ma) are all found in the zone between levels (1710-1520 m,well 6407/1-2). Together with the evidence summed up above, this indicates that the lower part of subzone IIa is late-middle Miocene (ca. 15 Ma) and the upper part of the zone is early late Miocene (ca. 9 Ma).

The extinction of both N. acostaensis and N. atlantiea (dextralis) occurs between 1460 and 1470 m (Fig. 6) in well 6407/1-2 and at 1575 m (Fig. 7) in well 6507/10-1 suggesting this to be the top of the Miocene. The Miocene/Pliocene boundary was placed in the North Atlantic at the change from dextral to sinistral coiling in N. atlantica (Poore and Berggren, 1975; Poore, 1979). Although this event is difficult to locate in the North Sea (King, 1989), it is placed close to the Miocene/Pliocene boundary. Spiegler and Jansen (1989) place the event just before the Miocene/Pliocene boundary as do Weaver and Clement (1987) who place the event at 7 Ma. According to King (1989) the presence of C. grossa indicates the presence of upper Pliocene sediments. However, C. grossa was found by Doppert (1980) to range throughout the Miocene and Pliocene. It disappears close to the Pliocene/Pleistocene boundary. C. grossa seems to be fairly abundant in this zone, the tests appear to be smaller than they were during the Pliocene (Assemblage Zone II and IV), and we therefore conclude that the tests are not fallout, but in situ late Miocene, agreeing with Doppert (1980) and not with King (1989). Bolboforma metzmacheri has its LAD at 1490 m (6407/1-2, Fig. 6) suggesting that the lower part of the zone (1530-1490 m) is of early late Miocene age (9.2-7.9 Ma, D. Spiegler, pers. commun., 1992) and from 1490 to 1460 m is of probable late Miocene age (7.9-5.5 Ma).

Age of subzone lid The lower boundary of this zone is defined by the extinction of typical Miocene species such as

187

PALEOENVIRONMENTS ON THE MID NORWEGIAN SHELF

N. acostaensis, N. atlantica (dex.), E. variabilis, T. gracilis and C. telegdi. Eidvin and Riis (1991) found the same zone in their material and suggested a possible early Pliocene age. However, the dating of this zone is uncertain. Zone A2 from the Voring Plateau (Osterman and Qvale, 1989) does indeed penetrate into the lower Pliocene.

Age of Assemblage Zone III This zone is difficult to date exactly. The presence of N. atlantica (sin.) can indicate a latest Miocene-Pliocene age. The presence of E. hannai and C. grossa may suggest a late Pliocene-early Pleistocene age for the sediments (King, 1989). However, as discussed above, C. grossa appears to occur in Miocene sediments. Small occurrences of

Florilus boueanus, Cibicides limbatosuturalis, Cancris auriculus and Textularia decrescens in the lowermost samples of the zone might indicate a late early Pliocene age (middle Pliocene, ca. 4 Ma) of the sediments (King, 1989), but these might be resedimented as they are only found in very small amounts ( < 1%). Thus either a middle Pliocene or a late Pliocene age of the sediments is possible.

Age of Assemblage Zone I V An influx of E. excavatum f. clavata in this zone indicates that the zone is probably deposited in glacial/cold environments and is most probably younger than 2.56 Ma age. The isotope data from the Voring Plateau suggest that a change from lighter to heavier values occurred at 2.56 Ma (Jansen et al., 1988). The presence of N. atlantica (sin.) indicates latest Miocene-Pliocene sediments. The continuous presence of both C. grossa and E. hannai indicates late Pliocene sediments according to King (1983, 1989). Subzones IVa-IVd are of late Pliocene age and subzone IVe is of Pleistocene age. According to Haflidason et al. (1991), C. grossa occurs in lower Pleistocene sediments from the mid Norwegian shelf. We place the boundary of the Pleistocene-late Pliocene at the level where

C. grossa decreases in abundance and where C. reniforme increases in abundance. Concluding remarks Evidence of lower Miocene sediments can be found in all three wells; however, these are difficult to date due to the lack of fossil data. Because of this we are unable to subdivide Assemblage Zone I into different zones and hiatuses. However, in well 6610/7-1, we find evidence of sediment as young as 16 Ma (G. praescitula). It would then appear that the uppermost sediments of this zone varies from 16 to 18 Ma. The hiatus observed between Assemblage Zone I and II has a minimum duration of 2.25 million years. Assemblage Zone II has its base at least at 13.75 Ma (B. rotunda). Its uppermost contact has been placed at) ca. 4 Ma. A small hiatus seems to exist between Assemblage Zone II and Assemblage Zone Ill/IV in wells 6507/10-1 and 6407/1-2. However, due to the large barren zone in well 6610/7-1 (Barren zone 1), it is difficult to say whether this extends over the whole study area. Eidvin and Riis (1991) found the same hiatus; however, unlike this work, Eidvin and Riis (1991) did not find sediments with an age of 4.5-2.5 Ma. We suggest that we find some evidence (E boueanus, C. auriculus and T. decrescens) of sediments of this age in well 6407/1-2 and 6610/7-1.

Stable isotope stratigraphy Several sources of errors are inherited when using stable isotope measurements on this material. Unlike the deep sea, continental shelf areas have both large temperature and salinity fluctuations. The isotopic composition of the tests from the shelf areas will thus vary considerably and are difficult to interpret. Furthermore, measuring stable isotopes at burial depths of up to 1800 m increase the likelyhood of diagenesis. Diagenesis may overprint or erase the isotope composition originally recorded by foraminifera and create anomalies in the record. Miller and Curry (1982), found that oxygen isotopic records at "deeply" buried ( > 400 m) sites may be depleted by up to 3%0 relative to data from unal-

188

tered samples. To test this diagenetic effect, N. barleeanum/affine and C. lobatulus were measured from five samples between 1550 and 1620 m. The average difference in isotope values between these two species, after correction, was -0.17%o+0.34. This variation is within the limits of the data and we can assume that the data are unaltered due to any diagenesis effect. In addition the carbon isotopes do not show any large anomalies. This too may indicate that the samples do not seem to have experienced much diagenesis. Carbon isotopes of pore waters are very light (McCorkle, 1985), and thus one would expect secondary calcite to have relatively low 613C values. Taking all these facts and problems into consideration, the oxygen isotope record can be roughly divided into four parts: middle Miocene-late Miocene, Early Pliocene, middle/late Pliocene and Pleistocene (Figs. 10-12), according to the time scale developed with the help of the biostratigraphy. Middle Miocene-late Miocene

During the middle-late Miocene (15-10 Ma), subzones IIa, lib and IIc, the oxygen isotope values increase from 3.3 to 4.45%0 (Figs. 10 and 12). This represents a decrease of approximately 4°C assuming constant ice volume and constant salinity. The same increase in isotopic signal has been documented in the deep sea around the middle-late Miocene (e.g. Miller et al., 1987). Buchart (1978) measuring molluscs from the North Sea also found a large paleotemperature decrease during the late Miocene. Early Pliocene

During the late Miocene-early Pliocene, the oxygen isotope signal seems to decrease slightly, from 3.95 to 3.3%0. This slight increase in isotope signal is also found in the North Sea (Buchart, 1978), and a number of deep-sea records (e.g. Keigwin, 1977; Jansen and Sjoholm, 1990). Middle Pliocene-late Pliocene

The isotope values become even lighter than during the early Pliocene. The average value during

D.A.R. POOLE AND T.O. VORREN

Assemblage Zone III is 2.46%0 increasing to 3.24%0 in subzone IVd. The oxygen isotope value in Assemblage Zone III, subzones IVa and IVb, has a tendency to become lighter in the upper part of the zones. This is not observed in subzones IVc or IVd. If we assume a salinity of 35%o and a constant ice volume at the present magnitude, the paleotemperature during Assemblage Zone III was approximately 7°C, and during subzone IVd was approximately 4°C. However, probably a large part of the signal is due to ice buildup after 2.57 Ma (Jansen et al., 1988, 1990; Miller et al., 1987). The 613C values also seems to change across the boundary from lighter toward heavier values. Pleistocene

The signal ranges from 3.7 to 1.6%o, equivalent to approximately 8°C temperature differences. During this time period, both large ice volume, salinity and temperature variations are to be expected on the shelf areas. The exact cause for these great shifts may be due to one of the above named parameters or a combination. Correlation

The biostratigraphy has been correlated to the seismic stratigraphy of Stuevold (1989), the lithostratigraphy of Dalland et al. (1988), and with supplementary data of our own including seismic correlations (see Fig. 2) and observations done under preparation of the samples (Fig. 13). We have further correlated our biostratigraphy to the holes of ODP Leg 104 from the Voring Plateau (Fig. 14). Miocene-lower Plioeene sediments

Well 6610/7-1, which lies on the northern part of the Nordland Ridge, contains a very thin layer of lower Miocene sediments. Wells 6507/10-1 and 6407/1-2, which are located over Haltenbanken, contain approximately 300 m and 250-300 m of Miocene-early Pliocene sediments, respectively (Figs. 6-8). Generally, there were no large prob-

PALEOENVIRONMENTS

ON THE MID NORWEGIAN

189

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Fig. 10. Oxygen isotope stratigraphy of well 6407/1-2. The circle represents Cibicides grossa, the square represents Cibicides lobatulus and the cross represents Nonion barleeanum/aj~ne. The paleotemperature is calculated from Shackleton's (1974) equation and Craig and Gordon's (1957) equation for the salinity (using a salinity of 35°/0o.)

190

D.A.R. POOLE AND T.O. VORREN

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191

PALEOENVIRONMENTS ON THE MID NORWEGIAN SHELF

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Fig. 13. Correlation of seismo-, bio- and lithostratigraphy for the three wells. The Top Hordaland (TH) reflector is equivalent to the base Pliocene (bP) reflector in Fig. 15. The Intra Hordaland reflector is equivalent to the base Miocene (bM) reflector in Fig. 15.

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lems dating the Miocene; however, the existence of early Pliocene sediments still remains uncertain.

Middle-upper Plioeene sediments Due to the lack of planktic foraminifera, it was difficult to date the Pliocene sediments. As mentioned above, we are uncertain as to whether Assemblage Zone III represents a late early Pliocene (middle Pliocene) or a late Pliocene age. The net thickness of the upper Pliocene wedges

varies over the area: to the north around well 6610/7-1 the thickness is about 600 m, whereas to the south (well 6407/1-2) the thickness rises to approximately 900 m (Fig. 6). The sediment source for the northern wedges has been in the northeast, whilst the sediment for the southerly wedges has been derived from the southeast (Stuevold, 1989). Correlating to Stuevold's work we can identify the intra Pliocene reflectors P1 5 and P1 11 for well 6407/1-2 and only P1 5 for well 6507/10-1 (Fig. 15). A good correlation between each seismic unit and each faunal zone is obvious. The P1 5 reflector marks the top of subzone IVb, and the P1 11 reflector marks the top of Assemblage Zone III. By correlating the wells seismically, we found that the Pliocene wedges found in well 6610/7-1 are stratigraphically older than the wedges found in the two southerly wells. This suggests that Stuevold's PI 10 reflector is older than the P1 11 reflector. The faunal zones also reflect this same tendency in that the Pliocene sediments in well 6610/7-1 contain very few arctic foraminiferal

192

D.A.R. POOLE AND T.O. VORREN 0

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2000L Fig. 14. C o r r e l a t i o n o f the t h r e e wells used in this s t u d y with O D P L e g 104 drilling 642 f r o m the V e r i n g Plateau.

species compared to the Pliocene sediments in the southerly wells. This will be discussed in more detail later. In well 6610/7-1 Barren zone 2, Figs. 8-14, is characterised by a sediment with a high percentage of sand. From the seismic, it appears that this zone corresponds to foreset layers of a delta (Fig. 15). Due to the lack of fossils, no concrete age of this delta can be given. However, it must be older than the oldest Pliocene wedges and younger than early Miocene (Assemblage Zone I). The delta is truncated by Pleistocene sediments to the east and Pliocene sediments to the west.

Discussion Late Oligocene?-early middle Miocene (30-16 Ma?) Assemblage Zone I represents the late Oligocene?-early Miocene in the study area. In wells 6407/1-2 and 6507/10-1 upper Oligocene?lower lower Miocene sediments are found, whereas in well 6610/7-1 a thin layer of upper early-early middle Miocene sediments seem to exist. The lithology is predominantly claystone. As was discussed earlier on, only a few specimens

P A L E O E N V I R O N M E N T S ON T H E M I D N O R W E G I A N S H E L F

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194

of planktic foraminifera (G. leroyi) can be found in the early Miocene sediments. The fauna is rich in siliceous microfossils, at least in the fraction larger than 100 lam. The predominance of siliceous fossils relative to calcareous fossils corresponds to results of studies from the Voring Plateau and from sites from Norwegian-Greenland Sea (Bohrman et al., 1990; Talwani et al., 1976). The surface waters were probably characterised by high fertility in the early Miocene Norwegian-Greenland Sea with a large amount of upwelling. Additional evidence for this is the high percentage of dinoflagellates and high total organic carbon values (Eldholm et al., 1987). Verdenius and Van Hinte (1983) suggested that the bottom water environment during this time was a harsh, low nutrient environment with little food supply. However, we believe that the lack of calcareous fossils together with the high productivity might suggest that a large amount of calcium carbonate dissolution occurred during this time both over the Voring Plateau (according to Osterman and Qvale, 1989) and the shelf areas. The decay of organic matter produced in the water column would enhance CO2 concentrations and thus reduce the pH. The presence of upper lower Miocene sediments in well 6610/7-1 suggests that the sea level has risen throughout the early Miocene, and that upper lower-lower middle Miocene sediments were deposited over the Nordland Ridge. The Voring Plateau became submerged during this time, probably due to a combination of subsidence (Skogseid and Eldholm, 1989) and sea-level change (Haq et al., 1987).

Late middle Miocene (14-10 Ma) The establishment of a calcium carbonate foraminiferal fauna during this time period in well 6407/1-2 (Assemblage Zone II, subzone IIa), together with the hiatus observed in wells 6407/12 and 6507/10-1 suggests that a dramatic change in oceanic circulation occurred. The presence of T. gracilis and C. dutemplei, which are extinct today, probably implies that during the middle Miocene the water mass circulation differed from modern circulation pattern although a CaCO 3 rich

D.A.R. P O O L E A N D T.O. V O R R E N

fauna dominated. Many changes in faunal distribution, as well as evolutionary originations and extinctions occurred between 16 and 13 Ma in the Pacific and North Atlantic (Woodruff, 1985; Miller and Katz, 1987). This environmental change from early to middle Miocene with a diversification of both planktic and benthic foraminifera corresponds closely to similar events recorded on the North Sea (King, 1989) and on the Voring Plateau (Osterman and Qvale, 1989). The isotope signal (Figs. 10-12) shows a gradual increase reaching a maximum after 10 Ma. This corresponds to a reported shift in benthic 6180 values (1.5-1%o) from the Pacific, Atlantic and Indian oceans in the early middle Miocene at approximately 15-10 Ma (Savin et al., 1981; Woodruff and Douglas, 1981, 1989; Miller and Fairbanks, 1983, 1985; Vincent et al., 1985; and others). This shift in 6180 has been related to ice buildup on the Antarctic continent (Shackleton and Kennett, 1975; Savin et al., 1975; Woodruff and Douglas, 1981, 1989; Miller et al., 1987) concurrent with a temperature decrease (Moore et al., 1987). Also related to this event is an increase in bottom water activity in the Atlantic due to the sinking of the Iceland-Faeroe Ridge due to lithospheric contraction (Schnitker, 1980; Vincent et al., 1985). To replace increasing volumes of outflowing cold, dense bottom waters, increasing amounts of surface waters must have been drawn north. This would have provided a more lucrative habitat and preservation situation for carbonate shells, thus the bloom. The delta sediments that are found in well 6610/7-1 (Barren zone 2) might have started to accumulate due possibly to a combination of relative sea-level fall and uplift of the mainland. However, it appears that these delta sediments might very well have been deposited any time between the middle Miocene and the early Pliocene.

Late Miocene (10-5.5 Ma) The dominant species during the late Miocene (Subzones IIb and IIc) include N. barleeanum/affine, C. teretis, E. nipponica and P. bulloides. The appearance of these modern species might be

195

PALEOENVIRONMENTS ON THE MID NORWEGIAN SHELF

due to the formation of a circulation pattern and water mass exchange in the North Atlantic that was similar to the present situation. The benthic assemblages, the seismic data, the lithology, and the abundance of planktic forms suggests an open shelf environment and the benthic contribution suggests that upper bathyal depths (130-500 m) prevailed over the Halten Bank area. The presence of E. nipponica and P. bulloides during the early late Miocene indicates that the water was relatively warm inasmuch as these forms are found in relatively warm Holocene sediments (Matoba, 1967; Sejrup et al., 1981; Hald and Steinsund, 1992), but also from Eemian sediments (Poole et al., 1991). However, during the course of the late Miocene, it appears that the relative temperature changed and became colder. Several observations indicate this trend: - The ratio of planktic to benthic foraminifera decreased throughout the Miocene as did the planktic diversity. - The isotope signal reaches a peak in heavy values during the late Miocene. P. bulloides became less abundant indicating a probable cooling of the water. - The increased presence of Cassidulina laevigata f. teretis and Cassidulina reniforme suggests relatively cold water temperatures (Murray, 1984; Mackensen et al., 1985; Mackensen and Hald, 1988; Hald and Steinsund, 1992; L. Polyak, pers. commun., 1993). The evidence above might suggest that a general cooling of waters in the study area occurred throughout the Miocene; however, periods of warm water influx have also occurred due to the presence of some warm water planktic species. This can represent a climatic cooling as was found by Berggren and Schnitker (1983), Murray (1984) and Locker and Martini (1989), among others. Although probably a substantial amount of the isotope signal represents a temperature decrease during the late Miocene, the oxygen isotope values may also reflect an ice expansion on Antarctica during the latest Miocene, including the development of the West Antarctic ice sheet (Hodell et al., 1986). It is during this period that the deposition of the Messinian evaporites in the Mediterranean -

occurred, and which in turn were related to these ice growth periods (Hodell et al., 1986). The sediments in wells 6407/1-2 and 6507/10-1 become coarser during this period and the percentage of planktic foraminifera decreases. Both these events might be due to larger sedimentation rates due to an exposed area over the Trondelag Platform and the northern part of the Nordland Ridge, however, a continued uplift of the area might also influence the sediment supply.

Early Pliocene (5.5-4? Ma) Subzone IId found in wells 6407/1-2 and 6507/10-1 (Figs. 6 and 7) and the Nonion affine Zone D of Eidvin and Riis (1991) in well 6506/121 probably represents the early Pliocene. This is rather speculative due to the insecure dating of this zone. Again the sequence does not appear on the Nordland Ridge. In the Northern Hemisphere there is evidence of warming during the early Pliocene (Funder et al., 1985; Cronin, 1988; Zubakov and Borzenkova, 1988). On the other hand, several studies have suggested an early northern hemisphere cooling and ice growth both from deep-sea sediments and land records (Einarsson et al., 1967; Einarsson and Albertsen, 1988; Jansen et al., 1990; among others). Our fossil data indicate a slight cooling during the early Pliocene. - Although there is a very small percentage of arctic cold water species (E. excavatum f. clavata and C. reniforme), the benthic fauna does change in character with many "warm" species (P. bulloides and Uvigerina) becoming less abundant and species typical of cold water environments such as Trifarina fluens start appearing. T. fluens is found in abundance in recent sediments from the eastern Barents Sea, where it dominates in the zone of mixture between Atlantic and Arctic waters (L. Polyak, pers. commun., 1993). - The isotope values decrease relatively to the late Miocene maxima, this might suggest either a warm period and/or less land ice on Antarctica and/or lower salinities of the water masses. As the fauna suggests that there was no temperature increase, and that Arctic water probably came into the study area at times, this might suggest that the

196

isotope change was due to salinity changes. However, there is evidence elsewhere of a change toward lighter 61so values during the early Pliocene at about 5.1-4.8 Ma, marking a glacial retreat and marine transgression that coincided with the termination of the salinity crisis in the Mediterranean (Hodell et al., 1986). This might then suggest that the isotope signal might reflect decreasing land ice on Antarctica. A similar episode can be found in the isotope values of Jansen et al. (1990) from the Voring Plateau where heavy 61so values (ca. 5%0) are found around 5.1-5 Ma, followed by an episode of lighter values at 4.6-4 Ma (ca. 4%0). In conclusion, the faunistic evidence suggests that the water temperatures were colder than during the previous faunal biochrons. The presence of T. fluens might even indicate the presence of Arctic waters in the area and possibly even fairly large expanses of sea-ice cover which could explain the crystalline clasts in our sample material and also the ice rafted debris found in the Voting Plateau holes between 4.5 and 3.7 Ma (Jansen et al., 1990). Comparing the isotopes to those on the Voring Plateau it would appear that similar lighter 6180 values relative to the late Miocene are found during the early Pliocene. This episode might suggest, as Hodell et al. (1986) state, that the isotope signal reflects a decrease of land ice on Antarctica. Middle Pliocene?-late Pliocene (4?-1.6 Ma) Middle-upper Pliocene sediments are represented by Assemblage Zone III and Assemblage Zone IV (subzones IVa-IVd). The dating of Assemblage Zone III is uncertain, and we conclude that either a late early Pliocene age (4 Ma?, middle Pliocene) or a late Pliocene age is appropriate. The age of Assemblage Zone IV is assumed younger than 2.56 Ma. The sediment geometry is characterised by large wedge buildups on the seismic profiles. Each faunal zone can be assigned to a particular phase of wedge buildup. The oldest unit is found in well 6610/7-1 in the north of the study area. The lithology changes from consolidated claystone (Miocene-lower Pliocene) to unconsolidated clay,

D.A.R. POOLE AND T.O. VORREN

silt, sand and gravel. Crystalline clasts were found throughout the whole sequence (Assemblage Zones III and IVa-d), but they increase in the younger wedges. A marked faunistic change occurs at the Assemblage Zone II/III boundary. Foraminiferal specimens tolerant towards harsher shallower habitats (C. lobatulus, A. beccarii and P. orbiculare, among others) take over from the deeper dwelling species (N. barleeanum/affine and E. nipponica, among others) found in the Miocene and early Pliocene. The Pliocene wedges can be divided into two main zones, i.e. Assemblage Zones III and IVa-d. Assemblage Zone III is dominated by C. lobatulus, E. groenlandicum and C. grossus, whereas Assemblage Zone IVa-d is dominated by E. excavatum f. clavata. The oxygen isotope data also show a large variation from heavy 4.45%0 to light 2.45%o. Although most of the species found in Assemblage Zone III are cosmopolitan, they are all typical of sub-arctic/temperate environments (Murray, 1991). Most previous reports of faunas similar to the Assemblage Zone III fauna are from the shallow neritic zone (less than 130 m). It contains certain species (Bulimina marginata and B. aeuleata, among others) which represent boreal shallow water (neritic) environments (Murray, 1991; and references therein). In Assemblage Zone IV, E. excavatum f. clavata becomes dominant. Elphidium exeavatum f. elavata has its main distribution in polar shelf waters and/or stressed shallow waters characterized by rapid changes in salinity and temperature (Cushman, 1948; Nagy, 1965; Osterman, 1984; Hald and Vorren, 1987). The Elphidium excavatum fauna occurs rarely in modern sediments, but has been reported in glacial marine sediments from all over the North Atlantic (Feyling-Hansen, 1972; Vilks and Rashid, 1976; Cronin, 1979; Scott and Medioli, 1980; Hald and Vorren, 1987; Hald et al., 1990; Poole et al., 1991). The oxygen isotope data indicate the same trend as the foraminiferal fauna. In Assemblage Zone III, there seem to be lighter ~taO values relative to Assemblage Zone IV. This agrees well with data from Jansen et al. (1988, 1990) from studies on the Voring Plateau who also have a period of light

197

PALEOENVIRONMENTS ON THE MID NORWEGIAN SHELF

6180 values at around the middle-late early Ptiocene (3.2-2.5 Ma). These light values suggest that during Assemblage Zone III, either the water temperature was warmer, there was less ice accumulated on land or/and the low values represent meltwater discharges from local glaciers or lower salinities. Due to the probable relatively shallow water conditions, lower salinities might have prevailed and might have important consequences on the isotope signal. Jansen et al. (1988) also suggested that lower salinities dominated during certain periods of the late Pliocene. It is possible that similar low salinity water masses prevailed also during middle Pliocene times. It is very difficult to deconvolve the isotope signal into its principal governing factors, but, the signal does indeed change across the Assemblage Zone III/IV boundary reflecting an important change in oceanographic conditions. We suggest that part of the isotope signal reflects an increase in ice volume and that the boundary between Assemblage Zones III and IV reflects an increase in ice buildup around 2.5 Ma. Throughout subzones IVa-d, there is a net increase in the oxygen isotope values, this probably reflects a temperature and ice volume effect. The increase does not seem to be gradual, and indeed there are periods where the average isotope value becomes lighter. The isotope values of Jansen et al. (1988) also are not gradual during the late Pliocene (Matuyama Chron), their isotope signal also varies largely and as mentioned above Jansen et al. (1988) suggested that lower salinities prevailed. The oxygen isotopes also indicate that within Assemblage Zone III and subzone IVa a shallowing (or temperature increase, or salinity decrease) seems to have occurred, probably due to sediment buildup within each wedge system. Due to the large sediment accumulations offshore, it appears that during Assemblage Zones III and IV, middle Pliocene-Pleistocene, there was extensive erosion of sediments/rocks onland. Probable due to glacial activity along coastal areas (Assemblage Zone IV) or fluvial erosion (Assemblage Zone III). One of the most controversial of Scandinavian geomorphological features is the strandflat (Reusch, 1894). The strandflat is developed along most of the Norwegian coast.

It is an uneven and partly submerged rock platform extending seawards from the coastal mountains. A wide range of processes have been suggested for the formation of the strandflat. These include marine abrasion (Reusch 1894), subaerial denudation (Ahlmann, 1919; Evers, 1962), glacial erosion (Holtedahl, 1929), frost shattering and sea-ice erosion (Nansen, 1904, 1922), or a combination of processes (Holtedahl, 1959; Klemsdal, 1982; Larsen and Holtedahl, 1985). We believe that it was during Assemblage Zone IV time that the frost shattering would have its greatest effect, and believe that the strandflat was initially formed during this period and that later glacial activity and planation molded the strandflat into its present state. To conclude, during the middle?-late Pliocene, large wedge systems were built up off the coast of mid Norway. From the fauna and the isotope signals, an important change in environment seems to have occurred during the deposition of these large wedges. It is difficult to ascertain the exact cause of this change; however, we believe that the change is due to an intensification of glaciations on the mainland and that the Assemblage Zone III was deposited in a non-glacial (sub-arctic) regime and Assemblage Zone IVa-d was deposited in near proximity to a glacier (arctic).

Pleistocene (1.6-Present) Pleistocene sediments are represented by a sequence that blankets the whole study area. The base of the Pleistocene is marked by an erosional unconformity, this is probably due to a number of glacial advances over the mid Norwegian shelf. Due to the fact that our samples are cuttings samples, no discussion is feasible on the number of glacial expansions. However, in our samples there is a general increase in the percentage of planktic foraminifera in this zone and a general increase in the number of benthic foraminifera (both arctic and boreal) relative to the late Pliocene. The zone is also characterised by sporadic occurrences of interglacial species (B. marginata and E. nipponica, among others) and the 8180 values are chaotic with both light and heavy values recorded (4-1.6%o), which probably represents a

198

combination of meltwater spikes and interglacial-glacial conditions. In some samples boreal benthic foraminifera dominate such as B. marginata. B. marginata is found today and is associated with interglacial conditions (Mackensen et al., 1985; Sejrup et al., 1989; Murray, 1991; among many others). Haflidason et al. (1991) found that the midNorwegian shelf showed sequences of short episodic glacigenic sedimentation and periods of more continuous marine sedimentation. This can be seen from the light 6180 values together with "warmer" faunal zones within this upper sequence. Haflidason et al. (1991) suggested that the study area was covered by five glacial expansions during the last 1.1 million years. The first appears to have expanded over the shelf area around 1.1 Ma; however, they have no older material and thus this is a minimum age. This agrees with Jansen et al. (1988) who suggested that a change toward larger glaciers occurred during the period 1.2-0.6 Ma. Interestingly, Spiegler and Jansen (1989) found larger accumulations of planktic foraminifera in the later interglacials than occurred during the earlier interglacials (2.5-1.2 Ma). This change in ice growth must have had a significant effect on the erosion and sedimentation on the continental shelf. From the sediment accumulations in the area it appears that the late Pliocene glaciers eroded far more effectively than the Pleistocene glaciations. However, further studies of the Pleistocene deposits further out in the Voring Basin and Plateau are needed to determine the extent of the Pleistocene deposition further offshore and thus evaluate the extent of this change.

Late Cenozoic sedimentation and uplift history No Tertiary sediments are found on the mainland of Norway. The large Neogene and Pleistocene accumulations that are found on the continental shelf together with the fairly good control over the habitats of foraminifera have provided us with a good basis for paleoenvironmental and depositional reconstructions. This will now be used to interpret the erosional history of the source area (Fig. 16).

D.A.R. POOLE AND T.O. VORREN

Sedimentation rates Using the dates obtained from the fossil record (using the exits of the Bolboforma species as discussed above), the net sedimentation rates for well 6407/1-2 were calculated (Table4). During the middle and late Miocene and the early Pliocene the average net sedimentation rate was approximately 2.5-3.0 cm/1000 yrs. There seem to have been fairly large changes in the sedimentation rate during the Miocene (Table 4), and these might correspond to changes in sea level (Haq et al., 1987) or uplift of the mainland, possibly resulting in deposition of the delta system documented in well 6610/7-1 (Barren zone 2). A dramatic change occurs during the middle-late Pliocene, with net sedimentation rates increasing tenfold to approximately 37.5 cm/1000 yrs. This increase in sedimentation rate is coeval with a several fold increase in sedimentation rate on the Bear Island fan along the continental shelf of the Barents Sea (Vorren et al., 1991). A similar increase in sedimentation rate is also found in the North Sea (Cameron et al., 1987) and from eastern Canada (Piper and Normark, 1989). Vorren et al. (1991) relate the increase to (1) glacio-eustatic sea-level lowering, (2) uplift of the shelf and adjacent areas in part due to isostatic readjustment, and (3) intensified erosional processes due to more extensive glaciations. Pliocene uplift The sediments deposited during Barren zone 1, well 6610/7-1, Assemblage Zones III and IV accumulated as wedges (Fig. 15). In the following discussion we have assumed that the oldest wedges (Barren zone 1 and Assemblage Zone III) were deposited during the Middle Pliocene (approximately 4 Ma) and that the younger wedges (Assemblage Zone IVa-d) are of late Pliocene age. Obviously the sediments were clastics transported from the mainland, witnessed by the content of crystalline rocks. We believe the wedge accumulations to be a result of an uplift and an increased erosional potential. It appears that the uplift was fairly rapid because the first wedge buildups (Barren zone 1, well 6610/7-1) were devoid of foraminifera, this may suggest that the earliest sedimentation rate was

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200

D.A.R. POOLEAND T.O. VORREN

TABLE 4 Sedimentation rate for certain intervals of well 6407/1-2. The dates that were used in the Miocene were taken from the Bolboforma (D. Spiegler, pers. commun., 1992) Age

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16 37.5 3.3 0.8 1.6 5.7 1.8 5 6.4

extremely high. From analysis of the foraminiferal faunas and their oxygen isotopes in the oldest wedges (Assemblage Zone III), it appears that the first wedges were probably deposited in a nonglacial regime (sub-arctic). This does not exclude mountain glaciations; however, the erosion has probably been predominantly fluvial. This suggests that the uplift occurred prior to any of the large glaciations. The uplifted Fennoscandian mainland might very well have acted as a positive feedback for glaciations in the area. Ruddiman (1988) and Ruddiman (1991) suggested that the late Cenozoic uplift of the Rocky Mountains and the Himalayan/Tibetan Plateaus induced global cooling and glaciations by changing the atmospheric circulation patterns. This tectonically induced cooling might very well have been aided by an uplift of Fennoscandia. An additional mechanism that could have had indirect effect on the erosion and sedimentation during this time could be related to the closing of the Panama strait. Around 4 Ma (Duque-Caro, 1990), the Panama strait closed and directed warm surface waters to the north. The result of this would be increased heat flux and water vapor transport northwards that would have caused additional moisture to fall over Fennoscandia. The additional water would consequently have eroded the uplifted terrain. A lowering of the sea level could have also influenced the sedimentation rate. However, we believe that such large accumulation

rates that are noted and the sediment type cannot alone be explained by lower sea levels. The later wedges in the study area have a fauna with a dominant glacial component (Assemblage Zone IV, Subzones IVa-d). The increasingly heavier isotope values also indicates that possibly the ice sheets had grown and may have reached the coastal areas. At this time, the glaciers probably had a greater erosional potential than during the Pleistocene as the environments seem not to fluctuate as much, and one might expect that the glaciers were probably more stable. This has been shown by ~180 values from the deep sea and from the isotope signals from the later wedges that gradually increase and show no large fluctuations as the isotope signal does in the Pleistocene. During the deposition of these sediments, considerable subsidence has occurred (Jackson and Hastings, 1986) and the differential loading has caused regional intrabasinal arching and local diapirism (Stuevold et al., in prep.).

Concluding remarks One of the greatest difficulties in discussing the late Neogene uplift of Fennoscandia is the problem in dating the oldest of the Pliocene wedges on shelf sediments proximal to coastal regions. As discussed above, the planktic and the benthic foraminiferal zonation is not as accurate as is required. However, one certain factor is that large climatic and oceanographic changes did indeed occur during the buildup of the Pliocene wedges, probably reflecting the intensification of Northern Hemisphere glaciations. We have discussed above the likelihood of an uplift prior to the onset of the large glaciations and that possibly, the glaciations were triggered by a combination of increased elevation of the hinterland and changes in the oceanographic situation due to plate reorganisations. The theory holds if the dating and paleoenvironmental reconstructions are correct. On the other hand, if the wedges are all of late Pliocene age as was suggested by Riis and Fjeldskaar (1993), much of the uplift of the mainland could be a consequence of glacial erosion and isostatic re-equilibration. This theory of uplift and erosion has also been suggested by Molnar and England (1990). They suggested that the uplift was triggered by increased erosion.

201

PALEOENVIRONMENTS ON THE MID NORWEGIAN SHELF

Conclusions Four main benthic assemblage zones were found with nine distinct subzones. These zones have been dated using a combination of planktic and benthic foraminifera and Bolboforma. However, similarities to other areas in the region especially the V~ring Plateau have been used to pinpoint special events. A possible scenario for the uplift and sedimentation history of the area is also described (Fig. 16).

Early Miocene A silica rich fauna dominated throughout the early Miocene. This was probably due to a combination of a high fertility of surface waters, upwelling and CaCO 3 dissolution.

Middle Miocene The establishment of a CaCO3 rich fauna can be associated to global paleoceanographic events such as the initiation of the Iceland-Faeroe overflow. It appears that the sea level had fallen during this time period and delta sediments accumulated on the shelf areas. This might be due to an uplift of the mainland.

Late Miocene The foraminiferal faunas that dominated are still existent today and we suggest that probably circulation patterns that are similar to present conditions prevailed. The fauna corresponds to an open shelf fauna (130-500 m). The temperature decreased throughout the late Miocene. It appears that warm atlantic waters flowed in the area. There does not seem to be any convincing evidence of coastal glaciations. Delta sediments continued to accumulate, probably due to continued uplift of the mainland.

Early Pliocene A further reduction in temperature occurred and consequently mountain glaciers might have started

to grow and possibly arctic waters entered the study area.

Middle~late Pliocene A large system of sediment wedges were deposited during the late early Pliocene (middle Pliocene)-late Pliocene. We believe that the wedges were first deposited in a non glacial regime. Evidence for this stems from analysis of the foraminiferal faunas and the isotope signal. We also suggest that the sediment was deposited in association with an uplift of the hinterland. Thereafter, during the late Pliocene the glaciers on land expanded and reached coastal regions. The foraminiferal fauna reflects proximal glacial conditions. Due to the extremely large sedimentation rates during the late Pliocene and the presumed strong erosion of the mainland, we suggest that the strandflat was probably created during the late Pliocene and was probably caused by large scale frost shattering.

Pleistocene The Pleistocene was an epoch of large fluctuations of continental ice sheets that seemed fairly stable. From the erosional unconformity on the top of the Pliocene wedges, it appears that glaciers moved out onto the shelf proper.

Acknowledgements The work was supported by grants from VISTA, a research cooperation between the Norwegian Academy of Science and Letters and Den Norske Stats Oljeselskap A S (Statoil). Statoil, British Petroleum (BP) and Saga Petroleum provided us with the data and valuable discussions. Dr. K.L. Knudsen, Dr. E. Jansen, Dr. M. Hald, T. Eidvin and two referees for Marine Geology critically read the manuscript and made many usefull suggestions for improvements. T. Dokken, E. Lebesbye, A. Rornes and S. Henriksen discussed earlier drafts of the paper. Mary Raste and Marit Berntsen (University of Tromso) and/~sa Knudsen (Statoil) helped prepare the samples. Oxygen isotope

202

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