Planktonic foraminifera of the Nkalagu Formation type locality (southern Nigeria, Cenomanian–Coniacian): biostratigraphy and palaeoenvironmental interpretation

Cretaceous Research 25 (2004) 191e209 www.elsevier.com/locate/CretRes

Planktonic foraminifera of the Nkalagu Formation type locality (southern Nigeria, CenomanianeConiacian): biostratigraphy and palaeoenvironmental interpretation Holger Gebhardt Institut fu¨r Geowissenschaften, Mikropala¨ontologie, Christian-Albrechts Universita¨t zu Kiel, Olshausenstraße 40, D-24118 Kiel, Germany Received 29 May 2003; accepted in revised form 18 November 2003

Abstract A study on the planktonic foraminifera of the southern Nigerian Nkalagu Formation (CenomanianeConiacian) was carried out. Four biostratigraphic zones are proposed for the (?)middle TuronianeConiacian interval in southern Nigeria: (1) Praeglobotruncana cf. stephani Zone (middle? Turonian); (2) Marginotruncana sigali Zone (late Turonian); (3) Dicarinella primitiva Zone (latest Turonian); and (4) Dicarinella concavata Zone (Coniacian). Based on planktonic/benthonic foraminiferal ratios and environmental index forms, a general deepening of depositional environments is indicated from late CenomanianeTuronian and Coniacian ages. Upper Cenomanian sediments were deposited on an inner shelf (0e70 m, 0e20% planktonic foraminifera; only one Heterohelix species occurs). During the (?)middle to early late Turonian, an upper bathyal environment of about 600 m water depth is indicated (46e94% planktonic foraminifera, with heterohelicids dominating and a relatively large number of keeled specimens). The middle late to latest Turonian interval is characterized by 20e71% planktonic foraminifera with heterohelicids dominating and very rare keeled specimens, pointing to an upper bathyal depositional environment (ca. 250 m water depth). A (deeper) upper bathyal environment (ca. 600 m water depth), dominated by heterohelicids but with up to 30% hedbergellids during the Coniacian, is indicated by 63e93% planktonic foraminifera with a relatively large number of keeled specimens. In general, an open marine deepwater environment (upper bathyal) is indicated by the (?)middle TuronianeConiacian planktonic foraminiferal faunas, further influenced by periods of eutrophication or (weak) salinity fluctuations. The (?)middle Turonian and latest late Turonian were time intervals of highest surface productivity in southern Nigeria. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Planktonic foraminifera; CenomanianeConiacian; Nigeria; Biostratigraphy; Palaeoenvironments

1. Introduction Planktonic foraminifera form the basis of many marine biostratigraphic zonations in the Cretaceous and Tertiary intervals (e.g., Caron, 1985; Bralower et al., 1995). However, long-ranging shallow-water associations dominate most marine Cretaceous deposits of the Benue Trough and keeled deep-water forms are restricted to the Nigerian coastal basins. Only the TuronianeConiacian of the Lower Benue Trough (Nkalagu Formation) yielded such forms, which are important for biostratigraphy and worldwide correlation of strata. Due to their

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relatively scarce occurrence in Nigeria (Petters, 1983a), a biostratigraphic zonation based on planktonic foraminifera for the Late Cretaceous was never attempted. The concentration of the usually larger tests of index forms using the dry sieving method has not been applied before. Due to the higher concentration of foraminiferal tests with the dry sieving of the samples studied, it was possible to gain sufficient material for a zonation of the ?middle TuronianeConiacian strata of the Lower Benue Trough (Band 18 and Band 20 sections; Fig. 1). In addition to the biostratigraphic study, the planktonic foraminiferal associations were analysed statistically and interpreted palaeoecologically. Further papers on planktonic foraminifera from the interval investigated in

192

H. Gebhardt / Cretaceous Research 25 (2004) 191e209

Fig. 1. Geology of the area investigated. A, important tectonic elements. B, geological map of Anambra Basin and Lower Benue Trough. C, geological map of the surroundings of the area investigated and location of the sections studied (geology according to Umeji, 1993; Isimkpuma Sandstone, Ezillo Silt and Nkalagu Shale are considered to be members of the Nkalagu Formation).

H. Gebhardt / Cretaceous Research 25 (2004) 191e209

Nigeria were published by Fayose and De Klasz (1976), Petters (1978a, 1980a,b), Perch-Nielsen and Petters (1981), Odebode (1982, 1983, 1986a,b), Nyong and Ramanathan (1985) and Gebhardt (1997). The sections investigated are situated on the northwestern flank of the Abakaliki Anticline (Fig. 1), a major tectonic structure in the Lower Benue Trough. The two most important sections crop out on faces of the NigerCem active quarries at the time of investigation. Annotation of the sections (Bands 18 and 20) is according to the NigerCem nomenclature, which numbers the successive limestone layers (bands) in the area. Detailed descriptions of the structural relationships in the region have been presented in previous studies by Agagu and Adighije (1983), Benkhelil (1988, 1989) and Ojoh (1990). Most of the previous authors have described the depositional environment of the entire Benue Trough as shallow marine and (in parts) anoxic (e.g., Petters, 1978b, 1982). However, more recent studies on sedimentological structures and facies indicate different depositional environments, at least in the lower part of the Benue Trough. This includes a new interpretation of the limestone layers as turbidites and new faunal analyses (benthonic foraminifera, ostracods; Gebhardt, 1999, 2000). The calcareous nannofossil content also emphasizes the open marine character of the deposits at Nkalagu (Gebhardt, 2001a). The depositional framework of the rocks investigated can be described as a deep embayment, which allowed for deep-water sedimentation (upper bathyal; Gebhardt, 2000). The area is of key importance for biostratigraphic studies to correlate strata of corresponding ages in the Middle Benue Trough, which were deposited under marginal marine (paralic) and continental conditions (e.g., Petters, 1982, 1983b; Obaje, 1994). A biostratigraphy based on planktonic foraminifera is needed, particularly in view of numerous boreholes drilled for petroleum exploration in southern Nigeria and the relative scarcity of macrofossils within the shales investigated. The deposits investigated were formed during the second of three major Cretaceous transgressions which flooded the Benue Trough (e.g., Adeleye, 1975; Petters, 1978b). Because of the lack of lithological differences, the former Eze-Aku and Awgu formations have been combined into the Nkalagu Formation by Petters and Ekweozor (1982). The Nkalagu Shales, the Ezillo Siltstones and the Isimkpuma Sandstones (Fig. 1) are considered here to be members of the Nkalagu Formation. The outcrops in the Nkalagu Quarries were chosen by Petters and Ekweozor (1982) to be the type locality for the Nkalagu Formation, and the Band 20 section as its type section. The 23.5-m-thick Band 18 section (Figs. 1, 2) begins with several thick limestone beds at its base, intercalated by thin shale layers. The limestone beds (mainly floatstones) show typical characteristics of turbiditic

193

sedimentation such as graded bedding and exotic components (e.g., transported shallow water bivalves). They represent the proximal fan facies and are very similar to the basal limestone beds in the Band 20 section. The top 20 m are built up of shales and silty shales, interrupted only by two marly layers which have been classified as planktonic foraminifera-dominated wackestones in thin sections (fine-grained turbidites; Oti, 1990; Gebhardt, 2000). The 26-m-thick Band 20 section (Figs. 1, 2) consists of shales to silty shales, intercalated by limestones and marlstones. A 6-m-thick stack of limestones close to the base of the section has been quarried for cement production. It is intercalated with thin shales or marl layers. Several centimetre- to metre-thick limestone and marlstone beds are interbedded with the shales towards the top of the section. The limestone and marlstone beds are interpreted as turbidites. They show typical characteristics in most cases (e.g., fining upward sequences, matrix-supported exotic components), although typical Bouma cycles are not present. This follows earlier interpretations by Banerjee (1981), Oti (1990) and Amajor (1992). Thick, coarse, massive beds represent proximal fan facies close to feeder channels while thin, finegrained, laminated layers represent distal facies (Gebhardt, 2000). A third section situated close to the Aboine River (Fig. 1) was also investigated but yielded only sparse and long-ranging planktonic foraminifera (see Table 1). This 14-m-thick section starts with sandstone beds (Isimkpuma Sandstone of Umeji, 1993), followed by predominating shales (sometimes silty shales) with intercalated sandstones (bioclastic quartz-arenites) and limestones (wackestones, oysterbeds; Ezillo Silt). Plant fragments are common in some places. Lawal (1991) dated this section as latest Cenomanian, based on the occurrence of characteristic pollen grains, spores and dinoflagellate cysts. He found the pollen grain Triorites africaensis to be indicative of this age. Zaborski (1987) discovered in abandoned open pit exposures on the western outskirts of Ezillo (see Fig. 1) an ammonite fauna which he regarded as close to the CenomanianeTuronian boundary. The fauna consists of Thomasites gongilensis, T. koulabicus, Wrightoceras wallsi, W. cf. munieri, Fagesia sp. and Paravascoceras? sp. Also west of the section investigated, Reyment (1955: fide Zaborski, 1987; Lawal, 1991) described a small fauna from Ezillo, which he assigned to the lower Turonian. Offodile and Reyment (1976) found a possible Allocrioceras annulatum close to or within this section; this species is typical for the upper Cenomanian of the Western Interior and elsewhere (Zaborski, 1987). Accordingly, a late (or latest) Cenomanian age for the Aboine River Section can be inferred with some confidence. Because this section is of little biostratigraphic interest as far as planktonic foraminifera are concerned, it is not figured.

Fig. 2. Occurrence of planktonic foraminifera in the samples of the Band 18 and 20 sections, biostratigraphical markers and planktonic foraminiferal zones (FO, first occurrence; LO, last occurrence).

195

H. Gebhardt / Cretaceous Research 25 (2004) 191e209

Table 1 Distribution of planktonic foraminifera in the Band 18 and Aboine River Sections; numbers indicate percentages; those in brackets indicate numbers of picked individuals Band 18 section Sample

2

4

7

9

10

11

12

13

14

15

5 (3)

7 (6)

3 (8)

6 (17)

5 (15)

3 (8)

90 (78)

82 (246) 1 (4)

90 (261)

85 (256) 1 (4)

7 (4)

3 (3)

84 (259) 2 (6) 3 (10) 7 (21)

4 (12) 1 (2) 77 (220) 5 (14)

2 (5)

86 (49) 2 (1)

5 (13) 1 (2) 91 (256) 1 (4)

87

1 (3) 307

299

17

18

19

20

3 (8) 0 (1) 82 (248) 1 (4)

3 (9) 2 (5) 76 (197) 8 (21)

12 (34) 2 (6) 76 (220) 4 (12)

4 (2)

3 (8) 1 (3) 79 (241)

13 (38) 1 (4)

10 (25) 0 (1)

6 (18)

Hedbergella delrioensis Hedbergella simplex Heterohelix globulosa Heterohelix moremani Heterohelix pulchra Heterohelix reussi Heterohelix reymenti Marginotruncana renzi Whiteinella baltica Whiteinella inornata Number of individuals picked

3

Sample

16

Hedbergella delrioensis Hedbergella simplex Heterohelix globulosa Heterohelix moremani Heterohelix pulchra Heterohelix reussi Heterohelix reymenti Marginotruncana renzi Whiteinella baltica Whiteinella inornata Number of individuals picked

nd (3)

9 (28) 1 (3)

2 (6)

89 (260)

4 (11) 1 (2) 1 (2)

12 (34) 1 (3)

6 (18) 2 (7)

9 (28) 2 (5)

281

1 (3) 293

1 (2) 287

291

301

21

22

2

8

0 (1) 57

Band 18 section

Aboine River Section

84 (41)

12 (6)

nd (1) nd (4)

nd (9)

nd (4)

nd (1)

4

10

4

1

12 (36) 5 (16) 1 (2)

303

258

290

49

2. Material and methods Eighty-eight samples were collected during a fieldtrip in 1996. The samples include limestones, marlstones and shales. Fifty-eight of them yielded planktonic foraminifera. About 1 kg of each sample was processed. The samples were dried, soaked in hydrogen peroxide (10%) and washed over a 0.063 mm sieve. This procedure was repeated several times when necessary. The residue was distributed on a black picking tray and at least 300 specimens were picked, whenever possible. The exact quantities of picked planktonic foraminifera are shown in Tables 1 and 2; all species are listed together with their absolute and relative occurrence in the samples. In order to concentrate the relatively large but scanty keeled planktonic foraminifera, dry sieving was done to retrieve the O0.250 mm fractions after picking of the O0.063 mm fractions. The O0.250 mm fractions were scanned particularly for index forms, but not picked completely in all cases (Table 3). The assemblages were analysed quantitatively and interpreted. No indication of displacement or reworking of planktonic foraminifera from older strata was seen (e.g., degree of alteration, index forms). However, corrosion or dissolution of calcareous foraminifera was observed at different levels (e.g., top of Band 18 section, top and base of Band 20 section). The content of calcareous microfossils in

306

these areas is distinctly reduced if compared with the unaltered areas and the recovered planktonic foraminifera show distinct signs of dissolution up to the complete dissolution of the tests (preserved as internal moulds; Gebhardt, 2000). All material is stored in the micropalaeontological collection of the Institut fu¨r Geowissenschaften of Christian-Albrechts Universita¨t zu Kiel.

3. Biostratigraphy Petters (1983a) decided not to propose a zonation for the Cretaceous rocks in the Benue Trough based on planktonic foraminifera. However, he assumed that it is possible to distinguish stages using them. On the basis of the Nkalagu material, the zonation for the tropics according to Caron (1985) is applicable to southern Nigeria to a large extent. Dupont (1996) has already shown this for Gabon. Fig. 2 shows the presence of planktonic foraminiferal species in the Band 18 and Band 20 sections, together with biostratigraphic markers and the zonation proposed here. Only a very few biostratigraphically non-diagnostic species occur in the Aboine River section (therefore, as noted above, no figure is provided; see Table 1). For comparison, further important planktonic foraminiferal zonations are shown in Fig. 3.

Sample

1

196

Table 2 Distribution of planktonic foraminifera in the Band 20 section; numbers indicate percentages; those in brackets indicate numbers of picked individuals 5

8

10

11

13

15

16

17

18

19

20

21

nd (2)

nd (6)

14 (14)

28 (26)

21 (57)

10 (29)

6 (17)

6 (12)

nd (9)

5 (9)

4 (4)

nd (3)

7 (6)

1 (3) 4 (12)

1 (4)

0 (1)

nd (1)

Hedbergella delrioensis Hedbergella flandrini Hedbergella holmdelensis Hedbergella planispira Hedbergella simplex Heterohelix cf. americana Heterohelix globulosa Heterohelix moremani Heterohelix pulchra Heterohelix reussi Heterohelix reymenti Marginotruncana cf. renzi Pseudoguembelina cf. costellifera Whiteinella baltica Whiteinella inornata Number of individuals picked

29

Sample

22

Hedbergella delrioensis Hedbergella flandrini Hedbergella holmdelensis Hedbergella planispira Hedbergella simplex Heterohelix cf. americana Heterohelix globulosa Heterohelix moremani Heterohelix pulchra Heterohelix reussi Heterohelix reymenti Marginotruncana cf. renzi Pseudoguembelina cf. costellifera Whiteinella baltica Whiteinella inornata Number of individuals picked

nd (2)

32

20

Sample

40

41

42

43

44

45

46

47

48

49

50

51

52

nd (7)

30 (34) 3 (3)

23 (66) 2 (5)

23 (22) 2 (2)

10 (29) 1 (3) 1 (3)

12 (33)

10 (29) 3 (9)

7 (19) 2 (5)

10 (31) 4 (12) 2 (6)

10 (28) 1 (4) 1 (4)

14 (40) 3 (9) 1 (4)

8 (24) 2 (5) 9 (28)

70 (211) 1 (3) 2 (6) 10 (29) 1 (3)

72 (196) 1 (4)

57 (160) 6 (17)

9 (25) 1 (4)

11 (32) 4 (12)

71 (210) 2 (5) 0 (1) 1 (4)

1 (3) 1 (3) 271

2 (5) 0 (1) 1 (3) 283

nd (5) nd (1)

1 (1)

nd (22)

85 (82)

61 (56)

64 (174) 1 (2)

82 (239) 1 (3)

78 (225) 1 (3)

76 (155) 3 (6)

nd (19)

86 (153) 1 (1)

nd (2)

1 (1)

4 (4)

8 (23)

5 (15) 1 (2)

12 (36) 2 (6)

14 (28) 1 (2)

nd (2)

8

30

97

92

271

292

290

204

23

25

28

29

31

32

33

35

nd (3)

12 (33)

6 (15)

3 (7)

2 (3)

3 (8)

2 (5)

1 (4) 2 (5)

1 (2) 0 (1)

1 (3)

nd (1)

nd (27)

nd (13)

73 (200) 0 (1)

nd (3)

nd (4)

11 (31) 0 (1)

76 (202) 3 (8) 1 (4) 11 (30) 2 (5)

75 (206) 5 (14) 1 (2) 15 (41)

nd (19)

8 (14) 1 (1)

78 (73) 4 (4) 1 (1) 9 (8) 3 (3)

31

178

94

25

36

37

38

39

4 (9)

3 (3)

3 (3)

1 (1)

nd (2)

0 (1)

0 (1) 0 (1)

1 (1)

75 (131) 3 (6)

0 (1) 76 (218) 4 (11)

88 (224) 1 (3)

84 (209) 2 (6)

nd (10)

12 (35) 4 (12)

8 (20) 1 (2)

8 (21) 2 (4)

78 (74) 7 (7) 2 (2) 9 (9) 2 (2)

95 (94) 2 (2)

17 (29) 3 (6)

80 (76) 3 (3) 1 (1) 9 (9) 2 (2)

175

285

255

250

95

97

99

nd (2) nd (1)

2 (2)

0 (1)

275

267

274

12

1 (3) nd (16)

nd (19)

57 (66) 1 (1)

nd (4)

nd (7)

9 (10) 1 (1)

51 (146) 5 (14) 2 (7) 13 (38) 2 (6)

57 (54) 1 (1) 16 (15) 1 (1)

68 (208) 0 (1) 1 (4) 16 (48) 3 (9)

62 (174) 0 (1) 1 (4) 21 (59) 3 (9)

68 (193) 1 (4) 16 (47) 1 (3)

78 (224) 5 (14) 1 (2) 7 (20)

1 (2)

20

33

115

1 (3) 285

95

305

280

285

287

301

6 (16) 295

H. Gebhardt / Cretaceous Research 25 (2004) 191e209

Hedbergella delrioensis Hedbergella flandrini Hedbergella holmdelensis Hedbergella planispira Hedbergella simplex Heterohelix cf. americana Heterohelix globulosa Heterohelix moremani Heterohelix pulchra Heterohelix reussi Heterohelix reymenti Marginotruncana cf. renzi Pseudoguembelina cf. costellifera Whiteinella baltica Whiteinella inornata Number of individuals picked

nd (28)

1 (3)

197

H. Gebhardt / Cretaceous Research 25 (2004) 191e209 Table 3 Numbers of planktonic foraminifera larger than 0.250 mm picked from samples from the Band 18 and 20 sections Band 18 section Sample Dicarinella concavta Dicarinella cf. hagni Dicarinella imbricata Dicarinella primitiva Hedbergella delrioensis Hedbergella flandrini Hedbergella holmdelensis Hedbergella planispira Hedbergella simplex Marginotruncana cf. pseudolinneiana Marginotruncana cf. renzi Marginotruncana schneegansi Marginotruncana sigali Marginotruncana cf. sinuosa Marginotruncana undulata Praeglobotruncana cf. stephani Praeglobotruncan austinana Whiteinella archaeocretacea Whiteinella baltica Whiteinella inornata

7

9

10

10

15

Band 20 section 11

12

13

14

15

51

1 2

3

12

16

17

18

20

13

15

16

22 10 13

12 21 2

1

4

28

2

7 3 1 21 7 11

25

4

4

3 5 2

19

7 6 2

6

1 2

2

1

1

7

1

2 1 5

3

17

3

8

47

48

49

50

51

52

1

1

1 10 4

3

1 4 3

1 6

2

2 18

12 29

14 28 59

35

38

39

40

1

1

1 9

21 4

3

2

42

43

44

45

1

3 8 2 21

Band 20 section Sample Dicarinella concavta Dicarinella hagni Dicarinella imbricata Dicarinella primitiva Hedbergella delrioensis Hedbergella flandrini Hedbergella holmdelensis Hedbergella planispira Hedbergella simplex Marginotruncana cf. pseudolinneiana Marginotruncana cf. renzi Marginotruncana schneegansi Marginotruncana sigali Marginotruncana cf. sinuosa Marginotruncana undulata Praeglobotruncana cf. stephani Praeglobotruncan austinana Whiteinella archaeocretacea Whiteinella baltica Whiteinella inornata

29

31

33

46

2

3

1

2

1 1

8 4 4

2

1 17

1

4

Praeglobotruncana cf. stephani Zone Category: Interval zone. Author: Gebhardt, this paper. Definition: Interval to the last occurrence of Praeglobotruncana cf. stephani, base not defined (see remarks). Age: ?middle Turonian. Remarks: Various authors (e.g., Robaszynski and Caron, 1979; Robaszynski et al., 1982; Weiss, 1982; Marks, 1984a; Caron, 1985; Loeblich and Tappan, 1988; Hart et al., 1989; Dupont, 1996) equate the last appearance of Praeglobotruncana stephani (or the genus Praeglobotruncana) with the end of the middle Turonian.

1 2

3 4

3 1

9

2

4

1

11

1 9 5

11 7

5 11

31 22

2 1

1

6

2 1 3 2

3 4

1

4 19 1 6

1 21

6 2

5 2 7 14 5

2 3

Because the usual index species for this interval, Helvetoglobotruncana helvetica, has not been found at Nkalagu, Praeglobotruncana cf. stephani has been chosen as index species for the middle(?) Turonian. Its last occurrence therefore marks the end of the middle(?) Turonian in southern Nigeria. The boundary between the middle(?) and upper Turonian is exposed in the middle portion of the Band 18 section (Unit 9). The base of this zone is not exposed in the NigerCem Plc.-quarries of Nkalagu. Further planktonic foraminiferal species occurring at Nkalagu within this zone are: Heterohelix globulosa, H. reussi, H. moremani, H. pulchra, H. reymenti, Hedbergella

198

H. Gebhardt / Cretaceous Research 25 (2004) 191e209

Fig. 3. Correlation of various important planktonic foraminiferal zones and first and last appearances of index species.

delrioensis, H. simplex, Whiteinella baltica, W. inornata, Marginotruncana cf. renzi, M. schneegansi, M. sigali, M. cf. pseudolinneiana and Dicarinella imbricata. In the Sergipe Basin of NE Brasil, the species Whiteinella baltica and Heterohelix moremani disappear at the top of the middle Turonian (H. aprica-H. (W.) baltica Zone; Koutsoukos and Bengtson 1993). These species however occur at Nkalagu upto the Coniacian (Dicarinella concavata Zone). The last occurrence of Helvetoglobotruncana helvetica does not coincide exactly with the last occurrence of Praeglobotruncana stephani in Europe (Marks, 1984a), where it disappears a little earlier. Marginotruncana sigali Zone Category: Partial range zone. Author: Barr (1972: fide Caron, 1985). Definition: Interval from the last occurrence of Helvetoglobotruncana helvetica (Praeglobotruncana cf. stephani at Nkalagu, see above) to the first occurrence of Dicarinella primitiva. Age: Late Turonian. Remarks: According to its definition, this zone also corresponds to the Marginotruncana schneegansi Zone of Premoli Silva and Boersma (1977), Robaszynski and Caron (1979, 1995) and Marks (1984a). The base of the zone is exposed at Nkalagu in the middle portion of the Band 18 section (Unit 9), whereas the top is exposed in the upper portion of the Band 20 section (Unit 32). In addition to the species already occurring in the Praeglobotruncana cf. stephani Zone, the following were found: Whiteinella archaeocretacea, Pseudoplanoglobulina austinana, Hedbergella planispira, Dicarinella cf. hagni, Pseudoguembelina cf. costellifera and Heterohelix cf. americana.

Dicarinella primitiva Zone Category: consecutive range zone. Author: Caron (1978). Definition: Interval from the first occurrence of Dicarinella primitiva to the first occurrence of Dicarinella concavata. Age: Latest Turonian. Remarks: Various former publications (e.g., Caron, 1978, 1985; Robaszynski and Caron, 1979; Birkelund et al., 1984; Marks, 1984a,b) equate the first occurrence of Dicarinella primitiva with the base of the Coniacian. However, later publications show the first occurrence of Dicarinella primitiva below the Turonian/Coniacian boundary (e.g., Robaszynski et al., 1990; Koutsoukos and Bengtson, 1993; Robaszynski and Caron, 1995; see Fig. 3). Robaszynski and Caron (1995) reported the simultaneous appearance of Dicarinella concavata and D. primitiva with the late Turonian ammonite Subprionocyclus neptuni. They concluded that a Dicarinella primitiva Zone is not needed and pointed to the difficulties concerning the definition of the Turonian/ Coniacian boundary based on planktonic foraminifera. However, the first occurrence of D. primitiva (Unit 33, Band 20 section) is recorded well below the first occurrence of D. concavata (Unit 34) at Nkalagu. Thus, it appears reasonable to maintain the Dicarinella primitiva Zone in order to achieve a higher biostratigraphic resolution. In the Tethyan Realm, the first occurrence of D. primitiva was close to the middle/late Turonian boundary (calibrated to ammonites; Robaszynski et al., 1990; Robaszynski and Caron, 1995). D. primitiva and D. concavata may have arrived later in Nigeria, similar to species of the Marginotruncana sinuosa group (see below).

H. Gebhardt / Cretaceous Research 25 (2004) 191e209

The definition of the Turonian/Coniacian boundary at Nkalagu as the transition between the Dicarinella primitiva Zone and the Dicarinella concavata Zone is certainly arbitrary, but appears justified in view of the occurrence of the Coniacian ammonite Prionocycloceras multicostatum in Units 32 and 33 (Gebhardt, 2001b), i.e., close to the Turonian/Coniacian boundary proposed here. However, the occurrence of ammonites in the sections investigated is too sporadic to use them as zonal markers. The first occurrence of species belonging to the Marginotruncana sinuosa group, as proposed by Birkelund et al. (1984), Marks (1984b) and Kauffman et al. (1996), cannot be utilized at Nkalagu because Marginotruncana sinuosa appears higher up-section (Band 20, Unit 39). Simultaneously with Dicarinella primitiva, Hedbergella flandrini occurs first in the Band 20 section and may be used as a supplementary indicator for this zone. In the Sergipe Basin of NE Brasil, Hedbergella simplex and Dicarinella primitiva disappear before the first occurrence of Dicarinella concavata (Koutsoukos and Bengtson, 1993). At Nkalagu however, both species also occur in the successive zone. Dicarinella concavata Zone Category: consecutive range zone. Author: Sigal (1955: fide Caron, 1985). Definition: Interval from the first occurrence of Dicarinella concavata to the first occurrence of Dicarinella asymetrica (see also remarks). Age: Coniacian. Remarks: The base of this zone is placed in the Coniacian by most authors. Robaszynski and Caron (1995), however, reported the simultaneous occurrence of Dicarinella concavata and D. primitiva and late Turonian ammonites (see remarks on the Dicarinella primitiva Zone). For reasons mentioned above, the Turonian/ Coniacian boundary may coincide with the base of the Dicarinella concavata Zone. The top of this zone is not exposed in the NigerCem Plc. quarries of Nkalagu. Besides the planktonic foraminiferal species already listed above, the following are new to this zone in addition to Dicarinella concavata: Hedbergella holmdelensis, Marginotruncana sinuosa and M. undulata. Hedbergella simplex is largely replaced by Hedbergella flandrini. Because of its higher frequency, H. flandrini may be used as a substitute for the less frequent Dicarinella concavata as an indicator for the Coniacian Stage in southern Nigeria.

4. Palaeoecologic interpretation 4.1. Depth of water The ratio between planktonic and benthonic foraminifera (P/B ratio, frequently expressed as a percentage of

199

planktonic foraminifera; see Figs. 4, 5; Table 4) is one of the most reliable proxies to estimate palaeo-water depths. It has been known for a long time that the percentage of planktonic foraminifera in modern sediments increases with water depth (e.g., Boltovskoy and Wright, 1976; Gibson, 1989; van der Zwaan et al., 1990, 1999). Gibson (1989) assumed that the relative difference between the higher rate of reproduction of planktonic species in open ocean areas and the higher rate of reproduction (density) of benthonic species in neritic areas is the main cause for the distribution observed. Also, the complex pelagic ecosystem requires a minimum water depth (the entire photic zone at least) to be fully functional (van der Zwaan et al. 1990). Therefore, the density of planktonic foraminifera depends on the proximity to open oceanic realms (Gibson, 1989). van der Zwaan et al. (1990, 1999) and Leckie et al. (1998) emphasized the importance of nutrients for the P/B ratio, in particular for benthonic foraminifera. The percentage of benthonic foraminifera is inversely proportional to depth because their rate of reproduction depends on the amount of organic matter reaching the sea floor. Benthonic foraminifera take up organic matter three times as effectively as planktonic foraminifera (van der Zwaan et al., 1990). Because the density of planktonic and benthonic foraminifera depends on the organic flux, and the amount of organic matter reaching the sea floor decreases with increasing water depth because of oxidation, the P/B ratio inevitably has to increase with depth (van der Zwaan et al., 1999). Other parameters such as temperature, salinity, substrate or circulation patterns may play a minor role. Variations in the P/B ratio can be described using organic matter flux-equations. In practice, interpretation of palaeo-water depths remains problematic. As a consequence of highly fluctuating values in different regions, Gibson (1989) proposed a minimum water depth, derived from superimposed gradients of all regions investigated. van der Zwaan et al. (1990) described the regression curves with a mathematical formula, which can be used to estimate the water depth: water depth ¼ eð3;58718Cð0;03534
%PlÞÞ (in which e Z base of natural logarithm and %Pl Z percentage of planktonic foraminifera). Leckie et al. (1998) reported values of 80e85% planktonic foraminifera for an upper bathyal, siliclastic basinal margin during the CenomanianeTuronian interval, which may decrease to 30e80% during periods of high benthonic growth. At Nkalagu, the water depths calculated according to the formula of van der Zwaan et al. (1990) and averaged values from the graphs of Gibson (1989) do not differ significantly. Both estimates point to inner shelf environments (0e20%, 0e70 m) for the Aboine River Section

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Fig. 4. Percentages of planktonic foraminiferal species in the Band 18 section.

and to upper bathyal depths (35e94%, ca. 250e600 m) for the Band 18 and 20 sections. Values at the base of the Band 18 section point to water depths around 200 m. They increase to around 600 m in the middle portion. The lower and middle portions of the Band 20 section show varying values around 250 m which increase distinctly to O600 m in its upper portion. The calculated values according to van der Zwaan et al. (1990) of the Band 18 and 20 sections are plotted in Fig. 6. Calculated values were rounded to 10-m intervals and samples with possible turbiditic admixtures were not used for the estimations. Further water depth interpretation of planktonic foraminiferal faunas may be made by comparison with recent morphotypes (e.g., Hemleben et al., 1989 for modern environments and Leckie, 1987, Leckie et al.,

1998, West et al., 1998, and Hart, 1999 for the Cretaceous) and that reproduction also took place in the deepest environments during their life cycle in the Cretaceous. This is supported by a few studies measuring stable isotope ratios of subsequent growth stages, assuming that the incorporation of C- and O-isotopes into calcitic tests was in equilibrium with the surrounding water (Corfield et al., 1990; Norris and Wilson, 1998 and in particular Houston et al., 1999). d13C is heavier in surface waters because of preferential incorporation of 12C into the biomass, and is lighter in deeper waters because of oxidation of the organic matter, while d18O is temperature dependent and represents surface and deep-water temperature gradients (higher surface temperatures (or freshwater runoff) and lower deep-water temperatures).

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201

Fig. 5. Percentages of planktonic foraminiferal species in the Band 20 section.

Isotope data of Heterohelix show the highest variability and the values measured even sometimes point to the deepest habitats (D’Hondt and Arthur, 1995; Huber et al., 1995, 1999; MacLeod et al., 2000). However, stable isotope values of the most frequent species at Nkalagu, Heterohelix globulosa, indicate surface habitats (Huber et al., 1995; MacLeod et al., 2000). Vital effects or growth below the thermocline may explain the values of other Heterohelix species (Huber et al., 1995). Hedbergella species show values farthest from benthonic foraminifera and may, therefore, be clear surface dwellers (Huber et al., 1995; Fassell and Bralower, 1999), while values of Whiteinella species point to intermediate habitats (Huber et al., 1995, 1999). Keeled genera (Dicarinella, Marginotruncana, and Praeglobotruncana at Nkalagu) often show oxygen values close to those of benthonic foraminifera, pointing to deeper water. However, such a relationship could be

shown only for the carbon values of Cenomanian species, not for Turonian species (Huber et al., 1999). Another approach is the successive appearance and disappearance of species or genera along gradients, e.g., water depth, with other factors also influencing species distribution. These may include the rise and fall of oxygen minimum zones (Jarvis et al., 1988; Leary et al., 1989; Koutsoukos and Hart, 1990; Hart, 1999) or nutrient supply (Premoli Silva and Sliter, 1999). Leckie (1987) distinguished between three main faunal groups: epicontinental shelf faunas, open marine shallow faunas and open marine deep faunas. Heterohelix species (together with Guembelitria, which does not occur at Nkalagu but which is frequent in the inland Upper Benue Trough; Gebhardt, 1997) are opportunists (r-strategists) indicating eutrophic or unstable conditions (salinity and/or oxygen fluctuations; Leckie, 1987; Nederbragt, 1991; Leckie et al., 1998; Nederbragt et al., 1998; West et al.,

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Table 4 Planktonic/benthonic foraminiferal ratio (as % planktonic foraminifera) of investigated samples Sample

P*

B*

%Pl

Sample

P*

B*

%Pl

B20/52 B20/51 B20/50 B20/49 B20/48 B20/47 B20/46 B20/45 B20/44 B20/43 B20/42 B20/41 B20/40 B20/39 B20/38 B20/37 B20/36 B20/35 B20/34 B20/33 B20/32 B20/31 B20/30 B20/29 B20/28 B20/27 B20/26 B20/25 B20/24 B20/23 B20/22 B20/21 B20/20 B20/19 B20/18 B20/17 B20/16 B20/15 B20/14 B20/13 B20/12 B20/11 B20/10 B20/9 B20/8

300 300 300 300 303 300 300 300 100 304 116 36 25 13 87 68 112 255 nd 170 300 99 nd 300 226 nd nd 300 nd 28 33 25 46 157 33 151 185 216 nd 300 nd 26 59 nd 36

118 87 37 43 173 52 148 23 22 39 18 100 100 100 105 104 43 106 nd 109 190 300 nd 158 129 nd nd 115 nd 74 70 73 75 182 68 252 156 66 nd 233 nd 107 89 nd 70

72 78 89 87 63 85 70 93 82 89 87 26 20 12 45 40 72 71 nd 61 61 25 nd 66 64 nd nd 72 nd 27 32 26 35 46 33 37 54 77 nd 56 nd 20 40 nd 34

B20/7 B20/6 B20/5 B20/4 B20/3 B20/2b B20/2a B20/1

nd nd 9 nd nd nd nd 30

nd nd 35 nd nd nd nd 65

nd nd 20 nd nd nd nd 32

B18/22 B18/21 B18/20 B18/19 B18/18 B18/17 B18/16 B18/15 B18/14 B18/13 B18/12 B18/11 B18/10 B18/9 B18/8 B18/7 B18/6 B18/5 B18/4 B18/3 B18/2 B18/1

12 4 219 66 108 178 179 142 308 290 265 117 257 311 nd 100 nd nd 59 nd 1 nd

97 99 39 26 104 102 103 28 25 48 43 24 52 19 nd 119 nd nd 33 nd 103 nd

11 4 85 72 51 64 63 84 92 86 86 83 83 94 nd 46 nd nd 64 nd 1 nd

AR/13 AR/12 AR/11 AR/10 AR/9 AR/8 AR/7 AR/6 AR/5 AR/4 AR/3 AR/2 AR/1

0 0 nd 0 nd 1 nd 0 nd 0 nd 4 nd

100 104 nd 47 nd 4 nd 12 nd 300 nd 103 nd

0 0 nd 0 nd 20 nd 0 nd 0 nd 4 nd

P, planktonic foraminiferal counts for P/B ratio; B, benthonic foraminiferal counts for P/B ratio; *, including unclassified specimens; nd, not determined.

1998; Premoli Silva and Sliter, 1999). Hedbergella and Whiteinella were open marine species and dominated when ideal conditions at greater depths did not exist (‘‘shallow’’ water depths or oxygen minimum zones; Leckie, 1987; Jarvis et al., 1988; Leary et al., 1989; Koutsoukos and Hart, 1990; Leckie et al., 1998; West et al., 1998; Premoli Silva and Sliter, 1999). Keeled genera (Dicarinella, Marginotruncana, Praeglobotruncana) were open marine deep-water species (oligotrophic, K-strategists; Leckie, 1987; Premoli Silva and Sliter, 1999),

probably occupying the deepest habitats (supported by d18O-data, but not by d13C-data; Huber et al., 1995, 1999). 4.2. Nutrient flux High nutrient flux and low competition promote the development of early reproductive stages in planktonic foraminifera (Lipps, 1979). This has been confirmed by laboratory experiments (Hemleben et al., 1989). Thus, high nutrient supply promotes r-strategists like, e.g., Heterohelix species, and may prevent the flourishing of keeled K-strategists like, e.g., marginotruncanids (Premoli Silva and Sliter, 1999). Small numbers of chambers in Heterohelix indicate early offspring and therefore point to higher nutrient flux and higher surface productivity.

5. Interpretation of sections The sole and sporadic occurrence of a single species (Heterohelix globosa) of stress tolerant, epicontinental shelf elements confirms the interpretation from the P/B ratio as inner shelf deposits for the Aboine River Section (0e70 m, 0e20% planktonic foraminifera). Planktonic foraminiferal faunas of the Band 18 and 20 sections (Tables 1, 2) are dominated by heterohelicids (Figs. 3, 4). Their proportions vary between 65 and 99%. Hedbergellids are also rather frequent. For the (?)middle to early late Turonian, a (deep) upper bathyal environment of about 600 m water depth is indicated (46e94% planktonic foraminifera, with heterohelicids dominating and a considerable number of keeled specimens). The middle late to latest Turonian interval is characterized by 20e71% of planktonic foraminifera with heterohelicids dominating and very rare keeled specimens, pointing to a (shallower) upper bathyal depositional environment (water depth ca. 250 m). Eventually, a (deeper) upper bathyal environment (water depth ca. 600 m), still dominated by heterohelicids, but with up to 30% of hedbergellids, was reached again during the Coniacian, indicated by 63e93% of planktonic foraminifera and the presence of a considerable number of keeled specimens. In general, an open marine deepwater environment (upper bathyal) is indicated by the planktonic foraminiferal faunas of the Band 18 and 20 sections, further influenced by periods of eutrophication or (weak) salinity fluctuations. The presence of keeled genera (Dicarinella, Marginotruncana, Praeglobotruncana) in the Band 18 and 20 sections point to much higher water depths, also confirmed by higher P/B ratios. Marginotruncana cf. renzi reaches 1% in two samples (lower portion of the Band 18 section and top of the Band 20 section). Although keeled species are rare, they can be found in

Fig. 6. Correlation of palaeo-water depth derived from P/B ratios in the Band 18 and 20 sections with the eustatic sea-level curve and systems tracts. Correlation based on planktonic foraminiferal zones.

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many samples if concentrated by dry sieving (Fig. 2, Table 3). As in this study, Leckie (1987) and Huber et al. (1999) also reported only very small percentages of keeled taxa in the O0.063-mm fraction of open ocean sites of early Late Cretaceous age. If conditions became more eutrophic, these forms may even have disappeared completely. The number of species in the O0.063-mm fraction at Nkalagu varies (1e18 species per sample) but are typical for open oceans in most samples (compare Leckie, 1987). However, the overall number of species is higher in the tropical Tethyan Realm (about double; compare, e.g., Robaszynski et al., 1990). The southern position of Nkalagu and the relatively unstable environmental conditions may account for small species numbers at Nkalagu. Because of the presence of keeled genera, the high percentages of heterohelicids is not due to shallow-water depths, but rather a result of high surface productivity. Also the high absolute number of planktonic foraminifera (although only roughly estimated) in the samples point in this direction. Such unfavourable conditions for keeled genera were, however, not bad enough to prevent their occurrence completely. Periods of especially high surface productivity within the sections investigated were found in the lower and middle portions of the Band 18 section and in the middle portion of the Band 20 section (corresponding to the (?)middle and latest Turonian; Figs. 4, 5). The high content of Heterohelix globulosa with relatively few, inflated chambers points to early reproductive stages and leads to the assumption of eutrophication during these periods. Upwelling can be assumed off southwest Africa (Roth and Bowdler, 1981; Roth and Krumbach, 1986), but calcareous nannofossil assemblages at Nkalagu are different (Gebhardt, 2001a). High fluctuations in the P/B ratio during transgressive phases are interpreted as unstable conditions in the upper water layers in the Western Interior Basin of North America. A combination of reduced salinity and high sedimentation rates, caused by increased fluvial input, may have led to such fluctuations (cf. West et al.,

1998). This is comparable to the situation in the Benue Trough, where brackish environments were widespread during the period investigated (compare Petters, 1978a,b, 1979, 1982, 1983b, 1995; Gebhardt, 1997). From sample 38 of the Band 20 section upward, the content of Heterohelix globulosa decreases significantly, coincident with a rapid increase in Hedbergella species. This may have been caused by a deepening of the upper surface of the oxygen minimum zone or the disappearance of reduced salinities combined with a general increase in water depth (increase in P/B ratio, more keeled taxa present). However, the rapid increase probably coincides with a local depositional gap (hiatus) and may have occurred more gradually than indicated by the fossil record.

6. Discussion The age determinations based on planktonic foraminifera of the Nkalagu Formation recovered from the type section and locality (this paper) is confirmed by cooccurring calcareous nannofossils (Perch-Nielsen and Petters, 1981; Gebhardt, 2001a) and inoceramid and ammonite faunas (Gebhardt, 2001b). All fossil groups combined, together with the planktonic foraminifera presented herein, produce a high level of confidence about the depositional ages of the rocks at Nkalagu. Because the ages of the deposits exposed at Nkalagu are now sufficiently well known, it was possible to correlate the water depths derived from the P/B ratios with the eustatic sea-level curve of Haq et al. (1987). Fig. 6 shows a good correlation of water depth and sealevel change at Nkalagu (Band 18, 20 sections). A general deepening of the area caused by tectonic subsidence is indicated by the shift from the inner shelf Aboine River section to the upper bathyal Band 18 and 20 sections during the early to middle Turonian, but variations in water depth within the (?)middle TuronianeConiacian interval were probably caused by sea-level changes.

Fig. 7. A, Heterohelix globulosa (Ehrenberg), sample B18/16, lateral view. B, Heterohelix moremani (Cushman), sample B20/45, lateral view. C, Heterohelix pulchra (Brotzen), sample B20/28, lateral view. D, Heterohelix reussi (Cushman), sample B18/16, lateral view. E, Heterohelix reymenti Odebode, sample B18/20, lateral view. F. Pseudoplanoglobulina austinana (Cushman), sample B18/16, lateral view. G, Pseudoguembelina cf. costellifera Masters, sample B20/51, lateral view. H, Hedbergella delrioensis (Carsey), sample B20/46, umbilical view. I, Hedbergella flandrini Porthault, sample B20/49, spiral view. J, Hedbergella holmdelensis Olson, sample B20/51, spiral view. K, Hedbergella planispira (Tappan), sample B20/51, spiral view. L, Hedbergella simplex (Morrow), sample B20/13, spiral view. M, Whiteinella archaeocretacea Pessagno, sample 20/45, spiral view. N, Whiteinella baltica Douglas and Rankin, sample B20/46, spiral view. O, Whiteinella inornata (Bolli), sample B18/14, spiral view. PeR, Praeglobotruncana cf. stephani (Gandolfi); P, sample B18/12, spiral view; Q, sample B18/13, lateral view; R, sample B18/13, umbilical view. S, Dicarinella cf. hagni (Scheibnerova), sample B20/43, spiral view. TeV, Dicarinella concavata (Brotzen); T, sample B20/43, spiral view; U, sample B20/ 43, lateral view; V, sample B20/43, umbilical view. W, Dicarinella cf. imbricata (Mornod), sample B18/12, spiral view. X, Marginotruncana cf. renzi (Gandolfi), sample B18/12, spiral view. YeAA, Dicarinella primitiva (Dalbiez); Y, sample B20/43, spiral view; Z, sample B20/43, lateral view; AA, sample B20/43, umbilical view. ABeAD, Marginotruncana sigali (Reichel); AB, sample B20/51, spiral view; AC, sample B20/51, lateral view; AD, sample B20/51, umbilical view. AE, Marginotruncana cf. pseudolinneiana Pessagno, sample B18/10, spiral view. AF, Marginotruncana schneegansi (Sigal), sample B18/12, spiral view. AG, Marginotruncana cf. sinuosa Porthault, sample B20/51, spiral view. AH, Marginotruncana undulata (Lehman), sample B20/51, spiral view. Scale bars represent 0.1 mm.

H. Gebhardt / Cretaceous Research 25 (2004) 191e209

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H. Gebhardt / Cretaceous Research 25 (2004) 191e209

Gebhardt (2000) discussed the possibility of sea-level changes as a triggering factor for turbidite sedimentation. Whereas in (classical) siliclastic shelves a sea-level lowering leads to an increased rate of turbidite sedimentation, the opposite is the case in carbonate environments: surface area for the carbonate factory increases with rising sea level, leading to higher production and thus higher frequencies and thicker layers of carbonate turbidites (Haak and Schlager, 1989; Schlager, 1992). Also the main components may vary with sea-level change: abiogenic matter dominates high-level turbidites while biogenic detritus is most common in turbidites deposited during sea-level lowstands (e.g., Haak and Schlager, 1989). Biogenic components dominate the Nkalagu turbidites (Gebhardt, 2000) and are most frequent and occur in thickest layers when P/B ratios are low, and sea-level lowstands are represented (Fig. 6). Thus, the sedimentary system at Nkalagu was similar to a siliclastic depositional system, and systems tracts according to Haq et al. (1987) may be applied to it (cf. Fig. 6).

niferal faunas of the Band 18 and 20 sections, further influenced by periods of eutrophication or (weak) salinity fluctuations. The (?)middle Turonian and latest Turonian were periods of highest surface productivity.

Acknowledgements I am grateful to Zentraleinrichtung Elektronenmikroskopie (ZELMI) and the geological photographic laboratory staff at the Technische Universita¨t Berlin and Christian-Albrechts Universita¨t zu Kiel, Germany, for the production of excellent SEM micrographs as well as to my dear colleague Mr. O.J. Ojo (University of Ilorin) for company during fieldwork. Fieldwork in the Nkalagu Quarry was kindly permitted by the management of Nigeria Cement Co. Plc. (NigerCem). I am particularly grateful to Mr. G.U. Obidike (Public Relations Officer), Mr. Osuji (Quarry Manager) and to Mr. G. Oko (Geologist) of the same company. E.A.M. Koutsoukos, an anonymous reviewer, and D.J. Batten provided valuable suggestions that improved the manuscript.

7. Conclusions 1. Based on planktonic foraminifera, four biostratigraphic zones are proposed for the (?)middle Turoniane Coniacian interval in southern Nigeria. The study of the Nkalagu Formation may allow a more exact age determinations of sedimentary successions in the Benue Trough and adjacent areas than before. The following zones are proposed: Praeglobotruncana cf. stephani Zone, (?)middle Turonian; Marginotruncana sigali Zone, late Turonian; Dicarinella primitiva Zone, latest Turonian; Dicarinella concavata Zone, Coniacian. 2. The sole and sporadic occurrence of Heterohelix globosa, a stress tolerant, epicontinental shelf element confirms the interpretation from the P/B ratio of inner shelf deposits for the Aboine River Section (0e70 m of water depth, 0e20% planktonic foraminifera). 3. For the (?)middle to early late Turonian, a (deep) upper bathyal environment of about 600 m water depth is indicated (46e94% of planktonic foraminifera, with heterohelicids dominating, and considerable numbers of keeled specimens). 4. The middle late to latest Turonian interval is characterized by 20e71% of planktonic foraminifera with heterohelicids dominating and very rare keeled specimens, pointing to a (shallower) upper bathyal depositional environment (water depth ca. 250 m). 5. A deep, upper bathyal environment (water depth ca. 600 m), dominated by heterohelicids but with up to 30% of hedbergellids was reached again during the Coniacian, indicated by 63e93% of planktonic foraminifera and a considerable number of keeled specimens. 6. In general, an open marine, deep-water habitat (upper bathyal) is indicated by the planktonic forami-

Appendix List of species with comments All species reported here have been well described in readily accessible literature. For this reason, species descriptions are not included and only where necessary, brief taxonomic remarks are given. 1. Dicarinella concavata (Brotzen, 1934), Fig. 7TeV 2. Dicarinella cf. hagni (Scheibnerova, 1962), Fig. 7S Remark: Specimens found at Nkalagu show a slightly inflated chamber similar to specimens from Gabon (Dupont, 1996). 3. Dicarinella cf. imbricata (Mornod, 1950), Fig. 7W Remarks: Specimens found at Nkalagu differ slightly from typical examples from the Tethyan Realm: the concaveeconvex character is not very pronounced, the last whorl shows only four to five chambers and the sutures of the umbilical side are slightly curved. However, imbrication is very prominent and the keels appear interrupted. 4. Dicarinella primitiva (Dalbiez, 1955), Fig. 7YeAA 5. Hedbergella delrioensis (Carsey, 1926), Fig. 7H 6. Hedbergella flandrini Porthault, 1970, Fig. 7I 7. Hedbergella holmdelensis Olsson, 1964, Fig. 7J 8. Hedbergella planispira (Tappan, 1940), Fig. 7K 9. Hedbergella simplex (Morrow, 1934), Fig. 7L 10. Heterohelix cf. americana (Ehrenberg, 1843) Remark: Earlier chambers are more inflated and increase in size more quickly than in typical specimens.

H. Gebhardt / Cretaceous Research 25 (2004) 191e209

11. Heterohelix globulosa (Ehrenberg, 1840), Fig. 7A 12. Heterohelix moremani (Cushman, 1938), Fig. 7B Remarks: Opportunistic species, which coped well with extreme environmental conditions (Nederbragt et al., 1998). Stable isotope data show values close to benthonic foraminifera (Huber et al., 1999): a deep marine habitat may therefore be possible. 13. Heterohelix pulchra (Brotzen, 1936), Fig. 7C 14. Heterohelix reussi (Cushman, 1938), Fig. 7D 15. Heterohelix reymenti Odebode, 1986, Fig. 7E 16. Marginotruncana cf. pseudolinneiana Pessagno, 1967, Fig. 7AE Remark: The specimen found shows only five and a half chambers in the last whorl. 17. Marginotruncana cf. renzi (Gandolfi, 1942), Fig. 7X Remarks: Specimens found at Nkalagu show only four to five chambers in the last whorl, corresponding to specimens figured by Petters (1980a). Not all specimens found at Nkalagu show merged keels on the last chambers. 18. Marginotruncana schneegansi (Sigal, 1952), Fig. 7AF 19. Marginotruncana sigali (Reichel, 1950), Fig. 7ABeAD 20. Marginotruncana cf. sinuosa Porthault, 1970, Fig. 7AG Remark: Specimens found at Nkalagu show more pustules on the spiral side than the holotype. 21. Marginotruncana undulata (Lehmann, 1963), Fig. 7AH 22. Praeglobotruncana cf. stephani (Gandolfi, 1942), Fig. 7PeR Remark: Specimens found at Nkalagu are slightly flatter than those figured by Caron (1985) and Ellis and Messina (1940 et seq.). 23. Pseudoguembelina cf. costellifera Masters, 1976, Fig. 7G Remark: The last chambers are broken and do not allow precise identification. 24. Pseudoplanoglobulina austinana (Cushman, 1938), Fig. 7F 25. Whiteinella archaeocretacea Pessagno, 1967, Fig. 7M 26. Whiteinella baltica Douglas and Rankin, 1969, Fig. 7N 27. Whiteinella inornata (Bolli, 1957), Fig. 7O

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