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Paleogene calcareous nannofossil magnetobiochronology: Results from South Atlantic DSDP Site 516
Marine Micropaleontology, 14 (1989) 119-152 Elsevier Science Publishers B.V., Amsterdam-- Printed in The Netherlands
119
Paleogene Calcareous Nannofossil Magnetobiochronology: Results from South Atlantic DSDP Site 516 W U C H A N G W E I and S H E R W O O D W. WISE, Jr. Department of Geology, Florida State University, Tallahassee, FL 32306 (U.S.A.) (Received September 30, 1988; revisedand acceptedOctober 28, 1988)
Abstract Wei, W. and Wise, S.W., Jr., 1989. Paleogene calcareous nannofossil magnetobiochronology: resultsfrom South Atlantic D S D P Site 516. Mar. Micropaleontol., 14: 119-152.
The detailed study of an expanded Paleogene section with abundant, moderate to well preserved calcareous nannofossils from South Atlantic DSDP Site 516 has resulted in a precise correlation of most calcareous nannofossil markers with the magnetostratigraphy.Many nontraditional datums have also been documentedand correlated to the magnetostratigraphy.Comparison of the results from Site 516 with those of previous studies from other areas enables a critical evaluation of the accuracy,synchroneityor diachroneity of the species events over geographically long distances. Of special significanceis the correlationfor the first time of the stratigraphic ranges of Chiasmolithus gigas and Rhabdosphaera gladius with the magnetostratigraphy.Other important results include the following:first occurrence (FO) of Cruciplacolithus primus, 66.3 Ma; FO Chiasmolithus danicus, 65.6-66.0 Ma; FO Prinsius martinii, 65.5-66.0 Ma; FO Heliolithus kleinpeUii, 59.8-61.6 Ma (probably diachronous); last occurrence (LO) of Tribrachiatus orthostylus, 51.0-54.8 Ma (unreliable); FO Chiasrnolithus gigas, 47.4 Ma; FO Reticulofenestra umbilica, 44.6 Ma; LO Chiasmolithus gigas, 44.4-46.8 Ma (diachronous);LO Nannotetrina fulgens, 44.2 Ma; LO Chiasmolithus grandis, 40.0-41.6 Ma (probablydiachronous);FO Chiasmolithus oamaruensis, 39.8-40.4 Ma (unreliable); FO Isthmolithus recurvus, 39.5 Ma; LO Reticulofenestra reticulata, 37.6 Ma; LO Discoaster saipanensis, 36.4 Ma; end acme of Ericsonia subdisticha, diachronous;FO and LO Sphenolithus distentus, unreliable; and LO Reticulofenestra bisecta, 24.0 Ma.
Introduction The rapid evolution and wide geographic distribution of calcareous nannofossils have made t h e m one of the best fossil groups for stratigraphic correlation of Mesozoic and Cenozoic marine sediments. The succession of m a n y Paleogene calcareous nannofossils is well known. However, problems still exist for m a n y of the traditional marker species (Beckmann et al., 1981; Perch-Nielsen, 1985; Martini and Miiller, 1986), and m a n y nontraditional marker species have been suggested by different authors
0377-8398/89/$03.50
but their stratigraphic values still await further testing. Moreover, the numerical ages, the synchroneity and diachroneity of these traditional and nontraditional markers world wide are poorly known. To date a modest number of studies have been made to correlate Paleogene calcareous nannofossil datums with magnetostratigraphy (Lowrie et al., 1982; Berggren et al., 1984; Poore et al., 1984; Manivit and Feinberg, 1984; Shackleton et al., 1984; Miller et al., 1985; Monechi and Thierstein, 1985; Monechi et al., 1985; Aubry et al., 1986; Backman, 1986, 1987; B a c k m a n and Hermelin, 1986). How-
© 1989 Elsevier Science Publishers B.V.
120
ever, each of these studies suffered to some degree from one or several of the following: incomplete sedimentary section, condensed section, poor paleomagnetic data, scarcity or poor preservation of the calcareous nannofossils. Consequently many of the traditional datums have only been correlated with magnetostratigraphy at one locality (see summery by Berggren et al., 1985) and the ages remain questionable when applied outside the original area studied (or even at a different site very nearby). Many other traditional datums and most nontraditional datums have not been calibrated with the geomagnetic polarity time-scale at all. Deep Sea Drilling Project (DSDP) Site 516 from the Rio Grande Rise, Southwest Atlantic recovered a rather complete and expanded Paleogene sedimentary sequence. The upper Eocene and Oligocene section is unique because it shows a relatively uniform sedimentation rate for over 350 m (see discussion below). Overall the Paleogene section contains abundant, moderate to well preserved calcareous nannofossils, and the magnetostratigraphy has been established (Berggren et al., 1984). Thus this site offers a rare opportunity to study the calcareous nannofossil abundance patterns in great detail and to tie the nannofossil datums to the magnetostratigraphy. The objectives of this study have been: (1) to determine semi-quantitatively the abundance patterns of the traditional marker species and a few nontraditional marker species from the Paleogene sediments in Hole 516F; (2) to establish correlations between the species events and the magnetostratigraphy; (3) to compare the results of this study with those of previous studies in other areas, to discuss the present status of the traditional and nontraditional marker species, and to evaluate the reliability and accuracy of the species events and their synchroneity and diachroneity in different regions.
Previous work Before DSDP Site 516 was cored, three other DSDP sites (21, 22, and 357) had been drilled on the Rio Grande Rise (Fig. 1 ). The calcareous nannofossil stratigraphy for the three sites was given by Bukry and Bramlette(1970), Gartner (1970), and Perch-Nielsen (1977). However, DSDP Sites 21 and 22 were interval cored, and large portions of Paleogene sediments were not recovered either at DSDP Site 357 (recovery was less than 40%; Maxwell et al., 1970; Perch-Nielsen et al., 1977). DSDP Site 516 is the only one on the Rio Grande Rise to recover a rathe~ complete Paleogene carbonate sequence. Unfortunately, no detailed calcareous nannofossil biostratigraphic data were published in the DSDP Initial Reports. Mostly core-catcher samples were examined, and the calcareous nannofossil assemblages and their abundance patterns were unreported (Barker et al., 1984).
Location and sedimentology DSDP Site 516 lies in 1313 m of water on the Rio Grande Rise at 30 ° 16.59'S, 35 ° 17.10'W (Fig. 1 ). Paleogeographic considerations reveal that there has been little change in latitude from the Paleocene to the present for Site 516 (Sclater et al., 1977). The Rio Grande Rise is an aseismic ridge, the origin of which, like its counterpart in the eastern South Atlantic, the Walvis Ridge, is still in debate (Barker, 1984). Three lithologic units can be recognized for the Paleogene sequence in Hole 516F. Cores 3 to 18 consist of brown and yellow foraminiferal-nannofossil chalks. Carbonate content ranges from 75 to 90% and the main biogenic components are nannofossils (70-90%), foraminifera (10%), and sponge spicules (10%). Cores 19 to 50 consist of upper Oligocene to middle Eocene light gray, nannofossil and nannofossil-foraminiferal chalks. The carbonate percentage is rather uniform throughout this unit (about 80% ), with nannofossils dominat-
121
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ah
2~4
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2
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I'20°
I~0 °
120 ° "
60 °
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Fig. 1. Locationof DSDP sites and land sectionsdiscussedin this paper. ing the components. Cores 51 to 89 are nannofossil and foraminiferal limestones. For detailed descriptions see Barker et al. (1984).
Methodology We recorded the abundance patterns of the calcareous nannofossils from Hole 516F because we believed that they would greatly facilitate the evaluation of the reliability and accuracy of the nannofossil datums identified. Backman and Shackleton {1983), and Backman (1986, 1987) have clearly demonstrated the importance of evaluating the species events by their abundance patterns. There are, however, drawbacks in Backman's method (see Wei, 1988), and we found the semi-quantitative method worked well for the present study. We observed the abundance of the calcareous nannofossils under the light microscope using a
magnification of 1600 and have recorded the abundances as follows: A = abundant, 1 or more specimens per field of view; C =common, 1 specimen per 2-10 fields of view; F=few, 1 specimen per 11-50 fields of view; R = r a r e , 1 specimen per 51-200 fields of view; N = n o t found in 200 fields of view. The calcareous nannofossil species investigated in this study are listed in Table I, and most of the species discussed and critical occurrences in some samples are documented with SEM and light micrographs in Plates I to IV. We presented our calcareous nannofossil stratigraphic results using the zonation of Okada and Bukry (1980) as a framework. We also numbered the zones with Martini's (1971) number codes. Instances where the differentiation of zones or subzones is impossible are discussed in the text. The magnetic chron terminology used follows
122 TABLE
I
Calcareous nannofossil species discussed in this paper
Biantholithus sparsus Bramlette and Martini, 1964. Plate III, 3 Blackites spinosus (Deflandre and Fert, 1954) Hay and Towe, 1962. Plate III, 15 Bramletteius serraculoides Gartner, 1969. Plate IV, 14, 15 Calcidiscusprotoanulus (Gartner, 1971 ) Loeblich and Tappan, 1978. Plate I, 10; Plate IV, 9, 10 Chiasmolithus altus Bukry and Percival, 1971. Plate IV, 21 Chiasmolithus bidens (Bramlette and Sullivan, 1961 ) Hay and Molder, 1967. Plate II, 13, 14; Plate III, 5 Chiasmolithus danicus (Brotron, 1959) Hay and Molder, 1967. Plate II, 10, 11 Chiasmolithus gigas (Bramlette and Sullivan, 1961 ) Radomski, 1968. Plate III, 12, 13 Chiasmolithus oamaruensis (Deflandre, 1954) Hay, Molder and Wade, 1966. Coccolithusformosus (Kamptner, 1963) Wise, 1973. Plate I, 7; Plate IV, 12, 13 Cruciplacolithus edwardsii Romein, 1979 Cruciplacolithus primus Perch-Nielsen, 1977. Plate II, 12 Cruciplacolithus tenuis (Stradner, 1961 ) Hay and Molder in Hay et al., 1967. Plate III, 4 Cyclicargolithus abisectus (Muller, 1970) Wise, 1973. Plate III, 2, right Cyclicargolithus floridanus (Roth and Hay in Hay et al., 1967) Bukry, 1971. Plate III, 2, left Discoaster barbadiensis Tan, 1927 Discoaster kuepperi Stradner, 1959 Discoaster lodoensis Bramlette and Riedet, 1954 Discoaster megastypus (Bramlette and Sullivan, 1961 ) Perch-Nielsen, 1986 Discoaster mohleri Bukry and Percival, 1971. Plate II, 2 Discoaster multiradiatus Bramlette and Riedel, 1954. Plate II, 4 Discoaster saipanensis Bramlette and Riedel, 1954. Plate I, 8; Plate III, 14 Discoaster sublodoensis Bramlette and Sullivan, 1961. Plate IV, 1 Discoaster tani nodifer Bramlette and Riedel, 1954 Ellipsolithus maceUus (Bramlette and Sullivan, 1961 ) Sullivan, 1964. Plate II, 9 Ericsonia subdisticha (Roth and Hay in Hay et al., 1967) Roth in Baumann and Roth, 1969. Plate I, 12; Plate IV, 7, 8 Fasciculithus tympaniformis Hay and Molder in Hay et al., 1967. Plate II, 5, 16 Helicosphaera compacta Bramlette and Wilcoxon, 1967. Plate IV, 19, 20 Helicosphaera recta Haq, 1966 Heliolithus hleinpellii Sullivan, 1964. Plate II, 6; Plate III, 7, 8 Heliolithus riedelii Bramlette and Sullivan, 1961. Plate III, 6 Isthmolithus recurvus Deflandre, 1954. Plate III, 16, 17 Lanternithus minutus Stradner, 1962 Nannotetrina fulgens (Stradner, 1960) Achuthan and Stradner, 1969 Nannotetrina cristata (Martini, 1958) Perch-Nielsen, 1971 Neococcolithes dubius (Deflandre, 1954) Black, 1967. Plate IV, 2 Prinsius bisulcus (Stradner, 1963) Hay and Molder, 1967. Plate II, 3 Prinsius martinii (Perch-Nielsen, 1969) Haq, 1971. Plate I, 4, 5 Prinsius tenuiculum (Okada and Thierstein, 1979) (Perch-Nielsen, 1984. Plate I, 1, 2; Plate II, 7, 8 Reticulofenestra bisecta bisecta (Hay, Molder and Wade, 1966) Roth, 1970, Plate IV, 22 Reticulofenestra bisecta filewiczii Wise and Wiegand in Wise, 1983. Plate III, 1 Reticulofenestra daviesii (Haq, 1968) Haq, 1971 Reticulofenestra reticulata (Gartner and Smith, 1967) Roth and Thierstein, 1972. Plate I, 9; Plate IV, 11 Reticulofenestra samodurovii (Hay, Molder and Wade) Roth, 1970. Plate I, 11 Reticulofenestra umbilica (Levin, 1965) Martini and Ritzkowski, 1968. Plate III, 11; Plate IV, 17 Rhabdosphaera gladius Locker, 1967. Plate IV, 4, 5 Sphenolithus ciperoensis Bramlette and Wilcoxon, 1967 Sphenolithus distentus (Martini, 1965 ) Bramlette and Wilcoxon, 1967. Plate IV, 18 Sphenolithus furcatolithoides Locker, 1967. Plate IV, 16 Sphenolithus predistentus Bramlette and Wilcoxon, 1967 Sphenolithus primus Perch-Nielsen, 1971 Toweius callosus Perch-Nielsen, 1971. Plate I, 3, 6 Tribrachiatus orthostylus Shamrai, 1963 Triquetrorhabdulus carinatus Martini, 1965 Zygodiscus sigmoides Bramlette and Sullivan, 1961. Plate II, 1 Zygrhablithus 5ijugatus (Deflandre in Deflandre and Fert, 1954) Deflandre, 1959
123
that of LaBrecque et al. (1983), that is, a magnetic chron is defined as from the youngest reversal boundary of a numbered anomaly to the youngest reversal boundary of the next older numbered anomaly. The chron unit is distinguished by prefixing a letter "C", and it can be subdivided into subchron by addition of an N or R suffix which refers to the polarity. The geomagnetic polarity time-scale used in this study is that of Berggren et al. (1985). This po-
larity time-scale, like that of Haq et al. (1987), integrated independent data sets from magnetostratigraphy, sea-floor spreading magnetic lineation patterns, biostratigraphy and isotopic ages, and has been used widely. The results from the present study thus can be easily compared with those of others. We constructed an age-depth curve for Hole 516F, using the magnetic polarity horizons which have been clearly identified and by as-
PLATEI 1, 2. Prinsius tenuiculum, X12,000, Sample 516F-89-2,12-13 cm. 3. Toweius callosus, X6,000, Sample 516F-83-6, 51-52 cm. 4, 5. Prinsius rnartinii, Xl0,000, Sample 516F-87-2, 88-89 cm. 6. Toweius callosus, X5,000, Sample 516F-83-6, 51-52 cm. 7. Coccolithus forrnosus, X4,500, Sample 516F-43-1, 16-17 cm. 8. Discoaster saipanensis, X6,500, Sample 516F-43-1, 16-17 cm. 9. Reticulofenestra reticulata, X6,500, Sample 516F-43-1,16-17 cm. 10. Calcidiscusprotoanulus, X8,500, Sample 516F43-1, 16-17 cm. 11. Reticulofenestra samodurovii, X8,500, Sample 516F-37-2, 72-73 cm. 12. Ericsonia subdisticha, X6,500, Sample 516F-37-2, 72-73 cm. PLATEII
1. Zygodiscus sigmoides, X6,500, Sample 516F-85-2, 52-53 cm. 2. Discoaster rnohleri, X5,500, Sample 516F-83-6, 51-52 cm. 3. Prinsius bisulcus, X10,500, Sample 516F-83-6, 51-52 cm. 4. Discoaster multiradiatus, X5,500, Sample 516F-83-6, 51-52 cm. 5. Fasciculithus tyrnpaniformis, X6,000, Sample 516F-85-2, 35-36 cm. 6. Heliolithus kleinpellii, X6,000, Sample 516F86-1, 52-53 cm. 7, 8. Prinsius tenuiculum, X3,400, Sample 516F-89-1,100-101 cm. 9. EUipsolithus rnacellus, X2,800, Sample 516F-88-3, 38-39 cm. 10, 11. Chiasrnolithus danicus, X3,400, Sample 516F-88-3, 38-39 cm. 12. Cruciplacolithus primus, X3,600, Sample 516F-89-4, 85-86 cm. 13, 14. Chiasrnolithus bidens, X2,400, Sample 516F-51-1,106-107 cm. 15. Cruciplacolithus edwardsii, X2,400, Sample 516F-87-5, 19-20 cm. 16. Fasciculithus tyrnpaniformis, X2,400, Sample 516F-17-2, 0-1 cm.
PLATEIII
1. Reticulofenestra bisecta filewiczii, X7,500, Sample 516F-19-2, 100-101 cm. 2. Cyclicargolithus floridanus on the left, Cyclicargolithus abiseetus on the right, X7,000, Sample 516F-15-5, 90-91 cm. 3. Biantholithus sparsus, X4,000, Sample 516F-89-3, 7-8 cm. 4. Cruciplacolithus tenuis, X4,000, Sample 516F-87-4, 46-47 cm. 5. Chiasmolithus bidens, X3,600, Sample 516F-87-4, 46-47 cm. 6. Heliolithus riedelii, X3,600, Sample 516F-84-3, 52-53 cm. 7, 8. Heliolithus kleinpellii, X3,600, Sample 516F-84-1, 24-25 cm. 9, 10. Campylosphaera dela, X4,000, Sample 516F-67-1, 39-40 cm. 11. Reticulofenestra urnbilica, X2,400, Sample 516F-66-1, 45-46 cm. 12, 13. Chiasrnolithus gigas, Xl,900, Sample 516F-66-1, 45-46 cm. 14. D/scoaster saipanensis, X2,400, Sample 516F-66-4, 89-90 cm. 15. Blackites spinosus, X3,600, Sample 516F-61-5, 32-33 cm. 16, 17. Isthmolithus recurvus, X3,000, Sample 516F-38-2, 6-7 cm. P L A T E IV
1. Discoaster sublodoensis, X2,400, Sample 516F-81-1,123-124 cm. 2. Neococcolithes dubius, X3,000, Sample 516F-66-4, 89-90 cm. 3. Helicosphaera lophota, X3,000, Sample 516F-62-4, 18-19 cm. 4, 5. Rhabdosphaera gladius, X2,400, Sample 516F-61-5, 32-33 cm. 6. Sphenolithus spiniger, X2,400, Sample 516F-59-1,112-113 cm. 7, 8. Ericsonia subdisticha, X3,000, Sample 516F-51-1,106-107 cm. 9, 10. Calcidiscusprotoanulus, X2,400, Sample 516F-40-2,105-106 cm. 11. Reticulofenestra reticulata, X2,400, Sample 52-1, 30-31 cm. 12, 13. Coccolithus [orrnosus, X2,400, Sample 516F-36-4, 23-24 cm. 14, 15. Brarnletteius serraculoides, X2,400, Sample 36-1, 39-40 cm. 16. Sphenolithus furcatolithoides, X2,400, Sample 516F-60-3, 19-20 cm. 17. Reticulofenestra umbilica, Xl,600, Sample 516F-36-1, 39-40 cm. 18. Sphenolithus distentus, X2,400, Sample 516F-31-1, 23-24 cm. 19, 20. Heliocosphaera compaeta, X2,400, Samp!e 516F-29-2, 33-34 cm. 21. Chiasrnolithus altus, X6,500, Sample 516F-16-1, 6-7 cm. 22. Reticulofenestra bisecta bisecta, X6,500, Sample 516F-19-1, 25-26 cm.
124
PLATE I
125
P L A T E II
d I
I
5
~3
51,
i6
126
P L A T E III
127
PLATE IV
5
0 9
14
6
10
128 signing numerical ages according to the polarity time-scale of Berggren et al. ( 1985 ) (Fig. 2 ). T h e d a t a points between 24 to 42 M a fall re-
m a r k a b l y close to a straight line. T h i s indicates t h a t the s e d i m e n t a t i o n rate was relatively uniform at this site from late Eocene t h r o u g h Oli(Ma)
2224 mbsf 200.
262030
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3234
365011 40 4244
52 54 56 50 60 62.
464050
T: t o p
220. 240. 260.
0? ~ T?0
B: b o t t o m Numbers o r e c h r o n numbers
26O" 300. 320" 340. 360 380 400 ' 420 • 440 ' 460, 400' 500. 520" 540' 560' 500' 600.
TIT = ~
620, 640,
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660, 680.
700' 720' 740'
010~
760'
760, 00~
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020 841~ 86W
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%
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oot
Fig. 2. Age-depth curve for the Paleogene section of Hole 516F, using the clearly identified polarity horizons and assigning numerical ages according to the time-scale of Berggren et al. ( 1985 ).
129
gocene time. Hiatuses, if present, must be minor because it is inconceivable that the hiatuses have been compensated perfectly by increases in sedimentation rate between control points so that there is no change in the slope of the agedepth curve over a period of 20 m.y. The apparent uniform sedimentation rate at this site for the upper Eocene through Oligocene section makes it unique for the biochronology study, because the datum levels identified in the sequence can be confidently and precisely tied to the geomagnetic polarity time-scale by simply reading off the straight age-depth line.
Calcareous nannofossil stratigraphy The abundance patterns of selected calcareous nannofossil species from Hole 516F are graphically presented in Fig. 3. Full species range charts and detailed discussions of the assemblages and their implication for paleoenvironments will be presented in a forthcoming paper. The accuracy and reliability of the species events can be judged by their abundance patterns near their first occurrences and last occurrences, i.e., those showing abrupt change near their appearances and extinctions are easier to determine and are more reliable, whereas those showing a long tail of abundance and occurring sporadically near their appearance or extinction may not have been reliably determined.
Cretaceous~Tertiary boundary The Cretaceous/Tertiary boundary lies between Samples 516F-89.5,131-132 cm and -894, 85-86 cm, the lower sample consisting of 100% uppermost Cretaceous nannofossils and the upper sample containing many Tertiary species (e.g., Cruciplacolithus primus, Zygodiscus sigmoides, and Markalius inversus ). This boundary is located within Subchron C29R, in agreement with the results obtained from other localities (Alvarez et al., 1977; Thierstein, 1982; Magaritz et al., 1985; Monechi et al., 1985). Rare
Biantholithus sparsus was found only in Sample 516F-89-3, 7-8 cm and its FO could not be used here to mark the Cretaceous/Tertiary boundary as suggested by Perch-Nielsen (1979).
Paleocene The first occurrence (FO) of Ellipsolithus maceUus is in Sample 516F-88-3, 38-39 cm (Plate II, 9), one core lower than reported in the Initial Reports (Berggren et al., 1984). Several marker species of various authors also occur first in this sample: Chiasmolithus danicus, Cruciplacolithus tenuis, and Prinsius martinii. Furthermore, the stratigraphic top of Prinsius tenuiculum is very sharp in Sample 516F-89-1, 10-11 cm, where it is very abundant. Due to the half core loss in Core 89, there is no direct proof that an unconformity occurs between these two samples, though extrapolation of sedimentation rates above and below this interval does suggest a hiatus. The base of Fasciculithus tympaniformis in Sample 516F-87-2, 0-1 cm defines the base of CP4; the first entry of Heliolithus kleinpeUii in Sample 516F-86-1, 52-53 cm indicates the CP4/ CP5 boundary and the first occurrence of Discoaster mohleri in Sample 516F-85-2, 35-36 cm marks the lower boundary of CP6. Rare and sporadic Discoaster nobilis, the FO of which was used as a datum to separate CP6 and CP7 in Okada and Bukry (1980), is present in a few samples along with Discoaster multiradiatus. Discoaster nobilis was not reported from Leg 73 materials from the southeast Atlantic Ocean (Percival, 1984) or from the Contessa section in Italy (Monechi and Thierstein, 1985). On the other hand, the FO of Heliolithus riedelii is the counterpart of the first D. nobilis in Martini's (1971) scheme. Rare H. riedelii was observed only in one sample (Sample 516F-84-3, 52-53 cm) at Site 516, which also contains rare Discoaster multiradiatus, therefore the sample should be assigned to Zone CP8. Thus neither
130
K e y to s y m b o l s *
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Age
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9-10 105-106 49-50
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105-106
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85-86
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36-37 69 I 70
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Fig. 3. Stratigraphic distribution pattern of selected Paleogene calcareous nannofossil species in Hole 516F.
131
]Abundant Age
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NP8
.
c~ CP5
NP7 NP6
CP3
NP5
NP4
CPla
NPt
Fig. 3. (continued)
.
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89-1,
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. 6466--24, 6 8 057-58 _3, CP4
~
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'ii
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132
Key
symbols
to
•
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m
!i"'
present
Not
Rare Few Common
oo
coro,
Abundant
Age
l~oc~ion
Interval
::, .c
¢i 111
(crn)
Zone
o. Miocene
CNlb
4-5,
.mo-io1
• ~-1, 89-70
NN1
5-3,
CNla
i00-i01
7-2,
I00-I01
8-2, 9-2, io-1, io-z,
2o-21 ioo-iol ioo-Iol 9o-91
. . n " " 1 .
}
.....
M
I
H
•
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'
r
,q
.
.
.
10-3,
90-91
~ ~ =_a
io-4, 11-i, 11-2.
9-1o 90-91 90-91
J -
ii-4,
i00-I01
12-1 • 13-i,
16-17 80-81
.
l
I{
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s
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13-3,
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11o-z11
.
|
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. ;
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90-91
..
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I. O l i g o c e n e
15-5, 15-;. 16-i.
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NP23
,% " CPI~ CP18 o. O l i g o c o n e
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l
CP16c
NP22
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17-3.
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. . "i
18-i,
90-91
. ~
19-1, 19-2, 19-3,
25-Z6
'
100-I01 80-81
19-,~,
18-19
2o-z. 20-2 20-4, 21-i, 21-4,
99-1oo 90-91 90-91 90-91
22-2,
~o-31 90-91
2,,-1. 25-1, 25-4, 28-2, 26-5.
,~z-,~2 90-91 81-82 lZO-lZz 11"~-118
28-2. 29-2, 30-2. ~i-,. 32-2(" 33-1, 33-2,
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Fig. 3. (continued)
the FO of
D. nobilis
nor the FO of
could be used for Hole interval
from Sample
516F
H. riedelii
material
516F-85-2,
35-36
and the cm to
Sample 516F-84-4, non-differentiated
20-21 cm was placed Zone CP6-7.
Discoaster diastypus,
in the
the FO of which defines
133 the base of CP9, was not detected in the material. Consequently CP8 and CP9 could not be differentiated for Hole 516F material. Tribrachiatus bramlettei is also absent (as it is in the Leg 73 material, see Percival, 1984). Many species have their last occurrences in Sample 516F83-1, 96-97 cm (e.g. Cruciplacolithus tenuis,
Discoaster multiradiatus, Fasciculithus tympaniformis, Neochiastozygus distentus, Prinsius bisulcus and Zygodiscus sigmoides) and many species occur first in Sample 516F-82-2, 113-114 cm (e.g. Campylosphaera dela, Chias-
molithus solitus, Discoaster barbadiensis, Neococcolithes dubius, Sphenolithus editus, Sphenolithus radians, and Tribrachiatus orthostylus). An unconformity must be present between these two samples or the interval must be very condensed.
Eocene The zonal marker for the base of CP10, D/s-
coaster lodoensis, was observed in Sample 516F82-1, 142-143 cm. The first occurrence of D/scoaster kuepperi was also found in this sample. Thus a brief hiatus is suggested right below this sample because normally the FO of D. kuepperi should be slightly higher in the section than the FO ofD. lodoensis. The first occurrences of both Chiasmolithus grandis and Coccolithus formosus in this sample also indicate the missing of the lower part of Zone CP10 and possibly upper part of Subzone CP9b (see Perch-Nielsen, 1985, p. 431 ). No Coccolithus crassus could be confidently identified in the material. The difficulty in using this species as a zonal marker and its unreliability have been discussed by Filewicz and Hill (1983), Applegate and Wise (1987), and Filewicz (written communication, 1987). The LO of T. orthostylus could not be used either (see discussion below). Consequently CP10 and C P l l cannot be differentiated for Hole 516F. The top of CP10-11 was tentatively drawn between Samples 516F-81-2, 107-108 cm and -811, 123-124 cm, because rare specimens of D/scoaster sublodoensis were recognized in the lat-
ter sample and the species occurs sporadically in Hole 516F sediment. The traditional middle to upper Eocene nannofossil datums were found in the following samples: the FO of Nannotetrina fulgens (base of CP13a) in Sample 516F-76°5, 121-122 cm; the FO of Chiasmolithus gigas (base of CP13b) in Sample 516F-74-1, 50-51 cm; the FO of Reticulofenestra umbilica (base of CP14a) in Sample 516F-68-1, 19-20 cm; the LO of Chiasmolithus solitus (top of CP14a) in Sample 516F51-1, 105-106 cm; the FO of Chiasmolithus oamaruensis {base of CP15a) in Sample 516F-471, 19-20 cm; the FO of Isthmolithus recurvus (base of CP15b) in Sample 516F-45-1, 6-7 cm; and the LO of Discoaster saipanensis (top of CP15b) in Sample 516F-38-2, 6-7 cm. It is worth noting that Chiasmolithus g/gas (Plate III, 12, 13 ) has a considerably long range (80 cm) in Site 516 and its upper range overlaps with the FO of Reticulo[enestra umbilica (Plate III, 11 ). This precludes use of subzones CP13b and CP13c as prescribed by Okada and Bukry's (1980) low latitude zonation. A similar situation has been reported by Applegate and Wise (1987) for material from the continental rise off New Jersey. Another significant observation is the finding of rare specimens of Rhabdolithus gladius in Sample 516F-61-5, 32-33 cm (Plate IV, 4, 5). The LO ofR. gladius has been used to mark the NP15/16 boundary in Martini's (1971) zonation and has been taken as equivalent to the LO of C. g/gas (top of CP13b) (Okada and Bukry, 1980; Perch-Nielsen, 1985; Martini and Miiller, 1986). The present finding contradicts this traditional correlation. Detailed discussions of C. g/gas and R. gladius in relation to the magnetostratigraphy are presented below. Discoaster bifax, the total range of which defines CP14a (Okada and Bukry, 1980), was not found at Site 516. Rare specimens of Discoaster praebifax were, however, found in a few samples in CP13. The current status of D. bifax and D. praebifax has recently been discussed by Wei and Wise (1989).
134
Oligocene The LO of Reticulofenestra umbilica in Sample 516F-33-1, 56-57 cm defines the top of the first calcareous nannofossil zone (CP16) in the Oligocene. The following species events occur progressively in this zone and may be useful datums for regional correlation: FO of Sphenolithus distentus, LO of Bramletteius serraculoides, LO of Lanternithus minutus, LO of Coccolithus formosus, and LO of Isthmolithus recurvus. The LO of C. formosus marks the CP16b/CP16c boundary. The Ericsonia subdisticha acme datum could not be used because it co-occurred with the LO of C. formosus in Site 516 (it occurred slightly higher than the LO of C. formosus at Sites 522 and 523; see Backman, 1987). The range of Sphenolithus distentus at Site 516 overlaps those of C. formosus and R. umbilica. A similar problem has been reported in Okada (1980), Bybell (1982, 1983 ), Siesser (1983), and Perch-Nielsen (1986). It is now clear that the FO of S. distentus is not a reliable datum for the CP17/CP18 boundary. Sphenolithus ciperoensis is rare in Hole 516F sediment and its first occurrence is stratigraphically too high to be used for the CP18/19 boundary at Site 516. Thus the interval from Samples 51632-2, 89-90 cm to -20-2, 90-91 cm was assigned to the CP17-19a combined zone. A succession of extinctions was found near the Oligocene/Miocene boundary: LO of Chiasmolithus altus followed by that of Helicosphaera
recta, Sphenolithus ciperoensis, Reticulo[enestra daviesii, Reticulofenestra bisecta bisecta, Zygrhablithus bijugatus, and Reticulofenestra bisecta filewiczii. The LO of Sphenolithus ciperoensis in Sample 516F-10-2, 90-91 cm defines the CP19/CN1 boundary. The Oligocene/Miocene boundary in terms of calcareous nannofossil stratigraphy is defined by the LO of R. bisecta (Berggren et al., 1985) and falls between Samples 516F-5-1, 69-70 cm and 516F4-5, 100-101 cm. An unconformity appears to be present between these two samples because there is an abrupt change in the abundance of
Cyclicargolithus abisectus and the upper part of Subchron C6CN seems missing. Assuming constant sedimentation rate above and below this unconformity, the hiatus would be about 0.6 m.y. in duration. The present study has improved the stratigraphic results of the initial study (Berggren et al., 1984 ) by identifying three hiatuses and by adjusting and locating more precisely most of the calcareous nannofossil zonal boundaries. Figure 4 provides an easy comparison of the results. Bio- and magneto-chronologic correlation Due to the relative nature of the conventional biostratigraphy, it cannot precisely correlate species events in a contemporary scale. Correlation of species events to a common geomagnetic time-scale will enable us to obtain information on their numerical ages, their synchroneity or diachroneity over geographic long distances. In the present study we have tied the calcareous nannofossil datums of Site 516 to Berggren et al. (1985) magnetic time-scale. We have also plotted on the same magnetic timescale all the published magnetobiostratigraphic data that are of moderate or good quality. Though the numerical magnetostratigraphic ages involve assumptions and interpretations that may not be agreed on by all investigations, and may be subject to revision in the future, the correlation of the species datums still holds true and the positions of the species events within the magnetic chron will be unchanged. The following is a discussion of some important Paleogene calcareous nannofossil datums and their calibration to the magnetic time-scale of Berggren et al. (1985).
Biantholithus sparsus, Z ygodiscus sigmoides and Cruciplacolithus primus As mentioned previously, rare specimens of Biantholithus sparsus were found in only one sample (516F-89-3, 7-8 cm). Its FO, which is
(i,a4)
BNN
sB
•
2.~.~
crab
22
.
j
6 t
N.2S g 1o
i
~7
'...2-' ~
,,
"~
.....
11
NP~4-21
IS
~
Np~s 17
II
-
2Z
I0
~
PIP23-24 ¢10
24
~
2s
i
~
e.Plb ~
e.ple
11
m
NP23.24
immE
=o
me
33
i
-
CPI~
NP22
:PI61 b
Np~I
34
NP22.2
36 37
CI3 m
~*
i
is
. . . . . NP22
--
NPI~-2
47
s~ i
-
~P~
[]
c1~
~s
-,~2
"°
71 -~--
imlmmm
c~o
;Tm
-
7~ $0
m
--
~
C24
?
~p~s
c~12
NPI~
.
.
.
Cruciplacolithus tenuis, Chiasmolithus danicus and Ellipsolithus macellus Due to core loss and possibly an unconformity between Samples 516F-88-3, 38-39 cm and -89-1, 10-11 cm, the FO's of Cruciplacolithus tenuis, Chiasmolithus danicus and Ellipsolithus macellus could not be precisely determined, but they fall within 65.6-66 Ma, extrapolating sedimentation rates from above and below these two samples. The age range of the FO of C. tenuis from Site 516 is in agreement with those from Sites 525 (65.8 Ma), 527 (65.9 Ma), and 528 (66 Ma) (Shackleton et al., 1984), but the FO's of C. danicus and E. macellus at Site 516 are older than those in Shackleton et al. (1984) (64.8 Ma and 63.8 Ma respectively). The first
.
NP~3-~, . . . . . NPI2
........... 2S
18
above those of Zygodiscus sigmoides and Cruciplacolithus primus at Site 516, could not be used to mark the base of C P l a (NP1) as suggested by Perch-Nielsen (1979). Martini and Miiller (1986) concluded that this datum is generally not applicable for defining the K / T boundary due to its scarcity in most areas. Both Z. sigmoides and C. primus occur first in Sample 5 1 6 F - 8 9 - 4 , 85-86 cm (Plate II, 1, 12), 1 m above the Iridium peak level (Hamilton, 1984) at this site. Assuming the Iridium peak level represents 66.4 Ma and sedimentation rate is constant within Chron C29, the FO ofZ. sigmoides and C. primus would be 66.3 Ma or older. At Site 524 the FO ofZ. sigmoides was found at 66.4 (Poore et al., 1984), in good agreement with our present result. The age of the first C. primus (66.1 Ma) as derived from Sites 525, 527 and 528 (Shackleton et al., 1984) seems too young compared with that from Site 516.
~
~27
21
~
C27
CP~
NP~.10
Np~
N~
Fig. 4. Peleogene magnetobiostratigraphy of Hole 516F: comparison of the results from this study and those of Berggren et el. (1984). The magnetostratigraphy is after Berggren et el. (1984). In the polarity column the stippled parts indicate lack of mR~etic data or uncertain polarity. Nannofossil zones are given in the zonations of Okada and Bukry (1980) and Martini (1971).
136
occurrences of C. danicus and E. macellus in Sample 516F-88-3, 38-39 cm have been documented in Plate II, 9-11, and we believe that the FO of E. maceUus is environmentally controlled, that is, the species may not be found in deep-water sediments. In other words, "the FO of E. macellus represents an inaccurate datum event" (Monechi et al., 1985, p. 793).
Fasciculithus tympaniformis The currently available magnetobiostratigraphic data show that the FO of F. tympaniformis (base of CP4) is located in the middle or lower part of Subchron C26E (Fig. 5). The age of this datum as determined at Site 516 is slightly older (at 62.7 Ma) than those from the previous studies for other regions. The good agreement of the age from the previous studies (62.1 Ma) makes the age from Site 516 abnormal. One possibility is that the sedimentation rate within Subchron C26R at Site 516 varied greatly and resulted in the incorrect assignment of the age. The substitute marker for the FO of F. tympaniformis as proposed by Perch-
Sites Ma
$"
Contessa
Latitudes 43°N
o
Bottaccione 43°N
577 Ii 527
528
516
32°N ~128°S i
28°S
30°S
Nielsen (1985), the FO of Sphenolithus primus, occurs 0.2 m.y. earlier at Site 516.
Heliolithus kleinpellii The age of the FO of Heliolithus kleinpellii (base of CP5 or NP6 ) at both Sites 516 and 527 is 61.6 Ma (Fig. 6). However, the results from other localities do not agree well, giving younger ages up to 59.9 Ma (Site 528) (Shackleton et al., 1984). Moreover, Heliolithus cantabriae, which occurs earlier than H. kleinpellii (Romein, 1979 ), grades into H. kleinpellii, and there is a considerable variation in forms within the helioliths as pointed out by Wise and Wind (1977, p. 296). The FO of H. kleinpellii should be used with caution and the age of 61.6 Ma as given in Berggren et al. (1985) should be considered tentative.
Discoaster mohleri The first non-birefringent discoaster, Do mohleri (base of CP6 or NP7) is easily recognized in most studies. Figure 7 shows that, except for Site 528, all sites gave an age of about 60.4 Ma for the FO ofD. mohleri. Shackleton et
Sites
59
o~
~
Ma
~
('3
Contessa Bottaccione
Latitudes 43~N
43~N
I 577 I 527
528
516
32°N i 28~S
28°S
30%
C25! 59
60
C25
i
61,
6O C26'
62
i
61
I C26
63
Fig. 5. Correlation of the FO of Fasciculithus tympanifor~ m/s with the magnetic time-scale of Berggren et al. ( 1985 ). Data are t a k e n from: Contessa a n d Bottaccione sections, Monechi a n d T h i e r s t e i n (1985); Site 577, Monechi et al. (1985); Sites 527 a n d 528, Shackleton et al. (1984); Site 516, this study.
62
63
---
Fig. 6. Correlation of the FO of Heliolithus kleinpellii with the magnetic time-scale of Berggren et al. (1985). Data sources are the same as in Fig. 5.
137
Ma
o~ ~"
=o
Sites Contessa Latitudes 43 °N
Bottaccione : 527
43° N
' 28° S
528 28°S
516 30 ° S
5559:
C5 60 61
II
iii
I
C ~E 62
I
Sites Latitudes
Contessa
43°N
577
: 527
28°N II 28°S i
528
524
516
28°S
290S
30°S
56. ; 57"
58
III!I
63 Fig. 7. Correlation of the FO ofDiscoaster mohleri with the magnetic time-scale of Berggren et al. (1985). Data sources are the same as in Fig. 5.
al. (1984) did not consider the result from Site 528 to be representative. Therefore, the FO of D. mohleri seems to be a sound datum at 60.4 Ma.
Discoaster multiradiatus
Discoaster multiradiatus has a wide geographic distribution, probably due to the relatively warm and equable climate world-wide during late Paleocene time. The species is easily identified, and results from the South Atlantic sites and the Contessa section agree well, giving an age of 59.2 Ma for the FO of D. multiradiatus (Fig. 8 ). Thus this datum is excellent for geographically long distance correlation. Furthermore, the number of rays in D. multiradiatus decreases progressively through time (Moshkovitz, 1967; Romein, 1979), and detailed morphometric study of this species from different areas should result in considerably finer stratigraphy using quantitative D. multiradiatus data.
Fig. 8. Correlation of the FO of Discoaster multiradiatus with the magnetic time-scale of Berggren et al. (1985). Data for Site 524 are taken from Poore et al. (1984). Refer to Fig. 5 for other data sources.
Tribrachiatus orthostylus Though the FO of Tribrachiatus orthostylus was not used in the zonations of Martini ( 1971 ) or Okada and Bukry (1980), its relative stratigraphic position in the lineage of Tribrachiatus has been studied in detail by Heckel (1968) and Romein (1979), and has been carefully recorded in many studies. Previous studies have indicated that the FO of T. orthostylus is just slightly below the LO of Tribrachiatus contortus and can be used to approximate the latter datum (Perch-Nielsen, 1985). It was so used in the zonation of Hole 516F material, because T. contortus was not found. The FO of T. orthostylus at Site 516 was recorded within the upper part of Subchron C24R at about 57 Ma (Fig. 9). Studies for the southeastern Atlantic (Shackleton et al., 1984 ), the North Pacific (Monechi et al., 1985) and the Contessa area (Lowrie et al., 1982; Monechi and Thierstein, 1985) also found the FO of T. orthostylus within the upper part of Subchron C24R, but at slightly different levels among the studies, giving an age range
138
from 56.1 to 56.4 Ma. Taking into consideration the uncertainty of variation in sedimentation rates within each site and among different sites, and the different abundance and preservation states of the calcareous nannofossils at different localities, the FO of T. orthostylus appears to be consistent and can be considered geologically synchronous in the mid-latitude areas. The LO of T. orthostyIus at Site 516 falls near the top of Chron C24, clearly lower than those observed in the other studies (Fig. 9). Though most previous studies (Shackleton et al., 1984; Monechi et al., 1985; Backman, 1986) suggested the LO of T. orthostylus to be near the top of chron C23 (around 54.0 Ma ), other stud-
~
Sites Contesssa Bottaccione Latitudes 43 N 43~
577 I 527 32¢N I 28%
528
516
28%
30%
ies (Lowrie et al., 1982; Monechi and Thierstein, 1985) recorded it as low as in Subchron C23R and as high as in Subchron C21R. It should be pointed out that Monechi and Thierstein (1985) misplaced the top of T. orthostylus for the Contessa Highway section in their fig. 6, because a few T. orthostylus were recorded in a higher sample (at 80 m), which was near the top of CP12 (see Monechi and Thierstein, 1985, fig. 7 and p. 429). Those few specimens of T. orthostylus may not be reworked, since Bukry (1973) also reported that this species ranges well into the middle Eocene. Perch-Nielsen {1985) has dashed the range of T. orthostylus up to CP13. It appears now that the LO of T. orthostylus is not a reliable datum for recognizing the NP12/NP13 boundary as Martini ( 1971 ) originally proposed.
Nannotetrina [ulgens
i Fig. 9. Correlation of the FO and LO of Tribrachiatus orthostylus with the magnetic time-scale of Berggren et al. (1985). Data are taken from: Contessa Section, Lowrie et al. (1982) in thinner line and Monechi and Thierstein ( 1985 ) in thicker line; Bottaccione Section, Monechi and Thierstein (1985); Site 577, Backman (1986) in thinner line and Monechi and Thierstein (1985) in thicker line; Site 527, Shackleton et al. (1984); Site 528, Backman (1986) in thinner line and Shackleton et al. (1984) in thicker line; Site 516, this study.
Both Martini (1971) and Okada and Bukry (1980) used the FO of Nannotetrina [ulgens as a zonal marker in the middle Eocene (NP14/ NP15 or CP12/CP13). Perch-Nielsen (1985) suggested use of the FO of the genus Nannotetrina (usually N. cristata) as a substitute marker for poorly preserved material. In Site 516 material, the first Nannotetrina to appear was N. cristata, followed by N. [ulgens 4 m above; the latter was used to define the base of CP13 (NP15). Figure 10 shows that the FO ofN. [ulgens at Site 516 agrees remarkably well with those from other studies, and its age at 49.6 Ma (middle of subchron C21N) can be considered relatively reliable. Site 523 only cored down to Chron C20, and the FO of N. [ulgens could not be determined there (see Poore et al., 1984). The LO of N. [ulgens has so far been correlated with the magnetostratigraphy only at Sites 516 and 523 and the age obtained from Site 516 is younger than that given by Poore et al. (1984) for Site 523. Backman (1987) reported the LO of Nannotetrina spp. at 44.2 Ma (subchron C19R), the same age as the LO ofN. fulgens at Site 516, where N. fulgens was also the last
139 Contesssa BoMaccionz, 527 528 43=N ! 28°S 28°S Latitudes 43=N I
Sites
523
516
28°S
30°S
Sites
~
Contesssa Bottaccion I 523
Latitudes 43°N
43°N
I
516
I 28°S
30°S
I
'7-1,,,
43
~2C
iiI _°'°
3" I m
¢2
Fig. 10. Correlation of the FO and LO of Nannotetrina fulgens with the magnetic time-scale of Berggren et al. (1985). Data are taken from: Contessa Section, Lowrie et al. (1982) in thinner line and Monechi and Thierstein (1985) in thicker line; Bottaccione Section, Monechi and Thierstein (1985); Sites 527 and 528, Shackleton et al. (1984); Site 523, Backman (1987) in thinner line and Poore et al. (1984) in thicker line; Site 516, this study.
Nannotetrina. The age ofLO ofN. fulgens (45.4 Ma) in Berggren et al. (1985) was taken from Poore et al. (1984) and therefore needs to be revised to 44.2 Ma using the new available data.
Chiasmolithus g/gas Chiasmolithus g/gas is easily recognized by its large size (19-27/lm), relatively small central opening and non-split crossbars. The total range of the species has been used by Okada and Bukry (1980) to define CP13b, but the scarcity of this species in most marine sediments has prevented its use in many localities. The FO of C. g/gas at Sites 516 was found within the lower part of Chron C20 (about 47.4 Ma) (Fig. 11). Site 523 terminated in Chron C20 and the FO of C. g/gas could not be determined. The synchroneity or diachroneity of the FO of C. g/gas
Fig. 11. Correlationof the FO ofReticulofenestraumbilica (solid lines) and the FO and LO of Chiasmolithusgigas (dashedlines) with the magnetictime-scaleof Berggrenet al. (1985). Data are taken from: Contessaarea, Lowrieet al. (1982); BottaccioneSection, Monechiand Thierstein (1985); Site523,Backman(1986) in thinnerlineand Poore et al. (1984) in thicker line and dashedline; Site 516, this study.
is difficult to assess at present because it has only been correlated to the magnetostratigraphy at one site (516). The upper range of C. g/gas overlaps with Reticulofenestra umbilica at Site 516 (see Plate III, 11-13), which contradicts at this latitude the low latitude zonation of Okada and Bukry (1980). A similar problem has been encountered in studies of material from Possagno, Italy (Proto Decima et al., 1975) and from the continental rise off New Jersey (Applegate and Wise, 1987), where they could not determine whether R. umbilica ranged too low or C. g/gas ranged too high. From Fig. 11 it is clear that the LO of C. g/gas is time transgressive and that it does range too high at Site 516.
Reticulofenestra umbilica The FO of R. umbilica has been used to mark the CP13/CP14 boundary (Okada and Bukry,
140
1980) and the LO of the species has been widely applied as an early Oligocene marker (Martini, 1971; Okada and Bukry, 1980). However, the use of this species as a marker depends crucially on the separation of this species from the very similar but smaller form termed Reticulofenestra samodurovii ( = Reticulofenestra dictyoda or Reticulofenestra coenura of some authors), which appeared earlier and persisted longer than R. umbilica. Morphometric study of R. samodurovii-R, umbilica in the early development of the lineage by Backman and Hermelin (1986) has demonstrated the rather gradual change in the mean size through time, and they suggested 14 #m as a cutoff for the two species. This size limit is what we have used in this study. The FO of R. umbilica at Sites 516 and 523 and in the Contessa Quarry section (Fig. 11) was located at the top of Chron C20 or near the bottom of Chron C19 at about 44.7 to 44.4 Ma. The level observed in the Contessa Road section appears too low compared with the results from the Quarry section (only 3 km away) and with the South Atlantic sites. Moreover, the entry level of Nannotetrina fulgens was one chron lower in the Road section than that in the Quarry section (see Lowrie et al., 1982 ). Diachroneity cannot account for the difference between these two sections 3 km apart, thus other reasons should be sought. The FO of R. umbilica determined in the Bottaccione section (Monechi and Thierstein, 1985) also appears to be too early and may possibly be due to use of a slightly different species concept. Assuming that the different results from the Contessa Road and Bottaccione sections are due to reasons other than diachroneity, we can consider the FO of R. umbilica as a good datum at 44.6 Ma (bottom of Chron C19) for the mid-latitude marine sediments. Therefore, the age for CP13/ CP14 boundary (46 Ma) in Berggren et al. (1985), which was based on Monechi and Thierstein's ( 1985 ) study of the Contessa Road section, should be revised to 44.6 Ma. The LO of R. umbilica at Site 516 was found in Sample 516F-33-1, 56-57 cm, where 3 speci-
mens ranging from 18 to 20 #m were spotted. Samples right above this level yielded specimens of only 12 #m or less and are not R. umbilica. Compared with its LO observed in other studies (Fig. 12), the level at Site 516 is basically in agreement with those determined for the North Atlantic Sites 558 and 563 and the South Atlantic Sites 522 and 523. The extinction level observed in the South Atlantic Site 528 (Shackleton et al., 1984) appears to be too high and probably is due to use of a slightly different species concept. The LO of R. umbilica probably should be located 2/3 distance down in Chron C12 at about 34.6 Ma. The reliability of this datum is, however, not high because there is a relatively long tail in its abundance near the extinction (see also Backman, 1987).
558
563
37°N
33°N I 26°S 28°S
I 522
523
528
516
28°S
30"S
m
34
35
35
37
38
c,5
Fig. 12. Correlation of the LO of Reticulofenestra umbilica with the magnetic time-scale of Berggren et al. (1985). Data are taken from: Site 558, Miller et al. (1985) in thinner line and Parker et al. (1984) in thicker line; Site 563, Miller et al. (1985) in thinner line and Parker et al. (1984) in thicker line; Site 522, Backman (1987) in thinner line and Poore et al. (1984) in thicker line; Site 523, Backman (1987); Site 528, Shackleton et al. (1984); Site 516, this study.
Rhabdosphaera gladius The range of Rhabdosphaera gladius is believed to be very short, and the LO of this spe-
141
cies has been used to define the top of NP15 (Martini, 1971). Until now the correlation of the top of NP15 with Okada and Bukry's (1980) zonation has not been clear. This is due mostly to the fact that R. gladius only occurs frequently in NW European basin sediments but is very rare or absent in other regions (Martini and Miiller, 1986); and the datum has never been correlated to the magnetostratigraphy before. At Site 516 rare specimens of R. gladius have been found in Sample 516F-61-5, 32-33 cm ( Plate IV, 4, 5 ), within the uppermost part of Chron C19, along with Lanternithus minutus and Reticulofenestra umbilica. This sample is above the LO of Nannotetrina fulgens, which occurred three cores below. The range overlap of R. gladius with L. minutus and with R. umbilica has been reported before (e.g. Berggren and Aubry, 1984). Though the precise FO and LO of R. gladius could not be determined due to the scarcity of this species in Site 516 sediments, its occurrence here can be correlated to the magnetostratigraphy. The LO of this species is clearly shown above the FO of R. umbilica and the LO of N. fulgens and should be 43.8 Ma or younger. Consequently the top of NP15 should not be correlated with the base of CP14a as PerchNielsen ( 1985 ) and Martini and Mtiller (1986) suggested, but should be stratigraphically higher.
Reticulofenestra reticulata
of the Eocene, this datum has been tied to the magnetostratigraphy only at Sites 522 (Backman, 1987) and 516 (the present study) (Fig. 13). Results from these two South Atlantic sites agree remarkably well, giving an age of 37.6 Ma (very bottom of Subchron C15N). Since the LO of R. reticulata has been consistently found slightly below the LO of Discoaster saipanensis in different regions (e.g., Barbados, Falkland Plateau, Site 549 in the Bay of Biscay, Sites 214, 216 and 217 in the Eastern Indian Ocean, Site 282 west of Tasmania, and Site 445 in the Philippine Sea; see Perch-Nielsen, 1986), and the LO of D. saipanensis can be demonstrated as synchronous in the lower and middle latitudes (see discussion below), we can consider the LO of R. reticulata as geologically synchronous also.
Ma
I
°~ E
g I ~
I
Sites 522 Latiludess 26 °S
516 30°S
36 C13 37
38 ¸
39
I
C15
C161
40 C171
Reticulofenestra reticulata has been reported from a variety of upper Eocene marine sediments, from shelf environments (e.g. Siesser, 1983) to pelagic environments (e.g. Gartner, 1974), and from low latitudes (e.g. Bukry, 1977) to high latitudes {e.g. Miiller, 1976; Wise, 1983). It is, therefore, a very promising species for correlation over different marine environments and latitudes. Though a number of authors (Miiller, 1970, 1979; Shafik, 1981; Martini and Miiller, 1986) have suggested the usefulness of the LO of R. reticulata as a datum near the top
41
42 C181 43"
44"
C191
Fig. 13. Correlation of the FO and LO of Reticulofenestra reticulata with the magnetic time-scale of Berggren et al. (1985). Data are taken from: Site 522, Backman (1987); Site 516, this study.
142
A progressive size change of R. reticulata through time has been observed at a number of sites from the South Atlantic and South Indian Ocean (Wei, unpubl, data), with the earlier ones being smaller and often showing an poorly developed central area structure whereas those near the extinction level reach maximum size and show distinct central area structure. This progressive size change may be one reason that the FO of R. reticulata has not been well agreed upon among different studies. A detailed morphometric study of R. reticulata through time (Wei, in prep.) should facilitate finer stratigraphy using quantitative R. reticulata data.
Chiasmolithus solitus There is considerable difficulty in separating Chiasmolithus solitus from Chiasmolithus bidens, which occurs much earlier and overlaps the range of the former species. Therefore the FO of C. solitus as well as the LO of C. bidens are not useful datums. The LO of C. solitus has, however, been applied to define the CP14a/CP14b (NP16/ NP17) boundary (Martini, 1971; Okada and Bukry, 1980 ). Since chiasmoliths are generally rare or absent in tropical waters, the LO of C. solitus may be difficult or impossible to recognize at low latitudes. Furthermore, questions have arisen as to the age of this datum. Wise and Mostajo (1983) found that the Subzone CP14a at Maurice Ewing Bank (South Atlantic) showed an exceptional sedimentation rate (70 m/m.y.) using the age of C. solitus from Bukry ( 1973 ) and suspected that C. solitus persisted longer in higher latitudes than in the tropics where Bukry's (1973) zonation is grounded. This is indeed the case. Calculation of sedimentation rates for several sites south of 40 °, using the age of 42.3 Ma as suggested by Berggren et al. (1985) for the LO of C. solitus, has given enormously high values for CP14a and has resulted in hiatuses right above this subzone (Wei and Thierstein, unpubl, data). This anomaly strongly suggests that the LO of C. so-
l s,,es
.
_~. ,,T
39
523 s,6 Latitudes 28 ~S
30 °S
C1E
40 C17 41
42 C18 43
44
Fig. 14. Correlation of the LO of Chiasmolithus solitus with the magnetic time-scale of Berggren et al. ( 1985 ). Data are taken from: Site 523, Poore et al. (1984); Site 516, this study.
litus in higher latitudes is younger than 42°3 Ma. Site 516 data suggest an age of 41.3 Ma (Fig. 14 ), 1 m.y. younger than that derived from Site 523 (Berggren et al., 1985). The timetransgression of the datum over different latitudes needs further documentation in order to refine the stratigraphy and to solve other paleoceanographic problems. Chiasmolithus grandis The LO of Chiasmolithus grandis is a substitute marker for the FO of Chiasmolithus oamaruensis at the CP14/CP15 boundary of Okada and Bukry (1980). At Site 516 the LO of C. grandis is only one sample lower than the FO of C. oamaruensis, which is rare at Site 516 and not reported at Site 522 (Percival, 1984). The LO of C. grandis was determined within Subchron C17N at Sites 516 and 523 but at different levels {40.0-40.6 Ma) (Fig. 15). In the Contessa area this datum was found within the upper part of Subchron C18N (about 41.6 Ma)
143 Sites Latitudes
a
39
Contessa
43 °N
Bottaccione It 523
516
43 =N
30°S
: 28°S
C16
Chron C17 at Site 516 (Fig. 16). At Site 523 Percival (1984) reported it very rare in one sample, absent in the next four samples up and few in the sixth sample. The age for the FO of C. oamaruensis in Berggren et al. (1985) was based on Percival's (1984) study of Site 523 material and is apparently not reliable.
40 IC17 Sites
~
41
42
C18
Botlaccione
43ON
558 37°N
563 I 33°1J
,,
522 26°S
523 28=S
516 30=S
|1-
T
43
Contesssa
Latituaes 43°N
34" 35£
Fig. 15. Correlation of the LO of Chiasrnolithus grandis with the magnetic time-scale of Berggren et al. (1985). Data are taken from: Contessa Section, Lowrie et al. (1982); Bottaccione Section, Monechi and Thierstein (1985); Site 523, Bacl~m~n (1987) in thinner line and Poore et al. (1984) in thicker line; Site 516, this study.
c1:
37"
Iic,, ai m
3g. =0~
I1ol, liII : ell
|
(Lowrie et al., 1982; Monechi and Thierstein, 1985). Such a lower level may be due to the poor preservation of the calcareous nannofossils in the Contessa area. Martini (1976) examined Sites 317 and 318 from the tropical Pacific, and concluded that the LO of C. grandis occurs consistently above the FO of Isthmolithus recurvus; therefore, he substituted the former for the latter to mark the N P 1 8 / N P 1 9 (see also Martini and Miiller, 1986). His correlation and results have been in conflict with most other studies, including the present one. Chiasrnolithus oamaruensis
Though the FO of C. oamaruensis has been used both in Martini's (1971) and Okada and Bukry's (1980) schemes to mark the NP17/ NP18 (CP14a/CP14b) boundary, the species is usually rare or absent in lower and middle latitude areas, and its application as a marker species is difficult in most cases. Only rare specimens of C. oamaruensis have been found in two adjacent samples near the middle of
Fig. 16. Correlation of the FO and LO of Chiasmolithus oamaruensis with the magnetic time-scale of Berggren et al. (1985). Data are taken from: Site 523, Poore et al. (1984); Site 516, this study.
Isthmolithus recurvus
The FO of Isthmolithus recurvus has been used as a zonal marker in the zonations of Martini (1971) and Okada and Bukry (1980), whereas the LO of I. recurvus was applied as a provincial zonal marker in the Oligocene sediment from the Falkland Plateau (Wise, 1983). The species is distinct but is easily subject to overgrowth and in that case different names may have been used by some authors (Martini, 1973). Isthmolithus recurvus is rare or absent in low-latitude areas (Bukry, 1978), more abundant in mid- a n d high-latitude sediments and has even been found in Antarctic shelf sediments (Wei et al., 1988). It is thus believed that the FO and LO of this species is only applicable
144
in the mid- and high-latitudes but not a useful datum in low-latitudes. The FO of/. recurvus in Hole 516F falls near the very bottom of Chron C16, in good agreement with the result from Site 523 (Backman, 1987) (Fig. 17). In the Contessa section and Bottaccione section Monechi and Thierstein (1985) found the base of I. recurvus to be just at the top of Chron C16, considerably higher than those at Sites 516 and 523. It appears that results from Sites 516 and 523 provide a better age estimate (39.5 Ma). The age used in Berggren et al. (1985) (37.8 Ma) was taken from Site 522 (Poore et al., 1984) and is apparently too young. Isthmolithus recurvus decreased from common within Subchron C13R to rare near the bottom of Subchron C13N in Site 516. This is basically the same pattern as observed at Sites 522 and 523 (Backman, 1987) and in the Contessa section (Monechi, 1986). The LO of I. recurvus at South Atlantic Sites 516, 522, and 523 and North Atlantic Sites 558,
~
Sites 523 Latitudes 28 °S
clE c1E c17
516 30°S
I
I
Fig. 17. Correlation of the FO and LO of I s t h m o l i t h u s rex c u r v u s with the magnetic time-scale of Berggren et al. (1985). Data are taken from: Contessa Section, FO-Monechi and Thierstein (1985), LO-Lowrie et al. (1982); Bottaccione Section, Monechi and Thierstein (1985); Sites 558 and 563, Miller et al. (1985) in thinner line and Parker et al. (1985) in thicker line; Site 522, Backman (1987) in thinner line and Poore et al. (1984) in thicker line; Site 523, Poore et al. (1984); Site 516, this study.
563 and the Contessa area are all within the lower part of Subchron C12R and it seems to be a good datum at 34.8 Ma for the mid-latitudes.
Discoaster saipanensis
barbadiensis
and
Discoaster
The Eocene/Oligocene boundary in terms of calcareous nannofossil stratigraphy is conventionally defined by the extinction of Discoaster barbadiensis and D. saipanensis (Berggren et al., 1985). However, several studies have shown that the boundary determined by these calcareous nannofossil datums is slightly older than that by the planktonic foraminifera datum (the last occurrence of Globorotalia cerroazulensis), and the time difference is estimated to be 0.4 m.y. at DSDP Site 592 (Martini, 1986; Martini et al., 1986). In Site 516 material the LO of D. saipanensis was found slightly above that of D. barbadiensis (Fig. 18), as was observed at Site 522 (Poore et al., 1984). This succession has also been observed in many other areas (Monechi, 1986). Where the extinction of D. saipanensis is found at the same level as D. barbadiensis, such as Sites 354 and 361 and Barbados (Beckman et al., 1981 ), a hiatus (or sampling gap) is believed to be the cause. From Fig. 18 it can be seen that the LO of D. saipanensis is a relatively reliable datum near the middle of Subchron C13R (about 36.5 Ma) which can be considered as geologically synchronous over the mid-latitudes. Discoaster barbadiensis and D. saipanensis are easily identified even when heavily overgrown. But like other discoasters, they decrease in abundance towards higher latitudes and their LO's have not been found useful for the Eocene/Oligocene boundary south of 60 ° (Wei and Wise, in prep.; Wei and Thierstein, in prep. ).
Ericsonia subdisticha Okada and Bukry (1980) used the end of the Ericsonia subdisticha acme to define the lower boundary of CP16b and the LO of Coccolithus
145 Sites
I°11o
Contessa !
522
528
516
28°S
30°S
i
Latitudes 4 3 ° N
I 26°S I I
m--
33
~ •
Sites Latitudes
Contessa 558
43°N
37°N
563 I 522
528
523
516
33=NII 26°S I I
28°S
28°S
30 oS
33.' • c12
:
Cl;
34;
34" 35
35"
3e
36 C1:
37 38
.: I IIIIIII I I
E
|B-
m
c1~ ~
Fig. 18. Correlation of the LO's of Discoaster barbadiensis (thicker lines) and Discoaster saipanensis (thinner lines) with the magnetic time-scale of Berggren et al. (1985). Data are taken from: Contessa Section, Lowrie et al. (1982); Site 522, Poore et al. (1984); Site 528, Shackleton et al. (1984); Site 516, this study.
formosus to define the upper boundary. At Site 516 the end of E. subdisticha acme coincided with the LO ofC. [ormosus, whereas at Sites 522 and 523 the former datum was stratigraphically higher than the latter (Backman, 1987), the reversal of the Okada and Bukry {1980) sequence. Since the LO of C. formosus can be shown below to be synchronous in the mid-latitudes, the end of E. subdisticha acme must be diachronous in different regions.
Coccolithus formosus The LO of Coccolithus formosus has been widely used to define the CP16b/CP16c (NP21/ NP22) boundary. This species is very distinct and can be identified even in poorly preserved assemblages (Wise, 1973). The currently available magnetobiostratigraphic data (Fig. 19) for the LO of C. formosus show remarkable agreement. It occurs near the bottom of Subchron C12R at 35 Ma. It can now be concluded that the extinction of C. formosus is geologically synchronous and is an excellent marker in the lower and mid-latitudes.
Fig. 19. Correlation of the LO of Coccolithus [ormosus with the magnetic time-scale of Berggren et al. (1985). Data are taken from: Contessa Section, Lowrie et al. (1982); Sites 558 and 563, Miller et al. (1985) in thinner lines and Parker et al. (1985) in thicker lines; Site 522, Backman (1987) in thinner line and Poore et al. (1984) in thicker line; Site 528, Shackleton et al. (1984); Site 523, Poore et al. (1984); Site 516, this study.
Sphenolithus distentus The FO of Sphenolithus distentus was used as a zonal marker for CP17/CP18 boundary in Okada and Bukry (1980) but not in Martini (1971). At Site 516 the FO ofS. distentus within Subchron C13N (Fig. 18) overlaps with the upper ranges of Reticulofenestra umbilica and Coccolithus formosus and is therefore in conflict with Okada and Bukry's (1980) zonation. The FO of S. distentus at Sites 522 (Percival, 1984 ) and 563 (Miller et al., 1985) and in the Contessa section (Lowrie et al., 1982) all were located within the lower part of Subchron C12R but at different levels in different sites (Fig. 20). Parker et al. (1985) reported the lower occurrence of the species at Site 563 as contamination based on the presence ofR. umbilica and C. formosus. After comparing the data from other sites, we can now determine that they were not contaminants. The age overlap problem has been reported in the studies for the Eastern U.S. (Bybell, 1982, 1983; Siesser, 1983) and Barbados (Perch-Nielsen, 1986). It is clear now that the FO of S. distentus is not a reliable datum.
146 Sites
Latitudes
Contessa 43°N
558 37 ° N
563 I 522 33 a NII 26 ° S I
528 2F.° S
516 30 °S
23-
!m-mm
24-" cxm 25;
_ 2S-
C7
27: ;
¢TAco
2a-'
30:
32:
! 332• 34"
C11
Sphenolithus ciperoensis
--
c~2
•
I
3S.E :
]
species near its upper range, especially outside tropical areas. The LO of Sphenolithus predistentus was closely associated with the LO of S. distentus at Sites 516 and 522 (Percival, 1984), 558 (Parker et al., 1984) and 588 (Martini, 1986). Correlation with the magnetostratigraphy at both Sites 516 and 522 has placed the LO of S. predistentus within the upper part of Subchron C9N (about 28.2 Ma). Therefore the LO of S. predistentus may be a good substitute for that of S. distentus as a datum because the former species is usually more abundant and its extinction level is easier to locate.
~ C13
37:
i Ig,o,-~ Fig. 20. Correlation of the FO and LO of Sphenolithus distentus with the magnetic time-scale of Berggren et al. ( 1985 ). Data are taken from: Contessa Section, Lowrie et al. (1982); Sites 558 and 563, Miller et al. (1985) in thinner lines and Parker et al. {1985) in thicker lines; Site 522, Poore et al. (1984); Site 528, Shackleton et al. (1984); Site 516, this study.
The LO of S. distentus defines the CP19a/ CP19b (NP24/NP25) boundary (Martini, 1971; Okada and Bukry, 1980). The currently available data show that this datum falls at different positions (as low as in Subchron C10N and as high as in Subchron C7N) in different areas (Fig. 20). Though Berggren et al. ( 1985 ) placed the LO of S. distentus at the uppermost part of Subchron C9N at Site 522, the species occurred sporadically up to Subchron C7N (see Percival, 1984, p. 412). The large discrepancy among the different studies casts doubt on the reliability and accuracy of this datum. One reason may be the general low abundance of the
The FO of Sphenolithus ciperoensis marks the lower boundary of Sphenolithus distentus Zone (CP19 or NP24). The species first occurs in low abundance and is sporadic at Site 516. The entry level is very close to that in the Contessa section (Lowrie et al., 1982), but much higher than those observed at Sites 522, 528, 558 and 563 (Fig. 21 ). The FO of S. ciperoensis at the latter sites does not agree well either, with two studies of the same site (558) yielding an age difference of about 1 m.y. It is apparent that the use of this datum for the recognition of CP18/ CP19 (NP23/NP24) boundary is difficult and not reliable. The age of the datum in Berggren et al. (1985) was based on data from Site 522 and should be used with caution. The LO of S. ciperoensis (top of CP19 or NP25) is one of the most commonly used datums to approximate the Oligocene/Miocene boundary. Sphenolithus ciperoensis, like other species of the genus, preferred warm waters and is rare or absent in high-latitude areas. Except for the rare occurrence of S. ciperoensis recorded in one sample in the upper part of Chron C6 at Site 522, which may well be reworked or contaminated, the LO of S. ciperoensis from six sources of studies for six sites over different ocean basins and hemispheres falls near the middle of Chron C7 (Fig. 21 ). It can be consid-
147 Sites Contessa 558 563 I 522 528 516 Latitudes 43 °N 37°N 33°N! 26°S 28°S 30°S I
I I
"t__=,
241 26 i
25
-
CeC
m i
28
29
C7
m
eB B i
~
~
,olml-
I
C1C
~']i
11
]
T
--
m m
°
Fig. 21. Correlation of the FO and LO of Sphenolithus ciperoensis with the magnetic time-scale of Berggren et al. (1985). Data are taken from: Contessa Section, Lowrie et al. (1982); Sites 558 and 563, Miller et al. (1985) in thinner lines and Parker et al. (1985) in thicker lines; Site 522, Poore et al. (1984); Site 528, Shackleton et al. (1984); Site 516, this study.
ered geologically synchronous (25.9 Ma) in the mid-latitudes.
dant Z. bijugatus up to the middle of Subchron C6CR and sporadic occurrences up to Chron C6A at Site 558. The sporadic occurrence may be considered as reworked, since Miller et al. (1985) studied the same material and placed the top of Z. bijugatus near the middle of Subchron C6CR, basically in agreement with the results from other sites. Percival (1984) reported sporadic occurrences of Zygrhablithus? sp. in a few samples in the Oligocene sediment at Site 522. Since he did not list Z. bijugatus, we assume that this Zygrhablithus? sp. is Z. bijugatus. The general scarcity of the species at that site can be accounted for by the water depth (over 4400 m), but the very abundant occurrence of the species in a few samples is probably related to fluctuations in productivity of the species.
558 563 37°14 33°N
516 3o°s
.:li__
-1=
25
i
~
Zygrhablithus bijugatus Along with the LO ofReticulofenestra bisecta, the LO of Zygrhablithus bijugatus has been used to define the Oligocene/Miocene boundary by some authors (e.g. Edwards, 1971; Edwards and Perch-Nielsen, 1975). Zygrhablithus bijugatus is a holococcolith and thus is usually not preserved in deep water sediments (Perch-Nielsen, 1985). It is also rare or absent in cool or cold waters (Wei and Wise, in prep.), probably due to preservation as well. On the other hand, Z. bijugatus is easily subject to overgrowth, for reasons poorly known at present. The top of Z. bijugatus at Sites 516 and 563 is relatively sharp at 24.7 Ma (in the middle of Subchron C6CR) (Fig. 22). Parker et al. (1985) reported abun-
Fig. 22. Correlation of the LO of Zygrhablithus bijugatus with the magnetic time-scale ofBerggren et al. (1985). Data are taken from: Sites 558 and 563, Miller et ell. (1985) in thinner lines and Parker et al. (1985) in thicker lines; Site 516, this study.
Reticulofenestra bisecta There are still disagreements among nannofossil experts on the use of the genera Dictyococcites and Reticulofenestra. For practical reasons we prefer to place Dictyococcites in synonym with Reticulofenestra and use here Reticulofenestra bisecta. There are some morphologic variations in the species of R. bisecta.
148
One type has a central area only partially closed by laths, revealing a perforated plate below when viewed distally. The other type has central area totally closed. Wise and Wiegand (in: Wise, 1983) erected the former as Reticulofenestra bisecta filewiczii and the latter, Reticulofenestra bisecta bisecta. We have recorded these two subspecies separately in our range chart and the LO of R. bisecta bisecta is below that of R. bisecta filewiczii, within Subchron C7R. Since most previous work did not differentiate the two subspecies, we take the LO of R. bisecta filewiczii as that of R. bisecta at Site 516 when comparing it with those from other studies. It is apparent from Fig. 23 that the LO of R. bisecta is geologically synchronous among the South Atlantic sites (516 and 522) and North Atlantic sites (558 and 563). The exit of R. bisecta at a lower level in Contessa Quarry section may not represent the true extinction because of poor preservation of the nannofossil assemblages there or due to other reasons. Biolzi (1985) made a detailed study of the Oligocene/Miocene boundary in many Atlantic, Mediterranean and Paratethyan sections. She found that in most sections examined the LO of R. bisecta was stratigraphically higher than the LO's of Heliocosphaera recta, Sphen-
Sites
Contessa
Latitudes 43°N
558
37°N
563
I
522
°' 33 N! 26°S
=
516
30°S
I
24
25
26
Fig. 23. Correlation of the LO of Reticulo[enestra bisecta with the magnetic time-scale of Berggren et al. (1985). Data are taken from: Contessa Section, Lowrie et al. (1982); Sites 558 and 563, Miller et al. (1985) in thinner lines and Parker et al. (1985) in thicker lines; Site 522, Poore et al. (1984); Site 516, this study.
olithus ciperoensis and Zygrhablithus bijugatus and in the other sections the LO of R. bisecta was at the same level with one or several of the latter datums. Each of these datums or a combination of these datums have been used to mark the Paleogene/Neogene boundary in different areas by different authors (see Biolzi, 1985; Berggren et al., 1985 for detailed discussion ). Mid-latitudes sediments appear to contain more abundant R. bisecta, whereas tropical areas seem not so favorable to R. bisecta (Roth and Thierstein, 1972; Perch-Nielsen, 1977; Biolzi, 1985 ). High-latitude sites also yield less abundant or rare R. bisecta (Wise, 1983; Wei and Wise, in prep. ). It is believed that the LO of R. bisecta is more reliable in approximating the Oligocene/Miocene boundary in mid-latitude sediments.
Concluding remarks The abundant, moderate to well preserved calcareous nannofossils in the expanded Paleogene section from Site 516 has enabled the relatively reliable and precise correlation of most traditional and nontraditional datums with the magnetostratigraphy. Critical evaluation of the currently available magnetobiostratigraphic data has revealed the accuracy, the reliability, the synchroneity and diachroneity of Paleogene calcareous nannofossil datums over geographically long distance. A summary of the results is presented in Table II. The detailed calcareous nannofossil stratigraphic results of this study and the evidence for the apparently complete upper Eocene-Oligocene section with a rather uniform sedimentation rate at this site should be valuable for many kinds of future studies, such as detailed taxonomic evolutionary studies, paleoclimatic and paleoceanographic studies.
149 TABLE II Summary of biochronological properties of Paleogene calcareous nannofossil species events
Species events
Ages (Ma)
Reference sites
LO R. bisecta LO Z. bijugatus LO S. ciperoensis LOS. distentus LO H. recta LO C. altus FO S. ciperoensis FO S. distentus LO R. umbilica FO C. abisectus LO/. recurvus End acme E. subdisticha LO C. formosus LO D. saipanensis LO D. barbadiensis LO R. reticulata FO/. recurvus FO C. oamaruensis LO C. grandis LO N. dubius LO C. solitus FO R. bisecta FOR. daviesii FOR. reticulata LO R. gladius LO N. fulgens LO C. gigas FOR. urnbilica FO C. gigas FO N. fulgens LOT. orthostylus FO D. multiradiatus FO D. rnohleri FO H. kleinpeUii FO F. tympaniformis FOE. rnacellus FO P. martinii FO C. danicus FOP. tenuiculum FO C. tenuis FO C. primus FO Z. sigrnoides
24.0 24.7-25.0 25.9 25.9-30.0 26.3 27.0 28.5-31.9 34.0-35.6 34.6 34.2 34.8-35.2 34.4-35.0 35.1 36.4 36.7 37.6 39.5 39.8-40.4 40.0-41.6 40.6 41.3 42.5 43.6 43.7 > 43.8 44.2 44.6-46.8 44.6 47.4 49.8 51.0-54.8 59.2 60.5 59.8-61.6 62.0 65.5-66.0 65.5-66.0 65.5-66.0 65.8 65.9 66.3 66.4
516, 522, 558, 563 516, 558, 563 516, 522,528, 558, 563, Contessa 516, 522, 528, 558, 563, Contessa 516 516 516, 522, 523, 558, 563, Contessa 516, 522, 558, 563, Contessa 516, 522,558, 563 516 516, 522, 523,558, 563 516, 522, 523 516, 522, 523, 528, 558, Contessa 516, 522 516, 522,528, Contessa 516, 522 516, 523 516, 523 516, 523 516 516 516 516 516 516 516, 523 516, 523 516, 523, Contessa 516 516. 527, 528, Contessa 527, 528, 577 516, 524, 527, 528, Contessa 516, 527, Contessa 516, 527, 528, 577, Contessa 516, 527, 528, 577, Contessa 516, Contessa 516 516 516 516, 525, 527, 528 516 516, 524
Remarks
consistent unreliable unreliable unreliable size > 14/Lm size > 8/~m diachronous very consistent
very distinct unreliable
diachronous
diachronous size > 14/~rn
unreliable diachronous? unreliable
FO denotes first occurrence and LO denotes last occurrence. Ages are given in the magnetic time-scale of Berggren et al. (1985). Reference site numbers are DSDP sites.
Acknowledgements A grant from the Hattner Fund made it possible for the first author to participate in the 1988 International Nannoplankton Association Meeting in Shanghai, China, where the major results of this study were presented. Discussion with Marie-Pierre Aubry, Katharina Perch-Nielsen and Tom Romein was helpful.
We thank John Firth for reviewing the manuscript. This study was supported in part by NSF grant DPP 8414268 to Wise. Wei has been supported by a Florida State University Dissertation Fellowship. Samples were provided by the National Science Foundation through the Ocean Drilling Program. Kim Riddle assisted in the SEM photography and Lei Shi helped to prepare the plates and figures.
150
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H., O'Connell, S. and Richardson, S.H., 1984. Accumulation rates in Leg 74 sediments. In: T.C. Moore, P.D. Rabinowitz et al., Initial Reports of the Deep Sea Drilling Project, 74. U.S. Government Printing Office, Washington, D.C., pp. 621-637. Shafik, S., 1981. Nannofossil biostratigraphy of the Hantkenina (foraminiferid) interval in the upper Eocene of southern Australia. J. Aust. Geol. Geophys., 6: 108-116. Siesser, W.G., 1983. Paleogene calcareous nannoplankton biostratigraphy: Mississippi, Alabama and Tennessee. Miss. Bur. Geol. Bull., 125, pp. 1-65. Thierstein, H.R., 1982. Terminal Cretaceous plankton extinctions: a critical assessment. In: L.T. Silver and P.H. Schultz (Editors), Geological Implications of Impacts of Large Asteroids and Comets on the Earth. Geol. Soc. Am. Spec. Pap., 190: 385-399. Wei, W., 1988. A new technique for preparing quantitative nannofossil slides. J. Paleontol., 62: 472-473. Wei, W. and Wise, S.W., Jr., 1989. Discoasterpraebi[ax: A possible ancestor of Discoaster bifax Bukry (Coccolithoporae). J. Paleontol., 63: 10-14. Wei, W. and Wise, S.W., Jr., submitted. Middle Eocene through Pleistocene calcareous nannofossils recovered by Ocean Drilling Program Leg 113 in the Weddell Sea. In: P.E. Barker, J.P. Kennett, et al., Proceedings of Ocean Drilling Program, Scientific Results, 113. Texas A&M University, Texas. Wei, W., Thierstein, H. and ODP Leg 119 Shipboard Scientific Party, 1988. Onset of continental glaciation on East Antarctica as dated by nannoplankton. Geol. Soc. Am. Annual Meeting, Abstr. Progr., v. 20, p. A222. Wise, S.W., Jr., 1973. Calcareous nannofossils from cores recovered during Leg 18, Deep Sea Drilling Project: biostratigraphy and observations on diagenesis. In: L.D. Kulm, R. von Huene et al., Initial Reports of the Deep Sea Drilling Project, 18. U.S. Government Printing Ofrice, Washington, D.C., pp. 569-615. Wise, S.W., Jr., 1983. Mesozoic and Cenozoic calcareous nannofossils recovered by Deep Sea Drilling Project Leg 71 in the Falkland Plateau region, Southwest Atlantic Ocean. In: W.J. Ludwig, V.A. Krasheninnikov et al., Initial Reports of the Deep Sea Drilling Project, 71. U.S. Government Printing Office, Washington, D.C., pp. 481550. Wise, S.W., Jr. and Mostajo, E.L., 1983. Correlation of Eocene-Oligocene calcareous nannofossil assemblages from piston cores taken near Deep Sea Drilling Sites 511 and 512, Southwest Atlantic Ocean. In: W.J. Ludwig, V.A. Krasheninnikov et al., Initial Reports of the Deep Sea Drilling Project, 71. U.S. Government Printing Office, Washington, D.C., pp. 1171- 1180. Wise, S.W., Jr. and Wind, F.H., 1977. Mesozoic and Cenozoic calcareous nannofossils recovered by DSDP Log 36 drilling on the Falkland Plateau, Southwest Atlantic sector of the Southern Ocean. In: P.E. Barker, I.W.D. Dalziel et al., Initial Reports of the Deep Sea Drilling Project, 36. U.S. Government Printing Office, Washington, D.C., pp. 269-492.
119
Paleogene Calcareous Nannofossil Magnetobiochronology: Results from South Atlantic DSDP Site 516 W U C H A N G W E I and S H E R W O O D W. WISE, Jr. Department of Geology, Florida State University, Tallahassee, FL 32306 (U.S.A.) (Received September 30, 1988; revisedand acceptedOctober 28, 1988)
Abstract Wei, W. and Wise, S.W., Jr., 1989. Paleogene calcareous nannofossil magnetobiochronology: resultsfrom South Atlantic D S D P Site 516. Mar. Micropaleontol., 14: 119-152.
The detailed study of an expanded Paleogene section with abundant, moderate to well preserved calcareous nannofossils from South Atlantic DSDP Site 516 has resulted in a precise correlation of most calcareous nannofossil markers with the magnetostratigraphy.Many nontraditional datums have also been documentedand correlated to the magnetostratigraphy.Comparison of the results from Site 516 with those of previous studies from other areas enables a critical evaluation of the accuracy,synchroneityor diachroneity of the species events over geographically long distances. Of special significanceis the correlationfor the first time of the stratigraphic ranges of Chiasmolithus gigas and Rhabdosphaera gladius with the magnetostratigraphy.Other important results include the following:first occurrence (FO) of Cruciplacolithus primus, 66.3 Ma; FO Chiasmolithus danicus, 65.6-66.0 Ma; FO Prinsius martinii, 65.5-66.0 Ma; FO Heliolithus kleinpeUii, 59.8-61.6 Ma (probably diachronous); last occurrence (LO) of Tribrachiatus orthostylus, 51.0-54.8 Ma (unreliable); FO Chiasrnolithus gigas, 47.4 Ma; FO Reticulofenestra umbilica, 44.6 Ma; LO Chiasmolithus gigas, 44.4-46.8 Ma (diachronous);LO Nannotetrina fulgens, 44.2 Ma; LO Chiasmolithus grandis, 40.0-41.6 Ma (probablydiachronous);FO Chiasmolithus oamaruensis, 39.8-40.4 Ma (unreliable); FO Isthmolithus recurvus, 39.5 Ma; LO Reticulofenestra reticulata, 37.6 Ma; LO Discoaster saipanensis, 36.4 Ma; end acme of Ericsonia subdisticha, diachronous;FO and LO Sphenolithus distentus, unreliable; and LO Reticulofenestra bisecta, 24.0 Ma.
Introduction The rapid evolution and wide geographic distribution of calcareous nannofossils have made t h e m one of the best fossil groups for stratigraphic correlation of Mesozoic and Cenozoic marine sediments. The succession of m a n y Paleogene calcareous nannofossils is well known. However, problems still exist for m a n y of the traditional marker species (Beckmann et al., 1981; Perch-Nielsen, 1985; Martini and Miiller, 1986), and m a n y nontraditional marker species have been suggested by different authors
0377-8398/89/$03.50
but their stratigraphic values still await further testing. Moreover, the numerical ages, the synchroneity and diachroneity of these traditional and nontraditional markers world wide are poorly known. To date a modest number of studies have been made to correlate Paleogene calcareous nannofossil datums with magnetostratigraphy (Lowrie et al., 1982; Berggren et al., 1984; Poore et al., 1984; Manivit and Feinberg, 1984; Shackleton et al., 1984; Miller et al., 1985; Monechi and Thierstein, 1985; Monechi et al., 1985; Aubry et al., 1986; Backman, 1986, 1987; B a c k m a n and Hermelin, 1986). How-
© 1989 Elsevier Science Publishers B.V.
120
ever, each of these studies suffered to some degree from one or several of the following: incomplete sedimentary section, condensed section, poor paleomagnetic data, scarcity or poor preservation of the calcareous nannofossils. Consequently many of the traditional datums have only been correlated with magnetostratigraphy at one locality (see summery by Berggren et al., 1985) and the ages remain questionable when applied outside the original area studied (or even at a different site very nearby). Many other traditional datums and most nontraditional datums have not been calibrated with the geomagnetic polarity time-scale at all. Deep Sea Drilling Project (DSDP) Site 516 from the Rio Grande Rise, Southwest Atlantic recovered a rather complete and expanded Paleogene sedimentary sequence. The upper Eocene and Oligocene section is unique because it shows a relatively uniform sedimentation rate for over 350 m (see discussion below). Overall the Paleogene section contains abundant, moderate to well preserved calcareous nannofossils, and the magnetostratigraphy has been established (Berggren et al., 1984). Thus this site offers a rare opportunity to study the calcareous nannofossil abundance patterns in great detail and to tie the nannofossil datums to the magnetostratigraphy. The objectives of this study have been: (1) to determine semi-quantitatively the abundance patterns of the traditional marker species and a few nontraditional marker species from the Paleogene sediments in Hole 516F; (2) to establish correlations between the species events and the magnetostratigraphy; (3) to compare the results of this study with those of previous studies in other areas, to discuss the present status of the traditional and nontraditional marker species, and to evaluate the reliability and accuracy of the species events and their synchroneity and diachroneity in different regions.
Previous work Before DSDP Site 516 was cored, three other DSDP sites (21, 22, and 357) had been drilled on the Rio Grande Rise (Fig. 1 ). The calcareous nannofossil stratigraphy for the three sites was given by Bukry and Bramlette(1970), Gartner (1970), and Perch-Nielsen (1977). However, DSDP Sites 21 and 22 were interval cored, and large portions of Paleogene sediments were not recovered either at DSDP Site 357 (recovery was less than 40%; Maxwell et al., 1970; Perch-Nielsen et al., 1977). DSDP Site 516 is the only one on the Rio Grande Rise to recover a rathe~ complete Paleogene carbonate sequence. Unfortunately, no detailed calcareous nannofossil biostratigraphic data were published in the DSDP Initial Reports. Mostly core-catcher samples were examined, and the calcareous nannofossil assemblages and their abundance patterns were unreported (Barker et al., 1984).
Location and sedimentology DSDP Site 516 lies in 1313 m of water on the Rio Grande Rise at 30 ° 16.59'S, 35 ° 17.10'W (Fig. 1 ). Paleogeographic considerations reveal that there has been little change in latitude from the Paleocene to the present for Site 516 (Sclater et al., 1977). The Rio Grande Rise is an aseismic ridge, the origin of which, like its counterpart in the eastern South Atlantic, the Walvis Ridge, is still in debate (Barker, 1984). Three lithologic units can be recognized for the Paleogene sequence in Hole 516F. Cores 3 to 18 consist of brown and yellow foraminiferal-nannofossil chalks. Carbonate content ranges from 75 to 90% and the main biogenic components are nannofossils (70-90%), foraminifera (10%), and sponge spicules (10%). Cores 19 to 50 consist of upper Oligocene to middle Eocene light gray, nannofossil and nannofossil-foraminiferal chalks. The carbonate percentage is rather uniform throughout this unit (about 80% ), with nannofossils dominat-
121
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ah
2~4
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2
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Fig. 1. Locationof DSDP sites and land sectionsdiscussedin this paper. ing the components. Cores 51 to 89 are nannofossil and foraminiferal limestones. For detailed descriptions see Barker et al. (1984).
Methodology We recorded the abundance patterns of the calcareous nannofossils from Hole 516F because we believed that they would greatly facilitate the evaluation of the reliability and accuracy of the nannofossil datums identified. Backman and Shackleton {1983), and Backman (1986, 1987) have clearly demonstrated the importance of evaluating the species events by their abundance patterns. There are, however, drawbacks in Backman's method (see Wei, 1988), and we found the semi-quantitative method worked well for the present study. We observed the abundance of the calcareous nannofossils under the light microscope using a
magnification of 1600 and have recorded the abundances as follows: A = abundant, 1 or more specimens per field of view; C =common, 1 specimen per 2-10 fields of view; F=few, 1 specimen per 11-50 fields of view; R = r a r e , 1 specimen per 51-200 fields of view; N = n o t found in 200 fields of view. The calcareous nannofossil species investigated in this study are listed in Table I, and most of the species discussed and critical occurrences in some samples are documented with SEM and light micrographs in Plates I to IV. We presented our calcareous nannofossil stratigraphic results using the zonation of Okada and Bukry (1980) as a framework. We also numbered the zones with Martini's (1971) number codes. Instances where the differentiation of zones or subzones is impossible are discussed in the text. The magnetic chron terminology used follows
122 TABLE
I
Calcareous nannofossil species discussed in this paper
Biantholithus sparsus Bramlette and Martini, 1964. Plate III, 3 Blackites spinosus (Deflandre and Fert, 1954) Hay and Towe, 1962. Plate III, 15 Bramletteius serraculoides Gartner, 1969. Plate IV, 14, 15 Calcidiscusprotoanulus (Gartner, 1971 ) Loeblich and Tappan, 1978. Plate I, 10; Plate IV, 9, 10 Chiasmolithus altus Bukry and Percival, 1971. Plate IV, 21 Chiasmolithus bidens (Bramlette and Sullivan, 1961 ) Hay and Molder, 1967. Plate II, 13, 14; Plate III, 5 Chiasmolithus danicus (Brotron, 1959) Hay and Molder, 1967. Plate II, 10, 11 Chiasmolithus gigas (Bramlette and Sullivan, 1961 ) Radomski, 1968. Plate III, 12, 13 Chiasmolithus oamaruensis (Deflandre, 1954) Hay, Molder and Wade, 1966. Coccolithusformosus (Kamptner, 1963) Wise, 1973. Plate I, 7; Plate IV, 12, 13 Cruciplacolithus edwardsii Romein, 1979 Cruciplacolithus primus Perch-Nielsen, 1977. Plate II, 12 Cruciplacolithus tenuis (Stradner, 1961 ) Hay and Molder in Hay et al., 1967. Plate III, 4 Cyclicargolithus abisectus (Muller, 1970) Wise, 1973. Plate III, 2, right Cyclicargolithus floridanus (Roth and Hay in Hay et al., 1967) Bukry, 1971. Plate III, 2, left Discoaster barbadiensis Tan, 1927 Discoaster kuepperi Stradner, 1959 Discoaster lodoensis Bramlette and Riedet, 1954 Discoaster megastypus (Bramlette and Sullivan, 1961 ) Perch-Nielsen, 1986 Discoaster mohleri Bukry and Percival, 1971. Plate II, 2 Discoaster multiradiatus Bramlette and Riedel, 1954. Plate II, 4 Discoaster saipanensis Bramlette and Riedel, 1954. Plate I, 8; Plate III, 14 Discoaster sublodoensis Bramlette and Sullivan, 1961. Plate IV, 1 Discoaster tani nodifer Bramlette and Riedel, 1954 Ellipsolithus maceUus (Bramlette and Sullivan, 1961 ) Sullivan, 1964. Plate II, 9 Ericsonia subdisticha (Roth and Hay in Hay et al., 1967) Roth in Baumann and Roth, 1969. Plate I, 12; Plate IV, 7, 8 Fasciculithus tympaniformis Hay and Molder in Hay et al., 1967. Plate II, 5, 16 Helicosphaera compacta Bramlette and Wilcoxon, 1967. Plate IV, 19, 20 Helicosphaera recta Haq, 1966 Heliolithus hleinpellii Sullivan, 1964. Plate II, 6; Plate III, 7, 8 Heliolithus riedelii Bramlette and Sullivan, 1961. Plate III, 6 Isthmolithus recurvus Deflandre, 1954. Plate III, 16, 17 Lanternithus minutus Stradner, 1962 Nannotetrina fulgens (Stradner, 1960) Achuthan and Stradner, 1969 Nannotetrina cristata (Martini, 1958) Perch-Nielsen, 1971 Neococcolithes dubius (Deflandre, 1954) Black, 1967. Plate IV, 2 Prinsius bisulcus (Stradner, 1963) Hay and Molder, 1967. Plate II, 3 Prinsius martinii (Perch-Nielsen, 1969) Haq, 1971. Plate I, 4, 5 Prinsius tenuiculum (Okada and Thierstein, 1979) (Perch-Nielsen, 1984. Plate I, 1, 2; Plate II, 7, 8 Reticulofenestra bisecta bisecta (Hay, Molder and Wade, 1966) Roth, 1970, Plate IV, 22 Reticulofenestra bisecta filewiczii Wise and Wiegand in Wise, 1983. Plate III, 1 Reticulofenestra daviesii (Haq, 1968) Haq, 1971 Reticulofenestra reticulata (Gartner and Smith, 1967) Roth and Thierstein, 1972. Plate I, 9; Plate IV, 11 Reticulofenestra samodurovii (Hay, Molder and Wade) Roth, 1970. Plate I, 11 Reticulofenestra umbilica (Levin, 1965) Martini and Ritzkowski, 1968. Plate III, 11; Plate IV, 17 Rhabdosphaera gladius Locker, 1967. Plate IV, 4, 5 Sphenolithus ciperoensis Bramlette and Wilcoxon, 1967 Sphenolithus distentus (Martini, 1965 ) Bramlette and Wilcoxon, 1967. Plate IV, 18 Sphenolithus furcatolithoides Locker, 1967. Plate IV, 16 Sphenolithus predistentus Bramlette and Wilcoxon, 1967 Sphenolithus primus Perch-Nielsen, 1971 Toweius callosus Perch-Nielsen, 1971. Plate I, 3, 6 Tribrachiatus orthostylus Shamrai, 1963 Triquetrorhabdulus carinatus Martini, 1965 Zygodiscus sigmoides Bramlette and Sullivan, 1961. Plate II, 1 Zygrhablithus 5ijugatus (Deflandre in Deflandre and Fert, 1954) Deflandre, 1959
123
that of LaBrecque et al. (1983), that is, a magnetic chron is defined as from the youngest reversal boundary of a numbered anomaly to the youngest reversal boundary of the next older numbered anomaly. The chron unit is distinguished by prefixing a letter "C", and it can be subdivided into subchron by addition of an N or R suffix which refers to the polarity. The geomagnetic polarity time-scale used in this study is that of Berggren et al. (1985). This po-
larity time-scale, like that of Haq et al. (1987), integrated independent data sets from magnetostratigraphy, sea-floor spreading magnetic lineation patterns, biostratigraphy and isotopic ages, and has been used widely. The results from the present study thus can be easily compared with those of others. We constructed an age-depth curve for Hole 516F, using the magnetic polarity horizons which have been clearly identified and by as-
PLATEI 1, 2. Prinsius tenuiculum, X12,000, Sample 516F-89-2,12-13 cm. 3. Toweius callosus, X6,000, Sample 516F-83-6, 51-52 cm. 4, 5. Prinsius rnartinii, Xl0,000, Sample 516F-87-2, 88-89 cm. 6. Toweius callosus, X5,000, Sample 516F-83-6, 51-52 cm. 7. Coccolithus forrnosus, X4,500, Sample 516F-43-1, 16-17 cm. 8. Discoaster saipanensis, X6,500, Sample 516F-43-1, 16-17 cm. 9. Reticulofenestra reticulata, X6,500, Sample 516F-43-1,16-17 cm. 10. Calcidiscusprotoanulus, X8,500, Sample 516F43-1, 16-17 cm. 11. Reticulofenestra samodurovii, X8,500, Sample 516F-37-2, 72-73 cm. 12. Ericsonia subdisticha, X6,500, Sample 516F-37-2, 72-73 cm. PLATEII
1. Zygodiscus sigmoides, X6,500, Sample 516F-85-2, 52-53 cm. 2. Discoaster rnohleri, X5,500, Sample 516F-83-6, 51-52 cm. 3. Prinsius bisulcus, X10,500, Sample 516F-83-6, 51-52 cm. 4. Discoaster multiradiatus, X5,500, Sample 516F-83-6, 51-52 cm. 5. Fasciculithus tyrnpaniformis, X6,000, Sample 516F-85-2, 35-36 cm. 6. Heliolithus kleinpellii, X6,000, Sample 516F86-1, 52-53 cm. 7, 8. Prinsius tenuiculum, X3,400, Sample 516F-89-1,100-101 cm. 9. EUipsolithus rnacellus, X2,800, Sample 516F-88-3, 38-39 cm. 10, 11. Chiasrnolithus danicus, X3,400, Sample 516F-88-3, 38-39 cm. 12. Cruciplacolithus primus, X3,600, Sample 516F-89-4, 85-86 cm. 13, 14. Chiasrnolithus bidens, X2,400, Sample 516F-51-1,106-107 cm. 15. Cruciplacolithus edwardsii, X2,400, Sample 516F-87-5, 19-20 cm. 16. Fasciculithus tyrnpaniformis, X2,400, Sample 516F-17-2, 0-1 cm.
PLATEIII
1. Reticulofenestra bisecta filewiczii, X7,500, Sample 516F-19-2, 100-101 cm. 2. Cyclicargolithus floridanus on the left, Cyclicargolithus abiseetus on the right, X7,000, Sample 516F-15-5, 90-91 cm. 3. Biantholithus sparsus, X4,000, Sample 516F-89-3, 7-8 cm. 4. Cruciplacolithus tenuis, X4,000, Sample 516F-87-4, 46-47 cm. 5. Chiasmolithus bidens, X3,600, Sample 516F-87-4, 46-47 cm. 6. Heliolithus riedelii, X3,600, Sample 516F-84-3, 52-53 cm. 7, 8. Heliolithus kleinpellii, X3,600, Sample 516F-84-1, 24-25 cm. 9, 10. Campylosphaera dela, X4,000, Sample 516F-67-1, 39-40 cm. 11. Reticulofenestra urnbilica, X2,400, Sample 516F-66-1, 45-46 cm. 12, 13. Chiasrnolithus gigas, Xl,900, Sample 516F-66-1, 45-46 cm. 14. D/scoaster saipanensis, X2,400, Sample 516F-66-4, 89-90 cm. 15. Blackites spinosus, X3,600, Sample 516F-61-5, 32-33 cm. 16, 17. Isthmolithus recurvus, X3,000, Sample 516F-38-2, 6-7 cm. P L A T E IV
1. Discoaster sublodoensis, X2,400, Sample 516F-81-1,123-124 cm. 2. Neococcolithes dubius, X3,000, Sample 516F-66-4, 89-90 cm. 3. Helicosphaera lophota, X3,000, Sample 516F-62-4, 18-19 cm. 4, 5. Rhabdosphaera gladius, X2,400, Sample 516F-61-5, 32-33 cm. 6. Sphenolithus spiniger, X2,400, Sample 516F-59-1,112-113 cm. 7, 8. Ericsonia subdisticha, X3,000, Sample 516F-51-1,106-107 cm. 9, 10. Calcidiscusprotoanulus, X2,400, Sample 516F-40-2,105-106 cm. 11. Reticulofenestra reticulata, X2,400, Sample 52-1, 30-31 cm. 12, 13. Coccolithus [orrnosus, X2,400, Sample 516F-36-4, 23-24 cm. 14, 15. Brarnletteius serraculoides, X2,400, Sample 36-1, 39-40 cm. 16. Sphenolithus furcatolithoides, X2,400, Sample 516F-60-3, 19-20 cm. 17. Reticulofenestra umbilica, Xl,600, Sample 516F-36-1, 39-40 cm. 18. Sphenolithus distentus, X2,400, Sample 516F-31-1, 23-24 cm. 19, 20. Heliocosphaera compaeta, X2,400, Samp!e 516F-29-2, 33-34 cm. 21. Chiasrnolithus altus, X6,500, Sample 516F-16-1, 6-7 cm. 22. Reticulofenestra bisecta bisecta, X6,500, Sample 516F-19-1, 25-26 cm.
124
PLATE I
125
P L A T E II
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126
P L A T E III
127
PLATE IV
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128 signing numerical ages according to the polarity time-scale of Berggren et al. ( 1985 ) (Fig. 2 ). T h e d a t a points between 24 to 42 M a fall re-
m a r k a b l y close to a straight line. T h i s indicates t h a t the s e d i m e n t a t i o n rate was relatively uniform at this site from late Eocene t h r o u g h Oli(Ma)
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129
gocene time. Hiatuses, if present, must be minor because it is inconceivable that the hiatuses have been compensated perfectly by increases in sedimentation rate between control points so that there is no change in the slope of the agedepth curve over a period of 20 m.y. The apparent uniform sedimentation rate at this site for the upper Eocene through Oligocene section makes it unique for the biochronology study, because the datum levels identified in the sequence can be confidently and precisely tied to the geomagnetic polarity time-scale by simply reading off the straight age-depth line.
Calcareous nannofossil stratigraphy The abundance patterns of selected calcareous nannofossil species from Hole 516F are graphically presented in Fig. 3. Full species range charts and detailed discussions of the assemblages and their implication for paleoenvironments will be presented in a forthcoming paper. The accuracy and reliability of the species events can be judged by their abundance patterns near their first occurrences and last occurrences, i.e., those showing abrupt change near their appearances and extinctions are easier to determine and are more reliable, whereas those showing a long tail of abundance and occurring sporadically near their appearance or extinction may not have been reliably determined.
Cretaceous~Tertiary boundary The Cretaceous/Tertiary boundary lies between Samples 516F-89.5,131-132 cm and -894, 85-86 cm, the lower sample consisting of 100% uppermost Cretaceous nannofossils and the upper sample containing many Tertiary species (e.g., Cruciplacolithus primus, Zygodiscus sigmoides, and Markalius inversus ). This boundary is located within Subchron C29R, in agreement with the results obtained from other localities (Alvarez et al., 1977; Thierstein, 1982; Magaritz et al., 1985; Monechi et al., 1985). Rare
Biantholithus sparsus was found only in Sample 516F-89-3, 7-8 cm and its FO could not be used here to mark the Cretaceous/Tertiary boundary as suggested by Perch-Nielsen (1979).
Paleocene The first occurrence (FO) of Ellipsolithus maceUus is in Sample 516F-88-3, 38-39 cm (Plate II, 9), one core lower than reported in the Initial Reports (Berggren et al., 1984). Several marker species of various authors also occur first in this sample: Chiasmolithus danicus, Cruciplacolithus tenuis, and Prinsius martinii. Furthermore, the stratigraphic top of Prinsius tenuiculum is very sharp in Sample 516F-89-1, 10-11 cm, where it is very abundant. Due to the half core loss in Core 89, there is no direct proof that an unconformity occurs between these two samples, though extrapolation of sedimentation rates above and below this interval does suggest a hiatus. The base of Fasciculithus tympaniformis in Sample 516F-87-2, 0-1 cm defines the base of CP4; the first entry of Heliolithus kleinpeUii in Sample 516F-86-1, 52-53 cm indicates the CP4/ CP5 boundary and the first occurrence of Discoaster mohleri in Sample 516F-85-2, 35-36 cm marks the lower boundary of CP6. Rare and sporadic Discoaster nobilis, the FO of which was used as a datum to separate CP6 and CP7 in Okada and Bukry (1980), is present in a few samples along with Discoaster multiradiatus. Discoaster nobilis was not reported from Leg 73 materials from the southeast Atlantic Ocean (Percival, 1984) or from the Contessa section in Italy (Monechi and Thierstein, 1985). On the other hand, the FO of Heliolithus riedelii is the counterpart of the first D. nobilis in Martini's (1971) scheme. Rare H. riedelii was observed only in one sample (Sample 516F-84-3, 52-53 cm) at Site 516, which also contains rare Discoaster multiradiatus, therefore the sample should be assigned to Zone CP8. Thus neither
130
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" " ~ "
a i
I
• "
...
1
" i
~
"
.
.
•
"
~
I
| ~
.
.
.
'
"'~'l
"
~
2-3
70-1, 34-35 70-2,
"
li ' 1 ~ 18~"
i"
i
•
~
.;
'
~
i I
.
i
iI i j .
69-2. 77-78
69-3,
.
u
.
" "
14-15
68-4'43-44 CP13c
~
.
~
66-3, 24-,_5 67-1, 67-3,
.
J
65-2.29-29 m. Eocene
"
i
54-1.46-46 54-3, 32-33 C P1 ~
'
34-35
I
"
71-3, 71-4, 71-5,
92-93 66-67 63-64
. . . . . . . . . .
72-i
72-4: 39-4o
49-50
~ :~:
73-2,
79-60
~
73-5,
44-45
.~ • , . ~ ~ , I
.
-
.
~..
• I
~
~ ~
, ." • I
~
. I
~: .
.
; •
.
J
. ~
~
~
~
. 4 •
. ~ - ~
~ •
.
. ~ ~
~ ~ ~
: :: .
~ l '
i
!
~
~
I i
~
~
J
•
- ~
.
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• I i
•
71-1. 13-14
.
-
I
l
i
.
. ~ . . ~
•
~. ~
~. o
Fig. 3. Stratigraphic distribution pattern of selected Paleogene calcareous nannofossil species in Hole 516F.
131
]Abundant Age
:°
core, section
Zone
....
NP14
CPll CP10
NP13 NP12
CF o
NP11
CP8
NP9
6o-61
74-~,
97-98
74-5,
46-47
74-6,
49-50
75-3,
71-72
75-5,
26-27
76-1,
24-25
',6-5,
~21-122
~1
77-1,
62-63 lo3-1o4
~
77-3, 77-4, 78-1.
110-111 30-31
78-4,
83-84
79-5,
61-62
~o-1
57-58
• 81-1,
•
NP8
.
c~ CP5
NP7 NP6
CP3
NP5
NP4
CPla
NPt
Fig. 3. (continued)
.
i
• • •
~- ~- ~-
~"
i ~
~..~
u
T
~ ~
~"
~
. . . .
|
|
N
-
a
R
8
|
.q • I"
~
~ ~ ~
. . ". . "
"
51-52 24-25
J | |
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. .
. .
. .
~ = D ~ .~
~ =~ a | ~ :
~
~ ~ . ~ "
,
,
.
•
•
~
~
52-53
•
•
I
i ~
" "
65-86
48-49
66-6,
67-68
66-7,
25-26
87-1,
88-89
87-3,
88-89
87-4, 67-5, 67-6,
46-47 18-2o
67-7, 88-1,
41-42 29-30
88-2,
74-75
.
86-3,
36-39
. .
...... =
• i
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.
.
.
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~ "
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i
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75-76
.
~ ~ : .~ .
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•
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.....
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H = = | •
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1
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t
i,
....
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.
I ~ j
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~u .
:
Q
10-11
.
: .
H
|
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..~
-
~
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i
~ ~._ ~.
. | • • i
::I
j
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r ~
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FI
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"--
~. ~ I ~ ;.; . I ~ . ~ I" :J
.
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.
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PN
~
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-. m. ~ S
.
.
. . . .
.=
.
.
~
•0
.
B
l .
lO~-1o2 35-36
~'
. . ~|
. _.
.
86-1,
86-5,
. ~ ~
. .u. . . . .
" "
"
~ ~
'
85-2,
66-4,
j
~
~
.
: :
.
:~
" " ;'= I
-
. . ! " "i
~~
. . . . .
~ • • • a . u.
" " ~ " " |
65-1,
lZ-~3
~.
n
83-6, 64-1,
7-8 85-86
~ ~ ~
" ~
~ ~ ~
89-3, .89-4,
• " B
~
30-31 142-143 113-114 96-97
89-2.
B"
.
81-3, 62-1, 82-2, 83-1,
89-1,
i
.
. 6466--24, 6 8 057-58 _3, CP4
~
~~ ~• ~ . . . .
20-21 67-68
~
,
107-108
67-68
~
• • i
~
123-124
84-4, 84-6,
'ii
~ "~ i
-
81-z,
64-5,
"~-"
(~ ~ ~ c~
II-
' ~ ' ' "
~4-~,
84-2, 65-66 64-3, 52-53 CP7
~
~ m ~ ~ ~ '~ ~ ~
74-~,-5o-51
CP12
" il iiii
~.~ ~ ~ - - ~
.
.
.
.
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|= ..
.
.
.
.
.
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.
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•
132
Key
symbols
to
•
" ~ - ~ -.~
m
!i"'
present
Not
Rare Few Common
oo
coro,
Abundant
Age
l~oc~ion
Interval
::, .c
¢i 111
(crn)
Zone
o. Miocene
CNlb
4-5,
.mo-io1
• ~-1, 89-70
NN1
5-3,
CNla
i00-i01
7-2,
I00-I01
8-2, 9-2, io-1, io-z,
2o-21 ioo-iol ioo-Iol 9o-91
. . n " " 1 .
}
.....
M
I
H
•
"
'
r
,q
.
.
.
10-3,
90-91
~ ~ =_a
io-4, 11-i, 11-2.
9-1o 90-91 90-91
J -
ii-4,
i00-I01
12-1 • 13-i,
16-17 80-81
.
l
I{
CPlgb
~_ i ' i . ~
s
{
.
_.m . e
:
.
.
= " i -
a ~ ~,
" .
' ;' • ~
i
" i
=:
'
"
!
s
.~
329o91
NP25
o. . . . . .
.
i , !
13-3,
BO-61
is-1.
lOO-lOl . ~
~s-z,
11o-z11
.
|
15-,~.
11o-n.1 90-91
. ;
_~ T
90-91
..
35-3o 6-;
•
? . 11{
.
~ ~
{~
I. O l i g o c e n e
15-5, 15-;. 16-i.
NP24 •,,
NP23
,% " CPI~ CP18 o. O l i g o c o n e
CP17
l
CP16c
NP22
•
17-3.
OO-lOl 13-i
. . "i
18-i,
90-91
. ~
19-1, 19-2, 19-3,
25-Z6
'
100-I01 80-81
19-,~,
18-19
2o-z. 20-2 20-4, 21-i, 21-4,
99-1oo 90-91 90-91 90-91
22-2,
~o-31 90-91
2,,-1. 25-1, 25-4, 28-2, 26-5.
,~z-,~2 90-91 81-82 lZO-lZz 11"~-118
28-2. 29-2, 30-2. ~i-,. 32-2(" 33-1, 33-2,
s5-8s 3n-3~ i**-145 72-73 89-90 56-57 39-40
B
|
~ ]
.
•
r,
•
•
; " "3= r t ~-=
~
•
.
:,
.
.
j
.~ . ~
.
.
t
= ~ • ~
~ ~, • ,.., • - '~ •
-
=i " '
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. .
I ~z | I
] ~ . . . "-- : " . . . ~ ]
, . • .
:
-
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.
". {
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I
j
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- 1 ~ . ~ I . W ,'
• . . . p ~ s
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, ~ ~ { ~
{ I'I I ' L u
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.
.
l
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" . . . . . ~ . • ': . . !
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u.
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} l ~ l ] {} . ~{ iI
i . ,-= • | • ".. =l ~"
. . • . ~i . • ~ • . : • ~;
{
C
. . . .
l'] { ] ] {
• • p ~, ~
• • . '
~, ":. r. ? "
. ~, -= . ~
. . .
• . • . •
. . . . . . ~ B . u, | 0 . : ~ •
i~ [ ~ U ~
= ~ | u | " ~, ~ {
-
Q
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t
-
-= I
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~ -~ i I -:
== [=
.
.
.
,~
.
. . . .
i-l'-1.
.L.
Fig. 3. (continued)
the FO of
D. nobilis
nor the FO of
could be used for Hole interval
from Sample
516F
H. riedelii
material
516F-85-2,
35-36
and the cm to
Sample 516F-84-4, non-differentiated
20-21 cm was placed Zone CP6-7.
Discoaster diastypus,
in the
the FO of which defines
133 the base of CP9, was not detected in the material. Consequently CP8 and CP9 could not be differentiated for Hole 516F material. Tribrachiatus bramlettei is also absent (as it is in the Leg 73 material, see Percival, 1984). Many species have their last occurrences in Sample 516F83-1, 96-97 cm (e.g. Cruciplacolithus tenuis,
Discoaster multiradiatus, Fasciculithus tympaniformis, Neochiastozygus distentus, Prinsius bisulcus and Zygodiscus sigmoides) and many species occur first in Sample 516F-82-2, 113-114 cm (e.g. Campylosphaera dela, Chias-
molithus solitus, Discoaster barbadiensis, Neococcolithes dubius, Sphenolithus editus, Sphenolithus radians, and Tribrachiatus orthostylus). An unconformity must be present between these two samples or the interval must be very condensed.
Eocene The zonal marker for the base of CP10, D/s-
coaster lodoensis, was observed in Sample 516F82-1, 142-143 cm. The first occurrence of D/scoaster kuepperi was also found in this sample. Thus a brief hiatus is suggested right below this sample because normally the FO of D. kuepperi should be slightly higher in the section than the FO ofD. lodoensis. The first occurrences of both Chiasmolithus grandis and Coccolithus formosus in this sample also indicate the missing of the lower part of Zone CP10 and possibly upper part of Subzone CP9b (see Perch-Nielsen, 1985, p. 431 ). No Coccolithus crassus could be confidently identified in the material. The difficulty in using this species as a zonal marker and its unreliability have been discussed by Filewicz and Hill (1983), Applegate and Wise (1987), and Filewicz (written communication, 1987). The LO of T. orthostylus could not be used either (see discussion below). Consequently CP10 and C P l l cannot be differentiated for Hole 516F. The top of CP10-11 was tentatively drawn between Samples 516F-81-2, 107-108 cm and -811, 123-124 cm, because rare specimens of D/scoaster sublodoensis were recognized in the lat-
ter sample and the species occurs sporadically in Hole 516F sediment. The traditional middle to upper Eocene nannofossil datums were found in the following samples: the FO of Nannotetrina fulgens (base of CP13a) in Sample 516F-76°5, 121-122 cm; the FO of Chiasmolithus gigas (base of CP13b) in Sample 516F-74-1, 50-51 cm; the FO of Reticulofenestra umbilica (base of CP14a) in Sample 516F-68-1, 19-20 cm; the LO of Chiasmolithus solitus (top of CP14a) in Sample 516F51-1, 105-106 cm; the FO of Chiasmolithus oamaruensis {base of CP15a) in Sample 516F-471, 19-20 cm; the FO of Isthmolithus recurvus (base of CP15b) in Sample 516F-45-1, 6-7 cm; and the LO of Discoaster saipanensis (top of CP15b) in Sample 516F-38-2, 6-7 cm. It is worth noting that Chiasmolithus g/gas (Plate III, 12, 13 ) has a considerably long range (80 cm) in Site 516 and its upper range overlaps with the FO of Reticulo[enestra umbilica (Plate III, 11 ). This precludes use of subzones CP13b and CP13c as prescribed by Okada and Bukry's (1980) low latitude zonation. A similar situation has been reported by Applegate and Wise (1987) for material from the continental rise off New Jersey. Another significant observation is the finding of rare specimens of Rhabdolithus gladius in Sample 516F-61-5, 32-33 cm (Plate IV, 4, 5). The LO ofR. gladius has been used to mark the NP15/16 boundary in Martini's (1971) zonation and has been taken as equivalent to the LO of C. g/gas (top of CP13b) (Okada and Bukry, 1980; Perch-Nielsen, 1985; Martini and Miiller, 1986). The present finding contradicts this traditional correlation. Detailed discussions of C. g/gas and R. gladius in relation to the magnetostratigraphy are presented below. Discoaster bifax, the total range of which defines CP14a (Okada and Bukry, 1980), was not found at Site 516. Rare specimens of Discoaster praebifax were, however, found in a few samples in CP13. The current status of D. bifax and D. praebifax has recently been discussed by Wei and Wise (1989).
134
Oligocene The LO of Reticulofenestra umbilica in Sample 516F-33-1, 56-57 cm defines the top of the first calcareous nannofossil zone (CP16) in the Oligocene. The following species events occur progressively in this zone and may be useful datums for regional correlation: FO of Sphenolithus distentus, LO of Bramletteius serraculoides, LO of Lanternithus minutus, LO of Coccolithus formosus, and LO of Isthmolithus recurvus. The LO of C. formosus marks the CP16b/CP16c boundary. The Ericsonia subdisticha acme datum could not be used because it co-occurred with the LO of C. formosus in Site 516 (it occurred slightly higher than the LO of C. formosus at Sites 522 and 523; see Backman, 1987). The range of Sphenolithus distentus at Site 516 overlaps those of C. formosus and R. umbilica. A similar problem has been reported in Okada (1980), Bybell (1982, 1983 ), Siesser (1983), and Perch-Nielsen (1986). It is now clear that the FO of S. distentus is not a reliable datum for the CP17/CP18 boundary. Sphenolithus ciperoensis is rare in Hole 516F sediment and its first occurrence is stratigraphically too high to be used for the CP18/19 boundary at Site 516. Thus the interval from Samples 51632-2, 89-90 cm to -20-2, 90-91 cm was assigned to the CP17-19a combined zone. A succession of extinctions was found near the Oligocene/Miocene boundary: LO of Chiasmolithus altus followed by that of Helicosphaera
recta, Sphenolithus ciperoensis, Reticulo[enestra daviesii, Reticulofenestra bisecta bisecta, Zygrhablithus bijugatus, and Reticulofenestra bisecta filewiczii. The LO of Sphenolithus ciperoensis in Sample 516F-10-2, 90-91 cm defines the CP19/CN1 boundary. The Oligocene/Miocene boundary in terms of calcareous nannofossil stratigraphy is defined by the LO of R. bisecta (Berggren et al., 1985) and falls between Samples 516F-5-1, 69-70 cm and 516F4-5, 100-101 cm. An unconformity appears to be present between these two samples because there is an abrupt change in the abundance of
Cyclicargolithus abisectus and the upper part of Subchron C6CN seems missing. Assuming constant sedimentation rate above and below this unconformity, the hiatus would be about 0.6 m.y. in duration. The present study has improved the stratigraphic results of the initial study (Berggren et al., 1984 ) by identifying three hiatuses and by adjusting and locating more precisely most of the calcareous nannofossil zonal boundaries. Figure 4 provides an easy comparison of the results. Bio- and magneto-chronologic correlation Due to the relative nature of the conventional biostratigraphy, it cannot precisely correlate species events in a contemporary scale. Correlation of species events to a common geomagnetic time-scale will enable us to obtain information on their numerical ages, their synchroneity or diachroneity over geographic long distances. In the present study we have tied the calcareous nannofossil datums of Site 516 to Berggren et al. (1985) magnetic time-scale. We have also plotted on the same magnetic timescale all the published magnetobiostratigraphic data that are of moderate or good quality. Though the numerical magnetostratigraphic ages involve assumptions and interpretations that may not be agreed on by all investigations, and may be subject to revision in the future, the correlation of the species datums still holds true and the positions of the species events within the magnetic chron will be unchanged. The following is a discussion of some important Paleogene calcareous nannofossil datums and their calibration to the magnetic time-scale of Berggren et al. (1985).
Biantholithus sparsus, Z ygodiscus sigmoides and Cruciplacolithus primus As mentioned previously, rare specimens of Biantholithus sparsus were found in only one sample (516F-89-3, 7-8 cm). Its FO, which is
(i,a4)
BNN
sB
•
2.~.~
crab
22
.
j
6 t
N.2S g 1o
i
~7
'...2-' ~
,,
"~
.....
11
NP~4-21
IS
~
Np~s 17
II
-
2Z
I0
~
PIP23-24 ¢10
24
~
2s
i
~
e.Plb ~
e.ple
11
m
NP23.24
immE
=o
me
33
i
-
CPI~
NP22
:PI61 b
Np~I
34
NP22.2
36 37
CI3 m
~*
i
is
. . . . . NP22
--
NPI~-2
47
s~ i
-
~P~
[]
c1~
~s
-,~2
"°
71 -~--
imlmmm
c~o
;Tm
-
7~ $0
m
--
~
C24
?
~p~s
c~12
NPI~
.
.
.
Cruciplacolithus tenuis, Chiasmolithus danicus and Ellipsolithus macellus Due to core loss and possibly an unconformity between Samples 516F-88-3, 38-39 cm and -89-1, 10-11 cm, the FO's of Cruciplacolithus tenuis, Chiasmolithus danicus and Ellipsolithus macellus could not be precisely determined, but they fall within 65.6-66 Ma, extrapolating sedimentation rates from above and below these two samples. The age range of the FO of C. tenuis from Site 516 is in agreement with those from Sites 525 (65.8 Ma), 527 (65.9 Ma), and 528 (66 Ma) (Shackleton et al., 1984), but the FO's of C. danicus and E. macellus at Site 516 are older than those in Shackleton et al. (1984) (64.8 Ma and 63.8 Ma respectively). The first
.
NP~3-~, . . . . . NPI2
........... 2S
18
above those of Zygodiscus sigmoides and Cruciplacolithus primus at Site 516, could not be used to mark the base of C P l a (NP1) as suggested by Perch-Nielsen (1979). Martini and Miiller (1986) concluded that this datum is generally not applicable for defining the K / T boundary due to its scarcity in most areas. Both Z. sigmoides and C. primus occur first in Sample 5 1 6 F - 8 9 - 4 , 85-86 cm (Plate II, 1, 12), 1 m above the Iridium peak level (Hamilton, 1984) at this site. Assuming the Iridium peak level represents 66.4 Ma and sedimentation rate is constant within Chron C29, the FO ofZ. sigmoides and C. primus would be 66.3 Ma or older. At Site 524 the FO ofZ. sigmoides was found at 66.4 (Poore et al., 1984), in good agreement with our present result. The age of the first C. primus (66.1 Ma) as derived from Sites 525, 527 and 528 (Shackleton et al., 1984) seems too young compared with that from Site 516.
~
~27
21
~
C27
CP~
NP~.10
Np~
N~
Fig. 4. Peleogene magnetobiostratigraphy of Hole 516F: comparison of the results from this study and those of Berggren et el. (1984). The magnetostratigraphy is after Berggren et el. (1984). In the polarity column the stippled parts indicate lack of mR~etic data or uncertain polarity. Nannofossil zones are given in the zonations of Okada and Bukry (1980) and Martini (1971).
136
occurrences of C. danicus and E. macellus in Sample 516F-88-3, 38-39 cm have been documented in Plate II, 9-11, and we believe that the FO of E. maceUus is environmentally controlled, that is, the species may not be found in deep-water sediments. In other words, "the FO of E. macellus represents an inaccurate datum event" (Monechi et al., 1985, p. 793).
Fasciculithus tympaniformis The currently available magnetobiostratigraphic data show that the FO of F. tympaniformis (base of CP4) is located in the middle or lower part of Subchron C26E (Fig. 5). The age of this datum as determined at Site 516 is slightly older (at 62.7 Ma) than those from the previous studies for other regions. The good agreement of the age from the previous studies (62.1 Ma) makes the age from Site 516 abnormal. One possibility is that the sedimentation rate within Subchron C26R at Site 516 varied greatly and resulted in the incorrect assignment of the age. The substitute marker for the FO of F. tympaniformis as proposed by Perch-
Sites Ma
$"
Contessa
Latitudes 43°N
o
Bottaccione 43°N
577 Ii 527
528
516
32°N ~128°S i
28°S
30°S
Nielsen (1985), the FO of Sphenolithus primus, occurs 0.2 m.y. earlier at Site 516.
Heliolithus kleinpellii The age of the FO of Heliolithus kleinpellii (base of CP5 or NP6 ) at both Sites 516 and 527 is 61.6 Ma (Fig. 6). However, the results from other localities do not agree well, giving younger ages up to 59.9 Ma (Site 528) (Shackleton et al., 1984). Moreover, Heliolithus cantabriae, which occurs earlier than H. kleinpellii (Romein, 1979 ), grades into H. kleinpellii, and there is a considerable variation in forms within the helioliths as pointed out by Wise and Wind (1977, p. 296). The FO of H. kleinpellii should be used with caution and the age of 61.6 Ma as given in Berggren et al. (1985) should be considered tentative.
Discoaster mohleri The first non-birefringent discoaster, Do mohleri (base of CP6 or NP7) is easily recognized in most studies. Figure 7 shows that, except for Site 528, all sites gave an age of about 60.4 Ma for the FO ofD. mohleri. Shackleton et
Sites
59
o~
~
Ma
~
('3
Contessa Bottaccione
Latitudes 43~N
43~N
I 577 I 527
528
516
32°N i 28~S
28°S
30%
C25! 59
60
C25
i
61,
6O C26'
62
i
61
I C26
63
Fig. 5. Correlation of the FO of Fasciculithus tympanifor~ m/s with the magnetic time-scale of Berggren et al. ( 1985 ). Data are t a k e n from: Contessa a n d Bottaccione sections, Monechi a n d T h i e r s t e i n (1985); Site 577, Monechi et al. (1985); Sites 527 a n d 528, Shackleton et al. (1984); Site 516, this study.
62
63
---
Fig. 6. Correlation of the FO of Heliolithus kleinpellii with the magnetic time-scale of Berggren et al. (1985). Data sources are the same as in Fig. 5.
137
Ma
o~ ~"
=o
Sites Contessa Latitudes 43 °N
Bottaccione : 527
43° N
' 28° S
528 28°S
516 30 ° S
5559:
C5 60 61
II
iii
I
C ~E 62
I
Sites Latitudes
Contessa
43°N
577
: 527
28°N II 28°S i
528
524
516
28°S
290S
30°S
56. ; 57"
58
III!I
63 Fig. 7. Correlation of the FO ofDiscoaster mohleri with the magnetic time-scale of Berggren et al. (1985). Data sources are the same as in Fig. 5.
al. (1984) did not consider the result from Site 528 to be representative. Therefore, the FO of D. mohleri seems to be a sound datum at 60.4 Ma.
Discoaster multiradiatus
Discoaster multiradiatus has a wide geographic distribution, probably due to the relatively warm and equable climate world-wide during late Paleocene time. The species is easily identified, and results from the South Atlantic sites and the Contessa section agree well, giving an age of 59.2 Ma for the FO of D. multiradiatus (Fig. 8 ). Thus this datum is excellent for geographically long distance correlation. Furthermore, the number of rays in D. multiradiatus decreases progressively through time (Moshkovitz, 1967; Romein, 1979), and detailed morphometric study of this species from different areas should result in considerably finer stratigraphy using quantitative D. multiradiatus data.
Fig. 8. Correlation of the FO of Discoaster multiradiatus with the magnetic time-scale of Berggren et al. (1985). Data for Site 524 are taken from Poore et al. (1984). Refer to Fig. 5 for other data sources.
Tribrachiatus orthostylus Though the FO of Tribrachiatus orthostylus was not used in the zonations of Martini ( 1971 ) or Okada and Bukry (1980), its relative stratigraphic position in the lineage of Tribrachiatus has been studied in detail by Heckel (1968) and Romein (1979), and has been carefully recorded in many studies. Previous studies have indicated that the FO of T. orthostylus is just slightly below the LO of Tribrachiatus contortus and can be used to approximate the latter datum (Perch-Nielsen, 1985). It was so used in the zonation of Hole 516F material, because T. contortus was not found. The FO of T. orthostylus at Site 516 was recorded within the upper part of Subchron C24R at about 57 Ma (Fig. 9). Studies for the southeastern Atlantic (Shackleton et al., 1984 ), the North Pacific (Monechi et al., 1985) and the Contessa area (Lowrie et al., 1982; Monechi and Thierstein, 1985) also found the FO of T. orthostylus within the upper part of Subchron C24R, but at slightly different levels among the studies, giving an age range
138
from 56.1 to 56.4 Ma. Taking into consideration the uncertainty of variation in sedimentation rates within each site and among different sites, and the different abundance and preservation states of the calcareous nannofossils at different localities, the FO of T. orthostylus appears to be consistent and can be considered geologically synchronous in the mid-latitude areas. The LO of T. orthostyIus at Site 516 falls near the top of Chron C24, clearly lower than those observed in the other studies (Fig. 9). Though most previous studies (Shackleton et al., 1984; Monechi et al., 1985; Backman, 1986) suggested the LO of T. orthostylus to be near the top of chron C23 (around 54.0 Ma ), other stud-
~
Sites Contesssa Bottaccione Latitudes 43 N 43~
577 I 527 32¢N I 28%
528
516
28%
30%
ies (Lowrie et al., 1982; Monechi and Thierstein, 1985) recorded it as low as in Subchron C23R and as high as in Subchron C21R. It should be pointed out that Monechi and Thierstein (1985) misplaced the top of T. orthostylus for the Contessa Highway section in their fig. 6, because a few T. orthostylus were recorded in a higher sample (at 80 m), which was near the top of CP12 (see Monechi and Thierstein, 1985, fig. 7 and p. 429). Those few specimens of T. orthostylus may not be reworked, since Bukry (1973) also reported that this species ranges well into the middle Eocene. Perch-Nielsen {1985) has dashed the range of T. orthostylus up to CP13. It appears now that the LO of T. orthostylus is not a reliable datum for recognizing the NP12/NP13 boundary as Martini ( 1971 ) originally proposed.
Nannotetrina [ulgens
i Fig. 9. Correlation of the FO and LO of Tribrachiatus orthostylus with the magnetic time-scale of Berggren et al. (1985). Data are taken from: Contessa Section, Lowrie et al. (1982) in thinner line and Monechi and Thierstein ( 1985 ) in thicker line; Bottaccione Section, Monechi and Thierstein (1985); Site 577, Backman (1986) in thinner line and Monechi and Thierstein (1985) in thicker line; Site 527, Shackleton et al. (1984); Site 528, Backman (1986) in thinner line and Shackleton et al. (1984) in thicker line; Site 516, this study.
Both Martini (1971) and Okada and Bukry (1980) used the FO of Nannotetrina [ulgens as a zonal marker in the middle Eocene (NP14/ NP15 or CP12/CP13). Perch-Nielsen (1985) suggested use of the FO of the genus Nannotetrina (usually N. cristata) as a substitute marker for poorly preserved material. In Site 516 material, the first Nannotetrina to appear was N. cristata, followed by N. [ulgens 4 m above; the latter was used to define the base of CP13 (NP15). Figure 10 shows that the FO ofN. [ulgens at Site 516 agrees remarkably well with those from other studies, and its age at 49.6 Ma (middle of subchron C21N) can be considered relatively reliable. Site 523 only cored down to Chron C20, and the FO of N. [ulgens could not be determined there (see Poore et al., 1984). The LO of N. [ulgens has so far been correlated with the magnetostratigraphy only at Sites 516 and 523 and the age obtained from Site 516 is younger than that given by Poore et al. (1984) for Site 523. Backman (1987) reported the LO of Nannotetrina spp. at 44.2 Ma (subchron C19R), the same age as the LO ofN. fulgens at Site 516, where N. fulgens was also the last
139 Contesssa BoMaccionz, 527 528 43=N ! 28°S 28°S Latitudes 43=N I
Sites
523
516
28°S
30°S
Sites
~
Contesssa Bottaccion I 523
Latitudes 43°N
43°N
I
516
I 28°S
30°S
I
'7-1,,,
43
~2C
iiI _°'°
3" I m
¢2
Fig. 10. Correlation of the FO and LO of Nannotetrina fulgens with the magnetic time-scale of Berggren et al. (1985). Data are taken from: Contessa Section, Lowrie et al. (1982) in thinner line and Monechi and Thierstein (1985) in thicker line; Bottaccione Section, Monechi and Thierstein (1985); Sites 527 and 528, Shackleton et al. (1984); Site 523, Backman (1987) in thinner line and Poore et al. (1984) in thicker line; Site 516, this study.
Nannotetrina. The age ofLO ofN. fulgens (45.4 Ma) in Berggren et al. (1985) was taken from Poore et al. (1984) and therefore needs to be revised to 44.2 Ma using the new available data.
Chiasmolithus g/gas Chiasmolithus g/gas is easily recognized by its large size (19-27/lm), relatively small central opening and non-split crossbars. The total range of the species has been used by Okada and Bukry (1980) to define CP13b, but the scarcity of this species in most marine sediments has prevented its use in many localities. The FO of C. g/gas at Sites 516 was found within the lower part of Chron C20 (about 47.4 Ma) (Fig. 11). Site 523 terminated in Chron C20 and the FO of C. g/gas could not be determined. The synchroneity or diachroneity of the FO of C. g/gas
Fig. 11. Correlationof the FO ofReticulofenestraumbilica (solid lines) and the FO and LO of Chiasmolithusgigas (dashedlines) with the magnetictime-scaleof Berggrenet al. (1985). Data are taken from: Contessaarea, Lowrieet al. (1982); BottaccioneSection, Monechiand Thierstein (1985); Site523,Backman(1986) in thinnerlineand Poore et al. (1984) in thicker line and dashedline; Site 516, this study.
is difficult to assess at present because it has only been correlated to the magnetostratigraphy at one site (516). The upper range of C. g/gas overlaps with Reticulofenestra umbilica at Site 516 (see Plate III, 11-13), which contradicts at this latitude the low latitude zonation of Okada and Bukry (1980). A similar problem has been encountered in studies of material from Possagno, Italy (Proto Decima et al., 1975) and from the continental rise off New Jersey (Applegate and Wise, 1987), where they could not determine whether R. umbilica ranged too low or C. g/gas ranged too high. From Fig. 11 it is clear that the LO of C. g/gas is time transgressive and that it does range too high at Site 516.
Reticulofenestra umbilica The FO of R. umbilica has been used to mark the CP13/CP14 boundary (Okada and Bukry,
140
1980) and the LO of the species has been widely applied as an early Oligocene marker (Martini, 1971; Okada and Bukry, 1980). However, the use of this species as a marker depends crucially on the separation of this species from the very similar but smaller form termed Reticulofenestra samodurovii ( = Reticulofenestra dictyoda or Reticulofenestra coenura of some authors), which appeared earlier and persisted longer than R. umbilica. Morphometric study of R. samodurovii-R, umbilica in the early development of the lineage by Backman and Hermelin (1986) has demonstrated the rather gradual change in the mean size through time, and they suggested 14 #m as a cutoff for the two species. This size limit is what we have used in this study. The FO of R. umbilica at Sites 516 and 523 and in the Contessa Quarry section (Fig. 11) was located at the top of Chron C20 or near the bottom of Chron C19 at about 44.7 to 44.4 Ma. The level observed in the Contessa Road section appears too low compared with the results from the Quarry section (only 3 km away) and with the South Atlantic sites. Moreover, the entry level of Nannotetrina fulgens was one chron lower in the Road section than that in the Quarry section (see Lowrie et al., 1982 ). Diachroneity cannot account for the difference between these two sections 3 km apart, thus other reasons should be sought. The FO of R. umbilica determined in the Bottaccione section (Monechi and Thierstein, 1985) also appears to be too early and may possibly be due to use of a slightly different species concept. Assuming that the different results from the Contessa Road and Bottaccione sections are due to reasons other than diachroneity, we can consider the FO of R. umbilica as a good datum at 44.6 Ma (bottom of Chron C19) for the mid-latitude marine sediments. Therefore, the age for CP13/ CP14 boundary (46 Ma) in Berggren et al. (1985), which was based on Monechi and Thierstein's ( 1985 ) study of the Contessa Road section, should be revised to 44.6 Ma. The LO of R. umbilica at Site 516 was found in Sample 516F-33-1, 56-57 cm, where 3 speci-
mens ranging from 18 to 20 #m were spotted. Samples right above this level yielded specimens of only 12 #m or less and are not R. umbilica. Compared with its LO observed in other studies (Fig. 12), the level at Site 516 is basically in agreement with those determined for the North Atlantic Sites 558 and 563 and the South Atlantic Sites 522 and 523. The extinction level observed in the South Atlantic Site 528 (Shackleton et al., 1984) appears to be too high and probably is due to use of a slightly different species concept. The LO of R. umbilica probably should be located 2/3 distance down in Chron C12 at about 34.6 Ma. The reliability of this datum is, however, not high because there is a relatively long tail in its abundance near the extinction (see also Backman, 1987).
558
563
37°N
33°N I 26°S 28°S
I 522
523
528
516
28°S
30"S
m
34
35
35
37
38
c,5
Fig. 12. Correlation of the LO of Reticulofenestra umbilica with the magnetic time-scale of Berggren et al. (1985). Data are taken from: Site 558, Miller et al. (1985) in thinner line and Parker et al. (1984) in thicker line; Site 563, Miller et al. (1985) in thinner line and Parker et al. (1984) in thicker line; Site 522, Backman (1987) in thinner line and Poore et al. (1984) in thicker line; Site 523, Backman (1987); Site 528, Shackleton et al. (1984); Site 516, this study.
Rhabdosphaera gladius The range of Rhabdosphaera gladius is believed to be very short, and the LO of this spe-
141
cies has been used to define the top of NP15 (Martini, 1971). Until now the correlation of the top of NP15 with Okada and Bukry's (1980) zonation has not been clear. This is due mostly to the fact that R. gladius only occurs frequently in NW European basin sediments but is very rare or absent in other regions (Martini and Miiller, 1986); and the datum has never been correlated to the magnetostratigraphy before. At Site 516 rare specimens of R. gladius have been found in Sample 516F-61-5, 32-33 cm ( Plate IV, 4, 5 ), within the uppermost part of Chron C19, along with Lanternithus minutus and Reticulofenestra umbilica. This sample is above the LO of Nannotetrina fulgens, which occurred three cores below. The range overlap of R. gladius with L. minutus and with R. umbilica has been reported before (e.g. Berggren and Aubry, 1984). Though the precise FO and LO of R. gladius could not be determined due to the scarcity of this species in Site 516 sediments, its occurrence here can be correlated to the magnetostratigraphy. The LO of this species is clearly shown above the FO of R. umbilica and the LO of N. fulgens and should be 43.8 Ma or younger. Consequently the top of NP15 should not be correlated with the base of CP14a as PerchNielsen ( 1985 ) and Martini and Mtiller (1986) suggested, but should be stratigraphically higher.
Reticulofenestra reticulata
of the Eocene, this datum has been tied to the magnetostratigraphy only at Sites 522 (Backman, 1987) and 516 (the present study) (Fig. 13). Results from these two South Atlantic sites agree remarkably well, giving an age of 37.6 Ma (very bottom of Subchron C15N). Since the LO of R. reticulata has been consistently found slightly below the LO of Discoaster saipanensis in different regions (e.g., Barbados, Falkland Plateau, Site 549 in the Bay of Biscay, Sites 214, 216 and 217 in the Eastern Indian Ocean, Site 282 west of Tasmania, and Site 445 in the Philippine Sea; see Perch-Nielsen, 1986), and the LO of D. saipanensis can be demonstrated as synchronous in the lower and middle latitudes (see discussion below), we can consider the LO of R. reticulata as geologically synchronous also.
Ma
I
°~ E
g I ~
I
Sites 522 Latiludess 26 °S
516 30°S
36 C13 37
38 ¸
39
I
C15
C161
40 C171
Reticulofenestra reticulata has been reported from a variety of upper Eocene marine sediments, from shelf environments (e.g. Siesser, 1983) to pelagic environments (e.g. Gartner, 1974), and from low latitudes (e.g. Bukry, 1977) to high latitudes {e.g. Miiller, 1976; Wise, 1983). It is, therefore, a very promising species for correlation over different marine environments and latitudes. Though a number of authors (Miiller, 1970, 1979; Shafik, 1981; Martini and Miiller, 1986) have suggested the usefulness of the LO of R. reticulata as a datum near the top
41
42 C181 43"
44"
C191
Fig. 13. Correlation of the FO and LO of Reticulofenestra reticulata with the magnetic time-scale of Berggren et al. (1985). Data are taken from: Site 522, Backman (1987); Site 516, this study.
142
A progressive size change of R. reticulata through time has been observed at a number of sites from the South Atlantic and South Indian Ocean (Wei, unpubl, data), with the earlier ones being smaller and often showing an poorly developed central area structure whereas those near the extinction level reach maximum size and show distinct central area structure. This progressive size change may be one reason that the FO of R. reticulata has not been well agreed upon among different studies. A detailed morphometric study of R. reticulata through time (Wei, in prep.) should facilitate finer stratigraphy using quantitative R. reticulata data.
Chiasmolithus solitus There is considerable difficulty in separating Chiasmolithus solitus from Chiasmolithus bidens, which occurs much earlier and overlaps the range of the former species. Therefore the FO of C. solitus as well as the LO of C. bidens are not useful datums. The LO of C. solitus has, however, been applied to define the CP14a/CP14b (NP16/ NP17) boundary (Martini, 1971; Okada and Bukry, 1980 ). Since chiasmoliths are generally rare or absent in tropical waters, the LO of C. solitus may be difficult or impossible to recognize at low latitudes. Furthermore, questions have arisen as to the age of this datum. Wise and Mostajo (1983) found that the Subzone CP14a at Maurice Ewing Bank (South Atlantic) showed an exceptional sedimentation rate (70 m/m.y.) using the age of C. solitus from Bukry ( 1973 ) and suspected that C. solitus persisted longer in higher latitudes than in the tropics where Bukry's (1973) zonation is grounded. This is indeed the case. Calculation of sedimentation rates for several sites south of 40 °, using the age of 42.3 Ma as suggested by Berggren et al. (1985) for the LO of C. solitus, has given enormously high values for CP14a and has resulted in hiatuses right above this subzone (Wei and Thierstein, unpubl, data). This anomaly strongly suggests that the LO of C. so-
l s,,es
.
_~. ,,T
39
523 s,6 Latitudes 28 ~S
30 °S
C1E
40 C17 41
42 C18 43
44
Fig. 14. Correlation of the LO of Chiasmolithus solitus with the magnetic time-scale of Berggren et al. ( 1985 ). Data are taken from: Site 523, Poore et al. (1984); Site 516, this study.
litus in higher latitudes is younger than 42°3 Ma. Site 516 data suggest an age of 41.3 Ma (Fig. 14 ), 1 m.y. younger than that derived from Site 523 (Berggren et al., 1985). The timetransgression of the datum over different latitudes needs further documentation in order to refine the stratigraphy and to solve other paleoceanographic problems. Chiasmolithus grandis The LO of Chiasmolithus grandis is a substitute marker for the FO of Chiasmolithus oamaruensis at the CP14/CP15 boundary of Okada and Bukry (1980). At Site 516 the LO of C. grandis is only one sample lower than the FO of C. oamaruensis, which is rare at Site 516 and not reported at Site 522 (Percival, 1984). The LO of C. grandis was determined within Subchron C17N at Sites 516 and 523 but at different levels {40.0-40.6 Ma) (Fig. 15). In the Contessa area this datum was found within the upper part of Subchron C18N (about 41.6 Ma)
143 Sites Latitudes
a
39
Contessa
43 °N
Bottaccione It 523
516
43 =N
30°S
: 28°S
C16
Chron C17 at Site 516 (Fig. 16). At Site 523 Percival (1984) reported it very rare in one sample, absent in the next four samples up and few in the sixth sample. The age for the FO of C. oamaruensis in Berggren et al. (1985) was based on Percival's (1984) study of Site 523 material and is apparently not reliable.
40 IC17 Sites
~
41
42
C18
Botlaccione
43ON
558 37°N
563 I 33°1J
,,
522 26°S
523 28=S
516 30=S
|1-
T
43
Contesssa
Latituaes 43°N
34" 35£
Fig. 15. Correlation of the LO of Chiasrnolithus grandis with the magnetic time-scale of Berggren et al. (1985). Data are taken from: Contessa Section, Lowrie et al. (1982); Bottaccione Section, Monechi and Thierstein (1985); Site 523, Bacl~m~n (1987) in thinner line and Poore et al. (1984) in thicker line; Site 516, this study.
c1:
37"
Iic,, ai m
3g. =0~
I1ol, liII : ell
|
(Lowrie et al., 1982; Monechi and Thierstein, 1985). Such a lower level may be due to the poor preservation of the calcareous nannofossils in the Contessa area. Martini (1976) examined Sites 317 and 318 from the tropical Pacific, and concluded that the LO of C. grandis occurs consistently above the FO of Isthmolithus recurvus; therefore, he substituted the former for the latter to mark the N P 1 8 / N P 1 9 (see also Martini and Miiller, 1986). His correlation and results have been in conflict with most other studies, including the present one. Chiasrnolithus oamaruensis
Though the FO of C. oamaruensis has been used both in Martini's (1971) and Okada and Bukry's (1980) schemes to mark the NP17/ NP18 (CP14a/CP14b) boundary, the species is usually rare or absent in lower and middle latitude areas, and its application as a marker species is difficult in most cases. Only rare specimens of C. oamaruensis have been found in two adjacent samples near the middle of
Fig. 16. Correlation of the FO and LO of Chiasmolithus oamaruensis with the magnetic time-scale of Berggren et al. (1985). Data are taken from: Site 523, Poore et al. (1984); Site 516, this study.
Isthmolithus recurvus
The FO of Isthmolithus recurvus has been used as a zonal marker in the zonations of Martini (1971) and Okada and Bukry (1980), whereas the LO of I. recurvus was applied as a provincial zonal marker in the Oligocene sediment from the Falkland Plateau (Wise, 1983). The species is distinct but is easily subject to overgrowth and in that case different names may have been used by some authors (Martini, 1973). Isthmolithus recurvus is rare or absent in low-latitude areas (Bukry, 1978), more abundant in mid- a n d high-latitude sediments and has even been found in Antarctic shelf sediments (Wei et al., 1988). It is thus believed that the FO and LO of this species is only applicable
144
in the mid- and high-latitudes but not a useful datum in low-latitudes. The FO of/. recurvus in Hole 516F falls near the very bottom of Chron C16, in good agreement with the result from Site 523 (Backman, 1987) (Fig. 17). In the Contessa section and Bottaccione section Monechi and Thierstein (1985) found the base of I. recurvus to be just at the top of Chron C16, considerably higher than those at Sites 516 and 523. It appears that results from Sites 516 and 523 provide a better age estimate (39.5 Ma). The age used in Berggren et al. (1985) (37.8 Ma) was taken from Site 522 (Poore et al., 1984) and is apparently too young. Isthmolithus recurvus decreased from common within Subchron C13R to rare near the bottom of Subchron C13N in Site 516. This is basically the same pattern as observed at Sites 522 and 523 (Backman, 1987) and in the Contessa section (Monechi, 1986). The LO of I. recurvus at South Atlantic Sites 516, 522, and 523 and North Atlantic Sites 558,
~
Sites 523 Latitudes 28 °S
clE c1E c17
516 30°S
I
I
Fig. 17. Correlation of the FO and LO of I s t h m o l i t h u s rex c u r v u s with the magnetic time-scale of Berggren et al. (1985). Data are taken from: Contessa Section, FO-Monechi and Thierstein (1985), LO-Lowrie et al. (1982); Bottaccione Section, Monechi and Thierstein (1985); Sites 558 and 563, Miller et al. (1985) in thinner line and Parker et al. (1985) in thicker line; Site 522, Backman (1987) in thinner line and Poore et al. (1984) in thicker line; Site 523, Poore et al. (1984); Site 516, this study.
563 and the Contessa area are all within the lower part of Subchron C12R and it seems to be a good datum at 34.8 Ma for the mid-latitudes.
Discoaster saipanensis
barbadiensis
and
Discoaster
The Eocene/Oligocene boundary in terms of calcareous nannofossil stratigraphy is conventionally defined by the extinction of Discoaster barbadiensis and D. saipanensis (Berggren et al., 1985). However, several studies have shown that the boundary determined by these calcareous nannofossil datums is slightly older than that by the planktonic foraminifera datum (the last occurrence of Globorotalia cerroazulensis), and the time difference is estimated to be 0.4 m.y. at DSDP Site 592 (Martini, 1986; Martini et al., 1986). In Site 516 material the LO of D. saipanensis was found slightly above that of D. barbadiensis (Fig. 18), as was observed at Site 522 (Poore et al., 1984). This succession has also been observed in many other areas (Monechi, 1986). Where the extinction of D. saipanensis is found at the same level as D. barbadiensis, such as Sites 354 and 361 and Barbados (Beckman et al., 1981 ), a hiatus (or sampling gap) is believed to be the cause. From Fig. 18 it can be seen that the LO of D. saipanensis is a relatively reliable datum near the middle of Subchron C13R (about 36.5 Ma) which can be considered as geologically synchronous over the mid-latitudes. Discoaster barbadiensis and D. saipanensis are easily identified even when heavily overgrown. But like other discoasters, they decrease in abundance towards higher latitudes and their LO's have not been found useful for the Eocene/Oligocene boundary south of 60 ° (Wei and Wise, in prep.; Wei and Thierstein, in prep. ).
Ericsonia subdisticha Okada and Bukry (1980) used the end of the Ericsonia subdisticha acme to define the lower boundary of CP16b and the LO of Coccolithus
145 Sites
I°11o
Contessa !
522
528
516
28°S
30°S
i
Latitudes 4 3 ° N
I 26°S I I
m--
33
~ •
Sites Latitudes
Contessa 558
43°N
37°N
563 I 522
528
523
516
33=NII 26°S I I
28°S
28°S
30 oS
33.' • c12
:
Cl;
34;
34" 35
35"
3e
36 C1:
37 38
.: I IIIIIII I I
E
|B-
m
c1~ ~
Fig. 18. Correlation of the LO's of Discoaster barbadiensis (thicker lines) and Discoaster saipanensis (thinner lines) with the magnetic time-scale of Berggren et al. (1985). Data are taken from: Contessa Section, Lowrie et al. (1982); Site 522, Poore et al. (1984); Site 528, Shackleton et al. (1984); Site 516, this study.
formosus to define the upper boundary. At Site 516 the end of E. subdisticha acme coincided with the LO ofC. [ormosus, whereas at Sites 522 and 523 the former datum was stratigraphically higher than the latter (Backman, 1987), the reversal of the Okada and Bukry {1980) sequence. Since the LO of C. formosus can be shown below to be synchronous in the mid-latitudes, the end of E. subdisticha acme must be diachronous in different regions.
Coccolithus formosus The LO of Coccolithus formosus has been widely used to define the CP16b/CP16c (NP21/ NP22) boundary. This species is very distinct and can be identified even in poorly preserved assemblages (Wise, 1973). The currently available magnetobiostratigraphic data (Fig. 19) for the LO of C. formosus show remarkable agreement. It occurs near the bottom of Subchron C12R at 35 Ma. It can now be concluded that the extinction of C. formosus is geologically synchronous and is an excellent marker in the lower and mid-latitudes.
Fig. 19. Correlation of the LO of Coccolithus [ormosus with the magnetic time-scale of Berggren et al. (1985). Data are taken from: Contessa Section, Lowrie et al. (1982); Sites 558 and 563, Miller et al. (1985) in thinner lines and Parker et al. (1985) in thicker lines; Site 522, Backman (1987) in thinner line and Poore et al. (1984) in thicker line; Site 528, Shackleton et al. (1984); Site 523, Poore et al. (1984); Site 516, this study.
Sphenolithus distentus The FO of Sphenolithus distentus was used as a zonal marker for CP17/CP18 boundary in Okada and Bukry (1980) but not in Martini (1971). At Site 516 the FO ofS. distentus within Subchron C13N (Fig. 18) overlaps with the upper ranges of Reticulofenestra umbilica and Coccolithus formosus and is therefore in conflict with Okada and Bukry's (1980) zonation. The FO of S. distentus at Sites 522 (Percival, 1984 ) and 563 (Miller et al., 1985) and in the Contessa section (Lowrie et al., 1982) all were located within the lower part of Subchron C12R but at different levels in different sites (Fig. 20). Parker et al. (1985) reported the lower occurrence of the species at Site 563 as contamination based on the presence ofR. umbilica and C. formosus. After comparing the data from other sites, we can now determine that they were not contaminants. The age overlap problem has been reported in the studies for the Eastern U.S. (Bybell, 1982, 1983; Siesser, 1983) and Barbados (Perch-Nielsen, 1986). It is clear now that the FO of S. distentus is not a reliable datum.
146 Sites
Latitudes
Contessa 43°N
558 37 ° N
563 I 522 33 a NII 26 ° S I
528 2F.° S
516 30 °S
23-
!m-mm
24-" cxm 25;
_ 2S-
C7
27: ;
¢TAco
2a-'
30:
32:
! 332• 34"
C11
Sphenolithus ciperoensis
--
c~2
•
I
3S.E :
]
species near its upper range, especially outside tropical areas. The LO of Sphenolithus predistentus was closely associated with the LO of S. distentus at Sites 516 and 522 (Percival, 1984), 558 (Parker et al., 1984) and 588 (Martini, 1986). Correlation with the magnetostratigraphy at both Sites 516 and 522 has placed the LO of S. predistentus within the upper part of Subchron C9N (about 28.2 Ma). Therefore the LO of S. predistentus may be a good substitute for that of S. distentus as a datum because the former species is usually more abundant and its extinction level is easier to locate.
~ C13
37:
i Ig,o,-~ Fig. 20. Correlation of the FO and LO of Sphenolithus distentus with the magnetic time-scale of Berggren et al. ( 1985 ). Data are taken from: Contessa Section, Lowrie et al. (1982); Sites 558 and 563, Miller et al. (1985) in thinner lines and Parker et al. {1985) in thicker lines; Site 522, Poore et al. (1984); Site 528, Shackleton et al. (1984); Site 516, this study.
The LO of S. distentus defines the CP19a/ CP19b (NP24/NP25) boundary (Martini, 1971; Okada and Bukry, 1980). The currently available data show that this datum falls at different positions (as low as in Subchron C10N and as high as in Subchron C7N) in different areas (Fig. 20). Though Berggren et al. ( 1985 ) placed the LO of S. distentus at the uppermost part of Subchron C9N at Site 522, the species occurred sporadically up to Subchron C7N (see Percival, 1984, p. 412). The large discrepancy among the different studies casts doubt on the reliability and accuracy of this datum. One reason may be the general low abundance of the
The FO of Sphenolithus ciperoensis marks the lower boundary of Sphenolithus distentus Zone (CP19 or NP24). The species first occurs in low abundance and is sporadic at Site 516. The entry level is very close to that in the Contessa section (Lowrie et al., 1982), but much higher than those observed at Sites 522, 528, 558 and 563 (Fig. 21 ). The FO of S. ciperoensis at the latter sites does not agree well either, with two studies of the same site (558) yielding an age difference of about 1 m.y. It is apparent that the use of this datum for the recognition of CP18/ CP19 (NP23/NP24) boundary is difficult and not reliable. The age of the datum in Berggren et al. (1985) was based on data from Site 522 and should be used with caution. The LO of S. ciperoensis (top of CP19 or NP25) is one of the most commonly used datums to approximate the Oligocene/Miocene boundary. Sphenolithus ciperoensis, like other species of the genus, preferred warm waters and is rare or absent in high-latitude areas. Except for the rare occurrence of S. ciperoensis recorded in one sample in the upper part of Chron C6 at Site 522, which may well be reworked or contaminated, the LO of S. ciperoensis from six sources of studies for six sites over different ocean basins and hemispheres falls near the middle of Chron C7 (Fig. 21 ). It can be consid-
147 Sites Contessa 558 563 I 522 528 516 Latitudes 43 °N 37°N 33°N! 26°S 28°S 30°S I
I I
"t__=,
241 26 i
25
-
CeC
m i
28
29
C7
m
eB B i
~
~
,olml-
I
C1C
~']i
11
]
T
--
m m
°
Fig. 21. Correlation of the FO and LO of Sphenolithus ciperoensis with the magnetic time-scale of Berggren et al. (1985). Data are taken from: Contessa Section, Lowrie et al. (1982); Sites 558 and 563, Miller et al. (1985) in thinner lines and Parker et al. (1985) in thicker lines; Site 522, Poore et al. (1984); Site 528, Shackleton et al. (1984); Site 516, this study.
ered geologically synchronous (25.9 Ma) in the mid-latitudes.
dant Z. bijugatus up to the middle of Subchron C6CR and sporadic occurrences up to Chron C6A at Site 558. The sporadic occurrence may be considered as reworked, since Miller et al. (1985) studied the same material and placed the top of Z. bijugatus near the middle of Subchron C6CR, basically in agreement with the results from other sites. Percival (1984) reported sporadic occurrences of Zygrhablithus? sp. in a few samples in the Oligocene sediment at Site 522. Since he did not list Z. bijugatus, we assume that this Zygrhablithus? sp. is Z. bijugatus. The general scarcity of the species at that site can be accounted for by the water depth (over 4400 m), but the very abundant occurrence of the species in a few samples is probably related to fluctuations in productivity of the species.
558 563 37°14 33°N
516 3o°s
.:li__
-1=
25
i
~
Zygrhablithus bijugatus Along with the LO ofReticulofenestra bisecta, the LO of Zygrhablithus bijugatus has been used to define the Oligocene/Miocene boundary by some authors (e.g. Edwards, 1971; Edwards and Perch-Nielsen, 1975). Zygrhablithus bijugatus is a holococcolith and thus is usually not preserved in deep water sediments (Perch-Nielsen, 1985). It is also rare or absent in cool or cold waters (Wei and Wise, in prep.), probably due to preservation as well. On the other hand, Z. bijugatus is easily subject to overgrowth, for reasons poorly known at present. The top of Z. bijugatus at Sites 516 and 563 is relatively sharp at 24.7 Ma (in the middle of Subchron C6CR) (Fig. 22). Parker et al. (1985) reported abun-
Fig. 22. Correlation of the LO of Zygrhablithus bijugatus with the magnetic time-scale ofBerggren et al. (1985). Data are taken from: Sites 558 and 563, Miller et ell. (1985) in thinner lines and Parker et al. (1985) in thicker lines; Site 516, this study.
Reticulofenestra bisecta There are still disagreements among nannofossil experts on the use of the genera Dictyococcites and Reticulofenestra. For practical reasons we prefer to place Dictyococcites in synonym with Reticulofenestra and use here Reticulofenestra bisecta. There are some morphologic variations in the species of R. bisecta.
148
One type has a central area only partially closed by laths, revealing a perforated plate below when viewed distally. The other type has central area totally closed. Wise and Wiegand (in: Wise, 1983) erected the former as Reticulofenestra bisecta filewiczii and the latter, Reticulofenestra bisecta bisecta. We have recorded these two subspecies separately in our range chart and the LO of R. bisecta bisecta is below that of R. bisecta filewiczii, within Subchron C7R. Since most previous work did not differentiate the two subspecies, we take the LO of R. bisecta filewiczii as that of R. bisecta at Site 516 when comparing it with those from other studies. It is apparent from Fig. 23 that the LO of R. bisecta is geologically synchronous among the South Atlantic sites (516 and 522) and North Atlantic sites (558 and 563). The exit of R. bisecta at a lower level in Contessa Quarry section may not represent the true extinction because of poor preservation of the nannofossil assemblages there or due to other reasons. Biolzi (1985) made a detailed study of the Oligocene/Miocene boundary in many Atlantic, Mediterranean and Paratethyan sections. She found that in most sections examined the LO of R. bisecta was stratigraphically higher than the LO's of Heliocosphaera recta, Sphen-
Sites
Contessa
Latitudes 43°N
558
37°N
563
I
522
°' 33 N! 26°S
=
516
30°S
I
24
25
26
Fig. 23. Correlation of the LO of Reticulo[enestra bisecta with the magnetic time-scale of Berggren et al. (1985). Data are taken from: Contessa Section, Lowrie et al. (1982); Sites 558 and 563, Miller et al. (1985) in thinner lines and Parker et al. (1985) in thicker lines; Site 522, Poore et al. (1984); Site 516, this study.
olithus ciperoensis and Zygrhablithus bijugatus and in the other sections the LO of R. bisecta was at the same level with one or several of the latter datums. Each of these datums or a combination of these datums have been used to mark the Paleogene/Neogene boundary in different areas by different authors (see Biolzi, 1985; Berggren et al., 1985 for detailed discussion ). Mid-latitudes sediments appear to contain more abundant R. bisecta, whereas tropical areas seem not so favorable to R. bisecta (Roth and Thierstein, 1972; Perch-Nielsen, 1977; Biolzi, 1985 ). High-latitude sites also yield less abundant or rare R. bisecta (Wise, 1983; Wei and Wise, in prep. ). It is believed that the LO of R. bisecta is more reliable in approximating the Oligocene/Miocene boundary in mid-latitude sediments.
Concluding remarks The abundant, moderate to well preserved calcareous nannofossils in the expanded Paleogene section from Site 516 has enabled the relatively reliable and precise correlation of most traditional and nontraditional datums with the magnetostratigraphy. Critical evaluation of the currently available magnetobiostratigraphic data has revealed the accuracy, the reliability, the synchroneity and diachroneity of Paleogene calcareous nannofossil datums over geographically long distance. A summary of the results is presented in Table II. The detailed calcareous nannofossil stratigraphic results of this study and the evidence for the apparently complete upper Eocene-Oligocene section with a rather uniform sedimentation rate at this site should be valuable for many kinds of future studies, such as detailed taxonomic evolutionary studies, paleoclimatic and paleoceanographic studies.
149 TABLE II Summary of biochronological properties of Paleogene calcareous nannofossil species events
Species events
Ages (Ma)
Reference sites
LO R. bisecta LO Z. bijugatus LO S. ciperoensis LOS. distentus LO H. recta LO C. altus FO S. ciperoensis FO S. distentus LO R. umbilica FO C. abisectus LO/. recurvus End acme E. subdisticha LO C. formosus LO D. saipanensis LO D. barbadiensis LO R. reticulata FO/. recurvus FO C. oamaruensis LO C. grandis LO N. dubius LO C. solitus FO R. bisecta FOR. daviesii FOR. reticulata LO R. gladius LO N. fulgens LO C. gigas FOR. urnbilica FO C. gigas FO N. fulgens LOT. orthostylus FO D. multiradiatus FO D. rnohleri FO H. kleinpeUii FO F. tympaniformis FOE. rnacellus FO P. martinii FO C. danicus FOP. tenuiculum FO C. tenuis FO C. primus FO Z. sigrnoides
24.0 24.7-25.0 25.9 25.9-30.0 26.3 27.0 28.5-31.9 34.0-35.6 34.6 34.2 34.8-35.2 34.4-35.0 35.1 36.4 36.7 37.6 39.5 39.8-40.4 40.0-41.6 40.6 41.3 42.5 43.6 43.7 > 43.8 44.2 44.6-46.8 44.6 47.4 49.8 51.0-54.8 59.2 60.5 59.8-61.6 62.0 65.5-66.0 65.5-66.0 65.5-66.0 65.8 65.9 66.3 66.4
516, 522, 558, 563 516, 558, 563 516, 522,528, 558, 563, Contessa 516, 522, 528, 558, 563, Contessa 516 516 516, 522, 523, 558, 563, Contessa 516, 522, 558, 563, Contessa 516, 522,558, 563 516 516, 522, 523,558, 563 516, 522, 523 516, 522, 523, 528, 558, Contessa 516, 522 516, 522,528, Contessa 516, 522 516, 523 516, 523 516, 523 516 516 516 516 516 516 516, 523 516, 523 516, 523, Contessa 516 516. 527, 528, Contessa 527, 528, 577 516, 524, 527, 528, Contessa 516, 527, Contessa 516, 527, 528, 577, Contessa 516, 527, 528, 577, Contessa 516, Contessa 516 516 516 516, 525, 527, 528 516 516, 524
Remarks
consistent unreliable unreliable unreliable size > 14/Lm size > 8/~m diachronous very consistent
very distinct unreliable
diachronous
diachronous size > 14/~rn
unreliable diachronous? unreliable
FO denotes first occurrence and LO denotes last occurrence. Ages are given in the magnetic time-scale of Berggren et al. (1985). Reference site numbers are DSDP sites.
Acknowledgements A grant from the Hattner Fund made it possible for the first author to participate in the 1988 International Nannoplankton Association Meeting in Shanghai, China, where the major results of this study were presented. Discussion with Marie-Pierre Aubry, Katharina Perch-Nielsen and Tom Romein was helpful.
We thank John Firth for reviewing the manuscript. This study was supported in part by NSF grant DPP 8414268 to Wise. Wei has been supported by a Florida State University Dissertation Fellowship. Samples were provided by the National Science Foundation through the Ocean Drilling Program. Kim Riddle assisted in the SEM photography and Lei Shi helped to prepare the plates and figures.
150
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