oligocene boundary

Palaeogeography, Palaeoclimatology, Palaeoecology, 36 (1981): 223--248 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

THE GEOLOGICAL BOUNDARY

223

EVENTS AT THE EOCENE/OLIGOCENE

CLAUDE CAVELIER 1, JEAN-JACQUES CHATEAUNEUF 1, CHARLES POMEROL 2, DOMINIQUE RABUSSIER 3, MAURICE RENARD 2 and COLETTE VERGNAUDGRAZZINI 3 1B.R.G.M., B.P. 6009, 45060 Orleans Cedex (France) :Laboratoire de G~ologie des Bassins S~dimentaires, Universit~ Paris VI, 4 place Jussieu, 75230 Paris Cedex 05 (France) 3Laboratoire de G~ologie Dynamique, Universit~ Paris VI, 4 place Jussieu, 75230 Paris Cedex 05 (France) (Received April 8, 1981)

ABSTRACT Cavelier, C., Ch~iteauneuf, J.-J., Pomerol, C., Rabussier, D., Renard, M. and VergnaudGrazzini, C., 1981. The geological events at the Eocene/Oligocene boundary. Palaeogeogr., Palaeoclimatol., Palaeoecol., 36: 223--24t~. At the end of the Eocene, series of geological events occurs on a worldwide scale, of which the intensity is comparable to the one occurring at the Cretaceous/Tertiary boundary. Oxygen and carbon isotopic variations in biogenic carbonates indicate a sudden drop in sea-water temperature as well as a modification of oceanic chemistry, which is corroborated by trace element analyses of the same carbonates. Simultaneously the C.C.D. drops and the oceanic sedimentation rate is considerably reduced. Many genera disappear (foraminifera, 13annoplankton, dinoflagellates), while on the continents the evolution of the microflora indicates a general cooling. Many groups among mammals become extinct before the Oligocene faunal renewal (the "break" of Stehlin). There is an important regression which explains that continuous sections are rare at the Eocene/Oligocene boundary. Gaps are c o m m o n in the deep oceanic domain. These dramatic events started in Europe at the boundary between the Priabonian, the Ludian, the Lattorfian or the Early Tongrian and the Stampian, the Rupelian or the Late Tongrian. INTRODUCTION The geological events which took place at the Cretaceous/Tertiary bounda r y are n o w w e l l d o c u m e n t e d (Symposium on the Cretaceous/Tertiary Boundary Events, Copenhagen, 1979). They include the massive extinction a n d t h e first a p p e a r a n c e o f n u m e r o u s species, a d e c r e a s e i n o c e a n i c sedimentation rates which affects carbonates to a greater degree than clay m i n e r a l s a n d w h i c h is r e s p o n s i b l e f o r n u m e r o u s h i a t u s e s , a g e n e r a l c l i m a t i c c o o l i n g a n d a d e c r e a s e i n t h e 5 13C o f m a r i n e c a r b o n a t e s ( L e t o l l e a n d R e n a r d , 1980). 0031--0182/81/0000--0000/$02.75 © 1981 Elsevier Scientific Publishing Company

224 In contrast to the very obvious break at the Maastrichtian/Danian boundary, that between the two periods of the Tertiary seems to be much more uncertain. During the 7th International Congress of the Regional Committee on the Stratigraphy of the Mediterranean Neogene, a proposal was made to place this boundary in the time interval between the beginning of Blow's P 22 Zone (Globigerina angulisuturalis zone) and the top of Zone N 5 (Globoquadrina dehiscens zone) (Circular No. 5, January, 1980) with the recommendation " t h a t it should be put near to or at the base of Martini's NN1 standard calcareous n a n n o p l a n k t o n zone". This proposition in which the "critical interval" is two biozones in duration -- that is, around 4--6 m.y. -- is still a matter of controversy among geologists. For Anglada (1978), for instance, the most suitable limit is at the top of Zone 19 (Pseudohastigerina barbadiensis zone). It seems impossible to consider seriously a limit on which there is disagreement to the extent of five zones and 10 m.y. Our purpose here is to point out that a much more distinct break occurs at the top of the Eocene. Most authors do not seem to have " d a r e d " to assert t h a t this was the main break within the Tertiary. Our proposal is, no doubt, an iconoclastic one, but is supported by numerous disciplines: palaeoclimatology, palaeontology, sedimentology, geochemistry, oceanography, etc. We propose to analyse the different geological events marking this break, but, first of all, we should like to point out the following methodology: if we compare how the specialists react in defining these stratigraphical limits, we see that those who examine the Oligocene/Miocene break follow the axiom " T h e Palaeogene/Neogene boundary must be placed within the time interval base of P 22--top of N 5" (W.G. on the Palaeogene/Neogene Boundary, Circular No. 5), whereas those who study the Eocene/Oligocene limit say: "We agree that there is a break, right enough, let us try to place it on the stratigraphical scale". THE CONTINENTAL ENVIRONMENT

Fauna The Swiss paleontologist Stehlin (1909) was the first to demonstrate the existence of an important break, the Grande Coupure, within the faunal succession of land mammals of the European Palaeogene. Established basically on the evolution of the larger mammals, the Grande Coupure lies stratigraphically between the Ludian (= Priabonian) and the Stampian (including the Sannoisian facies) in Western Europe (Cavelier, 1979). During the Tertiary, other " b r e a k s " have affected the mammals. Some of these are very important (for instance, those at the Paleocene/Eocene limit and at the beginning of the Burdigalian and of the Tortonian), but with the possible exception of the Paleocene/Eocene limit, none of these is as import a n t as the Grande Coupure. The Grande Coupure p h e n o m e n o n which has been re-examined in detail by Brunet (1979) and Cavelier (1979) has also been observed in the rodents

225 (Hartenberger, 1973; Vianey-Liaud, 1979) and in the reptiles (De Bonis et al., 1973; Rage, 1976). In Western Europe the Grande Coupure is shown mainly by the extinction of Eocene forms adapted to local conditions and their replacement by immigrants from the beginning of the Oligocene onwards. It is also n o t e w o r t h y that the mammal fauna shows (if we exclude the Paleocene) its least diversification in the Palaeogene, immediately on both sides of the Eocene/Oligocene boundary. In this sense it is a true "catastrophe", the extinctions having begun in a striking fashion before the end of the Eocene, in the Late Ludian, whereas the following renewal which produced a new fauna was still relatively undiversified. The Grande Coupure p h e n o m e n o n is not specifically European. While insufficiently examined in Asia, it is clearly apparent and very comparable in its effects in North America (Wilson, 1972). While the occurrence of immigration of Asiatic origin at the very beginning of the Oligocene in Western Europe and North America necessarily points to major palaeogeographical upheavals at the Eocene/Oligocene boundary ("bridges" over the Uralian Sea and the Bering Strait), the extinctions which occurred before or after this immigration and the low diversity of the first immigrants resulted from the separate p h e n o m e n o n of climatic deterioration, which seems to be proved by the evolution of continental associations through the Palaeogene.

Flora Floral evolution occurred in two of three stages in the Paris Basin and corresponds to the progressive replacement of the Late Cretaceous tropical flora by the m o r e temperate one of the Late Oligocene and the Neogene. As a result, tropical species including Normapolles and subtropical species were progressively replaced by the Arcto-Tertiary species which, as a group, consist of temperate forms. This diminution of the tropical flora is very progressive through the Paleocene and later through the Eocene, and no real discontinuity in the catastrophic sense of the word exists. The percentages of Normapolles in the Thanetian and the Sparnacian are still high but decrease considerably during the Cuisian. The Lutetian palm flora gradually replaces the Normapolles flora before being replaced in its turn by the subtropical association of the Late Eocene. At the end of the Ludian on the other hand (the level of the Marnes blanches de Pantin), most of the species present in the Late Eocene disappear owing to a climatic deterioration which may have been very rapid. The latter is announced by certain fluctuations at the base of the Upper Ludian, above all by the appearance of conifers, such as Tsuga, Picea or Sciadopitys. During the Early Stampian, a temporary climatic improvement allows certain taxa to be re-introduced. Some of these persist up to the following climatic deterioration, which occurred in the Late Oligocene. The curve of the variations in the absolute number of species

226

encountered in the Paris Basin formations (Ch~teauneuf, 1980) shows clearly the p r o f o u n d modifications of the vegetation cover at the Eocene/ Oligocene boundary (Fig.l). The average number of species present increases from 90 to 270 in the Late Eocene, but falls to 50 in the Marnes blanches before rising to 127 in the Stampian. Similar studies carried out on the European or American flora completely confirm these evolutionary tendencies. For instance, Reid and Chandler (1933), by comparing the percentages of ligneous plant genera through time, come to the conclusion, from the study of the macroscopic remains in England, that the most important evolution (decrease of the ligneous plants) t o o k place b e t w e e n the Early Eocene and the Late Eocene. Similarly Dorf (1963), comparing the Western European floras with that of the United States, concluded that at first the two floras had a similar evolution, but that later the tropical flora began to modify into a temperate one at the Eocene/ Oligocene limit. Mai (1964), discussing all the k n o w n elements of the Western European flora, showed that the holarctic and European temperate forms rapidly increased in importance from the beginning of the Oligocene. Schwarzbach (1968) even gave figures on the basis of the climatic requirements of certain taxa. For him the Eocene/Oligocene limit represents a passage through the 20 ° annual isotherm from higher temperature values to lower ones. Also in Europe, Krutzsch (1967), on the basis of the microflora, noted an increase in the proportion of the temperate flora, which is typical of the mid-Tertiary at the limit between the Eocene and the Oligocene. The pattern of evolution of the Northern Hemisphere flora leads, therefore, to the conclusion that at the Eocene/Oligocene boundary there was a general drop of temperature accompanied by a period of great drought which was associated with the disappearance or regression of most of the tropical species which were very widespread during the Late Cretaceous and Eocene (Ch~teauneuf, 1980). In the lacustrine and shallow-water environments, in spite of the fact that continental waters or water with a low salinity from shallow to very shallow depths act as a " b u f f e r " , the b o u n d a r y between the Ludian and Stampian is nevertheless well marked both in the molluscs (Cavelier, 1979) and in the algae (Characeae, Riveline, 1973) in relation to temperature. In b o t h cases most of the existing forms become extinct in the Late Ludian or, those which appeared later, at the beginning of the Stampian (Sannoisian facies). Relatively few species appear in the Early Stampian.

Fig.1. Total microfloral species numbers and inferred palaeoclimatic curve for the Late Eocene and Oligocene in the Paris Basin (after Chfiteauneuf, 1980).

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228 NERITIC ENVIRONMENT

Fauna and flora The catastrophic aspect associated with the Eocene/Oligocene boundary is no less important for the population ( b o t h fauna and flora) of the continental shelf. This p h e n o m e n o n is very striking in the large benthic Tethyan foraminiferids (s.1.). Well represented in the Priabonian, Discocyclina, Asterocyclina,

Aktinocyclina, Asterodiscus, Lockhartia, Fabiana, Gyroidinella, Orbitolites, Spiroclypeus gr. granulosus, Pellatispira madaraszi, Chapmanina gassinensis, etc., disappear just before the Eocene/Oligocene limit. Only t w o species of the large group of Priabonian Nummulites continue into the Stampian, while only N. intermedius and N. vascus make their first appearance. Somewhat later the lepidocyclinids appear. Although less obvious, the break is also to be seen in the small benthic foraminifera, at least at a regional level (Le Calvez, 1970) and in the ostracods, which Keen (1972) considered as a major important faunal modification at the beginning of the Stampian in Northwest Europe. In this same reference region, the combined Stampian and Rupelian mollusc faunas are much less diversified than that Of the Latdorfian (= Priabonian) of northern Germany and Belgium: of a b o u t 785 Latdorfian species no more than 75 occur also in the Stampian--Rupelian fauna, which are estimated at a b o u t 450 species (including the Parisian Stampian fauna with over 300 species). The study of the geographical distribution of European malacofauna during the Eocene (Chavan, 1946; Cavelier, 1979) shows that, after an almost exclusively S--N trending migration period indicating a warming up of the marine waters during the Early--Middle Eocene, a certain balance is reached during the Late Eocene and that the migration becomes distinctly N--S directed (= cooling of the waters) at,the very end of the Eocene (Late Priabonian). The Eocene/Oligocene break is seen both in the echinoderms of the Echinolarnpas group (Roman, 1977) and in the Madreporaria of the Mediterranean region (Barta-Calmus, 1977) (Figs.2 and 3). In the last two examples, a very sharp decrease in the number of species or genera marks the passage from the Eocene to the Oligocene, the diversity becoming high again during the Oligocene, however. Nevertheless, this is not always the case for groups who se diversity was already decreasing in the Eocene, like the Algae (Poignant, 1974; Deloffre and Genot, 1979) (Fig.4).

Palaeoternperatures Biological data At the present time, the 18--20°C January isotherm (tropics) corresponds approximately with the limit of occurrence of reef-building corals, larger

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Fig.2. Percentage of fossil Echinolampas species during the various periods of the Cenozoic. Single species numbers are shown by bold lines, ordinates on the left; cumulated species numbers are shown by dashed lines, ordinates on the right (the scale is half of that of the single numbers). Maximum diversity occurs during the MiddLe Eocene and Miocene, decrease usually occurring afterwards. Diversity increase was higher during the Middle Eocene (after Roman, 1977).

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foraminifera and mangroves in the marine environment. It is noteworthy that the 18--20°C isotherm, if defined as above, is located, in the middle of the Priabonian, at about latitude 50°N (Priabonian with Hantkenina, Globorotalia cerroazulensis, Nummulites fabianii). Towards the middle of the Early Stampian the same isotherm was at latitude 45°N (Cavelier, 1979). At the present day, this latitude difference corresponds to a 2.5°C temperature

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difference in the surface waters of August. The cooling of the marine waters at the very base of the Stampian was (probably) greater than the average difference of temperatures noted above and could have been up to 4 or 5°C.

Isotopic data Up to now, isotopic results published by different authors are from the Pacific Ocean (New Zealand, Australia) and the North Atlantic (Northern Europe). In these two oceans, the oxygen isotopic composition of the biogenic carbonates increases by more than 1% at the Eocene/Oligocene limit. This must correspond to a temperature drop of at least 4°C. In fact, the size o f the observed shift is difficult to interpret, since local phenomena may be superimposed on the actual temperature drop; moreover, the analysed material may be inhomogeheous and altered by diagenesis, etc. Indeed, the epicontinental marine fauna reacts very quickly to climatic variations but is directly subjected to the p h e n o m e n a of dilution, desalinisation, even confinement (e.g., in the Paris Basin). Very few isotopic analyses have been carried o u t on neritic fossil faunas and these are usually only related to molluscs (bivalves, gastropods). The bivalves precipitate their test carbonate in equilibrium with the surrounding sea water (Epstein et al., 1953; Buchardt, 1978), which makes it easier for analysis than foraminiferal tests. However, the results obtained b y the various authors mentioned above show that the isotopic compositions display large variation between different samples taken at the same horizon, and from a single population. This supports the suggestion that a metabolic fractionatign might also exist in the molluscan shell. Pacific. Dorman (1966) analysed a mollusc fauna from Australia (38°S). The Eocene/Oligocene boundary is indicated by three Notostrea tarda samples

231

stated to be of Eocene age and three of Oligocene age with no other stratigraphical details. As between the Eocene and the Oligocene, these samples show a temperature drop ranging from 4 to 7°C (T = 16.5°C in the Eocene, and 9.5°C < T < 12°C in the Oligocene). North Atlantic. Paris Basin: T h e n u m e r o u s analyses m a d e on mollusc shells (Letolle et al., 1 9 6 5 ; Tivollier and L e t o l l e , 1968) c a n n o t be i n t e r p r e t e d in t e r m s o f p a l a e o t e m p e r a t u r e and s h o w t h e i n f l u e n c e of the large rivers feeding t h e Parisian G u l f at t h e beginning o f t h e T e r t i a r y . T h e average 1%0 differe n c e o f 5~80 observed b e t w e e n the L u d i a n and S t a m p i a n deposits indicates t h e passage f r o m a lake, or at least a brackish facies, t o a m o r e t y p i c a l l y marine facies. Aquitaine: T h e Biarritz ( C a c h a o u ) section also c o r r e s p o n d s t o a coastal e n v i r o n m e n t deposit. Analysis (Rabussier-Lointier, 1980) o f the bulk carbonates (Fig. 5) shows a +2.5%o d i f f e r e n c e of ~ ~ O b e t w e e n t h e E o c e n e and Early Oligocene levels. 8 ~ B 0 % o vs F~D.B +2

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232

Northern Europe: Buchardt (1978) analysed mollusc shells from stratigraphical sequences in England, Holland, Germany, Denmark, and the south of Sweden. He found a high 5180 increase from the Lutetian to the Stampian, a b o u t 6%0 between the highest and the lowest values. The average curve (Fig.5), plotted by Buchardt, takes into account a possible 33--37%0 salinity variation which should cause a 1%o variation in the 5180. The observed values increase sharply near the Eocene/Oligocene boundary, where a variation of a b o u t 4%o occurs over 2 m.y. This increase corresponds to a temperature drop of at least 10°C. Such an abrupt variation is hard to explain, even for an epibathyal environment and it is likely that the signal m a y have been increased "artificially". In addition, the author analysed species coming from different latitudes, a fact which is not made clear on his published curve. Moreover, it is not clear whether gastropods or bivalves have been analysed and this raises the problem of the "fractionation e f f e c t " as well as that of the presence of aragonite. PELAGIC ENVIRONMENT

Biological data The planktonic organisms undergo particularly important changes at the Eocene/Oligocene boundary. In tropical and equatorial regions, in contrast to their high diversification in the Eocene, Oligocene planktonic foraminiferal associations are particularly impoverished. At the genus level, extinctions take place from the beginning of the Priabonian where representatives of the genera Truncorotaloides, Morozovella, etc., disappear, and are particularly important during the Late Priabonian with the extinctions of Acarinina, Hantkenina, Cribohantkenina, Porticulasphaera, Globigerinatheka and forms of the Turborotalia cerroazulensis group. Only the genera Turborotalia, Globigerina, Globorotaloides, Globigerinita, Cassigerinella, Chiloguernbelina and Pseudohastigerina cross the Priabonian/ Stampian boundary, the last two disappearing during the Stampian. On the whole, no specific evolution shows up during the period corresponding to the Priabonian (latter part of zone P 17) and the base of the Stampian (mid-zone P 18). The Oligocene planktonic foraminifera are cosmopolitan, little diversified and of simple morphology (Kennett et al., 1972). Nannoplankton evolution is similar to that of the foraminifera (Cavelier, 1979). The Oligocene nannoflora is particularly impoverished compared with that of the Middle--Late Eocene, the least diversification being at the beginning of the Stampian (zone NP 22). Practically no forms appear for the first time in the Late Priabonian--earliest Stampian interval (zones NP 21--NP 22). Fischer and Arthur (1977) (Fig.6) reviewed the diversity of the populations of pelagic organisms during the Tertiary and showed that the Eocene corresponds to a period of maximum diversification of the protists and the

233

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Arthur, 1977 ). chordates, either the whole of the phytoplankton, the dinoflagellates, and the planktonic foraminifera on the one hand, or the fish (sardines, mackerel, sharks) and the whales, on the other.

Isotopic data Stable isotope analyses of Cenozoic marine carbonates have multiplied since the study of the first core samples by Douglas and Savin (1973). Only two results have been published which relate to the Eocene/Oligocene boundary (Table I).

234 TABLE I Eocene/Oligocene limit: variation in the oxygen isotopic composition of foraminifera and bulk carbonates D.S.D.P. sites

Water depth (m)

Time interval

51aO variations during that time

A8 (%0)

Pacific 167

3176

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plankton benthos

--1.47 +0.37

--0.39 +1.95

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171

2283

P12-P13--P18 P 12-P 13--P 18

plankton benthos

--0.88 +0.7

+0.43 +1.88

+1.31 +1.17

44

1478

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277

1222

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2853

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bulk carbonates

1151

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--1.27

+ 0.04

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Oxygen isotopic composition Pacific. Th e first two curves were supplied by Douglas and Savin (1973). Their sampling is rather wide-spaced. Stratigraphical analysis was carried out by R o t h (1973) using t he nannoflora. At Site 167 in a subequatorial location (7°04 N, 167°49 W), 6180 values of the planktonic foraminifera increase f r om Zone P 14 to Zone P 17 by a b o u t 1%, those o f the benthic foraminifera by a b o u t 1.6%o from P 12 t o P 17. At Site 171 (19°7 N, 189°27 W), as at Site 167, 6180 values o f the planktonic foraminifera increase by about 1.3%o from Zones P12--13 t o P 18, those o f the benthic foraminifera by 1.2%o. At Site 44, close to Site 171 (19°18 N, 169°09 W), 5180 values of the planktonic foraminifera increase by about 1.5%o and those of t he benthic foraminifera b y 1.2%o (Fig.7). Site 277 in a subantarctic location (52°13 S, 166°11 E), was studied by Shackleton and K e n n e t t (1975). T h e 5180 figures of the planktonic and benthic foraminifera vary in the same way as those o f Site 44, by about 1.25%o (Fig.8). A comparable shift was observed by Margolis et al. (1975) f o r the isotopic co m pos i t i on o f coccoliths. Atlantic. Although an increase o f 5~80 at the Eocene/Oligocene b o u n d a r y is clearly shown b y a certain n u m b e r of the D.S.D.P. drillings (Sites 1 9 8 , 4 0 1 , 116, 366) (Fig.9), the f r e q u e n t existence of sedimentary hiatuses at the level

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+2,0

237

of this limit could hide the real extent of the isotopic shift (Sites 3 9 8 , 4 0 1 , 116). In all cases, however, this shift is around 1--1.5%o. Fig.10 illustrates the results obtained for Sites 116 (Rabussier-Lointier, 1980) and 366 (Vergnaud-Grazzini and Rabussier-Lointier, 1980). At Site 366, in a subequatorial location (5°40.7 N, 19°51.1 W), the stratigraphical study of the samples carried out by Krasheninnikov on the microfauna and by Bukry on the nannoflora (1977), checked by Muller (RabussierLointier, 1980) shows the absence of any hiatus. However, at Site 116 in a North Atlantic location (57°30 N, 15°55 W) (Berggren, 1972; Perch-Nielsen, 1972; Muller, in Rabussier-Lointier, 1980) the beginning of the Oligocene is marked by a sediment hiatus of about 15 m (Laughton, 1972). In these two sites, the same increase in the oxygen isotopic composition of the total carbonate fraction is to be seen between the top of the Upper Eocene (NP 20) and the Lower Oligocene (top of the NP 21 biozone). From around 1%o at Site 116, it reaches 1.2%o at Site 366 (see Fig.10).

Carbon isotopic composition As is the case with oxygen, the variations of carbon isotopic composition can be used as stratigraphical markers. Scholle and Arthur (1980) used them for the Cretaceous. Letolle and Renard (1980) applied this principle to the b o u n d a r y between the Cretaceous and the Tertiary, and that between the Paleocene and the Eocene, b o t h of which marked by abrupt 513C variations. The latter authors remark that the 513C variations in the carbonates can be correlated with global sea-level changes, the negative variations corresponding to regressive phases (Vail et al., 1977), the positive ones to transgressive

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238 phases ( F i g . l l ) . The Late Eocene is marked by a 0.5%o drop of 613C. After a slight increase in the values, this decrease continues up until the Middle Oligocene (Zone NP 24), the overall decrease being around 1%o.

Sedimentological data Oceanic sedimentation rates The estimation of oceanic sedimentation rates in the Atlantic, Pacific and Indian Oceans, carried out by Davies et al. (1977) on the basis of D.S.D.P. drilling data (Fig.12) clearly shows a sharp drop in sedimentation rates near the end of the Eocene. Whereas the Middle Eocene experiences high detritic sedimentation rates, the latter are very low in the late Eocene and Oligocene. Correlatively, the relative proportion of carbonates increases and the C.C.D. increases also. The considerable decrease in detritic supplies is probably the result of a decrease in rainfall on the continents. According to Davies et al. ( 1977), the pattern o f atmo spheric circulations and o f the associated evaporat i o n - p r e c i p i t a t i o n system replaced the oceanic currents as the major factor controlling the latitudinal heat transfer. Hiatuses (Fig.11) The number o f sedimentary hiatuses in the North Atlantic increases around the Eocene/Oligocene b o u n d a r y (Doche, 1976), although hiatuses are recorded as early as the Middle Eocene (Rabussier-Lointier, 1980). These hiatuses may be the result of erosion or a lack of deposition. The number of hiatuses varies from one ocean to another (Fischer and Arthur, 1977); t h e y are more c o m m o n in the Atlantic than in the Pacific, and more in the South Pacific than in the North Pacific. Their duration also depends on their geographical distribution: t h e y last longer in the western parts of the oceanic basins (Rona, 1973). A detailed study carried out in the South Atlantic (Rabussier-Lointier, 1980) shows clearly that the duration decreases from west to east; in this particular case, the observations seem to point to the fact that the sediments have been eroded by an oxygen increase in b o t t o m water circulation, taking into account the fact that deep Antarctic waters are deviated towards the western point of the South Atlantic. This means that a deep circulation already existed at that epoch. Its increased strength could be linked up with a quickening of glacial processes on the Antarctic continent. Moreover, it is shown t h a t there is an inverse correlation between the number o f hiatuses, the C.C.D. and sea-level variations: the transgressive periods corresponding to periods of continuous sedimentation and the regressive periods to periods of sedimentary hiatus. Variations o f the C.C.D. (Fig.11) Van Andel (1975) demonstrated at the level of the Eocene/Oligocene b o u n d a r y a decline of the C.C.D., which is abrupt in the Pacific and more gradual in the Atlantic and Indian Oceans (Fig.13). To explain the C.C.D.

239

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Trace elements in the carbonates Far less work has been done on the geochemistry of the trace elements in carbonates from the D.S.D.P. drillings t ha n on the oxygen and carbon isotopes. The p r es en t studies mainly concerning t he northeast Atlantic are insufficient to provide general conclusions. Strontium and magnesium. In the borings studied up to the present time, 398 C (west o f Portugal), 116 (Rockall Plateau), 400A (Bay of Biscay) (Renard et al., 1979b; Rabussier-Lointier, 1980), the Oligocene/Eocene transition is marked by a fall in Sr (Fig.14) and an increase in Mg. This contrasting behaviour of strontium and magnesium was linked with diagenesis b y Renard (1980). T he break in the strontium and magnesium distribution curves must, th er e f or e , correspond to an increasing occurrence of diagenesis p h e n o m e n a . The cause of this is still uncertain, since it does not seem t o correspond to an increase in the sedimentation rate and there is no p r o o f of a correlation with a possible acceleration in subsidence. Whatever the reason, the Eocene/Oligocene b o u n d a r y marks t he most i m p o r t a n t break in the Mg and Sr distribution curves -- t hat is, the zone in which diagenesis becomes t h e outstanding geochemical p h e n o m e n o n for the carbonates.

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Manganese. From the point of view of manganese distribution, the Eocene/ Oligocene boundary is very distinct in the North Atlantic since it separates a period of high content (Eocene) from one of low content (Oligocene). These phenomena were no doubt linked with volcanic activity along the mid-ocean ridges.

243 STRAIN DIRECTIONS AND TECTONICS While the oceanic expansion rates, which had been falling since the end of the Cretaceous, do not seem to show abrupt variations at the Eocene/Oligocene boundary, a modification in the direction of the strain fields in the Alpino-Mediterranean collision zone is observable (Letouzey and Tre'moli~res, 1980): in the Eocene and Late Miocene the strains usually have a NW--SE to NNW--SSE trend, whereas they tend to be NE--SW in the Oligocene. On the other hand, in the Late Eocene, after a compressive phase from the Ludian or the Middle Priabonian, a t e n d e n c y to relaxation occurs and becomes stronger in the Oligocene. The Eocene/Oligocene boundary corresponds, in fact, to an important tectonic phase in the T e t h y a n area; there is penetration and folding of the internal zones, the cover rocks of which are swept towards the west in the Franco-Italian Alps. One of the consequences of this major phase is the beginning of flysch supply to all the external zones of the Alpine chains. The more accentuated relief may have played its part in reducing the radiation absorbed by the earth (albedo effect), with a general cooling as a result. Moreover, it can be considered that, from this date onwards, there was no longer a deep passage between the Tethys and Atlantic (Rossignol and Auzende, in press). At the Eocene/Oligocene boundary, the T e t h y a n province is diminishing. At the same time, the warm east--west surface circulations must be diminishing. These concomitant events, Alpine orogenesis and reduction in importance of T e t h y a n influence, exaggerate the climatic zonation of the continental and marine regions of Europe. TENTATIVE INTERPRETATION AND CONCLUSIONS Examination of continental and marine populations indicates the existence of a major break between the Priabonian {= Ludian) and the Stampian, which defines the Eocene/Oligocene b o u n d a r y (Cavelier, 1979). In all the cases examined, the period close to this limit is characterised by a very undiversified population which contrasts with the highly varied associations of the Eocene. This very poor diversity results from the total sum of massive extinction which mainly took place at the end of the Priabonian and the beginning of the Stampian. Summarising, there was a diminution in the area of the tropics. At the same time, very few species appeared in the critical Late Priabonian--Early Stampian interval. During the Oligocene, certain groups redeveloped on a large scale, but, on the whole, the fauna and flora of this period are distinctly impoverished compared to those of the Eocene. Marine shelf sediments together with deepwater sediments show at the same time (between Zones P 17 and P 18 or between NP 21 and NP 22) changes in their geochemical characteristics (oxygen and carbon isotopic composition of the carbonates) and depositional

244

style (hiatuses, etc.) which can be related to a world-wide decrease in ocean temperatures. Can a deterioration in the climate alone {continental and oceanic cooling) account for this break or is it the result of a series of phenomena occurring by chance at about the same time? (Fig.15). Indeed, at first sight the evolution of the continental and marine flora seems to result from an overall cooling which could have begun long before the end of the Eocene, in fact from the Lower to Middle Eocene onwards. It is noted that the tropical flora was already retreating in Europe during the Paleocene and Eocene. The effect of this cooling on the fauna cannot have taken place, in fact, until a certain thermal threshold was reached -- at or near the Eocene/Oligocene boundary. It is this threshold which shows up in the curves of oxygen isotope composition. In the oceanic environment, the 1%o 5180 increase should indicate according to some authors (Shackleton and Kennett, 1975; K e n n e t t and Shackleton, 1976) a ca. 4--5°C temperature drop. This ocean cooling could result from the first formation of the sea ice around the Antarctic, and from the development of a deep heavy cold water circulation locally on the b o t t o m of the oceans. This hypothesis seems to be backed up by the upheavals noticed in all the benthic fauna and flora. However, it is possible that part of the 5180 increase is a result of a glacial effect: this effect usually corresponds to a preferential subtraction of the 5160 light isotope during glacial periods when water impoverished in heavy isotopes accumulates in the form o f ice on the continents. This ice accumulation produces an increase in the 180/160 ratio in the ocean. The appearance of an deepening of the CCD

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245

accumulation of ice on the Antarctic continent could thus have modified both the temperature and the isotopic composition of the oceanic waters. Lemasurier (1972) and Frakes (1978) both believe that important glaciations must have existed on the Antarctic continent from as early as the Eocene; moreover, the deep marine palaeoenvironment underwent upheavals from as early as the Early to Middle Eocene (benthic foraminifera: Corliss, 1979; S c h n i t k e r 1979; ostracods: Peypouquet and Benson, 1981; deep currents: Rabussier-Lointier, 1980). This first ice must have had a double influence on the climate: directly by the development of the ocean currents, indirectly by the increase in area of the emerged surfaces. The diminution of carbonate shelf deposits would be concurrent with the C.C.D. deepening and the lowering of the 513C;the hiatuses themselves would be the result of a strengthening of the deep circulation. Thus, the cooling revealed by the faunal and floral evolution as well as that of the oxygen isotopic composition, is associated with considerable sedimentological, tectonic and palaeogeographical modifications. A paroxysmal orogenic phase (Alps) together with an increase of emerged surfaces and of the albedo of middle latitudes by the slowing down of the deep exchanges between the Atlantic Ocean and the T e t h y s produced a climatic deterioration at the same time as a major regression (Fig.15). The Eocene/Oligocene boundary corresponds, therefore, to the acceleration of a climatic evolution which started in the ocean. A m o n g the causes bringing about this acceleration of the climatic evolution, we have proposed an important tectonic phase in the Alpine region and the cessation of the deep circulation between the Atlantic and the Tethys. Certain authors explain this also by a possible cosmic event, such as the impact of a giant meteorite followed by a darkening due to atmospheric dust (Hsii, 1980). The Popigay crater in the U.S.S.R. (100 km in diameter) dated at 3.8 m.y. by Napier and Clube (1979) could be evidence of this, or alternatively, a ring made up of several million tektites and microtektites comparable to those of Saturn which, according to O'Keefe (1980), encircled the earth around 34 m.y. resulting in the screening of sunlight. Obviously these p h e n o m e n a would not be the cause of the climatic change already apparent from the Middle Eocene on, but would have brought about an acceleration characterized by the crossing of some thermal thresholds with spectacular biological consequences. Whatever the causes, the geological events at the Eocene/Oligocene appear t o d a y as the most important during the course of the Tertiary and could justify the proposal to subdivide this super-system, like the Cretaceous, into two periods of approximately equal length (30--35 m.y.), Lower and Upper Tertiary, the former comprising two epochs (Paleocene and Eocene) and the second three (Oligocene, Miocene and Pliocene).

246 ACKNOWLEDGEMENT

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