Early Oligocene stratigraphic turnover on west Africa ... · The turnover observed in west Africa occurred at the Eocene-Oligocene transition, a time of rapid environmental evolution.

Stratigraphic turnover on west Africa margin page 1

Early Oligocene stratigraphic turnover on west Africa continental margin: a signature ofthe Tertiary greenhouse to icehouse transition?

Michel SéranneInstitut des Sciences de la Terre, de l’Eau et de l’Espace de Montpelliercase 060, CNRS/Université Montpellier 234095 Montpellier cedex 05, [email protected]

TERRA NOVA, 1999, vol.11, pp.135-140

Abstract: The post-rift stratigraphy on the west African margin is characterised by aggradation ofa carbonate ramp during late Cretaceous to Eocene and progradation of a terrigenous wedgefrom Oligocene to the Present. Such first-order structure has been attributed in the past togeodynamic forcing. However, comparison of the stratigraphic record of the margin with eustasy,d18O and 87Sr/86Sr curves, shows a close temporal relationship with the Tertiary climatecooling, an increase of continental weathering and a long-term sea-level lowering. We suggest thatthe transition from low-amplitude high-frequency sea-level changes during the greenhouse periodto high-amplitude high-frequency sea-level changes during the icehouse period may account for:1) the switch from an aggrading carbonate ramp to a prograding clastic wedge, and 2) theenhanced continental weathering and increased terrigenous influx to the margin.

Introduction

Preservation of thick sedimentary wedges on continental passive margins is mostly controlled bythermal subsidence. The stratal relationships result from the interplay between relative sea-levelchange and with sediment availability. Although it is commonly agreed that the latter depends on theclimatic conditions, and that environmental changes are expressed within sediment facies, lithology,faunal content, etc., climate is seldom viewed as a factor controlling first-order architecture on themargin.

Different segments of the west African continental margins (Fig 1) display similar first orderstratigraphic architecture in their sedimentary records (Fig 2). The post-rift succession ischaracterised by a late Cretaceous to Eocene aggradation interval, interrupted by a phase of erosionon the shelf. From the Oligocene to Present progradation typifies the system (Séranne et al., 1992).Previous interpretations have attributed this change to the subsidence-uplift history of the Africancontinent (e.g. Lunde et al., 1992; Walgenwitz et al., 1990; Walgenwitz et al., 1992), or to reducedaccommodation on the continental margin when it came close to achieving thermal equilibration(Séranne et al., 1992). In this paper we explore the relationship between the margin evolution andthe climatic changes that occurred during the Tertiary to determine wether the stratigraphic changeon the margin was caused by the transition from greenhouse to icehouse conditions at the Eocene-Oligocene transition. Such a climatically-driven process could account for the similar stratigraphicrecord of other Tertiary continental margins (Bartek et al., 1991).

Stratigraphic organisation and evolution of the west African margin:

The continental margin of west Africa resulted from the Neocomian rifting of Gondwanaand the opening of the South Atlantic ocean basin. Figure 2 shows seismic reflection profiles across


the west African continental margin. Segments presented at the same scale come from differentstructural settings: transform (Fig. 2a) or divergent margins with decollement level in the salt layer(Fig. 2 b, c, d, e), and display varying sedimentation rate: starved margins (Fig. 2 a, b), or thicksedimentary wedges located close to major river systems (Fig. 2 c, d, e). In spite of this variety, theydisplay a similar first order stratigraphic organisation.

The marine post-rift succession starts at the base with Aptian evaporites that accounts forimportant salt tectonics in the overlying Albian-Cenomanian (Duval et al., 1992). The Aptian -Eocene sequence corresponds to a 2-4 km-thick interval of aggradation across the margin, presentlytilted toward the basin. There is no evidence for acute shelf break within this interval, suggestingprogradation was minor compared to aggradation. Thin and discontinuous Palaeocene - Eoceneslope deposits indicate reduced sedimentation rates and deeper water carbonates (Teisserenc andVillemin, 1989) than on the shelf area, which, with the lack of an acute shelf-break suggests a rampprofile.

Around the whole Gulf of Guinea, a major unconformity truncates the aggradation interval,beneath the shelf-edge (Logar et al., 1983; Massala, 1993; McGinnis et al., 1993; Séranne et al.,1992; Spathopoulos, 1996; Tucker, 1992; Turner, 1995). In Congo, biostratigraphy indicates thepresence of several hiatuses within the late Eocene - early Miocene interval, amongst which twoForaminifer Zones (P16 and P17) seem to be consistently missing throughout the area (Massala,1993). Off southern Gabon, the shortest hiatus corresponds to the NP21/22 Nannofossil zone (J.Bignoumba, unpublished report), that correlates with the latest Eocene-earliest Oligocene (Berggrenet al., 1985).

The overlying sequence is made of terrigenous prograding clinoforms, mostly depositedseaward of an acute shelf break, that characterise little or lack of accommodation on the shelf. Afterthe development of the early Oligocene unconformity, deposition occurred in the slope and basin, incontrast to Late Cretaceous to Eocene time, when deposition was mostly on the shelf. Thisprograding interval corresponds to the generalised Neogene development of the major deltas aroundthe Gulf of Guinea: Niger/Benue (Doust and Omatsola, 1989), Ogooué, Zaire (Reyre, 1984), andKwanza (Lunde et al., 1992).

Tectonic forcing?

The change in sedimentary pattern that occurred at the Eocene-Oligocene transition has beenviewed as a response to geodynamic forcing. The major tectonic events documented in continentalAfrica (Guiraud et al., 1992) are recorded on the Gabon margin, where they correspond to basin-scale unconformities (Nzé Abeigne, 1997). Unlike the early Oligocene unconformity, none of theseregional unconformities marks a major and durable change of stratigraphic pattern. According toWalgenwitz et al. (1990; 1992) the post-Eocene subsidence off Angola represents an early Miocenetectonic event. However, the onset of renewed subsidence predates the Miocene, and results from anincreased accommodation due to withdrawal of the Aptian salt and to the early Oligocene erosionaltruncation (Lavier et al., 1995).

Post-Cenomanian epeirogenic uplift of Africa is ascribed to hot-spot activity beneath theEast African rifts, the Red Sea, and the Sahara. This led to important surface elevation (Sahagian,1988), increased continental denudation and hence to terrigenous sedimentation on the continentalmargins. Indeed, rifting occurred in north-east Africa throughout Neogene (Morley et al., 1992;Steckler et al., 1988), but the products of the rift flanks erosion were evacuated by the Nile and didnot reach the west African margin. By contrast, tectonic uplift related to late Miocene-Pliocenerifting and volcanism in the Tanganyika Rift located in the eastern Zaire drainage basin (Kampuzu etal., 1983), may have contributed to the increase of terrigenous sedimentation on the west Africanmargin. Most of the uplift of coastal Namibia and South Africa predates the Tertiary (Gilchrist et al.,1994), whilst seismic profiling across the off-shore Cameroon volcanic line (Fig. 1) indicates


magmatic-related uplift of the oceanic basement during the Miocene (Meyers and Rosendahl, 1991).Timing of these events does not correlate with the observed early Oligocene stratigraphic changes.

In conclusion, although several tectonic and magmatic events, significant enough to berecognised in the sedimentary record, have affected the west Africa continental margin, it appearsthat none of them is adequately located in time and space to account for the stratigraphic turnoverobserved on the margin. Besides, the invoked tectonic causes are unlikely to have resulted in anirreversible change of the parameters controlling sedimentation, as observed on the west Africamargin.

Tertiary climate change

The turnover observed in west Africa occurred at the Eocene-Oligocene transition, a time ofrapid environmental evolution. These changes were non-reversible: the sedimentary pattern switchedfrom aggradation across the shelf to progradation of a terrigenous wedge, at the Eocene - Oligocenetransition, and corresponds to the present-day situation.

The d18O measured in benthic and planktonic foraminifera can be used as a proxy for oceantemperatures and for presence of continental ice-caps (Miller et al., 1987). The 87Sr/86Sr measuredin the carbonate tests of foraminifera reflects the 87Sr/86Sr ratio of the ocean. Amongst the knowncauses of increase of this ratio, the riverine input of radiogenic-Sr-rich crustal material is the mostimportant (Elderfield, 1986; Raymo et al., 1988).

Comparing the evolution of d18O and the 87Sr/86Sr curve with the stratigraphy of westAfrica margin (Fig. 3), we note the following:

1- The essentially ice-free period (or greenhouse) during the Cretaceous, Palaeocene andEocene corresponds to the period of ramp aggradation in west Africa, greenhouse conditions areassociated with long-term high sea-level, culminating in the Cenomanian when the shorelinetransgressed onto the African continent (e.g. Sahagian, 1988) and references herein). Althoughfalling afterwards, long-term sea level remained high throughout late Cretaceous to Eocene withrespect to Oligocene to Neogene (Haq et al., 1988).

2- The major early Oligocene erosional unconformity corresponds to the first majoroccurrence of ice-cap on Antarctica, which removed large quantities of water from the ocean.Although there are evidence for temporary cooler intervals in mid-Cretaceous (Price et al., 1998)and sedimentological evidence for continental glaciers in the Eocene (Abreu and Anderson, 1998),the first occurrence of significant volume of continental ice on Antarctica can be precisely dated byhigh-resolution sampling at high latitude ODP sites, to the earliest Oligocene, and it is associatedwith a very rapid cooling (40-50 kyrs, Zachos et al., 1996). The amplitude of the d18O shiftcharacterising the event indicates a world-wide sea-level drop of 30 to 90 m (Miller et al., 1991).Consequently, by its amplitude and mostly by its rate of fall, the sea-level fall associated with thegrowth of Antarctica ice-sheet resulted in an unprecedented sub-aerial exposure of wide parts of theramp on the west African margin. This event is likely to have triggered deep incision and erosion ofshelfal deposits, and formation of canyons and sub-marine erosion in the slope, that characterise themajor early Oligocene unconformity.

3- Following a stable 87Sr/86S ratio during latest Cretaceous - Eocene (Elderfield, 1986),the increasing rate of 87Sr/86Sr during the Oligocene-Present interval (Fig. 3) suggests enrichmentof ocean waters by sialic material, corresponding to a period of enhanced continental erosion. Thisis in agreement with the progradation of terrigenous sediments delivered by the major deltas on thewest African margin, which indicates important erosion in Africa. The d18O increased over the same


period, witnessing cooling of the ocean and growing ice-caps. However, it records myrs-long pulses,suggesting alternating glaciated and ice-free periods during Oligocene- mid Miocene interval (Milleret al., 1991), and there is evidence for high-frequency (104 years) fluctuations indicating high-frequency growth and decay of ice-caps (Zachos et al., 1997). The curves of delta progradation (Fig.3) were computed for the Niger, Zaire and Kwanza rivers, they indicate the evolution of percentageof total (present) progradation with time. Niger and Zaire deltas show a marked increase inprogradation rate at 18 - 15 m.y., that correlates with an increasing rate of 87Sr/86Sr, and Zaire andKwanza deltas prograded faster during the Plio-Pleistocene global cooling and increasing rate of Srratio. In spite of the low resolution of the Neogene stratigraphy in the area, we may relate the mid-Miocene and Pliocene d18O shifts (cooling) and increasing rate of 87Sr/86Sr accumulation(enhanced continental erosion), with the increasing rate of progradation observed on the margin.

Greenhouse to icehouse transition

I propose that the changes observed on the stratigraphic record of south-west Africa areprimarily due to the transition of greenhouse period (Mesozoic - Eocene) to an icehouse period(starting in early Oligocene, and well established by middle Miocene).

High-frequency sea-level changes (in the Milankovitch bandwidth) took place duringMesozoic Greenhouse period, (e.g Goldhammer et al., 1990; Read, 1995), even though, in theabsence of process able to remove large quantities of water from the ocean (such as growth anddecay of ice caps), greenhouse high-frequency sea-level changes remained of low amplitude (0.1mto 10m) (Rowley and Markwick, 1992; Schulz and Schäfer-Neth, 1997). Consequently, the shelf ofsubsiding continental margins was seldom sub-aerially exposed and eroded (Fig. 4a). Furthermore,due to the ramp profile of the margin, during low sea level, one would expect narrower sub-aerialexposures than on a present-day type flat shelf. Finally, with low amplitude sea-level changes,sedimentary sequences tend to be stacked, resulting in aggradation of the shelf (Read et al., 1991;Steckler et al., 1993).

During icehouse periods, high-frequency sea-level changes are of high amplitude (Fig. 4b)because they were controlled by the growth and decay of ice sheets. Their amplitude is one order ofmagnitude higher than for greenhouse period (Miller et al., 1991). Consequently, due to the veryfast rate of sea-level fall, the shelf is periodically sub-aerially exposed and eroded during highfrequency seal-level falls. Highstand sediments that might be deposited on the shelf are flushedduring the successive high-frequency sea-level fall. Sediments by-pass the shelf and formprograding wedges in the slope where they accumulate and are preserved on a long-term time scale.Our data also indicate that icehouse period is characterised by increased influx of terrigenoussediments on the continental margin, which points to active continental erosional denudation. Thelong-term sea level lowering that took place over the whole Oligocene-Present interval is anexpression of the long-term increase of the continental ice volume. It induced a long-termdecreasing accommodation on the margin, which resulted in progradation.

Icehouse climatic conditions are controlled 1) by high latitudinal temperature gradient thatpromotes oceanic/atmospheric circulation and seasonality, and 2) alternating cooler/warmer anddryer/wetter periods due to high amplitude sea level changes, in relation to growth and decay of thepolar ice-caps. Consequently, the repeated climate changes that occurred during the icehouse periodmust be recorded in sediments (Sellwood and Price, 1993). Indeed, sedimentary records fromtropical Africa indicate successions of wetter and dryer climates, associated with alternating fasterand slower sediment accumulation rates during the Late Pleistocene (see Roberts and Barker, 1993,and references herein). The precipitation-sensitive vegetation may mediate the erosional response toclimate by reducing the erodibility of soils (Roberts and Barker, 1993). According to (Knox, 1972),a shift from dry to wet condition induces a rapid increase of the vegetal cover which decreaseshillslope erodibility: erosion rate peaks immediately after the onset of wet period and before the


vegetal cover has reached its maximum. Conversely, transition from wet (interglacial) to dry (glacial)periods is likely to favour wildfire activity on the adjacent continent, as observed in the Pleistocenerecord of the Zaire fan (Mikkelsen, 1984), which increases seasonal sediment run-off. Theseconsiderations suggest that the critical parameter in increasing continental erosion is the alternanceof wetter and dryer periods rather than the amount of precipitation. In support to this model,comparative analyses of the oxygen and strontium records during late Oligocene - mid-Miocene(Miller et al., 1991) suggest that the increase in frequency of glaciation/deglaciation caused anincrease in the flux of weathering products, which resulted in 87Sr/86Sr increase, while d18Oremained within 0.5%° (Oslick et al., 1994). These findings suggest that during periods dominatedby alternating climates, such as the icehouse period, even though the cyclicity might not have beenthat of the Pleistocene, the amount of erosion is related to the frequency of climatic changes(alternating wet/dry at low latitudes and glaciated/ice-free at high latitudes) rather than to the climaticparameters.

Discussion and conclusion

The switch from low-amplitude-greenhouse to high-amplitude-icehouse high frequency sea-level changes, proposed here, provides a mechanism for increased weathering during icehouse that isnot only valid for alpine glaciers and high latitude regions (Föllmi, 1995), but applies also toequatorial regions. The proposed climatically driven mechanism is active world-wide, and thus maybetter account for the dramatic post-Oligocene increase of 87Sr/86Sr ratio in oceans than a sourcerestricted to tectonically active regions (Raymo et al., 1988).

Data from the west African continental margin suggests that the observed irreversible switchfrom aggradation (late Cretaceous to late Eocene) to terrigenous clastic progradation seaward of anacute shelf-break (after early Oligocene) was initiated independently from tectonic forcing, but canbe explained by the transition from greenhouse to icehouse. Confirmation of this hypothesis willcome from higher resolution stratigraphy of the Tertiary interval, allowing a more accuratecomparison of observations from the African margin with global events, and from examples fromother continental margins, with simpler tectonic histories. The New Jersey continental margindisplays a similar evolution (Steckler et al., 1993), where the early Oligocene "siliciclastic switch"has also been attributed to the initiation of the icehouse period (Miller et al., 1987).

Acknowledgements

Data used in this study were provided by Elf-Gabon which is thanked for allowing this publication.N. Christie-Blick, N. Driscoll, L. Lavier, K. Miller, C. Nzé Abeigne and M. Steckler areacknowledged for discussions and review of the paper. B.W. Sellwood and an anonymous reviewerprovided insightful suggestions.

References cited

Abreu, V.S., and Anderson, J.B., 1998, Glacial eustasy during the Cenozoic: Sequence stratigraphicimplications: American Association ot Petroleum Geologists Bulletin, v. 82, p. 1285-1400.

Bartek, L.R., Vail, P.R., Anderson, J.B., Emmet, P.A., and Wu, S., 1991, Effect of Cenozoic ice sheetfluctuations in Antartica on the stratigraphic signature of the Neogene: Journal of GeophysicalResearch, v. 96, p. 6753-6778.

Berggren, W.A., Kent, D.V., Flynn, J., and Van Couvering, J., 1985, Cenozoic geochronology:Geological Society of America Bulletin, v. 96, p. 1407-1418.

Doust, H., and Omatsola, E., 1989, Niger delta, in Edwards, J.D., and Santogrossi, P.A., eds.,Divergent/passive margin basins, Volume Memoir 48: Tulsa, Ok., American Association ot


Petroleum Geologits, p. 201-238.

Duval, B., Cramez, C., and Jackson, M.P.A., 1992, Raft tectonics in the Kuanza Basin, Angola: Marineand Petroleum Geology, v. 9, p. 389-404.

Elderfield, H., 1986, Strontium isotope stratigraphy: Palaeogeography, Palaeoclimatology,Palaeoecology, v. 57, p. 71-90.

Föllmi, K.B., 1995, 160 m.y. record of marine sedimentary phosphorus burial: Coupling of climateand continental weathering under greenhouse and icehouse conditions: Geology, v. 23, p. 503-506.

Gilchrist, A.R., Kooi, H., and Beaumont, C., 1994, Post-Gondwana geomorphic evolution ofsouthwestern Africa: Implication for the controls on landscape development from observationsand numerical experiments: Journal of Geophysical Research, v. 99, p. 12211-12228.

Goldhammer, R.K., Dunn, P.A., and Hardie, L.A., 1990, Depositional cycles, composite sea-levelchanges, cycles stacking patterns, and hierarchy of stratigraphic forcing: Examples from theAlpine Triassic platform carbonates: Geological Society of America Bulletin, v. 102, p. 535-562.

Guiraud, R., Binks, R.M., Fairhead, J.D., and Wilson, M., 1992, Chronology and geodynamic setting ofCretaceous-Cenozoic rifting in West and Central Africa: Tectonophysics, v. 213, p. 227-234.

Haq, B.U., Hardenbol, J., and Vail, P.R., 1988, Mesozoïc and Cenozoïc chronostratigraphy and cyclesof sea-level change, in Wilgus, C.K., Posamentier, H., Ross, C.A., and Kendall, C.G.St.C, eds.,Sea-level changes - An integrated approach, Spec. Publ. 42, Soc. Ec. Paleont. & Mineral., p.71-108.

Kampuzu, A.B., Vellutini, P.-J., Caron, J.P.H., Lubala, R.T., Kanika, M., and Rumvegeri, B.T., 1983, Levolcanisme et l'évolution structurale du sud-Kivu (Zaïre): Bull. Centres Rech. Explor.-Prod. Elf-Aquitaine, v. 7, p. 257-271.

Knox, J.C., 1972, Valley alluviation in southwestern Wisconsin: Annals of the Association ofAmerican Geographers, v. 62, p. 401-410.

Lavier, L., Brigaud, F., and Steckler, M.S., 1995, Reconstruction of the Cenozoic development of theAngolan continental margin: EOS Trans. AGU, v. 76. Spring Meet. Suppl, p. S284.

Logar, J.F., Barnel, G., Benaboud, K., Bishop, G., Blankinsop, M., Catala, G., Cheeseman, K.,Desparmet, J.R., Dompnier, R., Fosset, C., Fraser, A., Guindy, A., Ichara, M., Mok, S., Paul, A.,Smith, P., Sodeke, T., Souhaité, P., Ukazim, I., Vu Hoang, D., Whittaker, S., and Westermann, M.,1983, Well Evaluation Conference - Afrique de l'Ouest 1983, Schlumberger.

Lunde, G., Aubert, K., Lauritzen, O., and Lorange, E., 1992, Tertiary of the Kwanza basin in Angola,in Curnelle, R., ed., Géologie Africaine: Coll. Géol. Libreville, Recueil des Communications,Volume Mémoire 13: Boussens, France, Elf Aquitaine, p. 99-117.

Massala, A., 1993, Le Crétacé supérieur et le Tertiaire du bassin côtier congolais - Biochronologie etstratigraphie séquentielle. [Doctorat thesis], Univ. Bourgogne, Dijon.

McGinnis, J.P., Driscoll, N.W., Karner, G.D., Brumbaugh, W.D., and Cameron, N., 1993, Flexuralresponse of passive margins to deep-sea erosion and slope retreat: Implications for relative sea-level change: Geology, v. 21, p. 893-896.

Meyers, J.B., and Rosendahl, B.R., 1991, Seismic reflection character of the Cameroon volcanic line:Evidence for uplifted oceanic crust.: Geology, v. 19, p. 1072-1076.


Mikkelsen, N., 1984, Diatoms in the Zaire deep-sea fan and Pleistocene paleoclimatic trends in theAngola basin and west equatorial Africa: Netherlands Journal of Sea Research, v. 17, p. 280-292.

Miller, K.G., Fairbanks, R.G., and Mountain, G.S., 1987, Tertiary oxygen isotope synthesis, sea levelhistory, and continental margin erosion: Paleoceanography, v. 2, p. 1-19.

Miller, K.G., Wright, J.D., and Fairbanks, R.G., 1991, Unlocking the ice houce: Oligocene-Mioceneoxygen isotopes, eustasy and margin erosion: Journal of Geophysical Research, v. 96, p. 6829-6848.

Morley, C.K., Wescott, W.A., Stone, D.M., Harper, R.M., Wigger, S.T., and Karanja, F.M., 1992,Tectonic evolution of the northern Kenyan Rift: Journal of the Geological Society, v. 149, p.333-348.

Nzé Abeigne, C.R., 1997, Evolution post-rift de la marge continentale sud Gabon: Contrôlestectonique et climatique sur la sédimentation [Doctorat thesis], Univ. Montpellier 2.

Oslick, J.S., Miller, K.G., Feigenson, M.D., and Wright, J.D., 1994, Oligocene-Miocene strontiumisotopes: stratigraphic revisions and correlations to an inferred glacioeustatic record:Paleooceanography, v. 9, p. 427-444.

Price,G.D., Valdes, P.J., and Sellwood, B.W., 1996, A comparison of GCM simulated Cretaceous'greenhouse' and 'Icehouse' climates: Implications for the sedimentary record, Palaeogeography,Palaeoclimatology, Palaeoecology, v. 142, p. 123-128.

Raymo, M.E., Ruddiman, W.F., and Froelich, P.N., 1988, Influence of late Cenozoic mountainbuilding on ocean geochemical cycles: Geology, v. 16, p. 649-653.

Read, J.F., 1995, Overview of carbonate platform sequence, cycle stratigraphic and reservoir ingreenhouse and ice-house worlds, in Read, J.F., Kerans, C., Weber, L.J., Sarg, J.F., and Wright,F.M., eds., Milankovitch sea-level changes, cycle, and reservoir on carbonate platforms ingreenhouse and ice-house worlds, Short Course N°35: Tulsa, Oklahoma, USA, Soc. Ec. Pal.Min., p. 1-102.

Read, J.F., Osleger, D., and Elrick, M., 1991, Two-dimensional modeling of carbonate ramp sequencesand component cycles, in Fransen, E.K., Watney, W.L., Kendall, C.G.S.C., and Ross, W., eds.,Sedimentary modelling: Computer simulations and methods for improved parameter definition,Volume Bulletin 233: Lawrence, Kansas, Kansas Geological Survey, p. 472-488.

Reyre, D., 1984, Caractères pétroliers et évolution géologique d'une marge passive. Le cas du bassinBas Congo - Gabon: Bull. Centres Rech. Explor.-Prod. Elf Aquitaine, v. 8, p. 303-332.

Roberts, N., and Barker, P., 1993, Landscape stability and biogeomorphic response to past and futureclimatic shifts in intertropical Africa, in Thomas, D.S.G., and Allison, R.J., eds., LandscapeSensivity: Chichester (UK), John Wiley and Sons Ltd, p. 65-82.

Rowley, D.B., and Markwick, P.J., 1992, Haq et al. Eustatic sealevel curve : Implications for sequestredwater volumes: The Journal of Geology, v. 100, p. 703-715.

Sahagian, D., 1988, Epeirogenic motions of Africa as inferred from Cretaceous shoreline deposits:Tectonics, v. 7, p. 125-138.

Schulz, M., and Schäfer-Neth, C., 1997, Translating Milankovitch climate forcing into eustaticfluctuations via thermal deep water expansion: a conceptual link: Terra Nova, v. 9, p. 228-231.

Sellwood, B.W., and Price, G.D., 1993, Sedimentary facies as indicators of Mesozoic palaeoclimates:


Phil. Trans. R. Soc. Lond., v. B.341, p. 225-233.

Séranne, M., Séguret, M., and Fauchier, M., 1992, Seismic super-units and post-rift evolution of thecontinental passive margin of southern Gabon: Bulletin de la Société Géologique de France, v.163, p. 135-146.

Spathopoulos, F., 1996, An insight on salt tectonics in the Angola Basin, South Atlantic, in Aslop, G.I.,Blundell, D.J., and Davison, I., eds., Salt tectonics, Special Publication 100: London, GeologicalSociety, p. 153-174.

Steckler, M., Berthelot, F., Lyberis, N., and Le Pichon, X., 1988, Subsidence in the Gulf of Suez:implications for rifting and plate kinematics: Tectonophysics, v. 153, p. 249-270.

Steckler, M.S., Reynolds, D.J., Coakley, B.J., Swift, B.A., and Jarrard, R., 1993, Modelling passivemargin sequence stratigraphy, in Posamentier, H.W., Summerhayes, C.P., Haq, B.U., and Allen,G.P., eds., Sequence stratigraphy and facies associations, Special Publication No18: Oxford,International Association of Sedimentologists, p. 19-41.

Teisserenc, P., and Villemin, J., 1989, Sedimentary basin of Gabon - Geology and oil systems, inEdwards, J.D., and Santogrossi, P.A., eds., Divergent/passive margin basins, Memoir 48: Tulsa,Ok., American Association ot Petroleum Geologits, p. 177-199.

Tucker, J.W., 1992, Aspects of the Tano Basin stratigraphy revealed by recent drilling in Ghana., inCurnelle, R., ed., Géologie Africaine: Coll. Géol. Libreville, Recueil des Communications,Mémoire 13: Boussens, France, Elf Aquitaine, p. 153-156.

Turner, J.P., 1995, Gravity-driven structures and rif basin evolution: Rio Muni Basin, offshoreEquatorial West Africa: American Association ot Petroleum Geologists Bulletin, v. 79, p. 1138-1158.

Walgenwitz, F., Pagel, M., Meyer, A., Maluski, H., and Monié, P., 1990, Thermo-chronologicalapproach to reservoir diagenesis of the offshore Angola basin : a fluid inclusion, 40Ar-39Arand K-Ar investigation: American Association of Petroleum Geologists Bulletin, v. 74, p. 547-563.

Walgenwitz, F., Richert, J.P., and Charpentier, P., 1992, Southwest border of African plate; thermalhistory and geodynamical implications, in Poag, C.W., and de Graciansky, P.C., eds., Geologicevolution of Atlantic continental rises: New York, Van Nostrand Reinhold, p. 20-45.

Zachos, J.C., Flower, B.P., and Paul, H., 1997, Orbitally paced climate oscillations across theOligocene/Miocene boundary: Nature, v. 388, p. 567-570.

Zachos, J.C., Quinn, T.M., and Salamy, K.A., 1996, High-resolution (104 years) deep-seaforaminiferal stable isotope records of the Eocene-Oligocene climate transition:Paleoceanography, v. 11, p. 251-266.





Early Oligocene stratigraphic turnover on west Africa ... · The turnover observed in west Africa occurred at the Eocene-Oligocene transition, a time of rapid environmental evolution.

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