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Early stratigraphic turnover on west continental margin: a signature of the Tertiary greenhouse to icehouse transition?

Michel Séranne Institut des Sciences de la Terre, de l’Eau et de l’Espace de Montpellier case 060, CNRS/Université Montpellier 2 34095 Montpellier cedex 05, France [email protected]

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

Abstract: The post- stratigraphy on the west African margin is characterised by aggradation of a carbonate ramp during late to and progradation of a terrigenous wedge from Oligocene to the Present. Such first-order structure has been attributed in the past to geodynamic 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 climate cooling, an increase of continental weathering and a long-term sea-level lowering. We suggest that the transition from low-amplitude high-frequency sea-level changes during the greenhouse period to 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) the enhanced continental weathering and increased terrigenous influx to the margin.

Introduction Preservation of thick sedimentary wedges on continental passive margins is mostly controlled by thermal subsidence. The stratal relationships result from the interplay between relative sea-level change and with availability. Although it is commonly agreed that the latter depends on the climatic 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 the margin. Different segments of the west African continental margins (Fig 1) display similar first order stratigraphic architecture in their sedimentary records (Fig 2). The post-rift succession is characterised by a late Cretaceous to Eocene aggradation interval, interrupted by a phase of erosion on 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 African continent (e.g. Lunde et al., 1992; Walgenwitz et al., 1990; Walgenwitz et al., 1992), or to reduced accommodation 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 and the climatic changes that occurred during the Tertiary to determine wether the stratigraphic change on 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 stratigraphic record 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 Gondwana and the opening of the South basin. Figure 2 shows seismic reflection profiles across

Stratigraphic turnover on west Africa margin page 1 the west African continental margin. Segments presented at the same scale come from different structural 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 thick sedimentary wedges located close to major river systems (Fig. 2 c, d, e). In spite of this variety, they display a similar first order stratigraphic organisation. The marine post-rift succession starts at the base with Aptian that accounts for important 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, presently tilted toward the basin. There is no evidence for acute shelf break within this interval, suggesting progradation was minor compared to aggradation. Thin and discontinuous Palaeocene - Eocene slope deposits indicate reduced sedimentation rates and deeper water carbonates (Teisserenc and Villemin, 1989) than on the shelf area, which, with the lack of an acute shelf-break suggests a ramp profile. 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 the presence of several hiatuses within the late Eocene - early interval, amongst which two Foraminifer 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 (Berggren et al., 1985). The overlying sequence is made of terrigenous prograding clinoforms, mostly deposited seaward of an acute shelf break, that characterise little or lack of accommodation on the shelf. After the development of the early Oligocene unconformity, deposition occurred in the slope and basin, in contrast to Late Cretaceous to Eocene time, when deposition was mostly on the shelf. This prograding interval corresponds to the generalised Neogene development of the major deltas around the Gulf of Guinea: Niger/Benue (Doust and Omatsola, 1989), Ogooué, Zaire (Reyre, 1984), and Kwanza (Lunde et al., 1992). Tectonic forcing? The change in sedimentary pattern that occurred at the Eocene-Oligocene transition has been viewed as a response to geodynamic forcing. The major tectonic events documented in continental Africa (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 these regional unconformities marks a major and durable change of stratigraphic pattern. According to Walgenwitz et al. (1990; 1992) the post-Eocene subsidence off represents an early Miocene tectonic event. However, the onset of renewed subsidence predates the Miocene, and results from an increased accommodation due to withdrawal of the Aptian salt and to the early Oligocene erosional truncation (Lavier et al., 1995). Post-Cenomanian epeirogenic uplift of Africa is ascribed to hot-spot activity beneath the East African , the Red Sea, and the Sahara. This led to important surface elevation (Sahagian, 1988), increased continental denudation and hence to terrigenous sedimentation on the continental margins. 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 did not reach the west African margin. By contrast, related to late Miocene-Pliocene rifting and volcanism in the Tanganyika Rift located in the eastern Zaire drainage basin (Kampuzu et al., 1983), may have contributed to the increase of terrigenous sedimentation on the west African margin. Most of the uplift of coastal Namibia and South Africa predates the Tertiary (Gilchrist et al., 1994), whilst seismic profiling across the off-shore volcanic line (Fig. 1) indicates

Stratigraphic turnover on west Africa margin page 2 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 be recognised in the sedimentary record, have affected the west Africa continental margin, it appears that none of them is adequately located in time and space to account for the stratigraphic turnover observed on the margin. Besides, the invoked tectonic causes are unlikely to have resulted in an irreversible change of the parameters controlling sedimentation, as observed on the west Africa margin. Tertiary climate change The turnover observed in west Africa occurred at the Eocene-Oligocene transition, a time of rapid environmental evolution. These changes were non-reversible: the sedimentary pattern switched from aggradation across the shelf to progradation of a terrigenous wedge, at the Eocene - Oligocene transition, and corresponds to the present-day situation.

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

Comparing the evolution of d18O and the 87Sr/86Sr curve with the stratigraphy of west Africa margin (Fig. 3), we note the following: 1- The essentially ice-free period (or greenhouse) during the Cretaceous, Palaeocene and Eocene corresponds to the period of ramp aggradation in west Africa, greenhouse conditions are associated with long-term high sea-level, culminating in the Cenomanian when the shoreline transgressed onto the African continent (e.g. Sahagian, 1988) and references herein). Although falling afterwards, long-term remained high throughout late Cretaceous to Eocene with respect to Oligocene to Neogene (Haq et al., 1988). 2- The major early Oligocene erosional unconformity corresponds to the first major occurrence 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 by high-resolution sampling at high latitude ODP sites, to the earliest Oligocene, and it is associated with a very rapid cooling (40-50 kyrs, Zachos et al., 1996). The amplitude of the d18O shift characterising 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 the growth of Antarctica ice-sheet resulted in an unprecedented sub-aerial exposure of wide parts of the ramp on the west African margin. This event is likely to have triggered deep incision and erosion of shelfal deposits, and formation of canyons and sub-marine erosion in the slope, that characterise the major 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 enrichment of ocean waters by sialic material, corresponding to a period of enhanced continental erosion. This is in agreement with the progradation of terrigenous delivered by the major deltas on the west African margin, which indicates important erosion in Africa. The d18O increased over the same

Stratigraphic turnover on west Africa margin page 3 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 (Miller et 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 percentage of total (present) progradation with time. Niger and Zaire deltas show a marked increase in progradation rate at 18 - 15 m.y., that correlates with an increasing rate of 87Sr/86Sr, and Zaire and Kwanza deltas prograded faster during the Plio-Pleistocene global cooling and increasing rate of Sr ratio. 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 are primarily 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 during Mesozoic Greenhouse period, (e.g Goldhammer et al., 1990; Read, 1995), even though, in the absence of process able to remove large quantities of water from the ocean (such as growth and decay of ice caps), greenhouse high-frequency sea-level changes remained of low amplitude (0.1m to 10m) (Rowley and Markwick, 1992; Schulz and Schäfer-Neth, 1997). Consequently, the shelf of subsiding 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-aerial exposures 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 of magnitude higher than for greenhouse period (Miller et al., 1991). Consequently, due to the very fast rate of sea-level fall, the shelf is periodically sub-aerially exposed and eroded during high frequency seal-level falls. Highstand sediments that might be deposited on the shelf are flushed during the successive high-frequency sea-level fall. Sediments by-pass the shelf and form prograding 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 terrigenous sediments on the continental margin, which points to active continental erosional denudation. The long-term sea level lowering that took place over the whole Oligocene-Present interval is an expression of the long-term increase of the continental ice volume. It induced a long-term decreasing accommodation on the margin, which resulted in progradation. Icehouse climatic conditions are controlled 1) by high latitudinal temperature gradient that promotes oceanic/atmospheric circulation and seasonality, and 2) alternating cooler/warmer and dryer/wetter periods due to high amplitude sea level changes, in relation to growth and decay of the polar ice-caps. Consequently, the repeated climate changes that occurred during the icehouse period must be recorded in sediments (Sellwood and Price, 1993). Indeed, sedimentary records from tropical Africa indicate successions of wetter and dryer climates, associated with alternating faster and 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 to climate 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 decreases hillslope erodibility: erosion rate peaks immediately after the onset of wet period and before the

Stratigraphic turnover on west Africa margin page 4 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 Pleistocene record of the Zaire fan (Mikkelsen, 1984), which increases seasonal sediment run-off. These considerations suggest that the critical parameter in increasing continental erosion is the alternance of 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 an increase in the flux of weathering products, which resulted in 87Sr/86Sr increase, while d18O remained within 0.5%° (Oslick et al., 1994). These findings suggest that during periods dominated by alternating climates, such as the icehouse period, even though the cyclicity might not have been that 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 climatic parameters. 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 is not only valid for alpine glaciers and high latitude regions (Föllmi, 1995), but applies also to equatorial regions. The proposed climatically driven mechanism is active world-wide, and thus may better account for the dramatic post-Oligocene increase of 87Sr/86Sr ratio in oceans than a source restricted to tectonically active regions (Raymo et al., 1988). Data from the west African continental margin suggests that the observed irreversible switch from aggradation (late Cretaceous to late Eocene) to terrigenous clastic progradation seaward of an acute shelf-break (after early Oligocene) was initiated independently from tectonic forcing, but can be explained by the transition from greenhouse to icehouse. Confirmation of this hypothesis will come from higher resolution stratigraphy of the Tertiary interval, allowing a more accurate comparison of observations from the African margin with global events, and from examples from other continental margins, with simpler tectonic histories. The New Jersey continental margin displays 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 are acknowledged for discussions and review of the paper. B.W. Sellwood and an anonymous reviewer provided insightful suggestions. References cited

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