The offshore east African rift system: new insights from the Sakalaves seamounts (Davie Ridge,SW Indian Ocean)

The offshore branch of the East African Rift System (EARS) has developed during Late Cenozoic time along the eastern Africa continental margin. While Neogene–Pleistocene extensional tectonic deformation has been evidenced along the northern segment of the Davie Ridge, the spatial extent of deformation further south remains poorly documented. Based on recent and various oceanographic datasets (bathymetric surveys, dredge samples and seismic profiles), our study highlights active normal faulting, modern east–west extensional tectonic deformation and Late Cenozoic alkaline volcanism at the Sakalaves Seamounts (18°S, Davie Ridge) that seem tightly linked to the offshore EARS development. In parallel, rift‐related tectonic subsidence appears responsible for the drowning of the Sakalaves Miocene shallow‐water carbonate platform. Our findings bring new insights regarding the development of the EARS offshore branch and support recent kinematic models proposing the existence of a plate boundary across the Mozambique Channel.     

Holocene and Pleistocene fringing reef growth and the role of accommodation space and exposure to waves and currents (Bora Bora,Society Islands,French Polynesia)

Holocene fringing reef development around Bora Bora is controlled by variations in accommodation space (as a function of sea‐level and antecedent topography) and exposure to waves and currents. Subsidence ranged from 0 to 0·11 m kyr^?1, and did not create significant accommodation space. A windward fringing reef started to grow 8·7 kyr bp , retrograded towards the coast over a Pleistocene fringing reef until ca 6·0 kyr bp , and then prograded towards the lagoon after sea‐level had reached its present level. The retrograding portion of the reef is dominated by corals, calcareous algae and microbialite frameworks; the prograding portion is largely detrital. The reef is up to 13·5 m thick and accreted vertically with an average rate of 3·12 m kyr^?1. Lateral growth amounts to 13·3 m kyr^?1. Reef corals are dominated by an inner Pocillopora assemblage and an outer Acropora assemblage. Both assemblages comprise thick crusts of coralline algae. Palaeobathymetry suggests deposition in 0 to 10 m depth. An underlying Pleistocene fringing reef formed during the sea‐level highstand of Marine Isotope Stage 5e, and is also characterized by the occurrence of corals, coralline algal crusts and microbialites. A previously investigated, leeward fringing reef started to form contemporaneously (8·78 kyr bp ), but is thicker (up to 20 m) and solely prograded throughout the Holocene. A shallow Pocillopora assemblage and a deeper water Montipora assemblage were identified, but detrital facies dominate. At the Holocene reef base, only basalt was recovered. The Holocene windward–leeward differences are a consequence of less accommodation space on the eastern island side that eventually led to a more complex reef architecture. As a result of higher rates of exposure and flushing, the reef framework on the windward island side is more abundant and experienced stronger cementation. In the Pleistocene, the environmental conditions on the leeward island side were presumably unfavourable for fringing reef growth.     

Sealevel history recorded in the Pleistocene carbonate sequence in IODP Hole 310‐M0005D,off Tahiti

Material cored during the Integrated Ocean Drilling Program (IODP) Expedition 310 ‘Tahiti Sea Level’ revealed that the fossil reef systems around Tahiti are composed of two major stratigraphic sequences: (i) a last deglacial sequence; and (ii) an older Pleistocene sequence. The older Pleistocene carbonate sequence is composed of reef deposits associated with volcaniclastic sediments and was preserved in Hole 310‐M0005D drilled off Maraa. Within an approximately 70‐m‐thick older Pleistocene sequence (33.22–101.93 m below seafloor; 92.85–161.56 m below present sealevel) in this hole, 11 depositional units are defined by lithological changes, sedimentological features, and paleontological characteristics and are numbered sequentially from the top of the hole downward (Subunits P1–P11). Paleowater depths inferred from nongeniculate coralline algae, combined with those determined by using corals and larger foraminifers, suggest two major sealevel rises during the deposition of the older Pleistocene sequence. Of these, the second sealevel rise is associated with an intervening sealevel drop. It is likely that the second sealevel rise corresponds to that during Termination II (TII, the penultimate deglaciation, from Marine Isotope Stages 6 to 5e). Therefore, the intervening sealevel drop can be correlated with that known as the ‘sealevel reversal’ during TII. Because there are limited data on the Pleistocene reef systems in the tropical South Pacific Ocean, this study provides important information about Pleistocene sealevel history, the evolution of coral reef ecosystems, and the responses of coral reefs to Quaternary climate changes.     

Postglacial growth history of a French Polynesian barrier reef tract, Tahiti, central Pacific

The internal structure and growth pattern of Tahiti reefs over the last 14 ka is reconstructed using sedimentological, morphological and palaeobiological data coupled with radiometric dates in drill cores through the modern barrier reef. Flooding of the volcaniclastic deposits or the karst surface of a Pleistocene reef started at ≈ 14 ka BP, and coral growth began shortly after inundation. The sequence in the Tahiti barrier-reef edge has formed predominantly through long-term keep-up growth controlled by stable environmental conditions, while the adjacent backreef deposits did not start to accumulate before sea-level stabilization, around 6 ka. The dominance of Porites communities and the coeval occurrence of branching gracile Lithophyllum in the lowermost part of the postglacial reef sequence (14–11 ka) suggest the prevalence of uniformly moderate- to low-energy conditions and/or growth in slightly deeper waters all over the drilled area during the early reef stages. During the last 11 ka, the reef frameworks developed in a high-energy environment, at maximum water depths of 5–6 m, and were dominated by an Acropora robusta/danai–Hydrolithon onkodes association; the local interlayering of other coralgal assemblages (dominated by tabular Acropora or domal Porites ) reflects distinct diversification stages, resulting either from the palaeotopographic control of the substrate or from slight and episodic environmental changes.     

Origins and development of Holocene coral reefs: a revisited model based on reef boreholes in the Seychelles, Indian Ocean

 Until recently, concepts of coral reef growth and accumulation have been predominantly based on a Darwinian model. In this, the upwards and outwards growth of a reef core (a coral framework) takes place over a foreslope consisting of reef talus, with the simultaneous filling of the back-reef lagoon by reef-derived debris. The principal adaptations of this pattern relate to the influence of relative changes in sea level and commonly ignore oceanographic factors such as storm frequency and severity. Boreholes through the outer edge of a fringing reef in the Seychelles, western Indian Ocean, reveal a record of Holocene sediment accumulation first established approximately 8 ka ago. Faunal and floral associations show that growth of this body began in relatively deep water but that this shallowed to <5 m within 1 ka. Subsequent accumulation was of “keep-up” style but, as the rate of sea-level rise slowed, shoaling became more frequent and aggradation was limited by reducing accommodation space. Constructional facies are characterised either by massive corals, including Leptastrea, Porites and faviids, or by branching corals, typically Acropora of the danai-robusta group. Coral surfaces may be encrusted by red algae, foraminifera and vermetids, and are commonly bored by filamentous algae, clionids and molluscs. However, detrital facies are volumetrically dominant, and the paucity of a constructional framework requires a re-evaluation of models of reef accretion. New models relate the geometry of accretion to the interplay between extreme storm events and fairweather hydrodynamic conditions. These suggest that a contiguous framework forms in areas of moderate fairweather energy without extreme storm events. Severe storms destroy the continuity of reef structures and generate increasing volumes of coarse detritus. Low storm severity, coupled with low fairweather hydrodynamic energy, may promote the accumulation of fine-grained reef-derived sediments that inhibit framework growth. While ecology reflects year-by-year sea conditions, lithology and structure are controlled by exceptional storms, with the effects of changing sea level superimposed. Received: 30 November 1998 / Accepted: 4 November 1999     

Sedimentology of coexisting travertine and tufa deposits in a mounded geothermal spring carbonate system,Obruktepe, Turkey

The recent discoveries of deeply buried Cretaceous reservoir bodies in the Atlantic Ocean revealed that relationships between the distribution of spring carbonate deposits and faults are poorly understood. The well‐exposed Quaternary deposits at Obruktepe (Denizli Basin, Turkey) provide an opportunity to reconstruct the three‐dimensional sedimentary architecture of such a system. Integration of sedimentological, lithofacies and geochemical analyses reveals complexity in the lateral relationships between sedimentary environments, faults and geothermal spring carbonates. Five environmental systems are distinguished based on the lithofacies analysis: (i) vent; (ii) smooth slope; (iii) travertine‐terrace; (iv) tufa‐barrage; and (v) flood systems. Encrusting, baffling and settling sedimentary processes are reflected in data acquired at several scales, from lithofacies observations to the morphology and arrangement of geobodies, together with microfabrics and stable carbon and oxygen isotope data. Mean values of +4·9‰ δ¹³C and −8·74‰ δ¹⁸O Vienna PeeDee Belemnite reflect geothermal circulation of springwaters. The environmental distribution and lithofacies indicate a lateral continuum between travertine and tufa deposits within this hot spring system. This finding supports two depositional models in which water flow variation is the main control on both CaCO₃ precipitation and the resulting formation of travertine and tufa. The proposed models address the factors responsible for the development of these complex mound‐shaped carbonate spring deposits, and how they are related to fluid circulation at depth and in association with faults.     

Sea-Level Changes and δO Record for the Past 34,000 yr from Mayotte Reef, Indian Ocean

New sea-level and δ¹⁸O curves for the past 34,000 yr, based on uranium–thorium chronology, are proposed for the southwestern part of the Indian Ocean. The archives include cores drilled from onshore coral reefs and submersed samples from foreslope corals of Mayotte in the Comoro Islands. The Mayotte sea-level curve shows a lowstand of 145 ± 5 m below the present level during the last glacial maximum dated at 18,400 yr. This lowstand is supported by the maximum¹⁸O enrichment in the coral colonies. The residual signal (Δδ¹⁸O), controlled by sea-surface temperature changes, indicates that surface waters 18,400 yr ago were approximately 5°C cooler than present. The deglacial sea-level rise is clearly recorded, with a mean rate of about 1.7 cm yr⁻¹between 18,400 and 10,000 yr ago. The deglaciation phase is characterized by a strong¹⁸O depletion marked by two pulses related to meltwater discharges into the North Atlantic Ocean but also characterized by responses specific to the tropical Indian Ocean.     

Late Quaternary barrier and fringing reef development of Bora Bora (Society Islands,south Pacific): First subsurface data from the Darwin‐type barrier‐reef system

The universally known subsidence theory of Darwin, based on Bora Bora as a model, was developed without information from the subsurface. To evaluate the influence of environmental factors on reef development, two traverses with three cores, each on the barrier and the fringing reefs of Bora Bora, were drilled and 34 uranium‐series dates obtained and subsequently analysed. Sea‐level rise and, to a lesser degree, subsidence were crucial for Holocene reef development in that they have created accommodation space and controlled reef architecture. Antecedent topography played a role as well, because the Holocene barrier reef is located on a Pleistocene barrier reef forming a topographic high. The pedestal of the fringing reef was Pleistocene soil and basalt. Barrier and fringing reefs developed contemporaneously during the Holocene. The occurrence of five coralgal assemblages indicates an upcore increase in wave energy. Age–depth plots suggest that barrier and fringing reefs have prograded during the Holocene. The Holocene fringing reef is up to 20 m thick and comprises coralgal and microbial reef sections and abundant unconsolidated sediment. Fringing reef growth started 8780 ± 50 yr bp ; accretion rates average 5·65 m kyr^?1. The barrier reef consists of >30 m thick Holocene coralgal and microbial successions. Holocene barrier‐reef growth began 10 030 ± 50 yr bp and accretion rates average 6·15 m kyr^?1. The underlying Pleistocene reef formed 116 900 ± 1100 yr bp , i.e. during marine isotope stage 5e. Based on Pleistocene age, depth and coralgal palaeobathymetry, the subsidence rate of Bora Bora was estimated to be 0·05 to 0·14 m kyr^?1. In addition to subsidence, reef development on shorter timescales like in the late Pleistocene and Holocene has been driven by glacioeustatic sea‐level changes causing alternations of periods of flooding and subaerial exposure. Comparisons with other oceanic barrier‐reef systems in Tahiti and Mayotte exhibit more differences than similarities.     

Microbialite development patterns in the last deglacial reefs from Tahiti (French Polynesia; IODP Expedition #310): Implications on reef framework architecture

The widespread occurrence of microbialites in the last deglacial reef frameworks (16–6 Ka BP) implies that the accurate study of their development patterns is of prime importance to unravel the evolution of reef architecture through time and to reconstruct the reef response to sea-level variations and environmental changes.The present study is based on the sedimentological and chronological analysis (¹⁴C AMS dating) of drill cores obtained during the IODP Expedition #310 “Tahiti Sea Level” on the successive terraces which typify the modern reef slopes from Tahiti. It provides a comprehensive data base to investigate the microbialite growth patterns (i.e. growth rates and habitats), to analyze their roles in reef frameworks and to reconstruct the evolution of the reef framework architecture during sea-level rise.The last deglacial reefs from Tahiti are composed of two distinctive biological communities: (1) the coralgal communities including seven assemblages characterized by various growth forms (branching, robust branching, massive, tabular and encrusting) that form the initial frameworks and (2) the microbial communities developed in the primary cavities of those frameworks, a few meters (1.5 to 6 m) below the living coral reef surface, where they heavily encrusted the coralgal assemblages to form microbialite crusts. The dating results demonstrate the occurrence of two distinctive generations of microbialites: the “reefal microbialites” which developed a few hundred years after coralgal communities in shallow-water environments, whereas the “slope microbialites” grew a few thousands of years later in significantly deeper water conditions after the demise of coralgal communities.The development of microbialites was controlled by the volume and the shape of the primary cavities of the initial reef frameworks determined by the morphology and the packing of coral colonies. The most widespread microbialite development occurred in frameworks dominated by branching, thin encrusting, tabular and robust branching coral colonies which built loose and open frameworks typified by a high porosity (> 50%). In contrast, their growth was minimal in compact coral frameworks formed by massive and thick encrusting corals where primary cavities yielded a low porosity (~ 30%) and could not host a significant microbialite expansion.     

Impact of environmental parameters on coral reef development and drowning: Forward modelling of the last deglacial reefs from Tahiti (French Polynesia; IODP Expedition #310)

The sedimentological and chronological analysis of the last deglacial reef sequences of Tahiti (French Polynesia), drilled during the Integrated Ocean Drilling Program Expedition 310, provide a high‐resolution data set allowing a well‐constrained forward modelling study. This study represents the first attempt to model in three dimensions the coral reef development of Tahiti during the last deglacial sea‐level rise (23 000 to 6000 cal yr bp ) using the software dionisos developed by IFP Energies nouvelles. It allows the testing of the reconstructed last deglacial sea‐level curve and the different environmental parameters (for example, wave energy and sediment fluxes) that could have influenced the reef development. These last deglacial reef sequences form two prominent ridges occurring seaward of the living barrier reef that consist of successive submerged reefs. These reefs have been prone to drowning because the window of maximum carbonate production rate is inhibited by high water turbidity (sediment supply from a nearby river), shallow depth of wave action and substrate availability. These factors, combined with rapid sea‐level rise, have driven the growth of retrograding reef pinnacles. Local factors (substratum nature, sediment supply and wave energy) were the main processes that induced the drowning of the inner ridge, whereas interplay of local and global factors (acceleration of the sea‐level rise) was responsible for the drowning of the outer ridge. This particular acceleration of sea‐level rise of 16 m between 14·6 ka and 14 ka bp corresponds to meltwater pulse 1A.