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PROPOSED DRILL SITES
Tahiti (17°50'S, 149°20'W; S = 142 km2) is the largest of the Society Islands and is composed of twin shield volcanoes that were active at 1.367 ± 0.016 to 0.187 ± 0.003 Ma. Subsidence rates of 0.25 mm/y have been deduced from dating of aerial lava underlying the Pleistocene reef sequence. This island is surrounded by discontinuous fringing reefs that grade locally into a chain of barrier reefs, commonly interrupted and locally enclosing a narrow lagoon. The barrier reef complex includes, from land seaward: a back-reef zone that corresponds to a 1 km wide bay, reaching a maximum depth of 20 m, a relatively narrow reef-flat zone (130 m in maximum width), and an outer-reef slope that consists of coral-built spurs and grooves. In the northwest, the reef foreslope gently deepens seaward to a depth of 15 m and then steepens sharply to 50 m and forms an almost vertical wall between 50 and 100 m (Table T1; Fig. F2). Before our site survey data (October 2002), the occurrence of two prominent terraces at 50 m and 90100 m, dipping seaward and exhibiting living coral reef was demonstrated through a survey by the submersible Cyana (Salvat et al., 1985) and bathymetric data from Port Autonome Papeete, Service Hydrographique et Océanographique de la Marine (SHOM), and Institut Français de Recherche pour lExploitation de la Mer (IFREMER).
Scientific Achievements on Existing Holes
The first drilling operations on Tahiti reefs were carried out in Papeete Harbor, where a 40 m long core penetrated the detrital talus close to the reef edge (Core ETM) and 20 m long cores penetrated large patch reefs (Cores PAD3 and PPTD5) (Montaggioni, 1988; Camoin and Montaggioni, 1994) (Fig. F2).
Subsequently, the barrier reef edge facing Papeete has been cored at several points by a Sedidrill drilling machine and coring system in 1992 and 1995 (Institut de Recherche pour le Développement [IRD, formerly ORSTOM] and Programme National Récifs Coralliens [PNRCO] programs). Cores with diameters ranging from 48 to 64 mm were recovered from two vertical (Holes P6 and P7) and three inclined (Holes P8, P9, and P10; 30°33° from vertical) drill holes (Fig. F3). Holes P6, P7, and P8 are located on the outer barrier reef-flat, whereas Holes P9 and P10 were drilled on the edge of Papeete Pass (see Camoin et al., 1999; Cabioch et al., 1999). Recovery in these P-series cores was dependent upon framework type and on the size of internal cavities but ranged from 50% to 95%; sections with poor or no recovery generally correspond to unconsolidated sands. During drilling, the tube barrel was advanced in 1.5 m increments; core depths were estimated with ±0.3 m accuracy.
Except for the Core P6, the P-series cores penetrated the entire postglacial carbonate sequence beneath the reef edge and reached the antecedent substrate. The volcanic bedrock that lies beneath the reef has a general southwestnortheast slope of 6° and dips gently seaward (average = 10°). Below the barrier reef, the depth of the basaltic or terrigenous substrate ranges from 85.4 m (Core P9) to 114.0 m (Core P7), deepening toward the northeast. In Cores P9 and P10, the basalt is overlain by brownish clay including pebbles of volcanic rocks that may correspond to the weathering profile of the basalts. Correlations between the drill holes on the barrier reef and those in Papeete Harbor (Cores ETM, PAD3, and PPTD5) indicate that the terrigenous deposits form a 1845 m thick wedge below the modern back reef zone, thinning both seaward and landward (Camoin et al., 1999).
The overlying sequence may be subdivided into two units separated by a clear unconformity with evidence of subaerial exposure:
1. The lower unit, up to 30 m thick, is probably late Pleistocene in age and occurs in Cores P7 and P8 at 87.0 and 92.0 m deep, respectively, and is interpreted as having been deposited in a reef-flat environment and capped by a karst surface. It is absent in the area of Holes P9 and P10, where the volcanic substrate is much shallower.
2. The upper unit encompasses the last 13.85 k.y. and exhibits stratigraphically consistent ages with no reversals (Bard et al., 1996). It ranges in thickness from 85.5 to 92.5 m in Cores P9 and P8, respectively, and is primarily composed of in situ 0.16 m thick coral frameworks alternating locally with unconsolidated coral rubble and skeletal sands, terrigenous sands, and silts (Camoin et al., 1999; Cabioch et al., 1999). It corresponds to the longest continuous postglacial reef sequence described so far (Montaggioni et al., 1997). The reconstructed sea level curve for the past 13.70 k.y. is based on more than 100 U-Th dates confirming that the samples used for this study are of high quality and remained closed geochemical systems in the past, thus implying that the measured ages are accurate.
Available Site Survey Data at Tahiti
Before the SISMITA cruise (October 2002), bathymetric seafloor mapping and seismic lines at depths >250 m were carried out on the northern side of the island by the Washington in 1982, whereas SeaBeam data were collected during the ETM 19 cruise in 19831987 (Jean Charcot) (Voisset, 1990). Available data at depths >500 m around Tahiti include bathymetric and acoustic data collected during the Zone Economique de Polynésie Française (ZEPOLYF) (Sichoix and Bonneville, 1996) and POLYDRAG1 (1997) oceanographic cruises (Atalante and Alis, respectively), whereas available data at depths <250 m were restricted to bathymetric data around Papeete (IFREMER and SHOM) (i.e., in the vicinity of the proposed drilling Site TAH-01A).
The SISMITA cruise was carried out in October 2002 (1824 October), using the Alis, owned by IRD. Shipboard participants included G. Camoin (Centre National de la Recherche ScientifiqueCentre de Recherche en Gestion [CNRS-CEREGE]), G. Cabioch (IRD), F. Gallois (IRD), B. Hamelin (CEREGE), and G. Lericolais (IFREMER). During this cruise we planned to acquire very high resolution seismic and multibeam bathymetric data at depths ranging from 50 to 250 m around Tahiti, especially in the three proposed drilling areas: offshore Papeete-Faaa (proposed Site TAH-01A), Tiarei (proposed Site TAH-02A), and Maraa (proposed Site TAH-03A) (Camoin et al., 2003). The major objectives were the following:
- To determine the thickness and geometry of the postglacial (late PleistoceneHolocene) reef sequence and, if possible, of the underlying carbonate units;
- To identify unconformities in the carbonate units overlying the volcanic basement;
- To evaluate the transitional patterns between the outer-reef slope and the downslope environments; and
- To determine the morphology of the volcanic substrate.
Accurate navigation information has been obtained from a Global Positioning System (GPS). Other information regarding water currents has also been acquired.
Simrad EM 1002, 100 kHz data have been acquired around Tahiti and Moorea at depths ranging from 3040 to 500700 m. The three target zones in Tahiti have been mapped in great detail.
Very High Resolution Seismic Data
IFREMER/SIG seismic equipment: asynchronic THR seismic, sediment sounder, and sparker data have been collected in the three target zones in water depths ranging from 40 to 500 m (see maps of the seismic grids and profiles):
- Along lines longitudinal and perpendicular to the axis of the barrier reef and
- within grids of 6 km x 1 km (Tiarei area), 8.5 km x 1 km (offshore Papeete-Faaa), and 9 km x 1 km (Maraa area).The seismic lines are located at a minimal distance of 250 m from the barrier reef, and the distance between the lines forming the grid were fixed to 200400 m in order to crosscut the proposed drilling sites. All potential drilling target zones have crossing seismic lines.
A total of 120 seismic lines have been carried out in the three target zones. In several lines, the hard bottom over most of the outer shelf inhibited subbottom penetration, rendering interpretation of the recorded data problematic, although some internal structure is revealed. This illustrates the difficulty of imaging drilling targets in reef environments.
The morphology of the fore-reef slope as obtained from echo and seismic profiling exhibits consistent features around Tahiti, especially in the three drilling target zones. The most significant morphological features were imaged, mapped, and ground-truthed by a series of 38 dredgings at depths ranging from 50 to 500 m. The maps and the seismic lines provided illustrate these features (see bathymetric maps and seismic lines in Figs. F4, F5, F6, F7, F8, F9, F10, F11, F12, and F13:
- An extensive terrace recorded at 5060 m water depth is characterized by build-ups corresponding to relict reefs. The terrace is gently inclined seaward down to 90 m. The reef sequence deposited on top of this terrace forms a sedimentary wedge that pinches out at 90 m deep in the Tiarei area.
- A narrower terrace typically characterizes depths ranging from 75 to 90100 m. It displays abundant build-ups that are interpreted as drowned reefs. In the Tiarei area (proposed Site TAH-02A), the height of these build-ups ranges from 30 m (base = 100 m, top = 70 m below sea level) to 45 m (base = 90 m, top = 45 m below sea level). There is a clear break in slope at 90100 m, where the slope steepens sharply to form a cliff.
- The transition zone between 90100 and 200250 m may correspond either to an almost vertical wall or to a steep slope. A significant break in slope is observed at 120130 m. This zone is generally composed of laterally discontinuous ledges and gives the appearance of being highly stratified down to a depth of 200 m. Build-ups as high as 45 m (base = 135 m, top = 90 m below sea level) on the slope are interpreted as relict reef ridges.
- A major reflector, possibly corresponding to the top of the volcanic basement, has been imaged from 120150 m water depth close to the cliff, up to 80 m below the 60 m terrace.
Dredging carried out on the successive reef terraces and slopes at depths ranging from 50 to 500 m yielded reef material including coral colonies and fragments, coralline algal encrustations, and microbial crusts (Camoin et al., in press). Datable material has been obtained at all depths within the bathymetric range of the development of the postglacial reef sequence (i.e., to 150 m below the present sea surface). It has served to establish a preliminary chronological frame based on the radiometric dating (U/Th and 14C) of recovered corals and other datable organisms (Camoin et al., in press).
Drilling offshore Barbados (Fairbanks, 1989) demonstrated that the reef sequence corresponding to the last deglaciation developed only on slopes and therefore forms discontinuous successive terraces of various lateral extent.
Recovery of the whole postglacial reef sequence requires the drilling of successive reef terraces that occur seaward of the living barrier reef.
Studies and surveys around Tahiti have demonstrated the occurrence of successive reef terraces at various depths, 100, 90, 60, and 4050 m (see above), which therefore correspond to drilling targets. Thus, at each site, we propose to sample a transect of several offshore drill holes in order to recover the entire postglacial reef sequence.
The numerical model simulating reef building used to study the effect of sea level jumps on the reef geometry in Tahiti has demonstrated that only offshore drilling can recover the corals that were living during the period between the LGM and the MWP-1A event (Fairbanks, 1989) at 13.80 ka (see Bard et al., 1996, and subsequent unpublished tests with the same model). This reconstruction is supported by dating of reef deposits related to the LGM and early deglacial stages in continuous drill holes with seaward deviations of 3045°, 300 m in length, carried out on the northeast rim of the Mururoa atoll (French Polynesia; ages ranging from 15.55 ± 0.08 ka to 23.51 ± 0.07 ka) (Camoin et al., 2001), which then demonstrate the feasibility of our investigations on the Tahiti slopes.
Based on the results of previous scientific drilling and bathymetric and seismic data acquired during the SISMITA cruise, we propose to drill a transect of holes in three areas around Tahiti: offshore Papeete-Faaa (proposed Site TAH-01A), Tiarei (proposed Site TAH-02A), and Maraa (proposed Site TAH-03A). The involved water depths range from 40 to 310 m with one exception concerning the drilling of a keep-up reef in the Tiarei area (water depth = 25 m). The detailed drilling plan is described below. Based on the results of drilling carried out on the Papeete Reef and on the results of dredging carried out during the SISMITA cruise, the expected lithologies for the postglacial reef sequence and the underlying Pleistocene carbonate units include reef frameworks (corals, algal crusts, etc.), lithified limestones, and carbonate sands. The depth of penetration below seafloor to reach the volcanic basement should be <100 m at all sites. Furthermore, the volcanic basement will be penetrated at all sites.
Bathymetric and seismic data obtained for the three potential drilling sites, coupled with the results of previous scientific drilling and dredging, demonstrate the feasibility of our project and that the proposed drilling plan is the best to recover the entire postglacial reef sequence. The location of the proposed drill holes and the expected thickness of the carbonate sequence have been determined on the basis of the available site survey data (seismics, bathymetry, and dredging) and the data acquired on the drill holes carried out on the Papeete Reef.
The exact location of the drill holes will be determined during the cruise by checking the nature and morphology of the seafloor with a remotely operated vehicle (ROV) and/or video. Ideally, a circle of ~100 m around the spots indicated below would give enough flexibility to reach the best targets. A 150 m circle around the spots indicated has been accepted by the Environmental Pollution and Safety Panel (EPSP).
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