A major objective of Leg 166 will be to establish if there is advection of fluids in the margin of the GBB. This will be achieved through a combination of temperature measurements and the analysis of fluids.
Temperature measurements will be made using the APC heat-flow coring shoe (ADARA) tool during APC operations. ADARA measurements will be made on all sites on the first APC core starting on Core 4 to a depth of 100 meters below seafloor (mbsf). Below this depth to APC refusal, ADARA samples will be taken every third core. During XCB coring, temperature measurements will be made using the water sampler temperature probe (WSTP) on every fifth core starting with the first XCB core.
Interstitial Water Measurements
To achieve one of the primary objectives of Leg 166, high-resolution sampling of interstitial waters will be required. Interstitial water samples will be taken by squeezing 5-cm whole-round samples every core over the upper 100 m of the first hole at each site Below 100 m, pore waters will be collected from whole-round samples every third core. In addition, 5-cm whole-round samples will be taken every section in the first two cores at the fluid flow sites, BT1/F1, BT2, F3, F4, F5, and F6. In the deeper, more consolidated sections obtained using XCB 10-cm whole-round sections may be necessary. These samples may be combined with physical property samples taken on vertically orientated cores.
Physical Property Samples
To study the origin of bedding and seismic reflections and the correlation between slope carbonates and physical properties, high-quality, reliable sonic velocities are required. For evaluating anisotropy, vertical samples are required. One hundred vertically orientated minicores will be taken from 7-cm whole sections of selected lithologies. Where possible, these will be shared with the interstitial water whole round samples. Samples will be taken to study the relationship among form of slope curvature, sediment composition, and failure of carbonate slopes. Twenty whole-round samples with a length of 15 to 20 cm will be obtained to measure triaxial shear strength and consolidation for the modeling of slope failure conditions.
X-ray diffraction (XRD) analysis will be performed on selected samples analyzed for head-space analysis. Quantitative XRD will be performed using standards supplied by the University of Miami X-ray diffraction laboratory.
X-ray fluorescence will be performed on every third core below 100 mbsf to obtain representative major-and minor-elemental analysis, primarily for calibration to the GLT.
PROPOSED SITESThe proposed sites (Table 2; Figs. 1, 2) are located on the basinward part of the existing seismic line. Six sites, arranged in two transects, are planned for the fluid-flow objectives. BT3 also is the site where the paleoceanographic objectives are addressed.
Sea-level and Paleoceanographic Objectives
The location of the Bahamas Transect along the western margin of the GBB fulfills the requirements for a transect suitable to answer sea-level-related questions. The margin is constructed by stacked prograding sequences with a sedimentation rate high enough to be recorded as seismic sequences. The internal architecture of the sequences is well defined and allows an interpretation of highstand vs. lowstand deposits. Thus, a transect of cores will recover the vertical and lateral facies variations in each sequence that potentially give the record of sea-level fluctuations along the margin. Two boreholes (Unda and Clino) on the platform top give the record in the proximal part of the sequences. The holes drilled during Leg 166 will provide the sedimentary and stratigraphic record of the more distal portions of the prograding sequences.
The primary sites for the sea-level objectives consist of a four-site transect (BT-1 through BT-4) and a lateral site (BT-20). The four transect sites are positioned on the basinward part of the existing seismic line in order to have a direct correlation to the shallow-water drill sites, Unda and Clino. One site (BT-20) is placed on a strike line 5 km to the north of BT-2 to evaluate the lateral variation of facies and sediment thickness in the individual sequences.
BT-1/F-1: (24°34.12' N; 79°13.64' W)
BT-1 currently is positioned at a water depth of 290 m at the crossing of seismic Lines 106 and 123, approximately 850 m from the modern platform edge on the upper slope. Operational safety considerations may require relocation of this site to deeper water. The site will penetrate the thickest portion of the prograding Neogene sequences seen on the seismic line; the total thickness is approximately 1035 m. The upper Pliocene/Quaternary package alone is estimated to be about 240 m thick. The high sedimentation rates allow for a detailed analysis of the high-frequency climate and sea-level changes. The objectives of BT-1 are (1) to date precisely the sequence boundaries, (2) to determine the facies within the different systems tracts, especially the nature of the onlapping units that are currently interpreted as lowstand deposits, (3) to evaluate the fluid flow in this uppermost part of the slope, and (4) to retrieve a high-resolution record of climate and sea-level fluctuations for the Quaternary and late Pliocene.
To achieve these objectives, two APC/XCB cores are planned through the unconsolidated sediment; one core for high-resolution stratigraphy, and one core for interstitial-water analysis. Possible hard layers encountered will be penetrated with the XCB. Rotary drilling will be used to deepen the hole to a target depth of 1035 m, if necessary. BT-1 also serves as the proximal site for the fluid flow transect (i.e., it also is site F-1).
BT-2: (24°32.71' N; 79°15.58' W)
BT-2, at the crossing of seismic Lines 106/108A, is located approximately 2.5 km further basinward than BT-1. It is positioned on the middle and lower slopes of the sequences. The middle and lower portions of the slopes generally are characterized by abundant mass-gravity flow deposits (e.g., turbidites). The seismic data show a stacking of seismic reflections with continuous reflections alternating with channelized and discontinuous reflection patterns, indicating redeposition. The timing of the main deposition of redeposited carbonates still is controversial. Classical sequence stratigraphic models predict an increase of mass gravity flow deposits during sea-level lowstands (e.g., Sarg 1988) whereas carbonate sedimentologists point out that offbank transport of sediment off carbonate platforms is highest during sea-level highstands when the platform is flooded (Droxler and Schlager, 1985; Schlager 1991).
The main objective of this site is to evaluate facies of lowstand vs. highstand deposits, in particular, to determine the respective amount of redeposited strata in both systems tracts. Additional objectives are (1) to refine the ages of the sequence boundaries, (2) determine the nature of a prominent unconformity on top of a low-amplitude to transparent seismic zone at 0.75 s two-wat traveltime (twt) that is interpreted to be the top of the early Pliocene, and (3) produce a high-resolution isotope stratigraphy of the Neogene to Holocene. Furthermore, this site serves as the distal site for the fluid-flow transect. Interstitial waters in combination with heat-flow and temperature measurements should help assess the direction of possible fluid flow through this portion of the margin. This site also is designed as a calibration site in which an extended downhole logging program will be run to produce an optimal correlation between the cores and the geophysical data. To achieve this goal a VSP experiment will be performed.
BT-3: (24°30.26' N; 79°19.34' W)
BT-3, at the crossing of seismic Lines 106/125, is positioned on an undisturbed portion of the lower slope of the western GBB at a water depth of 630 m. At this site the core mostly will penetrate the lower slope portions of the prograding sequences. We expect to find a higher content of microfossils in these foresets than in the proximal sites BT-1 and BT-2. Therefore, the main objective of this site in regards to sea level is to assemble a data set suitable to compare the sedimentary record with the d18O record of the Neogene to Recent sea-level fluctuations. To assure a precise correlation between the seismic and sedimentary record, a VSP experiment will be performed at this site.
By deepening BT-3, the Lower Tertiary and Upper Cretaceous section will be penetrated. A major seismic reflection horizon at a depth of 1413 m (i.e., 2.0 s twt) is interpreted to be the MCSB which is a major regional reflection horizon in the seaways of the Bahamas archipelago and the Gulf of Mexico. The section between the base of the Neogene and the MCSB gives the opportunity to address several paleoceanographic objectives: (1) to acquire a low-latitude record of the Paleogene "Doubthouse" and its transition into the Neogene "Icehouse;" (2) determine the onset of Florida Current; and (3) potentially to sample the K/T boundary.
In addition, during Late Cretaceous-Paleocene/Eocene Cuba collided with the southern end of the Bahamas archipelago. The Paleocene/Eocene section is expected to record this plate-tectonic event by changes in the subsidence rate. The primary goal is to determine the onset of the collision, which is still a matter of debate. Because of time constraints, the primary target horizon was set at the K/T boundary, which is estimated to be approximately 1000 mbsf. If time permits, the hole will be deepened to the MCSB to assess the cause of the platform demise in the middle Cretaceous.
BT-4: (24°28.36' N; 79°22.17' W)
BT-4, at crossing of seismic Lines 106/101B, approximately 12 km from the modern platform margin, is the basinward end of the Bahamas Transect. It is positioned to penetrate a broad channel-like feature that fills the depression between the eastern end of a broad mound in the axis of the Florida Straits and the distal portions of the prograding clinoforms. The mound and the channel-like feature are interpreted to be sediment drifts deposited by the Florida Current. These drift deposits overlay, interfinger, and onlap the distal lobes of the prograding sequences. The position of the site at the periphery of the Florida Current allows us to address the three main objectives: (l) to retrieve the facies of the basinal deposits within the prograding sequences; (2) to assess and date the onset, deposition, and erosional gaps associated with the Florida Current, and (3) to establish the relationship between sea-level fluctuations and the changes in the vigor of the Florida Current.
BT-20: (24°35.41' N; 79°15.56' W) at crossing 112/108A
BT-20 is positioned 5 km north of BT-2 on the middle slope of the western margin. The site is chosen to evaluate the lateral facies variations in the prograding sequence. Because carbonate margins generally act as a sediment source along their entire length, the prograding clinoforms are less likely to develop individual lobes than siliciclastic fans. The strike lines along the margin confirm this general pattern but also display small mounds that indicate the existence of small coalesced lobes. The main objectives of this site are (1) to assess the lateral continuity, or the lack thereof, of facies within the prograding carbonate sequences and (2) to test the consistency of the ages of the sequence boundaries in this lateral position.
For the fluid-flow objectives it is proposed to sample along two transects away from the platform edge. The first transect will partially coincide with the sea-level transect and utilize BT-1 = F-1 as the starting point and BT-2 as end point of the transect. F-3 (24°33.28'N; 79°14.95'W) is located between BT-1 and BT-2 on the same dip line. The main goal is to retrieve pore-water chemistry as an indication for the fluid flow through the margin.
A second transect is planned further south on the margin of the western GBB where the onlapping sediment package is somewhat thinner. The reason for choosing a second transect is to assess possible variations in the fluid-flow pattern. The chosen sites are:
F-4: (23°36.92' N; 79°02.66' W) at crossing FS4/FS7 (operational safety conditions may require relocating this site to deeper water).
F-5: (23°36.80' N; 79°03.36' W) at crossing FS4/FS2
F-6 (23°36.64' N; 79°05.01' W) at crossing FS4/FS1
Sea-level Objectives and Paleoceanographic Objectives
BT-10: (24°36.40' N; 79°13.75' W)
BT-10 is the alternative site to BT-1. The site is located on the crossing of Lines 112/123 in 310 m of water. Objectives and drilling strategies are the same as for BT-1.
BT-20 (Hole C): (24°32.71' N; 79°15.58' W)
BT-20 (Hole C) is designed as an alternate hole to BT-2. If the target depth of 852 m is not reached in BT-2, BT-20 will be deepened to a depth of 822 m, the base of the Neogene.
BT-21: (24°22.96' N; 79°15.52' W) at crossing 126/108B
BT-21 is an alternative site to BT-2. Objectives and drilling strategies are the same as for BT-2.
BT-30 : (24°33.00' N; 79°19.05' W) at crossing 112/125
BT-31: (24°32.60' N; 79°45.70' W) at crossing 132/131
BT-30 and BT-31 are alternative sites to BT-3. BT-30 is further to the north along the same strike line as BT-3. BT-31 is located on the western side of the Straits of Florida on the buried eastern edge of a former larger Cay Sal Bank. The target horizon, the MCSB, is seen as part of a drowned platform at about 2 s (twt) in a water depth of approximately 845 m. Thus, BT-31 is not part of a transect and the sea-level objectives could not be fully addressed.
BT-40: (24°20.02' N; 79°21.95' W) at crossing 115/101B
BT-41: (24°33.34' N; 79°22.05' W) at crossing 128/101A
BT-40 and BT-41 are alternative sites to BT-4. They are located north and south, respectively, of BT-4 but on the same strike line. Objectives and strategies are the same as for BT-4.
F-2: (24°33.70 N; 79°14.23 W) at crossing 119B/106
F-21 (24°36.00 N; 79°14.56) at crossing 130/112.
F-2 and F-21 are secondary sites to F-3.