Leg 166-Bahamas

The primary objective of Leg 166 was to address fundamental questions regarding sea level. To attain this objective, five sites in the Straits of Florida were drilled during Leg 166, completing a transect through prograding carbonate sequences formed in response to sea-level fluctuations along the western margin of the Great Bahama Bank. Two boreholes drilled previously on the western Great Bahama Bank as part of the Bahamas Drilling Project represented the shallow-water sites of the transect. The primary goal of the transect was to document the platform-margin record of the Neogene-Holocene sea-level changes by determining the ages of the major unconformities and comparing the timing of these unconformities with ages predicted from the oxygen isotopic record of glacioeustasy. Core borings along the complete transect document the facies variations associated with oscillations of sea level and, thus, the sedimentary response of the carbonate environment to sea-level changes. The correlation between the two independent records of sea-level changes--sequence stratigraphy and oxygen isotope proxy--has the potential to evaluate rate and amplitude of eustatic vs. relative sea-level changes and to establish a causal link between glacioeustasy and the stratigraphic pattern.

Leg Objectives

Objective: The main objective of the proposed drilling transect along the western margin of the Great Bahama Bank was to study the record of Neogene-Quaternary sea-level fluctuations in the prograding sequences. Within this sea-level objective were the following goals:

Determine the timing of the sequence boundaries and relative sea-level fluctuations to acquire the necessary database for the possible global synchroneity of these fluctuations.

Conclusion: Calcareous nannofossils and planktonic foraminiferal biostratigraphic datums were used to establish an age-depth relationship for each of the sites drilled as part of the Bahamas Transect (Sites 1003-1007) on the leeward slope of GBB. Many biohorizons from both groups were identified, establishing a framework inter-site correlation, which was transferred into time using the Geomagnetic Polarity Time Scale. The latter step was the most important to obtain absolute ages to fulfill one of the objectives of this leg: testing the age consistency of the sequence boundaries observed in the seismic records and logging data in combination with lithologic changes observed in the recovered sedimentary section. The overall consistency in the relative position of planktonic foraminiferal and nannofossil datums means that a reliable framework for inter-site correlation has been established for the Bahamas Transect sites. This framework gives the opportunity to date the lithologic changes and seismic sequences identified along the transect, which provide critical information for understanding the role of sea-level change and its effect on the development of the Bahamian Platform.

Sequence analysis was performed on the seismic data prior to drilling. The reflections identified as sequence boundaries were traced to the slope and basinal areas of the Bahamas Transect Leg 166 sites. By reaching the Oligocene at Site 1007, a complete record for the sequence stratigraphic architecture is available for the entire Neogene. We identified 17 sequences in the Neogene section. At the Leg 166 sites, most of the sequence boundaries are conformable; however, truncation is observed in the more proximal sites.

The time/depth conversion obtained from vertical seismic profile experiments allowed us to correlate the sequence boundaries to the cores. All sequence boundaries coincide, within a seismic resolution of about 10 m, with a facies change indicative of a sea-level change. The exact position of the boundaries is recorded on the log data, which have better resolution than the seismic data. The age at the correlative depth was calculated using biostratigraphic datums and extrapolating sedimentation rates between datums. Although this procedure carries the uncertainty of both the exact position of the biostratigraphic datums and the seismic resolution, the sequence boundaries showed consistent ages along the seismic reflections. This result is exciting as it confirms one of the major assumptions of sequence stratigraphy that seismic reflections are time lines.

The ages of the sea-level changes in the Neogene were estimated from the ages of the sequence boundaries. In the proximal sites where several of the sequence boundaries are associated with erosion that occasionally is detected by a hiatus, the age of the boundary was taken from the distal conformable locations. In light of the resolution problem, the ages of the sequence boundaries probably have an error bar of approximately 0.2 m.y. Nevertheless, the ages indicate that the seismic sequences along the Bahamas Transect record most of the known major (third-order) sea-level changes in the Neogene. Shore-based log-seismic correlation and detailed biostratigraphic analysis will refine the age model for these sea-level changes.

Determine the stratigraphic response of carbonates to sea-level changes of variable frequency by analyzing the facies of the stacked depositional sequences. Special emphasis was placed on documenting the amount and nature of lowstand deposits in carbonates and the hierarchical stacking of high-frequency cycles into seismic sequences.

Conclusion: Overall core recovery was sufficient (55.3%) to document the facies successions throughout the cores. The facies successions contain indications of sea-level changes on two different scales. First, there are high-frequency alternations of layers containing more platform-derived material with layers consisting of more pelagic sediments. In the Pleistocene and Pliocene periplatform section, these alternations are reflected in the ratio between neritic components and nannofossils, and mineralogically between aragonite and low Mg-calcite. In previous studies these cycles have been shown to correlate to the orbitally forced, high-frequency sea-level changes in the Quaternary. We found similar dm- to m-scale high-frequency changes in facies in the Miocene sections in all the holes. The Miocene cycles display a decimeter-thick alternation of light-colored, bioturbated, well-cemented biowackestones and less cemented, darker biowackestones with compacted burrows. The Miocene alternations are interpreted to reflect changes in the rate of neritic input (metastable aragonite and high-Mg calcite). Thus, the well-cemented wackestones represent higher neritic input and have a greater diagenetic input than the less-cemented intervals with less neritic input. Because of the color changes and the cementation differences, these cycles are also recorded in the color reflectance and the logs. Formation Micro-Scanner data provided a continuous record of these cycles at two sites, which will be analyzed to determine the frequency of these cycles in the Miocene.

A repetitive pattern of facies succession on the order of tens to hundreds of meters yields a sedimentary record of longer term sea-level changes. These larger scale patterns are imaged in the seismic sequences. Similarly, as in the small-scale cycles, changes in the amount of platform-derived material indicate periods of high and low sea level. Platform exposure during low sea level is reflected in reduced sedimentation rates, occasionally leading to hardground formation on the slope. In addition, erosion of the platform margin during these periods is documented by the deposition of coarse-grained packstones and floatstones in pelagic-rich background sediments. Erosional truncation is also observed in the proximal slope sites, which leads to hiatuses in the biostratigraphic successions. Cycles are best developed during sea-level rises and usually comprise the middle part of the sequences. Redeposition of platform carbonates occurs again in the upper portion of the sequences. This pattern is best developed in the Miocene when the platform had a ramp-like morphology. In the Pliocene and Pleistocene, the bulk of the sediments, especially in the more proximal locations, are dominated by thick successions of fine-grained material, but turbidites occur preferentially in the upper portions of the sequences. More mass gravity flow deposits are found in the distal sites, indicating that the steep upper slopes are bypassed by these flows.

Retrieve the low-latitude isotopic signals of the Ice House World in the Neogene and Quaternary and compare them with the stratigraphic record, potentially to document a causal link between eustasy and sequence stratigraphic pattern.

Conclusion: To compare the sedimentary record of the sea-level changes with the oxygen isotope proxy, a basinal hole (Site 1006) was drilled. The core far exceeded expectations, as most conventional nannofossil and planktonic foraminifers were found in the Pleistocene to middle Miocene section. In addition, recovery was very high and the preservation of the foraminifers was good throughout the recovered section. Thus, all the prerequisites are in hand to establish a oxygen isotopic record of the sea-level changes in the same transect. The correlation of the sedimentary and the isotopic record will enable us to assess the causal relationship between sea-level changes and the sequence stratigraphic pattern, thereby fulfilling the third objective within the sea-level theme.

Estimate the magnitude and rate of sea-level changes using age and recovered facies for a precise subsidence analysis.

Conclusion: Estimating the rate and magnitude of sea-level changes will require shore-based analyses, including further refinement of the age model (detailed biostratigraphic analysis and paleomagnetic, O and Sr-isotopic analyses) and detailed facies analyses of core samples and logs and log-seismic correlation.

Objective: The goal was to assess the processes responsible for fluid circulation in platforms by sampling slope sediments and analyzing their pore-water chemistry.

Conclusion: One of the primary objectives of Leg 166 was to investigate the possibility of fluid-flow processes through the margin of the Great Bahama Bank. The association of the fluid chemistry and heat-flow sampling program with the sea-level objectives means that changes in fluid chemistry cannot only be examined as a function of age and depth, but also within a sequence stratigraphic framework. This approach is therefore fundamentally different from other investigations of pore-water profiles along carbonate platforms that in most instances only drilled single holes.

The clearest evidence that there is active recharge of fluids through the margin of GBB is derived from the non-steady state profiles of both conservative and non-conservative elements in the upper 100 mbsf porewater profiles obtained from both the northern (Sites 1003-1007) and southern transect (Sites 1008-1009). A zone has been identified (the flushed zone) that is confined to the upper 40 mbsf, in which there is an absence of geochemical gradients in both conservative and non-conservative constituents. The geochemical data in this interval show essentially no change from bottom seawater concentrations for all of the normally measured cations and anions. Sites 1006 and 1007, which are situated further from the platform, also have a flushed zone, but it is reduced in thickness and exhibits small, but nevertheless, significant increases in Sr2+. The majority of these sites also showed irregular temperature profiles in this portion of the sedimentary column. A similar finding of an upper flushed zone approximately 40 m thick is also seen at the sites drilled along the more southward transect. We propose that the absence of geochemical gradients and the irregular temperature profiles support the notion that there is advection of seawater through this portion of the sedimentary column. The presence of small increases in Sr2+ at the more distal sites suggests that fluid flow here is reduced relative to the platform. Below the flushed zone there is a sharp change in the concentrations of both conservative and non-conservative elements. The SO42- concentration decreases sharply, whereas alkalinity and Sr2+ increase. The nature of these gradients are not steady state and reflect depression by a downward-advecting fluid. An unexpected finding at all the sites drilled was the increase in Cl- concentration with increasing depth. This increase is probably a result of the diffusion of Cl- and Na+ from an underlying brine or evaporite deposits.

Based on the evidence from the seven sites drilled during Leg 166, there is clear evidence that a mechanism exists that produces active exchange between the upper 40 m of sediments and the bottom waters. At the present time we do not know the precise mechanism involved in the flushing mechanism, only that it exists. The observations are consistent, however, with water being drawn into the platform by the mechanism known as Kohout convection. In this mechanism the temperature difference between the platform interior and the adjacent seaways causes underpressure to be developed within the platform, drawing water through the flanks of the platform.

Objective (secondary): Since the middle Cretaceous, several major changes occurred in the Earth's climate, fauna, and ocean circulation. The sediments in the seaways of the Bahamian archipelago potentially record most of these events, many of which are important global problems (for example, the onset of the Gulf Stream Current, the record of the Paleogene "Doubthouse" Earth, the influence of the Cuban collision, the K/T boundary, and the mid-Cretaceous drowning of the megabank).

Conclusion: Although operational time constraints precluded recovery of extensive Paleogene and older sediments, an excellent Neogene sedimentary section was recovered that will allow many of the paleoceanographic objectives to be pursued in post-cruise studies. In particular, the sedimentary section at Site 1006 consists of mixed pelagic and bank-derived carbonates, with varying amounts of clay material thought to have been derived from Cuba and Hispaniola. The excellent continuous sedimentary sequence recovered contains abundant, well-preserved foraminifers. Furthermore, the abundant pelagic biogenic components are less diluted by platform-derived material and the microfossil preservation is less affected by diagenesis than in the upper slope sites (Sites 1003, 1004, and 1007). The expanded Pliocene and upper Miocene sequence combined with the excellent preservation will allow this site to become a classic site for upper Neogene paleoceanography in the low-latitude Atlantic. In addition, changes in the composition of the sediments are postulated to vary in conjunction with variations in the strength of the Florida Current. These variations would be able to be correlated with changes in sea level as recorded by the prograding and regressive sequences at platform sites and the oxygen isotopic signature of the foraminifers.

To Semiannual Report Contents

To Program Updates, Leg Highlights Contents