166 Preliminary Report


Ocean Drilling Program Leg 166 drilled a total of 17 holes at seven sites (Sites 1003 through 1009) on the western flank of the Great Bahama Bank, recovering almost 3 km of core ranging in age from late Oligocene to Holocene. Leg 166 was designed to address two important geologic themes: (1) causes and effects of eustatic sea-level fluctuations and (2) fluid-flow processes in the margins of isolated platforms. To address fundamental questions regarding sea level fluctuations, five sites in the Straits of Florida were drilled (Sites 1003-1007), 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 (Clino and Unda) drilled previously on the western Great Bahama Bank as part of the Bahamas Drilling Project represent the shallow-water sites of the transect. The primary goal of the transect was to document the record of the Neogene-Holocene sea-level changes by determining the ages of the major unconformities in the sedimentary record and comparing the timing of these unconformities with ages predicted from the oxygen isotopic record of glacio-eustasy. Core borings from the complete transect document the facies variations associated with oscillations of sea level, and, therefore, 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 glacio-eustasy and the stratigraphic pattern.

With the sedimentary sequence recovered in advanced hydraulic piston coring, extended core barrel, and rotary core barrel drilling, and the abundance of biostratigraphic markers, it was possible to define the ages of the Neogene sequence boundaries and to test their consistency with ages determined in the boreholes previously drilled on the platform. This yielded an excellent correlation between sites, documenting the age consistency of the sequence boundaries and chronostratigraphic significance of the seismic reflections. By reaching the Oligocene at Site 1007, a complete record for the sequence stratigraphic architecture is available for the entire Neogene. At all five transect sites (Sites 1003, 1004, 1005, 1006, and 1007), alternating high (up to 15 to 20 cm/k.y.) and low sedimentation rates (<2 cm/k.y.) reflect a long-term pattern of (1) bank flooding (0.5-2 m.y.), (2) concomitant shedding to the slope and periods of exposed banks, (3) a shutdown of shallow-water carbonate production, and (4) largely pelagic sedimentation. The pulses of bank-derived material coincide with the prograding pulses seen in the seismic data as sequences, which with their geometries indicate base-level lowerings as a result of sea-level falls.

Within these long-term changes, high-frequency sea-level changes are recorded in decimeter scale depositional cycles. The ages of 17 seismic sequence boundaries (SSB) were determined. On the slope sites, several of these sequence boundaries coincide with biostratigraphically detected hiatuses indicating erosion. In some cases, slightly older ages of the SSB, when compared to the more distal sites on the slope, also suggest erosional downcutting. Therefore, to determine the ages of the sea-level changes, the ages of the SSBs were taken at the conformable portion of the sequences. They yielded the following preliminary ages: 0.1, 0.6, 1.7, 3.1, 3.6, 5.6, 8.7, 9.4, 10.5, 12.1, 12.5, 15.1, 16.0, 18.2, 19.4, 23.2, and 23.7 Ma. A comparison of seismic sequences with the global sea-level curve indicates that nearly all major (third-order) sea-level changes are recorded along the Bahamas Transect. This cyclic sedimentation is best developed in the Pleistocene and Miocene. The Pleistocene cycles display the characteristics of well-documented periplatform aragonite cycles where an increase in aragonite content coincides with sea-level highstands. 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 potential and have a greater diagenetic input than the less cemented intervals with less neritic input. Facies and diagenesis act in concert to produce petrophysical differences within the sedimentary section. Thus, log data closely image the sedimentary and stratigraphic record, and the sequence boundaries can be clearly identified on these data. Consequently, the logging data may also be used to fill in gaps in core recovery.

The second objective of Leg 166 was to investigate the possible fluid flow within carbonate platforms. To achieve this objective, the sites of the transect were complemented by three additional shallow holes on the upper slope (Sites 1004, 1008, and 1009). The composition of the interstitial fluids retained in the sediment are used to assess rate and flow mechanisms through the bank.

In the upper 20-40 m of all sites, pore-water chemistry profiles display near-normal seawater concentrations of all constituents. The zone is thickest and most pronounced in the proximal, upper slope sites (Sites 1003, 1004, and 1005). This zone probably experiences pervasive flushing of seawater that prevents diffusional gradients from developing between the overlying seawater and the underlying, more saline fluid. Below the flushed zone, diffusive processes dominate the gradients of conservative elements, whereas non-conservative constituents show changes that are locally controlled by reactions such as organic matter remineralization and the recrystallization of carbonate minerals. These reactions are a result of the high in situ abundance of organic material trapped within these marginal sediments during rapid Pliocene-Pleistocene platform shedding events. The organic matter provides the fuel that drives the early carbonate alteration reactions in these shallow sediments. Superimposed on these downhole trends below the flushed zone, changes in gradients (e.g., localized increases in SO4 and Cl-) at similar depths provide strong evidence that flow is at least partially horizontal and related to stratigraphic boundaries. The deep interstitial waters show concentrations of Cl- and other minor elements that are elevated relative to seawater (nearly twice that of normal seawater at Site 1003). The source of this high Cl- is at present not known, but it could result from upward migration of a high salinity brine from an evaporitic deposit deeper in the Straits of Florida or beneath the Great Bahama Bank.

166 Introduction

166 Table of Contents

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