The Bahamas Transect has three objectives: (1) to retrieve the record of NeogeneQuaternary sea-level fluctuations in prograding carbonates; (2) to study the fluid flow through the margin of an isolated platform; and (3) to assess the paleoceanographic changes in the Straits of Florida since the middle Cretaceous.
Sea -level Objective
The main objective of the proposed drilling transect along the western margin of Great Bahama Bank is to study the record of NeogeneQuaternary sea-level fluctuations in the prograding sequences. Within this sea-level objective are the following goals.
1. Determine the timing of the sequence boundaries and relative sea-level fluctuations to acquire the necessary data base for the possible global synchroneity of these fluctuations.
2. Determine the stratigraphic response of carbonates to sea-level changes of variable frequency by analyzing the facies of the stacked depositional sequences. A special emphasis will be placed on documenting the amount and nature of lowstand deposits in carbonates and the hierarchical stacking of high-frequency cycles into seismic sequences.
3. Retrieve the low-latitude isotopic signals of the Ice House World in the Neogene and Quaternary and compare it with the stratigraphic record potentially to document a causal link between eustasy and sequence stratigraphic pattern.
4. Estimate magnitude and rate of sea-level changes using age and recovered facies for a precise subsidence analysis.
Fluid -flow Objective
There is increasing evidence of active fluid movement deep within carbonate platforms. This evidence was derived from BDP results and also from recent ODP legs on the Queensland Plateau and in the Pacific (Elderfield et al., 1993; Paull et al., 1995). Based on drilling on the western portion of the Great Bahama Bank, it is now known that the majority of the alteration from metastable carbonates to low-Mg calcite and dolomite occurred in a regime dominated by seawater rather than meteoric fluids (Melim et al., in press), suggesting the presence of massive seawater circulation in the subsurface of the Bahamas. Further evidence for circulation was derived from the analysis of borehole fluids retrieved during the drilling. The chemical analyses of borehole fluids reveal that in some areas there is a rapid buildup of Sr, resulting from the recrystallization of carbonate, yet in other areas the Sr content is not elevated. The absence of gradients, despite substantial carbonate recrystallization, indicates areas in which active circulation is taking place, whereas in other areas where Sr content is elevated, circulation is stagnant or not as active. In summary, the samples collected during the BDP confirmed the existence of fluid movement, but there is still insufficient information to resolve the fundamental question about the flow mechanism within the platform. The goal is to assess the processes responsible for fluid circulation in platforms by sampling slope sediments and analyzing their pore-water chemistry.
Since the middle Cretaceous, several major changes occurred in Earth 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:
1. The onset of the Gulf Stream current: The Bahamas Transect is located on the periphery of the modern-day Florida Current/Gulf Stream current that influences global climate and ocean circulation. The timing of the onset of this circulation, however, still is not precisely determined. Leg 101 documented the existence of the Florida Current from the Oligocene onward (Austin et al., 1988), but it probably started sometime in the Late Cretaceous. The breakup of the carbonate-platform system on the Nicaraguan Rise in the middle to late Miocene might have strengthened the current (Buffler et el., 1994). Drift deposits on seismic lines are observed to interfinger with the distal portions of the prograding sequences. Thus, the sediments retrieved in distal holes of the Bahamas Transect potentially record the onset and changes in strength of the Florida Current.
2. The record of the Paleogene "Doubthouse" Earth: The latest Paleocene to Middle Eocene is an interval that has at least two unconformity-generating events. However, this time interval is controversial, with climatic interpretations ranging from glacial to nonglacial. A comparison of the delta 18O proxy of sea-level change for this interval with the isotopic values of the overlying Neogene can test their similarity with both nonglacial and glacial events. Because these unconformities occur at a time of relatively frequent magnetic polarity changes, they can be dated accurately and are suitable to evaluate synchroneity.
3. The influence of the Cuban collision: The collision of Cuba with the North American plate in the (?) Late Cretaceous/early Cenozoic to Late Eocene transformed the Bahamian archipelago into a foredeep. The onset of the collision and the timing of the subsequent collisional events are not well constrained. In the relatively distal location of the Bahamas transect, these tectonic events probably are recorded in increased subsidence because of loading, and will be observed in the sedimentary record as variations in sediment thickness with an increase of mass-gravity flows.
4. The K/T boundary: The K/T boundary potentially is preserved in the Straits of Florida, although the boundary was not preserved at ODP Site 627 north of Little Bahama Bank.
5. The middle Cretaceous drowning of the megabank: Toward the end of the Early Cretaceous, a large megabank was established along the eastern and southern margins of the North American continent. This "megabank" segmented and partially drowned in the Albian to Cenomanian. Documenting the extent of the megabank, as well as the cause and timing of its disintegration, was one of the major objectives of Leg 101. The drilling was intended to resolve the controversy between scientists who interpreted the modern platform configuration as a reflection of the Early Jurassic rift topography (Mullins and Lynts, 1977), and those who viewed the modern archipelago as a remnant of a large megabank that drowned in the middle Cretaceous (Sheridan et al., 1981, Ladd and Sheridan, 1987; Schlager and Ginsburg, 1981). The debate continues in the wake of Leg 101 because drilling failed to reach the Mid-Cretaceous Sequence Boundary (MCSB) in the Straits of Florida and Northeast Providence Channel. Drilling the MCSB along the Bahamas transect would add important information to the controversy and would achieve a goal that was intended, but never attained during, Leg 101. Time limitations, however, might prevent Leg 166 from reaching this target horizon.
To understand the sedimentary response to sea-level changes and achieve a well-dated record, a transect of holes is needed from the shallow platform (proximal) to the adjacent deep water (distal). The two core borings, Unda and Clino, in conjunction with three shallow borings on islands (McNeill and Ginsburg, 1992) on the platform provide the record in the proximal part of the sequences (Fig. 2). The deeper water sites (relative to the BDP holes) are planned to be drilled by the JOIDES Resolution during Leg 166. A transect of four sites is planned to retrieve the sedimentary record of the sequences in several locations. An additional lateral site is intended to test continuity and lateral facies variations of the prograding lobes. The thick Quaternary sediment package on the leeward slope of the GBB will provide a high-resolution record of the climate and short-term sea-level fluctuations in the Quaternary. The target horizon is the base of the Neogene. Successful drilling of the deep-water sites by the JOIDES Resolution will generate a data set that can be compared with the NJ/MAT of Leg 150 for a test of the synchroneity of sea-level changes.
A deep drill hole penetrating the MCSB is required to answer the paleoceanographic objectives. The most basinward drill site of the proposed northern transect described above is situated on the periphery of the Florida Current, permitting the determination of the influence of the Florida Current on the deposition of the sequences and possible changes of its intensity during the Neogene. The periphery of the Florida Current is the ideal location for answering questions regarding evolution of the current. Unfortunately, such a location is prone to have large hiatuses due to winnowing, thus it is less suitable for all the other objectives. Therefore, a location closer to the platform, the second most distal site, is chosen as the location for a deep hole. The target depth is approximately 1400 m.
It is assumed that in areas where unconsolidated muddy sediments are onlapping onto older cemented rocks, fluids are moving into or out of the platform and may partially move through the sediment. Coring the sediment and squeezing its waters can provide an idea of the composition of these fluids. As one progresses away from the platform, the influence of advection probably will lessen and the pore water chemistry will become dominated by diffusion. It is proposed to sample interstitial waters along two transects away from the platform edge. The first transect will partially coincide with the northern transect and utilize the same sites as the two most proximal sites. Penetration will be approximately 100300 m depending on thickness of the unconsolidated sediment at each location. The second transect, consisting of three sites, will be located further to the south, where the Holocene sediment wedge is somewhat thinner (Fig. 1).
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166 Table of Contents