166 Preliminary Report


Scientific drilling during Ocean Drilling Program (ODP) Leg 166, the Bahamas Transect, addressed two important geologic themes: (1) causes and effects of eustatic sea-level fluctuations, and (2) fluid-flow processes in continental margins. These themes were identified as first-order priorities by several planning groups of ODP (COSOD II, 1988; JOI/USSAC Workshop El Paso [Watkins and Mountain, 1990]; JOI/USSAC Workshop Arrowhead [Kastner and Brass, 1990]; and the Long Range Planning Documents, [JOI, Inc., 1990, 1996]). Specific approaches and drilling strategies were recommended by the ODP planning structure to gather the data necessary to answer questions pertaining to these themes. Based on this mandate, Leg 166 was designed as an integrated investigation of the designated themes within the context of a carbonate platform setting.

Leg 166 is the second transect (Leg 150 on the siliciclastic New Jersey margin was the first) in the ODP effort to examine the record of sea-level changes within the paradigm of sequence stratigraphy following the criteria established by the aforementioned planning groups. During Leg 166, the JOIDES Resolution drilled a transect of five sites (Sites 1003, 1004, 1005, 1006, and 1007) on the western flank of the Great Bahama Bank (GBB) (Figs. 1, 2). Sites 1008 and 1009, drilled further south on the western margin, were dedicated primarily to addressing fluid-flow objectives.

ODP Leg 166 was devoted to obtaining a transect of cores to ascertain the timing and amplitude of Neogene sea-level changes. The GBB is an ideal location to assess sea-level changes because it combines three independent ways of measuring sea-level changes: (1) it is a flat-topped platform on a passive continental margin, and because it is level, cores drilled into it should record sea-level variations with accuracy, (2) the prograding sequences on its margin should record sea-level changes in their stratigraphic pattern, and (3) the correlative deep-water deposits encode the delta18O proxy of sea-level changes in their foraminiferal assemblages. Correlation of the glacio-eustatic oxygen isotopic record with the sequence stratigraphic pattern of sea-level change can potentially document a causal link between glacio-eustasy and the stratal pattern. This correlation should also provide insights into how high-frequency sea-level fluctuations are recorded in these sediments and how the stacking of these high-frequency cycles combines to produce the lower-order seismic sequences. There is increasing evidence that there is active fluid movement deep within carbonate platforms, as demonstrated by the results from ODP Leg 133 on the Queensland Plateau, Australia (Elderfield, et al., 1993), ODP Leg 143 in the Pacific (1993; Paull et al.,1995), and from the Bahamas Drilling Project (Swart et al., in press a, in press b). Evidence for fluid flow can be observed in temperature and geochemical profiles. Temperature measurements in the sediments can detect nondiffusive geothermal gradients that indicate water movement either into or out of the formation (e.g., Davies, McKenzie, Palmer-Julson, et al., 1991). In addition, geochemical interstitial water measurements can show the influence of waters other than those indigenous to the formation. In a manner analogous to the temperature profiles, a uniform profile with depth could indicate fluid movement into the formation. Conversely, a concave upwards profile could imply fluids moving out of the formation. Lateral fluid movement may be indicated by erratic changes in the nature of the geochemical or temperature profiles.

The sea-level-related objectives of ODP Leg 166 were to:

1. determine the timing of the sequence boundaries and relative sea-level fluctuations. The ages of these sequences will contribute to the database that is necessary eventually to evaluate the global synchrony of sea-level fluctuations in the Neogene;

2. determine the stratigraphic response of carbonates to sea-level changes by analyzing the facies of the recovered depositional sequences. A special emphasis was placed on documenting the amount and nature of lowstand deposits in carbonates and on the hierarchical stacking of high-frequency cycles into seismic sequences;

3. retrieve low-latitude isotope signals of the Icehouse World in the Neogene and Quaternary and compare it with the stratigraphic record to evaluate whether there is a causal link between eustasy and sequence stratigraphic pattern; and

4. estimate the magnitudes and rates of sea-level changes using ages and recovered facies for a precise subsidence analysis.

To accomplish the fluid-flow objectives, drilling was designed to:

1. assess the possibility that there is fluid flow into and out of the platform along the margins, and

2. test the proposed mechanisms of fluid flow.

To address these topics, seven sites were drilled on the western flank of GBB (Figs. 1, 2).

Site 1003 (proposed Site BT-2) is one of five sites along a transect connecting the shallow bank with the deeper water areas. Site 1003 is located on the middle slope of the prograding western margin of Great Bahama Bank, approximately 4 km from the platform edge and 12.6 km from the borehole Clino, which was drilled on the banktop from a self-propelled jack-up barge. The primary objectives of Site 1003 were to penetrate to the base of the Neogene to evaluate facies of lowstand vs. highstand deposits and to determine the respective amount of redeposited strata in both systems tracts. Additional objectives were (1) to refine the ages of the sequence boundaries, (2) to determine the nature of a prominent unconformity on top of a low-amplitude to transparent seismic zone at 0.75 s two-way traveltime (TWT) that is interpreted to be the top of the early Pliocene, (3) to produce a high-resolution isotope stratigraphy of the Neogene to Holocene, and (4) to evaluate fluid flow in the middle of the lower slope.

Site 1004 (proposed Site F-3) is located between Sites 1003 and 1005 on the same dip line. The primary objective of Site 1004 was to obtain heat-flow and interstitial water geochemistry measurements to be used in conjunction with data obtained from Sites 1003, 1005, 1006, and 1007 to ascertain possible fluid flow and diagenetic reactions in the carbonate margin. In addition, this site serves as a location for the study of high-resolution sea-level changes during the Pleistocene, adding information to the comprehensive sea-level objectives of Leg 166.

Site 1005 (proposed Site BT-1A/F-1) is the most proximal site of five sites along the Bahamas Transect from the western margin of GBB into the Straits of Florida. Operational safety considerations required relocating Site 1005 to deeper water than the originally proposed Prospectus location (BT-1/F-1). Site 1005 is located in 352 m water depth, approximately 1150 m from the modern platform edge on the upper slope, 30 shot points SW of the crossing of seismic lines 106 and 107. It is positioned on the thickest part of the prograding Neogene sequences seen on the seismic line. The target depth of 700 m was designed to penetrate seven seismic sequences. The sea-level objectives of Site 1005 were (1) to date the sequence boundaries precisely; (2) to determine the facies within the different systems tracts, especially the nature of the onlapping units that were interpreted as lowstand deposits; and (3) to retrieve a high-resolution record of climate and sea-level fluctuations for the Quaternary and late Pliocene. This site served also as the proximal site for the fluid-flow transect.

Site 1006, (proposed Site BT-5), is located in 650 m of water approximately 30 km from the western platform edge of Great Bahama Bank. It is the most distal of the five transect sites positioned at the crossing of seismic lines 105 and 106. Drilling at Site 1006 was designed to core and log a thick sequence of drift deposits, which are thought to extend through the Neogene with relatively few hiatuses. There were two principal objectives at this site. First, the sediments at Site 1006 were believed to have a greater pelagic component than the proximal sites (Sites 1003, 1004, 1005, and 1007) of the transect, so they should be well suited for dating and establishing an oxygen and strontium isotope stratigraphy for the Neogene, and thereby provide an independent indicator of sea level. These datums could be traced on the seismic reflections to the distal lobes of the prograding margins in the proximal sites. This approach will help date the proximal sites where the chronostratigraphy is occasionally unclear because of poor recovery, diagenesis, and/or dilution by neritic sediments. Second, changes in the sediment composition were postulated to vary in conjunction with variations in the strength of the Florida Current. These variations would correlate with changes in sea level, as recorded by the prograding and regressive sequences at platform sites and the oxygen isotopic signature of the foraminifers.

Site 1007 (proposed Site BT-3), at the crossing of seismic lines 106 and 102B, was located on the toe of the western Great Bahama Bank slope at a water depth of 647 m. The target depth was the base of the Neogene which was estimated to be at approximately 1230 mbsf. The site was positioned to penetrate the thin basinal portions of 16 prograding sequences and, in the upper part, a thick onlapping wedge that could be either a drift deposit or an accumulation of mass-gravity flows. To the west, the distal portions of the prograding clinoforms interfinger with the drift deposits of Site 1006. Site 1007, therefore, was the link between this basinal site and the proximal slope sites to the east. In addition, a higher content of microfossils with good preservation was expected than in the more proximal Sites 1005, 1004, and 1003. Therefore, the main sea-level objectives of this site were (1) to precisely date the sequence boundaries, (2) to determine the facies in the distal portions of carbonate sequences, and (3) to assemble a data set suitable to compare the sedimentary record with the oxygen isotope record of the Neogene to Holocene sea-level fluctuations. Site 1007 was also a intermediate site for the fluid-flow studies. Interstitial water chemistry and in situ temperature measurements at this site should provide information about the fluid movement in the lower slope, and, by comparison with the three previously drilled proximal sites, assess lateral changes in fluid chemistry. A logging suite, including a vertical seismic profile experiment, was performed for optimal correlation between the cores and the seismic data.

A second transect was drilled further south on the margin of the western GBB. The primary objective of the second transect was to assess possible variations in the fluid-flow patterns. Site 1008 (proposed Site F-6) was drilled in 437.1 m of water, and Site 1009 (proposed Site F-4A) in 307.9 m of water. Operational safety considerations required relocating Site 1009 to deeper water than the proposed Prospectus location (F-4). Prospectus Site F-5 was not drilled as a result of operational time constraints.

166 Overview

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