166 Prospectus


Regional Geologic Setting

The Bahamas archipelago is a carbonate province consisting of several isolated platforms situated on the southern end of the eastern continental margin of the United States. The margin formed in the Jurassic as a result of the opening of the Atlantic and underwent collision with the Caribbean plate during the late Cretaceous, culminating in the Eocene. During this time, the Bahamas were part of the foreland basin, which resulted in some folding, faulting, and increased subsidence in areas close to the plate boundary (Ball et al. 1985; Walles, 1993). With the shift of the plate boundary south of Cuba at the end of the Eocene, Cuba remained welded to the southern part of the American continental margin and tectonic activity ceased.

The foundation of the Bahamian platforms was and still is a subject of debate that centers around two major questions: (1) whether the Bahamian basement is continental or oceanic and (2) whether the modern platform pattern still reflects the graben and horst topography that formed during the Jurassic rift stage (Mullins and Lynts, 1977; Ladd and Sheridan, 1987; and references in both). The character of the basement underlying the platforms is unknown from drilling, but refraction seismic data indicate a 12- to 14-km-thick crust with a wave velocity of 7.2 km/s (Sheridan, 1972). These velocities can be interpreted as derived from either altered continental crust or oceanic crust.

Two fundamentally different concepts were proposed to explain the origin of the pattern of the Bahamian platforms. The graben hypothesis assumed that the modern array of platforms reflects the block-faulted topography of the Jurassic rift stage in which the intra-platform seaways are remnants of the grabens, whereas the platforms are located on ancient horsts (Mullins and Lynts, 1977). The megabank hypothesis proposed that the Bahamian platforms were part of a much larger extensive shallow-water carbonate platform under which inherited rift topography was buried in an early stage of platform evolution (Sheridan et al., 1981). This megabank was drowned during the middle Cretaceous, and only small, isolated platforms reestablished and became the foundation of the modern platforms (Schlager and Ginsburg, 1981). One of the objectives of ODP Leg 101 was to test these hypotheses, but coring results did not entirely settle the debate. They did, however, confirm the existence of a larger platform in the Bahamas/Blake Plateau area at the end of the Early Cretaceous, but also a deep-water reentrant at the location of the modern Northeast Providence Channel (Austin, Schlager, et al., 1988).

In the late 1980's, seismic reflection profiles across the top of the GBB revealed the anatomy of part of the bank, and to a large extent, settled the debate. The seismic profiles show that the modern pattern is neither the reflection of a topography that formed in the Jurassic nor in the middle Cretaceous, but is modified continuously by destructive and prograding processes (Eberli and Ginsburg, 1987, 1989). The profiles document that tectonism controlled the initial pattern of the platforms, and progradation subsequently filled some of the seaways. In addition, seismic stratigraphy indicates a repetition of the process of tectonic segmentation and depositional coalescence which continuously modifies the platform pattern (Eberli and Ginsburg, 1987, 1989). The repetition of segmentation implies that tectonic events related to the American/Cuban collision are important in the development of the GBB. The productive bank, however, covers faults and buries the tectonic relief by the infilling of intraplatform seaways and bank margin progradation. In the GBB, extensive progradation advanced the modern western bank edge to 27 km basinward over the original fault (Eberli and Ginsburg, 1989). The Bahamas Transect is located on this western prograding margin of the GBB (Figs. 1, 2).

Results of the Bahamas Drilling Project

BDP operations on the western margin of the GBB aimed to address (1) the nature of progradational facies, and the source of sediment, (2) the timing and the rate of progradation, (3) the role of sea level in controlling progradation, and (4) the cause of seismic reflections in such a pure carbonate environment.

The Seismic Record

The holes of the BDP were positioned on a multichannel seismic grid that crossed the GBB platform margin (Fig. 1). The geometry and internal architecture of eight sequences seen on this line (Fig. 3) is interpreted to reflect the following general relative sea-level history. A lowering of sea level during the deposition of the basal sequence h at the middle/upper Miocene boundary(?) shifted the platform edge more than 10 km basinward. Two subsequent sea-level changes created little accommodation space on the platform during the rise, but deep incisions were created during the falls of sea level. Thus, the three basal sequences are thought to be deposited during a long-term relative lowstand of sea level. A major backstep of the margin is observed in the following sequence, indicating a high-amplitude sea-level rise (during the late early Pliocene) with which the platform could not keep pace. With a decrease of the rate of rise, the platform was able to fill all the accommodation space in an aggradational/progradational pattern. The subsequent sea-level rises overstep the platform only slightly. As a result, a situation similar to a forced regression was created that led to the major prograding pulses seen in the upper Pliocene and Pleistocene sequences c, b, and a.

The Sedimentary Record

The two shallow-water cores (Unda and Clino) of the BDP were positioned on the prograding western margin, where a high sedimentation rate yields a high chronostratigraphic resolution. Clino penetrated mostly inclined slope deposits, whereas Unda, the more proximal site, penetrated platform deposits and a buried platform margin. Continuous cores were made using a wireline system with a triple-tube core barrel 3.05 m (10 ft) long that recovered cores of 6.3 cm (2.5 in) in diameter. Core Clino was drilled 677.71 m (2222.0 ft) below the mud pit datum (7.3 m, 24 ft above sea level). Recovery in Clino averaged 80.8%. Core Unda was drilled 454.15 m (1489 ft) below the mud pit (5.2 m, 17 ft above sea level). Recovery in Unda averaged 82.9%.

Detailed sedimentologic studies in two core borings revealed multiple, punctuated depositional sequences with different frequencies that are interpreted as the sedimentary record of relative sea-level changes (Fig. 4). In the shallow portions, caliche crusts, karst, and black pebble conglomerates are taken as indicators of exposure horizons, whereas, in the slope section, changing sediment composition is correlated to fluctuations in sea level.

Unda consists of three successions of shallow-water platform sands and reefal deposits (108.1­8.6 m, 354.7­292.8 m, 453­443.5 m), making up ~40% of the core, that alternate with sand and silt-sized deeper marginal deposits (Fig. 4). The upper two platform/reefal intervals show evidence of repeated episodes of shoaling and/or subaerial exposure. The intervals between the three platform/reefal units consist of fine-grained skeletal to mixed skeletal/nonskeletal silts and sands. The intervals are arranged in several successions that are interpreted as depositional sequences reflecting changes in relative sea level. The tops of five of the successions contain marine Fe/Mn hardgrounds or condensed layers with concentrations of phosphate and/or reworked benthic foraminifers.

In Clino, a single unit of platform/reefal sediments overlying a thick succession of slope sediments was recovered (Fig. 4). The shallow platform section (98.5­21.6 m) has at least seven parasequences, each of which is capped by a horizon of subaerial exposure. In the reefal unit (197.4­98.5 m), there is an upward progression from deep reef/fore slope to fore reef to reef crest and, finally, to back reef. The nearly 480 m of slope sediments (676.6­197.4 m), consists predominantly (80%) of monotonous background sediment of fine sand- to silt-sized skeletal and non-skeletal grains interrupted by 12 intervals of coarse-grained skeletal sands; five of these intervals are associated with marine hardgrounds or firm grounds. The alternating intervals of skeletal interruptions, overlain by intervals of background sediment, reveal a pattern of three larger depositional sequences with an average thickness of ~170 m, each containing two to three smaller scale sequences with a thickness ranging from 25 to 90 m. The sediment composition of the interruptions with reworked foraminifers, lithoclasts, and coarse skeletal debris indicates deposition during sea-level lowstands. Flooding events are expressed as marine hardgrounds and/or deposits of reworked material. Sedimentation during times when the platform is flooded is characterized by the fine-grained mixed skeletal and peloidal packstones and wackestones.

The Correlation Between Cores and Seismic Sections

A vertical seismic profile and a synthetic seismic profile derived from the sonic and density logs were used to tie the cores Unda and Clino to the seismic sequences. Within the resolution of the seismic reflection, all but one seismic sequence boundary correlated with a lithologic facies change (Figs. 4 and 5). The lithologic indications of sea-level changes coincide with the interpretation drawn from the sequence stacking pattern. At the platform interior site Unda, four sequence boundaries are associated with subaerial exposure horizons. Within the slope section Clino, the inferred positions of the seismic sequence boundaries coincide with a hardground and/or an erosional contact (Fig. 4). Both surfaces are overlain by coarse-grained redeposited grainstones and packstones that probably were deposited during relative lowstands of sea level.

Chronostratigraphy of the Core Borings

High-resolution chronostratigraphy is obtainable in prograding-margin carbonate through a combined age-dating approach involving micropaleontology, strontium isotope stratigraphy, and magnetostratigraphy. This integrated dating showed that an understanding of the slope dynamics and depositional system is critical toward interpretation of the biostratigraphic data, as extreme dilution by platform-derived sediments occurs during margin progradation. As such, the occurrence and highest abundance of microfossils is restricted to the thin units of pelagic-rich sediment deposited during temporary intervals when platform sediment supply was shut down. Consequently the conventional open-ocean biostratigraphic approach must be used with caution because of the possibility of time erroneous first and last appearance datums (FADs and LADs, respectively). These biostratigraphic datums can be either premature LADs or delayed FADs resulting from the overwhelming dilution of shallow-platform-derived sediment. Although low in abundance, microfossils found in the platform-rich prograding units usually appear to be good indicators of depositional age and still provide a most powerful tool for age determination.

The data from BDP cores define six units in one hole and five in the other, bracket the biozones present and their ages, and show that they are correlative between the holes. The ages range from a maximum of ~12.2 Ma to a minimum of ~1.6 Ma, but they include numerous periods of inferred erosion and/or nondeposition. The largest condensed interval/hiatus (~1.2 Ma) occurs at the Miocene/Pliocene boundary.

The biozones range sequentially from middle Miocene Globoratalia fohsi robusta Zone N12 to at least the uppermost Pliocene part of G. crassaformis viola Subzone N22, but the foraminifers indicate that deposition was not continuous. Recognition of G. tosaensis tosaensis Zone N21 is very tentative and suggest that the biozone may have accumulated on the shelf, but its absence on the slope is consistent with a widespread regional unconformity.

Sedimentation rates and positions of series boundaries vary widely in both holes. At the margin, the Miocene/Pliocene boundary is placed at a depth of 542 to 532 m in a condensed section; the lower/upper Pliocene boundary occurs at or near 444 m; and the Pliocene/Pleistocene boundary occurs within the top 366 m of the hole, a reasonable assessment considering an exceptionally high rate of sedimentation (536 m/m.y.) for the interval. On the bank top, the Miocene/Pliocene boundary lies between 295 and 278 m; the lower/upper Pliocene boundary is more precisely placed at or near a depth of 236 m; and the Pliocene/Pleistocene boundary lies within the top 110 m in the hole. Bank-top sedimentation rates ranged from near zero at the condensed interval to a late Pliocene high of 279 m/m.y.

The availability of a high-resolution chronostratigraphy enables the development of well-constrained platform evolution and sea-level records. Three major progradational episodes were delineated using seismic stratigraphy, lithostratigraphy, and depositional age information. Progradation occurred during the late Miocene, late early Pliocene, and latest Pliocene, a time period considered a "lowstand" on much of the shallow platform. In the Pliocene shelf/ramp setting, margin progradation initiates during the highstand but also occurs in a forced regression-type situation during a fall in sea level. Rapid reef progradation occurred near the end of the Pliocene and early Pleistocene when the platform had infilled the proximal slope sufficiently to provide a near-horizontal migration surface. The transformation from a shelf/ramp platform topography to a horizontal top platform started during the late Pliocene and culminated in the middle Pleistocene. As a result, the distinction between highstand and lowstand was much less distinct during much of the late Cenozoic because the GBB had a gently dipping shelf/ramp morphology. The steep platform margin and the associated abrupt on/off nature of sea-level highstand/lowstand seen today, and through much of the middle and late Pleistocene, are fairly recent developments. Pre-middle/upper Pleistocene basin/periplatform deposits around the Bahamas should be interpreted with this new understanding of platform topography.

Ages of Sequence Boundaries

Sequence boundaries (SB) identified in multi-channel seismic lines before drilling (Fig. 4), and refined after coring and well-logging, can now be examined in a chronostratigraphic framework. The age of seismic reflections forming sequence boundaries are summarized below in Table 1.

Table 1 Ages of seismic sequence boundaries.

Sequence Depth Estimated age of
Boundary 1 (m/ft below mud pit) seismic sequence boundary

Unda Clino

SB-7 g/h 441.9/1450 NR middle Miocene <12.6 Ma
SB-6 f/g 365.8/1200 NR late Miocene, 5.9­8.9 Ma
SB-5 e/f 292.6/960 545.6/1790 Miocene/Pliocene, 5.35­4.71 Ma
SB-4 d/e 149.4 /490 377.9/1240 late Pliocene, ~2.1­2.48 Ma
SB-3 c/d 50.3/165 118.9/390 early Pleistocene, 1.66­0.83 Ma
SB-2 b/c 30.5/100 33.5/110 late Pleistocene, <0.83 Ma
SB-1 ------- 21.3/70 late Pleistocene, <0.83 Ma
Note: NR = not recorded

Relevance of the BDP to Questions of Sea-Level Changes

Clino and Unda, in conjunction with other shallow core borings in the Bahamas, provide a first step in deciphering the record, timing, and magnitude of sea-level change. The oldest sea level event recorded is a middle Miocene (<12.6 Ma) highstand event that flooded an existing shallow platform and resulted in slope/open-shelf facies near Unda. This highstand flooding gave way to a prolonged upper middle Miocene condensed/hiatal interval, capped by shallow-water facies and a distinct discontinuity surface. These middle Miocene sea-level changes occurred before the latest Miocene (Chron 3An, 5.9 Ma). A subsequent late Miocene event deposited shallow-water reefal sediment near Unda, perhaps as part of a major rise in eustatic sea level. Similar upper late Miocene age deposits have been recovered and dated from much shallower portions of other Bahamian platforms. The early Pliocene consisted of a major sea-level rise that forced eastward backstepping of the shallow-water platform. The subsequent highstand resulted in major progradation of the western margin of the GBB. The late Pliocene again saw progradation of the margin and the westward shift of the clinoform depocenter. This late Pliocene highstand event was interrupted by a fall in sea level, temporarily reducing sediment production on the platform and resulting in a marine hardground on the proximal slope. At Clino, deposition of a pelagic-rich unit occurred during this drop in sea level, followed by extremely high sedimentation related to the westward shift of the upper-shelf zone of sediment production. This lowstand is correlative to the buildup of Northern Hemisphere glaciers and the worldwide drop in eustatic sea level. The late Pliocene/Pleistocene lowstand is consistently recorded in Bahamian platforms as well as around the world. The numerous subaerial exposure horizons in the upper portion of both Unda and Clino provide a record of the frequent middle and late Pleistocene highstand flooding events.

Seismic reflections and major sequence boundaries appear to be synchronous within the age-dating resolution of the chronostratigraphy of the BDP (Fig. 5). Especially well-constrained is the late Miocene sequence boundary, the late Pliocene sequence boundary, and a Pliocene/Pleistocene sequence boundary. The late Miocene and late Pliocene sequence boundaries both represent periods of erosion and/or nondeposition on the slope, and can be tied to major changes in sea level.

As recommended by the SL-WG, a complete transect from the shallow to the distal part of a continental margin is required to collect the necessary data to address questions surrounding sea-level changes. The results of the BDP summarized above provide the shallow-water sites for such a transect of a carbonate platform. Thus, one of the primary objectives of Leg 166 is to provide a transect from the proximal to the distal part of the Bahamas platform so that questions surrounding sea-level changes can be fully addressed.

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