Sedimentary patterns and geometries of the Bahamian ... - CiteSeerX

Sedimentology (1999) 46, 1127±1143

Sedimentary patterns and geometries of the Bahamian outer carbonate ramp (Miocene±Lower Pliocene, Great Bahama Bank) CHRISTIAN BETZLER*, JOHN J. G. REIJMER , KARIN BERNETà, GREGOR P. EBERLI à and FLAVIO S. ANSELMETTI§ *Geologisch-PalaÈontologisches Institut, Senckenberganlage 32±34, 60054 Frankfurt am Main, Germany (E-mail: [email protected]) Geomar, Wischhofstr. 1±3, 24148 Kiel, Germany (E-mail: [email protected]) àRSMAS-MGG, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, USA (E-mail: [email protected], [email protected]) §Geologisches Institut ETHZ, Sonneggstrasse 5, 8092 ZuÈrich, Switzerland (E-Mail: ¯[email protected]) ABSTRACT

Core, logging and high-resolution seismic data from ODP Leg 166 were used to analyse deposits of the Neogene (Miocene±Lower Pliocene) Bahamian outer carbonate ramp. Ramp sediments are cyclic alternations of light- and dark-grey wackestones/packstones with interbedded calciturbidite packages and minor slumps. Cyclicity was driven by high-frequency sea-level changes. Light-grey layers containing shallow-water bioclasts were formed when the ramp exported material, whereas the dark-grey layers are dominantly pelagic. Calciturbidites are arranged into mounded lobes with feeder channels. Internal bedding of the lobes shows a north-directed shingling as a result of the asymmetrical growth of these bodies. Calciturbidite packages occur below and above sequence boundaries, indicating that turbidite shedding occurred during third-order sealevel highstands and lowstands. Highstand turbidites contain shallow-water components, such as green algal debris and epiphytic foraminifera, whereas lowstand turbidites are dominated by abraded bioclastic detritus. Gravity ¯ow depocentres shifted from an outer ramp position during the early Miocene to a basin ¯oor setting during the late Miocene to early Pliocene. This change was triggered by an intensi®cation of the strength of bottom currents during the Tortonian, which was also responsible for shaping the convex morphology of the outer ramp. The Miocene and Lower Pliocene of the leeward ¯ank of Great Bahama Bank provides an example of the poorly known depositional setting of the outer part of distally steepened carbonate ramps. The contrast between its sedimentary patterns and the well-known Upper Pliocene±Quaternary slope facies associations of the ¯at-topped Great Bahama Bank shows the strong control that the morphology of a carbonate platform exerts on the depositional architecture of the adjacent slope and base-of-slope successions. Keywords Bahama carbonate platform, carbonate ramp, cyclicity, Neogene, sea-level changes, turbidites.

INTRODUCTION Distally steepened ramps, in contrast to homoclinal ramps, may bear major amounts of gravity ¯ow deposits in the outer ramp facies (Burchette & Wright, 1992). However, little is known about Ó 1999 International Association of Sedimentologists

depositional geometries and sea-level-controlled stacking patterns of such ramps. Detailed core data, logging data, excellent biostratigraphic control and high-resolution seismic pro®les were acquired before and during ODP Leg 166 for the Neogene outer ramp deposits of Great Bahama 1127

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Bank; these provide an ideal opportunity for investigating such a distally steepened ramp setting. The aim is to reconstruct lateral and vertical facies changes of this outer ramp and to show its sedimentary evolution. High-resolution seismic pro®les have enabled us to map the lateral extension and geometries of gravity ¯ow deposits in detail. The signatures of third-order and higher order sea-level ¯uctuations are discussed, and an overview of the sedimentary regime in this largely uninvestigated depositional setting is provided. GEOLOGICAL SETTING The Bahamas archipelago with Great Bahama Bank consists of several carbonate platforms. The Cenozoic progradation of the leeward ¯ank of Great Bahama Bank was of the order of 27 km (Austin et al., 1986; Eberli & Ginsburg, 1987, 1989). The platform growth occurred in pulses during sea-level highstands; each pulse resulted in an unconformity-bounded depositional sequence, the boundaries of which were generated during sea-level lowstands. Material, which now forms the slope of the bank, was largely provided by shallow-water carbonate particles produced on the bank top. This sediment export pattern is intimately linked to the con®guration of Great Bahama Bank as a ¯at-topped platform during Late Pliocene and Quaternary times (Eberli & Ginsburg, 1987, 1989). Beach & Ginsburg (1980), Schlager & Ginsburg (1981) and Reijmer et al. (1992) have pointed out

that this recent con®guration and most of the related processes acting on the Bahama carbonate platform can only be projected back to the Late Pliocene. Based on sediment compositional changes in boreholes across the Great Bahama Bank, they postulated a Pliocene change in platform geometry. During older stages of platform growth, the Bahama Bank had a different geometry, which, according to the above authors, was similar to a reef-rimmed atoll. The change in geometry has been veri®ed by seismic data (Eberli & Ginsburg, 1987, 1989), which show a Pliocene turnover from a distally steepened ramp to a ¯attopped platform. The turnover in platform geometry is recorded in slope sediments at the Clino borehole (Fig. 1), located in the inner platform where sea-leveldriven sedimentary cycles show only minor compositional changes in the Pliocene ramp stage. Bioclastic packstones characterize both lowstand and highstand deposits. The ¯at-topped platform exported peloidal micrites during highstands and coarse-grained sparites with calcite grains, algae and carbonate lithoclasts during lowstands (Kenter et al., 1999; Westphal et al., 1999). Two other boreholes also provide a limited record from the ramp deposits (Fig. 1): ODP Site 626 in the Straits of Florida (Austin et al., 1986; Schlager et al., 1988) and the Great Isaac Well on western Great Bahama Bank (Schlager et al., 1988). In the Straits of Florida drill site, Miocene sediments are winnowed carbonate sands and carbonate debris ¯ows/calciturbidites. The Great Isaac Well recovered deep-water slope and debris-apron deposits of the platform. During ODP Leg 166, seven sites were drilled at the

Fig. 1. (A) Location of ODP Leg 166 drill sites on the leeward ¯ank of Great Bahama Bank. Position of 100 m isobath delimiting areas of shallow-water carbonates is shown. Square indicates position of ODP Leg 166 Site map (B). (B) Position of boreholes from ODP Leg 166 and seismic lines studied. Ó 1999 International Association of Sedimentologists, Sedimentology, 46, 1127±1143

Neogene Bahamian ramp

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Fig. 2. Seismic pro®le for Line 106 and Western Line (above) and interpreted cross-section (below). Letters refer to depositional sequences (modi®ed after Eberli et al., 1997b).

leeward ¯ank of Great Bahama Bank (Eberli et al., 1997a). Five sites (1003±1007) in the Straits of Florida (Figs 1 and 2) are analysed in this study. METHODS Descriptions of the lithologies rely on core descriptions presented in Eberli et al. (1997a), as well as post-cruise thin-section, smear slide, scanning electron microscopy, X-ray powder diffraction, carbonate content and geophysical log data analyses. Pelagic carbonates are termed ooze or chalks. For lithologies dominated by shallow-water components, the Dunham (1962) classi®cation was applied. In addition, three classes are differentiated: unlithi®ed, partially lithi®ed and lithi®ed. Large-scale geometries and seismic responses of analysed lithologies were obtained from multichannel, high-resolution seismic data (45 cubic inch GI-airgun, 50±500 Hz) shot on R/V Lone Star in co-operation with Rice University (Eberli et al., 1997a). The biostratigraphy of the presented deposits is extensively discussed in the site reports of the individual drill holes by Eberli et al. (1997a).

LITHOFACIES AND LITHOSTRATIGRAPHY A preliminary delimitation of sediment types and sedimentary units in the drill holes was presented by Eberli et al. (1997a). Here, post-cruise analysis of thin sections and geophysical logs are incorporated. As such, the limits of some sedimentary units (Fig. 3) are shifted with respect to the initial descriptions, as additional calciturbidite packages were identi®ed in the logs. The sedimentary successions at the outer ramp Sites 1003, 1004, 1005 and 1007 are dominated by the input of platform-derived components, whereas basinal Site 1006 shows more pelagicdominated sedimentation. In the following, sedimentary units of the outer ramp sites are discussed. For detailed descriptions of lithofacies, the reader is referred to Eberli et al. (1997a). Three large-scale sedimentary units could be distinguished: (1) Oligocene to lowermost Pliocene; (2) lower Pliocene; and (3) upper Pliocene to Pleistocene.

Oligocene to lowermost Pliocene A major element of the Oligocene to lowermost Pliocene sedimentary unit 1 is an alternation of light-grey and dark-grey wackestones and

Ó 1999 International Association of Sedimentologists, Sedimentology, 46, 1127±1143

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Fig. 3. Synthesis of lithological successions at ODP Sites 1003±1007. Numbers refer to sedimentary units, letters to sequence boundaries. Vp: velocity log (km s±1); c ray: gamma ray log (CPS units). Age assignments after Eberli et al. (1997a).

packstones (Fig. 4). The light-grey wackestones/ packstones are uncompacted throughout the succession and contain shallow-water allochems (such as Amphistegina, Cibicides and red algal

debris) and planktonic foraminifera. The darkgrey wackestones/packstones contain clay. At Sites 1003 and 1005, siliciclastic contents in these sediments are between 2% and 10%, at Site 1007



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dark-grey to the light-grey intervals in these cases is again gradational, as in the symmetric cycles. Intercalated in these deposits are calciturbidites and some slump horizons (Fig. 3). The turbiditic packstones to ¯oatstones contain shallow-water bioclasts and planktonic foraminifera; an overview of representative compositional changes in the calciturbidites is provided in Fig. 5. Slumped deposits occur as isolated intervals. The slumps consist of contorted light-grey and dark-grey wackestones/packstones and calciturbidites. A subdivision of sedimentary unit 1 into different subunits (Fig. 3) relies on the occurrence of hardgrounds/®rmgrounds, on the position of calciturbidites and on coarseningupward trends of the light-grey wackestones/ packstones (Eberli et al., 1997a).

Lower Pliocene At Sites 1003 and 1005, lower Pliocene sedimentary unit 2 consists of an interval with poorly differentiated unlithi®ed to partially lithi®ed mudstones to wackestones (Fig. 3) with planktonic and benthic foraminifera, calcareous nannoplankton, minor diatoms and radiolarians. Sediments in general contain more carbonate mud than the deposits of the underlying unit. At Site 1007 (Fig. 3), the corresponding interval is ooze and chalk.

Upper Pliocene to Pleistocene

Fig. 4. Core photograph showing alternation of lightgrey and dark-grey wackestones/packstones (Site 1003, 1078á9±1088á5 m below sea ¯oor; core 1003C-66R).

between 10% and 20%. Carbonate components are ®ne-grained bioclasts, as well as planktonic and benthic foraminifera. No shallow-water bioclasts occur in the dark-grey intervals. In most cases, the rhythmic change between lithologies is gradational and symmetric (Fig. 4). However, there are also strongly asymmetric cycles, within the upper part of which an intensely bioturbated horizon occurs. Such layers are sharply overlain by an interval of light-grey wackestones/packstones that grade upwards into dark-grey wackestones/packstones. The transition from the

The upper Pliocene to Pleistocene sedimentary unit 3 is a succession of unlithi®ed to partially lithi®ed peloidal mudstones and wackestones with calciturbidites, slumps, ooze and chalk. Calciturbidites consist of unlithi®ed to partially lithi®ed packstones to ¯oatstones with shallowwater bioclasts, blackened components and lithoclasts. The composition of slumped horizons is polymict, with interbeds of wackestones, packstones and ¯oatstones. SEDIMENT GEOMETRIES Large-scale depositional geometries of the sediments were analysed in seismic pro®les obtained from the Leg 166 site survey and older industrial seismic data imaging the entire Bahama Transect (Western Line; Fig. 1). Sequence stratigraphic analyses on these data were performed by Eberli & Ginsburg (1987, 1989) and Eberli et al. (1997a,b). Most of the seismic sequences were


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Fig. 5. Thin-section point-counting data (300 points) from samples of upper Miocene sequence k at Site 1003. Note that shallow-water indicators (green and red algae, intraclasts) only occur in the uppermost samples of the depositional sequence. K and I are sequence boundaries. Benth. hyal., hyaline benthic foraminifera; benth. mil., miliolid benthic foraminifera; benth. aggl., agglutinated benthic foraminifera.

identi®ed below the Great Bahama Bank. Erosional truncations and onlap geometries de®ne the sequence boundaries. These geometries show that the boundaries were produced during sealevel lowstands. Sequence boundaries are termed `A' to `Q', the overlying respective seismic sequences `a' to `q' (Eberli et al., 1997a,b). Calculated depth positions of individual sequence boundaries were presented in Eberli et al. (1997a); Fig. 3 shows the positions of these geophysically de®ned limits in the individual holes. In order to correlate depositional geometries, as observed in the seismic lines, with the facies in the drilled holes, the time±depth conversion curves calculated from check-shot survey data (Eberli et al., 1997a) were used. These ensured that the position and lateral extension of individual sedimentological units could be traced precisely in the seismic pro®les. Here, emphasis lies on turbidite and slump deposits. These interpretations provide a minimum estimate of the occurrence of these facies in the Leg 166 transect, because only the geometries in the lateral continuation of the facies drilled in the individual holes were mapped. Using this method, we are also aware that the seismic pro®les only image large-scale geometrical features (Eberli et al., 1994; Sta¯eu & Schlager, 1995; Anselmetti et al., 1997). Resolution of the seismic lines is in the order of 5±12 m. To provide an overview of the changes in the ¯ank dip of the platform, slope angles were calculated (Table 1, Fig. 6). These angles refer to

the dip of the sequence boundaries. As Line 106 crosses the slope of Great Bahama Bank at an angle of »48° to the strike of the slope, depths of sequence boundaries were projected onto a line perpendicular to the ¯ank before calculating individual angles. As such, these angles are true dip values.

Geometry of depositional sequences Miocene and Pliocene slope angles reached a maximum value of »4° (Fig. 6). The distal part of the Early and Middle Miocene ramp (sequences p to k) had a concave geometry, which changed to a convex surface during the Late Miocene and Early Pliocene (sequences i to f ). During the Late Pliocene, the turnover from a ramp geometry to a ¯at-topped platform occurred (Fig. 6). Great Bahama Bank aggraded during the formation of sequences p±n (Figs 2 and 6). The overlying sigmoidal sequences m±g are arranged in a prograding to of¯aping pattern. The upper sequences f±a ®nally record the increased steepening of the strongly prograding margin of Great Bahama Bank (Fig. 6). An overview of the depositional geometries in Line 106 along the transect between Sites 1005 and 1006 is provided in Fig. 7. The sedimentation along the ¯ank of Great Bahama Bank in this line is re¯ected in a stack of wedge-shaped depositional sequences in the proximal part of the drillhole transect. Distally, between Sites 1006 and 1007, these prograding clinoforms inter®nger



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Table 1. True dip angles of sequence boundaries along the Bahama Transect. Site 1007

Site 1003

Site 1004

Site 1005

Sequence boundary

Depth (mbsf)

Angle (°)

Depth (mbsf)

Angle (°)

Depth (mbsf)

Angle (°)

Depth (mbsf)

Angle (°)

A B C D E F G H I K L M N O P Sea ¯oor (mbsl)

ND ND 35 210 210 310 365 420 490 600 670 810 900 960 1020 650á3

ND ND 1á4 3á3 2á8 2á3 2á6 2á6 1á9 1á4 1á4 0á8 0á6 0á3 0á3 2á4

8 25 100 145 175 315 350 400 520 670 740 915 1025 1105 1165 481á4

3á2 1á3 0á7 1á3 1á4 1á2 1á4 1á2 2á7 ND ND ND ND ND ND 3á5

15 65 150 185 ND ND ND ND ND ND ND ND ND ND ND 418á9

3á2 2á2 1á7 1á4 ND ND ND ND ND ND ND ND ND ND ND 3á4

20 90 185 225 255 400 430 485 550 ND ND ND ND ND ND 350á7

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND 46á8

mbsf, metres below sea ¯oor; mbsl, metres below sea level. ND, not determined.

with continuous re¯ections, which, in part, display an onlap or downlap con®guration against the prograding margin deposits. According to Eberli et al. (1997a), these deposits are drift deposits, which are arranged into drift wedges in some sequences (e.g. in sequence f ). Sequences p to i are characterized by a slight basinward decrease in thickness. In the slope part of these sequences, downlapping re¯ections occur above the different sequence boundaries. The angle of slope increases gently upsection throughout this package (Table 1). Sequence h shows a similar trend of thickness distribution to the foregoing sequences, but it dips with a slightly increased angle (2á6°). Sequence g has an opposite trend in thickness distribution and shows an increase basinwards. Sequence f has a complex internal geometry. Similar to sequence g, it thickens between Sites 1007 and 1006. But in contrast to sequence g, in which some re¯ections can be traced from the drift deposits to the distal part of the prograding bank deposits, major sequence-bounding and intrasequential unconformities occur. The unconformity at the lower bounding sequence boundary is not visible in Line 106, but it is well displayed in Line 108 (Fig. 9). The boundary is characterized by deeply incised canyons, which are oriented downslope. Locally, most of the underlying sequence g was eroded. The erosion at the

upper sequence boundary (E), resulting in the formation of deeply incised canyons, is well displayed in Line 106 (Figs 7 and 8), in Line 108 (Fig. 9) and in Line 102 (Fig. 10). One of the intrasequential unconformities is shown in Fig. 7 between Sites 1006 and 1007. It is characterized by an onlap con®guration of re¯ectors of a slopeward-thinning body onto the distal part of a prograding lower slope wedge. Another sequence-internal unconformity is shown in Fig. 8, a detailed view of Line 106 between Sites 1005 and 1007. This unconformity is characterized by erosional truncation of re¯ectors and deeply incised canyons. The position of this level in the boreholes corresponds to intervals within sedimentary unit 2 (Sites 1003, 1005 and 1007) in which clear downhole changes in the velocity logs are present (Fig. 3); at Site 1007, this boundary is represented by a hardground. Thus, this unconformity may represent an additional sequence boundary, which we term E2. Sequence e is very thin on the slope part of the transect. A view along strike (Line 108, Fig. 9) shows that it is not continuous along the margin of Great Bahama Bank. The sequence merely ®lls the incised canyons of sequence boundary E. Between Sites 1007 and 1006, deposits of sequence e ®ll-in a slope-parallel depression with an erosive base at the toe-of-slope. This results in a convex-upward geometry, which inverts the


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Fig. 6. (A) Evolution of slope angles of the leeward ¯ank of Great Bahama Bank. Note that the depositional relief is projected on a line normal to the Holocene platform edge. Dashed lines indicate where the calculation of angles is based on seismic data alone. Depth conversions in these cases were performed for individual boundaries (in parentheses) using data from Eberli et al. (1997a,b). (B) Location of turbidite and drift depocentres along the ¯ank.

pre-existing relief (Fig. 6). In the median part of this body, re¯ections show a ¯at, concave depression. Sequence d is again very thin on the slope part of the transect. Between Sites 1003 and 1007, the sequence thickens, but it thins out towards Site 1006. In the lower part of the sequence, drift deposits onlap the basal sequence boundary. At

Site 1007, slope angles through sequences f to d increase from 2á3° to 3á3° (Table 1). At Site 1003, values are more uniform (1á2° and 1á4°). The upper sequences c±a record the increased steepening of the strongly prograding margin of Great Bahama Bank. Sequence a is not seismically resolvable at Site 1007, because it thins below seismic resolution.



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Fig. 7. Seismic Line 106 (above) and interpretation (below). The grey stippled area in the interpretation shows the position of drift deposits; the white arrow points to an intrasequence unconformity in sequence f (see text for explanation).

Facies geometries Figure 8 shows a detailed view of the slope part of Line 106, with the facies geometries of redeposited sediments such as turbidites and slumps. The positions of these deposits in the individual lithological successions at the different sites are presented in Fig. 3. The interpretation of the seismic line was not performed below re¯ector P, as the sea-bottom multiple crosses the line at this level and partly masks the primary re¯ection pattern. The sea-bottom multiple also inhibits a detailed correlation of the lower part of Site 1003 with Site 1005.

Much of the Miocene turbidite packages in the analysed seismic lines have mounded lobate depositional geometries. A good example of this arrangement is provided in sequence m, SW of Site 1007 (Figs 8±10). At this site, sequence m forms the lower part of sedimentary unit 1, subunit 3, with a lower turbidite-poor and an upper turbidite-dominated interval. The turbidite succession can be subdivided into three individual bodies. On a seismic scale, these bodies pinch out updip from Site 1007. The lower two bodies are up to 1 km wide and 20±30 m thick, mounded structures. Re¯ections in these mounded zones are discontinuous. In the central


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part of the mounds, ¯at to slightly concave re¯ections occur. Geometries of re¯ections indicate that individual turbidite bodies prograde. For

example, in sequence k, SW of Site 1003, a downlapping occurs within a turbidite package, indicating a progradational pattern (Fig. 8).


Neogene Bahamian ramp Fig. 8. Proximal (slope) part of Seismic Line 106 with line drawing showing interpretation of geometries of turbidite and slump packages. Note the presence of interpreted slumps and associated faults. Similar faults were observed by Austin et al. (1988) and Harwood & Towers (1988) in other seismic lines of the western ¯ank of Great Bahama Bank. These faults probably represent detachment surfaces produced by downslope movements of sediment. Such slope adjustment processes were also described from the Miocene of the northern ¯ank of Little Bahama Bank (Harwood & Towers, 1988). The arrows in the line drawing point to features discussed in the text. CI, canyon incision in sequence f; PT, prograding turbidite package in sequence k; ML, mounded lobes in sequence m; CR, convoluted re¯ections in slump of sequence n.

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In addition to down- and updip terminations, wedging out of the turbidite bodies also occurs parallel to strike (Figs 9 and 10). Figure 10 shows that, 1±1á5 km S of Site 1007, the two lower bodies in sequence m have shingled to sigmoidal internal geometries. The same depositional geometries also occur in the turbidite systems of sequences l and k. The dominant direction of lateral accretion of all of these bodies is towards the north; only very minor south-directed package-internal dips occur. Pliocene and Pleistocene depositional systems with turbidites are restricted to the uppermost part of the successions in sequences c±a. The geometries of these deposits, as re¯ected in the seismic lines, are those of steeply inclined

Fig. 9. Seismic Line 108 (above) and interpretation (below). See text for discussion. Ó 1999 International Association of Sedimentologists, Sedimentology, 46, 1127±1143

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Fig. 10. Seismic Line 102 (above) and interpretation (below). Arrow indicates shingled internal geometry of a turbidite body. See text for discussion.

prograding wedges (Fig. 8), which are representative of the recent leeward ¯ank of Great Bahama Bank (Wilber et al., 1990). Along strike (Fig. 9), these turbidite packages display a pronounced lateral continuity. No major package-internal structures occur in the seismic line. South of Site 1003, some discontinuous, slightly inclined to curved re¯ectors may indicate the position of very shallow, incised channel complexes.

Slump deposits occur in different positions along the transect of drill holes (Fig. 8). The slumps of sequences n and i are clearly related to the occurrence of turbidite depositional systems. In sequence n at Site 1007, the interval of the seismic line that corresponds to the slump of sedimentary unit 1, subunit 1 (Fig. 8), appears as a zone with curved and convoluted re¯ections. The same type of geometrical expression of


Neogene Bahamian ramp slumped intervals is in sequence d at Site 1007 and in sequences d and c at Sites 1003±1005. The slumps in the basal part of sequence i at Site 1007 (sedimentary unit 1, subunit 4; Fig. 3) show a different geometry (Fig. 8). This interval is characterized by sigmoidal, downlapping re¯ections. Re¯ections of `background' deposits are not laterally traceable over greater distances. The seismic image of these sediments is characterized by strong re¯ections with a non-continuous, partly chaotic seismic facies. This re¯ection pattern is caused by small-scale faults (Fig. 8). These faults probably represent detachment surfaces produced by downslope movements of sediment. Such slope adjustment processes have also been described from the Miocene of the northern ¯ank of Little Bahama Bank (Harwood & Towers, 1988).

INTERPRETATION AND DISCUSSION

`Background' sedimentation According to Eberli et al. (1997a), the Miocene and lower Pliocene alternation of light-grey and dark-grey wackestones/packstones re¯ects sealevel changes with a frequency of 20 kyr (Bernet & Eberli, 1999). As such, the cycles can be treated as high-frequency sequences (Mitchum & van Wagoner, 1991), as genetic sequences in the sense of Homewood et al. (1992) or as elementary sequences (Pasquier & Strasser, 1997). However, the exact translation of the signal of sea-level ¯uctuations into the cycles still needs discussion. Sea-level changes in a ramp setting may trigger only minor differences in the character of lowstand and highstand sediments (Burchette & Wright, 1992), because facies belts can shift

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down- and upramp hand-in-hand with sea-level. Potential areas for shallow-water carbonate production may not be reduced signi®cantly during lowstands. This pattern should also apply to the Bahamian ramp. Taking the slope angle values of Table 1 and the ramp pro®les of Fig. 6, the zone of shallow-water carbonate production during the formation of Miocene sequences p±i occupied approximately the inner 500 m of the ramp pro®le (