Marine Geology, 44 (1981) 1--24 1 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

BAHAMA CARBONATE PLATFORMS - THE DEEP AND THE PAST

WOLFGANG SCHLAGER AND ROBERT N. GINSBURG

Comparative Sedimentology Laboratory, University of Miami, Fisher Island Station, Miami Beach, FL 33139 (U.S.A.)

(Accepted for publication March 12, 1981)

ABSTRACT

Schlager, W. and Ginsburg, R.N., 1981. Bahama carbonate platforms -- the deep and the past. In: M.B. Cita and W.B.F. Ryan (Editors), Carbonate Platforms of the Passive-Type Continental Margins, Present and Past. Mar. Geol., 44 : 1--24.

Carbonate deposition prevailed for 150 Ma in the Bahama--Florida segment of the West Atlantic margin and provides an interesting test for actualism. Present-day facies patterns on the banks are controlled by the backward decrease of waves and tides and the concomitant increase of temperature and salinity variation. In the troughs, influx of sand and rubble from gravity flows varies with topography and distance from shallow-water sources and allows one to define facies belts: a rhythmic sequence of ooze and graded beds on the basin floors, subdivided into a basin-margin belt of coarse, thick turbidites and basin interior with fine turbidites; slope facies change with increase in height and declivity from accretionary to by-pass to erosional regimes.

Stratigraphic history of the Bahamas is not simply a projection of the "Holo-Scene" back in time. Both long-ten-n natural evolution (decrease in subsidence, upbuilding of the banks, submarine erosion) and outside factors (climate, eustacy) have caused significant changes. Since the Jurassic, the Bahamas seem to have evolved from a clastics-evaporite province to a single carbonate-evaporite platform and finally to an array of platforms and troughs. During the platform-trough stage, the rate of upbuilding of the platforms decreased, submarine canyon erosion increased. Platform flanks steepened as they grew higher and changed from acczetionary to by-pass to erosional slopes. A change imposed by exlzaneous factors occurred in the Pliocene, when the Great Bahama Bank changed from a giant reef-rimmed atoll to a flat platform covered by oolites and peloid sands.

The Bahamas share both long-term trends as well as random changes by extraneous factors with other platforms in the geologic record. Compared to ancient platforms, how- ever, they are unusually long-lived and at a more advanced stage of growth, they are deeply dissected by erosion, their flanks are unusually high and steep and the troughs very narrow. The Neogene platform sequence is strongly controlled by eustatic sea-level fluctu- ations.

INTRODUCTION

The Recent sediments of the Bahama Banks are a widely accepted standard for the interpretation of comparable ancient deposits. However, because there is in this day and age a natural tendency to exaggerate and extrapolate models as well as reputations, it is useful to examine critically just how standard are the Recent sediments.

0025--3227/81/0000--0000/$02.75 © 1981 Elsevier Scientific Publishing Company

Carbonate research in the Bahamas over the last decade focused on two areas: the pre-Holocene stratigraphy of the platforms, and the sediments and stratigraphy of the troughs. As it turned out, the troughs with their deeply eroded flanks and seismic records vastly superior to the banks, were also windows into the past. This two-prong approach created an interesting test for the principle of actualism: to what extent provide the modem Bahamian sediments -- so often used as a key to ancient carbonates -- a key to their own past and how does the answer to this question impact on the interpreta- tion of ancient platforms? We hope to demonstrate that Bahamian strati- graphy is not simply the "Holo-Scene" projected back in time. The strict analogue approach, that looks for exactly congruent patterns does not lead very far. From this perspective, the Holocene is the key to the Late Pleisto- cene, nothing more. Beyond the last 150,000 years there is evidence of major changes in style of deposition that are produced either as a natural evolution (shoaling, growth of barriers, shifts from deposition to erosion) or as a result of changing external conditions: tectonic, eustatic, climatic.

THE PRESENT

Holocene platform sediments

Holocene bank sediments are undoubtedly the best-studied part of Baha- mian geology. The classical papers by Illing (1954), NeweU and Rigby (1957), Cloud (1962) and Purdy (1963), outline sediment types and facies patterns of northwestern Great Bahama Bank already in considerable detail. Later refinements and coverage of other areas were considered by Enos (1974). Bathurst (1971) presented a comprehensive summary of environments and depositional processes.

Production and dispersal of sediment on the platforms is mainly a function of organic growth, pre-Holocene topography and water circulation by wind and tides. The basic facies pattern is well displayed by Great Bahama Bank (Figs.l, 2). The bank is rimmed by a narrow (1--10 kin) belt of skeletal sands with reefs and islands of eolianites preferentially on the windward (east-fac- ing} side. A blanket of peloidal sands covers the interior, patch reefs seem to be localized where hard substrate is available. Oolite sand shoals form where tidal currents are reinforced either by Pleistocene ridges along the bank mar- gins (e.g., Cat Cay) or by resonance effects at the ends of deep-water troughs (e.g., south of Tongue of the Ocean or north of Exuma Sound, Ball, 1967a). Pleistocene islands (mainly ridges of eolianites) profoundly modify this simple pattern. In their lee they create a protective haven for mud deposition in form of shallow-marine packstones and wackestones, passing shoreward into muddy tidal deposits. On the ocean-facing side, the islands favor reef growth by restricting the runoff of saline, warm bank water inimical to coral growth (Ginsburg and Shinn, 1964). Andros Island is a case in point (Fig.2). On its ocean-facing east side grows one of the most luxuriant barrier reefs in the Bahamas, interrupted only where the island is breached by tidal channels

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Fig.1. Location map of Bahamas. Contours in meters after King (1969). Shaded: deep- water facies maps shown in Fig.4; large circles: deep wells; small circles: shallow core borings.

that funnel inimical bank water across the reef tract. On the west side, Andros casts a protective shadow with muddy deposits all across the bank. Similar mud shadows are found in the lee of Eleuthera and, according to Enos {1974}, also leeward of Long Island and Cat Island. Peloids for the interior sand blanket are derived either from oolite shoals and the skeletal sand belt by micritization of grains or from the mud belts leeward of the islands by cementation and hardening of fecal pellets. In stabilized environ- ments, the pellets become cemented together into grapestone lumps. On the open banks with peloidal sand, conditions are good enough to sustain patch reefs wherever hard substrate is available, either exposed Pleistocene bedrock or hardened crusts within the Holocene sequence.

Cores from the Holocene show that all thick sediment accumulations display a distinct vertical sequence that commonly consists of a shoaling-up- ward sequence terminating in the supratidal zone (see Shinn et al., 1969, for examples of muddy deposits; Harris, 1979, for an oolite shoal). The leeward bank margins have evolved from a narrow belt of fringing reefs to wider belts of skeletal sands that buried the early Holocene reefs (Hine and Neumann, 1977; Palmer, 1979).

Fig.2. Surface sediments of Great Bahama Bank. (After Enos, 1974.) Note shadow of muddy sediments in lee of islands.

Quaternary deep-wa ter sediments

Compared to shallow-water banks, the deep-water troughs remained nearly terra incognita until the early 1970's. Early on, Globigerina ooze and graded sands (turbidites) had been recognized as the most common sediment types (Ericsson et al., 1952; Rusnak and Nesteroff, 1964). It also was noted that the ooze often contained over 50% of clay-size, mostly bank-derived aragonite needles, and 10--20% magnesian calcite, also of neritic origin (Kier and Pilkey, 1971). Schlager and James (1978) proposed the term "periplatform ooze" for this sediment. In closely-spaced cores, Bornhold and Pilkey (1971), Sieglie et al. (1976) and Bennetts and Pilkey (1976) studied individual tur- bidites and revealed their lobe-shaped geometry. Subsequently, the basic facies patterns of the troughs were documented by a combination of 3.5-kHz seismic profiling and piston coring (Mullins et al., 1979; MuUins and Neumann, 1979; Schlager and Chermak, 1979; Crevello and Schlager, 1981). Figs.3 and 4 show characteristic examples of facies patterns in the troughs.

Tongue of the Ocean (Fig. 3, 4a). Tongue of the Ocean represents the simplest case. While the periplatform ooze is spread evenly throughout the basin, gravity-flow deposits vary in response to relief and distance from source. The flows are triggered on the uppermost slope by slumping or

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Fig.3. Gullied slope and basin floor of Tongue of the Ocean (TOTO) in 3.5-kHz profiler records. Interpretation of echo types in terms of sediment facies is based on piston cores. (After Schlager and Chermak, 1979.)

collapse of the reef wall, they by-pass the lower slope where they erode a system of shallow gullies, and deposit graded sand and mud on the basin floor. Relatively thick layers of coarse to medium sand are restricted to a belt along the basin margin, only the very distal turbidites of fine sand and mud reach the basin interior. The seismic tool responds to these facies changes by changes in echo character (Fig.3). The gullied slopes produce a series of overlapping hyperbolae (with conformable subbottom reflections where the sediment is mud). In the basin margin belt of coarse and thick sand layers, the echo is opaque, with extended sea-bottom return and no sub- bottom reflections. The basin interior appears layered. Schlager and Chermak (1979) attributed the opaque echo to the high impedance contrast in the shallow subsurface between coarse sand layers and intervening ooze. Basin- ward, this impedance contrast decreases as the sand layers become finer and thinner. Thus, subbottom reflections begin to form as interference patterns. Because sand is delivered all along the platform margin and because the gullied slopes lack a dendritic drainage system that would funnel sediment into large gullies, deep-sea fans are absent in this facies pattern. Rather, the platform acts as a line source of material creating a continuous belt of over- lapping small turbidites at the toe-of-slope. Two more facies belts above the gullied slope, a near-vertical escarpment {James and Ginsburg, 1980; Goreau and Land, 1974) footed by talus slope, are almost certainly the result of the rise and fall of Quaternary sea-level. They should not be considered typical for platform margins in general.

6

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Exurna Sound (Fig. 4b). In Exuma Sound, the facies pattern is dominated by one Late Pleistocene debris flow that covered the entire basin floor with a meter-thick layer of graded sand and rubble (Crevello, 1978; Crevello and Schlager, 1981). The concentric facies pattern of small turbidites is present' but appears in the seismic map only in the southeastern part, where the debris flow is too thin to mask it. Exuma Sound also shows that small plat~ forms (such as the one around Cat Island) or isolated spurs of platforms shed less debris and produce a less pronounced basin margin belt of proximal tur- bidites.

Northwest Providence Channel (Fig. 4c). This channel shows a composite facies pattern. A canyon flanked by gullied slopes in the east passes westward into a U-shaped basin. The slopes rimming this basin are much gentler than those of Exuma Sound or Tongue of the Ocean and gradually pass into the basin floor. Gravity flows deposit part of their load on the lower slope and the basin-margin belt of coarse sand and opaque echo onlaps onto the lower slope. The Providence Channels, unlike Tongue of the Ocean or Exuma Sound are open seaways, where winnowing by contour currents and sea-floor lithification produce widespread opaque echos on the upper slopes (Mullins and Neumann, 1979; Mullins et al., 1980).

High-stand vs. low-stand sedimentation. Superimposed on the regional facies patterns are changes caused by sea-level fluctuations. The Holocene package is normally less than 1 m thick, sometimes completely missing. High- frequency seismic surveys as well as most sedimentologic studies lumped together several high-stand and low-stand deposits. The facies patterns des- cribed above are thus averaged over several high-stand and low-stand deposits.

Fig.4. Sediment facies in three Bahamian troughs based on piston cores and 3.5-kHz seis- mic surveys. a. Tongue of the Ocean (TOTO). Simple concentric pattern ,,f three facies belts, gullied by-pass slope, basin margin with ooze and proximal turbidites and basin interior with ooze and very fine distal turbidites. Note good correlation between opaque echo and the occurrence of coarse sand and rubble on basin margin. Bank edge and gullied slope act as line source of sand and thus create a continuous belt of sand lobes along basin margin. Major canyons and fans (point-source systems) are absent in Bahamian troughs. b. Exurna Sound. From N.W. to S.E., a Pleistocene debris flow and turbidity current swept across the basin and covered it with a meter-thick layer of coarse sand and rubble, indicated by opaque echo; toward the S.E., (i.e., in flow direction), coarse material of this flow is confined to the axial valley; outside the valley layered echo (thin turbidites); basin margin belt of opaque echo, (coarser turbidites), is narrow and disappears where flanking platforms are small and shed little debris, (e.g., western and eastern spurs of Cat Island platform), (after Crevello and Schlager 1980). c. Northwest Providence Channel shows a composite facies pattern. Western part is U- shaped basin, eastern part is V-shaped canyon, flanked by gullied slopes of erosional and by-pass type with hypecbolic echos. Basin in the west shows flat floor with layered echo (distal turbidites), and relatively gentle, depositional slopes, which are subdivided into lower slope of proximal turbidites and current-swept upper slopes with hard grounds and sand. Contour currents also scour parts of basin floor and favour growth of lithotherms. (After Mullins and Neumann, 1979.)

It must be emphasized, however, that alternate flooding and exposure of the platforms causes significant changes in trough sedimentation by varying the input and dispersal of bank-derived sediment (Fig.5;Kier and Pflkey, 1971; Lynts et al., 1973). Rates of deposition are higher during high-stands because of increased input of aragonite mud winnowed from the flooded banks and because turbidity currents increase in size and number when the bank edge reservoirs of loose sand fill up (Crevello and Schlager, 1981). During the low- stands the exposed banks produce no fine sediment and shed little coarse debris because of the rapidity of meteoric lithification that turns the deposits of the previous high-stands into hard rock in less than 10 000 years. The short duration of high-stand flooding and rapid lithification of exposed bank sediments reduce the input of coarse debris into the basins (Crevello and Schlager, 1981). This may explain why sediments in Bahamian troughs in spite of the height of the surrounding slopes, lack the large amounts of coarse sand and rubble commonly found in ancient deposits.

Sedimentation in a Bahamian trough is thus the opposite of a deep sea adjacent to a siliclastic shelf, where lowered sea level increases erosion and delivery of terrigenous material to the deep depocenter.

THE PAST

Only Holocene and Late Pleistocene are exposed on land in the Bahamas. Our knowledge of the pre.Quaternary stratigraphy is based primarily on the following data: (1) four deep exploration wells drilled between 1947--1971

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Fig.5. Sedimentation in TOTO during glacial and interglacial periods. Exposure of plat- forms during ghclaks reduces neritic input and thus zates of deposition. Similarly, per- centage of clay-size aragonite decreases during glacials. (Sea level after Moerner, 1971, other data from Lynts et al., 1973.)

(Fig.l; Meyerhoff and Hatten, 1974; Paulus, 1972); (2) DSDP Hole 98 in the Northeast Providence Channel (Hollister/Ewing et al., 1972); (3) shallow- water wells by the Bahamian government and three core borings by the University of Miami (Fig.l; Beach and Ginsburg, 1980); (4) dredge hauls and submersible dives on the eroded platform flanks (D'Argenio et al., 1975; Ryan, 1980; Freeman-Lynde et al., 1979); (5) seismic surveys in the deep- water troughs (Ball, 1967; Mullins and Lynts, 1976; Sheridan et al., 1981). We will review separately the long-term history of the platforms, the short- term (Plio--Pleistocene) record of the platforms and the history of the troughs.

Long-term history of the platforms

The four Bahamian deep test wells as well as dredge hauls along the ocean- facing Blake--Bahama escarpment, clearly show that neritic conditions similar to today's persisted at least since the Late Cretaceous on the platforms.

But over the last 150--180 Ma we notice a trend in the Bahamas, from clastics to restricted carbonates to normal marine carbonates. Clastic se- quences of Triassic through mid-Jurassic age underneath the carbonate plat- forms, have been found in boreholes and outcrops around the margins of the Bahama archipelago: red arkosic volcaniclastics in Great Isaac well (Tarot and Hatfield, 1975), sandstone, shale and salt in the Remedios zone of northern Cuba (Punta Alegre and San Cayetano formations, Meyerhoff and Hatten, 1974, p.437; Khudoley, 1967, p.671). Seismic profiles indicate widespread occur- rence of these rocks in the northern Bahamas and the Blake Plateau (Sheridan et al., 1981). The clastics are unconformably overlain by (mainly dolomitic) carbonates and anhydrite of Late Jurassic to Early Cretaceous age. Seismics and wireline logs reveal a layer-cake stratigraphy with excellent lithologic correlation over 150 000 square kilometers in Jurassic--Early Cretaceous time {Sheridan et al., 1981; Tator and Hatfield, 1975). It seems the Bahamas, Florida and northern Cuba were then welded together to a single carbonate platform. The restriction in the interior of this platform is probably a func- tion of its huge size. Eastward, that is towards the open Atlantic, the carbonate-evaporite province with highly restricted conditions gives way to open-marine but still neritic environments in the Long Island well and in out- crops on the Blake-Bahama escarpment (Meyerhoff and Hatten, 1974; D'Argenio et al., 1975; Freeman-Lynde et al., 1979).

During the Late Cretaceous--Tertiary interval, the platform margins appear to have been more or less stationary. Wells Andros-1 and Long Island-l, drilled 14 km and 7 km, respectively, from the present edge indicate no sig- nificant changes in environment over this period (Goodell and Garman, 1969; Paulus, 1972, pp.883--885; Meyerhoff and Hatten, 1974). Had the margins progmded significantly as in many ancient platforms, the drill would have crossed the ancient margin facies; had the outer part of the platform been drowned and the margin retreated like in West Florida in the Mid-Cretaceous, deeper-water deposits would have been penetrated. Stratigraphic sections of

30

the Bahama escarpment also suggest gradually retreating or stationary bank margin during the Late Cretaceous--Tertiary interval (Ryan, 1980).

Plio--Pleistocene depositional facies, stratigraphy and neotectonics

In the past several years it has become clear that Plio--Pleistocene deposits, some 50 mthick, are not at all a simple repetition of the Holocene facies.

Marginal reefs play a much larger role in the Plio--Pleistocene than in the Holocene. As indicated earlier in the description of the Holocene facies, shallow reefs are now developed preferentially on the windward margins of Great Bahama Bank and are absent or at best poorly represented on the lee- ward margins. Quite in contrast, the banks are rimmed with reefs on both sides during the Plio--Pleistocene. Cant (1977) found that reef limestone forms a substantial part of the buried Pleistocene margins of Mayaguana and other isolated banks of the southeastern Bahamas. More recently, Beach and Ginsburg (1980), who studied a cross-section of borings across northwestern Great Bahama Bank, found that reefal limestones rimmed both the leeward and windward margins during the Plio--Pleistocene. These findings support Newell's prediction that the Great Bahama Bank is indeed a gigantic Late Tertiary atoll mantled by Quaternary oolite (Newell, 1955}.

A second change in style of deposition was discovered in the borings across Great Bahama Bank. As shown in Fig.6, there is a major change in the composition of sediment in the interior of the platform at about 40 m below present sea level; below, skeletal debris and branched corals predominate; above, there are increasing amounts of non-skeletal grains, peloids and ooids,

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Fig.6. Grain type of Plio--Pleistocene sediments in boreholes across Great Bahama Bank. Skeletal sands (with reefs) dominate in Early Pliocene, below 40--60 m; non-skeletal material dominates in Late Pliocene---Pleistocene deposits. The stratigraphic turning point occurs at ca. 3 Ma and coincides with an increase in amplitude and frequency of sea-level fluctuations due to the onset of glaciation in the Northern Hemisphere. (After Beach and Ginsburg, 1980; Beach, 1980.)

II

grain types that typify the Holocene and exposed Late Pleistocene lime- stones. Beach and Ginsburg (1980} designate this rather abrupt change in composition from skeletal to non-skeletal limestone as the base of their proposed Lucayan Limestone (Late Pliocene--Pleistocene). They dated the change in composition as Middle Pliocene based on the disappearance of a branched coral and the upward increase in the frequency of horizons indica- tive of subaerial exposure (glacial low stands).

The change in style of deposition over the interior (skeletal to non-skeletal) is seen as a result of the transformation of broad, atoll-like platforms (Plio- cene), rimmed by reefs and having a relatively deep lagoon towards the flat. topped platform rimmed by oolitic eolianites seen today (Fig.7, and Beach and Ginsburg, 1980). This change in style coincides with the onset of large- scale glaciation in the Northern Hemisphere (Shackleton and Opdyke, 1977) but whether it is the result of changes in the relative position of sea level, a change in the rate of sea-level rise, or some associated effect has not been established.

The second turning point in the Quaternary history of the Bahamas, is the sudden appearance of oolitic eolianites, dunes of lime sand that developed preferentially on the windward margins in the Pleistocene and are the present-day islands (Fig.7). Beach (1980) suggests that this change is the result of progressive shoaling of the bank that provides the peloids and ooids for the eolianites. The shoaling may have been inherent, simply the result of progressive accumulation of sediment or it may have been dictated by an appropriate position of sea level.

The Lucayan Limestone defined by Beach and Ginsburg (1980} is inter- preted as having accumulated in depths comparable to those of its Holocene counterparts, i.e., less than 5--7 m. Therefore, the thickness of this formation is a first-order measure of rate of subsidence. Pierson and Beach (1980) reconnoitered the regional variations in thickness of the Lucayan limestone in existing core borings. They found that this formation is consistently some

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Fig.7. Two schematic drawings depicting the change in topography and sediment facies during the Pliocene. Early Pliocene platform resembled a giant, reef-rimmed atoll with a 15--20 rn deep lagoon, Late P1iocene--Pleistocene platform was flat-topped and covered by peloidal sand blankets. Sand shoals and eollanites controlled the margins.

12

40 m thick east--west across the central Bahamas--Andros, New Providence, San Salvador. To the north, on Little Bahama Bank, the Lucayan is somewhat thinner, 24 m., and towards the southeast the thickness varies from bank to bank: 24 m on Great Inagua, 9 m on Mayaguana. If one accepts the consis- tently shallow-water environment for the Lucayan Limestone, and interprets the observed variations as variations in rates of subsidence, it seems that there are significant variations that in turn may reflect block faulting or gentle warping of the Banks that is going on today.

Evolution of the troughs

Even more than on the banks, conditions in the troughs, that is topo- graphy, depositional environments and facies, have changed and evolved through time. Quality and type of information on the troughs is different from that on the banks. Ground truth is scarce and restricted to one bore hole, DSDP 98, and outcrops on erosional slopes; seismic, on the other hand is of much better quality than on the banks and contributed substantially to our knowledge of trough history. A third approach is of significance and may be called "comparative anatomy" of troughs. Comparison of modern Bahamian troughs reveals systematic differences that indicate, so we believe, evolutionary trends that can be projected back (and forward) in time and guide interpretation of other data.

Basins and canyons among Bahamian troughs. A glance over contour maps (Andrews et al., 1970; Athearn, 1962; Belding and Holland, 1970; Hurley et al., 1962; King, 1969) shows that some Bahamian troughs are flat-floored, U-shaped basins, e.g., Exuma Sound, southern TOTO, Columbus Basin, others are deeply incised, V-shaped canyons, such as the Providence Channels (Fig.8). Both basins and canyons are flanked by steep slopes (overall dips of 5--7 ° ) that are riddled with gullies. The platform edge above the gullied slopes is an almost vertical escarpment, footed by ungullied talus slope. Several authors (e.g., Hess, 1960;-Gibson and Schlee, 1967; Andrews et al., 1970; Schlager et al., 1976} pointed to the essentially erosional character of gullied slopes and canyons and concluded that besides upbuilding of the banks, erosion by gravity flows is the most important factor in shaping the troughs. For the Tongue of the Ocean and the Providence Channels, Hooke and Schlager (1980) proposed that long-term headward erosion by gravity flows has gradually transformed the flat-floored basins into V-shaped canyons. The pattern in other troughs is consistent with this hypothesis. All flat basin floors seem to be connected with the abyssal plain of either the Atlantic or the Gulf of Mexico through a V-shaped canyon that cuts headward into the flat floor like a rejuvenated river into an alluvial valley. The system of gullied platform slopes and V-shaped canyons can be looked upon as a drainage system that disposes of the excess sediment of the platforms by carrying it to the flat basin floors and the abyssal plains as the ultimate repositories.

flal basin floor ~ . [ ~,~:~'J~=~ I I

13

Fig.8. Carbonate platforms, U-shaped flat-floored basins, V-shaped canyons and abyssal plains of the Florida--Bahama region. Basins are Straits of Florida (SF), southern TOTO, Exuma Sound (EX), Columbus Basin (CB). Similar in depth, but not surrounded by growing platforms, is Blake Plateau (BP). Major canyons are Northwest Providence Channel (NWPC), Northeast Providence Channel (NEPC), Old Bahama Channel (OB) and northern TOTO. (After Schlager and Chermak, 1979.) Printed with permission of Society of Economic Paleontologists and Mineralogists.

Accretionary, erosional and by-pass slopes. In agreement with the concept of headward erosion, the height of platform slopes in the Bahamas increases from the inner parts of the archipelago outward to the abyssal plains of Atlantic and Gulf of Mexico. As they become higher, the slopes steepen (Fig.9) and change from depositional slopes to by-pass slopes and finally to erosional slopes (Fig.10). Other effects may be superimposed on this pattern, for instance, the difference between windward and leeward exposure of a slope (Hine and Neumann, 1977; Mullins and Neumann, 1979) or erosion and deposition by bottom currents (Mullins and Neumann, 1979). The con- trast between windward and leeward flanks of the Bahama platforms seems to be more pronounced along the bank margin and on the uppermost slope. For the lower slopes, Boardman (1978) has shown that sediment input and dispersal are rather similar on windward and leeward slopes in the Northwest Providence Channel and the same symmetry prevails in southern Tongue of the Ocean (Schlager and Chermak, 1979).

Depositional slopes are found along the western flanks of Little and Great Bahama Bank (Mullins and Neumann, 1979, Fig. 12) and in the western parts of Northwest Providence Channel (Mullins and Neumann, 1979, figs. 19 and 23). In Northwest Providence Channel and on the slopes facing Blake Plateau, some by-passing is indicated by hardgrounds (Mullins et al., 1980). Typical by-pass slopes are the gullied flanks of Tongue of the

14

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Fig.9. Profiles of Bahamian platform slopes show increase in declivity with increasing height. Concomitantly, the depositional regime changes from accretion to by-passing of sediment in turbidity currents and finally to erosion. Observations based on submersible dives, coring, dredging, bottom surveys and seismic profiles. Slope contours after Belding and Holland (1970).

Ocean, where erosion by turbidity currents is partly balanced by perennial sedimentation and has cut nowhere deeper than Neogene. The dominant sediment is periplatform ooze (Schlager and James, 1978; Hooke and Schlager, 1980) and the turbidite sequence of the basin floor onlaps on the toe-of-slope (unpublished seismic lines by W. Schlager, M. Ball, A. Chermak). Impressive examples of erosional slopes with truncated seismic reflectors and/or outcrops of pre-Tertiary rocks are the flanks of Northeast Providence Channel (Mullins and Lynts, 1976; Sheridan et al., 1981) and the ocean- facing Blake-Bahama Escarpment in the east (Freeman-Lynde et al., 1979; Ryan, 1980).

History of a trough. Based on the principle of headward erosion by turbidity currents (Hooke and Schlager, 1980) and the concomitant changes in slope regime along with our knowledge on the rate and total amount of platform subsidence (Fig. 11), we can formulate a model of trough evolution, leading from wide, shallow basins with gentle depositional slopes to narrow canyons with deeply eroded flanks. Fig. 12 shows such a reconstruction for Northeast Providence Channel, the trough where most data are available and narrow constraints can be put on the timing of events. The succession of facies at DSDP Site 98 (Fig.13, and Hollister/Ewing et al., 1972), is of particular significance here and has been re-examined by one of us (W. Schlager). The hole is located on a steep by-pass slope, some 600 m above the canyon floor, and by-pass conditions had prevailed throughout the Tertiary producing a

15

EROSION /

turbMity ca rrents ~ / ~ / contour current5

BY-I~551NG ~ _ mud, hardgzounds / / gullies w 5and

ACCRETION slumps, grawty flows ~

Fig.lO. Schematic cross-sections of accretionary, by-passing and erosional slopes forming the flanks of the Bahama platforms. Accretionary slopes are thought to be dominated by slumping and debris flows with minor contributions from turbidity currents; most sedi- ment stays on the slope, causing it to prograde basinward. By-pass slopes develop gradually from accretionary slopes as the profile steepens and turbidity currents increase in vigor and carry most of their load to the basin floor; erosion by turbidity currents is minor and largely counterbalanced by perennial sedimentation of fine material. Erosional slopes evolve from by-pass slopes as the profile steepens and the erosive action of undersaturated turbidity currents out-paces perennial sedimentation. In the Bahamas, scouring by contour currents and carbonate dissolution may also contribute to erosion of the lower slopes.

sequence of carbonate ooze with small glauconite-covered hardgrounds and overall rates of deposition of 4 mm/1000 yr. for the interval Maastrichtian-- Recent. The few cores recovered from the Late Cretaceous (Campanian) show rhythmic alternations of turbidites (graded calcarenites with platform material) and pelagic ooze, characteristic of deposits of basin floors or very gentle depositional slopes in modern Bahamian troughs. Rates of deposition are ca 10 mm/1000 yr. if one assumed that the hole bottomed at the Campanian--Santonian boundary; they could be much higher if the Creta- ceous cores represent only part of the Campanian section. We explain this change from basin floor to by-pass slope by the combined effect of canyon erosion that cuts into the basin floor and upbuilding of the platform flanks that increases the overall slope angle.

16

\ \ \

\

3urassic \ l . . . . , , ~ = -

<

i CAY SAL OREAF iSAAC

I CA', 5AL ~nferred Jur~sS~,C subsidence I

oet~ceou~ [ ce.oi~ic

:',, ,A N~,;?05

-.....

r 5" km "~,

Ma BP i fSO 160 11,0 f20 lO0 0 80 oo ~o 20

Fig.11. Subsidence curves of Bahamas based on four deep wells. Overall trend shows pro- gressive decrease in rates, probably caused by decay of the thermal component of sub- sidence. Irregular patterns of Andros well possibly due to its location near major fault (Sheridan et al. 1981). Decrease in subsidence increases likelihood of platforms being exposed during eustatic falls of sea level; frequency and duration of exposure periods can thus be expected to have increased with time. Data from Paulus (1972), Meyerhoff and Hatten (1974), Khudoley (1967).

Pia/%orrn ÷ Canyon HE PROVIDENCE CHANNEl Cenozoic

Platform *Basw Late Oetac

/V/egaD latforrn 3u~ -Neocomian

reZLef

Drowning

Fig.12. Evolution of NE Providence Channel shown in schematic crogs-sections NW--SE across DSDP site 98. We speculate that the trough was estebli~ed in Early Cretaceous time by partial drowning of a large carbonate platform; depositional relief (sand shoals, reef belts) or block-faulting governed location of drowned area, By Late Cretaceous time, rapid subsidence and upbuilding of adjacent banks had created a U-shaped basin, 1500 m deep and with depositional slopes. During the Cenozoic, slopes steepened and turned into by-pass and erosional slopes; headward erosion by turbidity currents transformed the flat basin floor into a V-shaped canyon.

17

kn ,¢

3(A)O-

3500.

~ , / I)5DP 98

~ 2 : L Provzdence Channe[

~ ~ o ~ /Tertzary 5lopeprofl/e5 " l , / / Late Cre[oceous

\

\ \ ! \x / /

I young canyon fdl l l %. I

Fig.13. Geology of DSDP hole 98 in NE Providence Channel. Located on steep southern flank of the canyon, the hole shows pelagic chalk and turbidites with neritic material (Late Cretaceous) overlain by the pelagic ooze and chalk (Cenozoic). We speculate that headward-cutting erosion in Providence Channel transformed the fiat basin floor into a by-pass-slope and so caused the change in facies.

The concept of progressive headward erosion does not explain the initia- tion of troughs. In our model, we assume that embryonic troughs were established by partial drowning of an originally continuous platform that included Florida and the Bahamas. Early and mid-Cretaceous saw widespread drowning of platforms in the Atlantic-Caribbean region (Schlager, 1981) and are the most likely time intervals for establishment of the first Bahamian troughs. Sheridan et al. (1981) have tentatively identified a prominent seismic reflector in the Providence Channels as a drowned mid-Cretaceous platform underlying the present troughs. Late Cenomanian pelagic deposits in a piston core from Northeast Providence Channel show that at least this part of the trough was a deep-water area by the end of the mid-Cretaceous (Meyerhoff and Hatten, 1974, p.441).

Mullins and Lynts (1976) argue against a continuous platform under- pinning the troughs. In their opinion, the troughs originated as tectonic grabens in the rifting stage of the continental margin and remained topo- graphic depressions ever since. This hypothesis faces a number of formidable

18

objections: (1) The Remedios and Cayo-Coco zones of northern Cuba, general- ly considered a continuation of the Bahamas, show Middle to Late Cretaceous deep-water and shallow-water carbonates juxtaposed and jointly overlying a continuous carbonate--evaporite sequence analogous to the "megabank" pos- tulated for the Bahamas (Meyerhoff and Hatten, 1968). (2) The Blake Plateau immediately north of the Bahamas is underlain by a continuous carbonate platform of Early Cretaceous age, again suggesting that the present-day pat- tern of platforms and troughs in the Bahamas is a Cretaceous or even younger feature. (3) Seismic surveys in the Straits of Florida show no displacement between reflectors in the Straits of Florida and onshore boreholes (Sheridan et al., 1981). (4) The present-day geometry of bank margins and troughs show a dominance of curved, often semi-circular patterns with little resem- blance to fault systems.

THE MODERN BAHAMAS AS MODELS OF CARBONATE PLATFORMS

The modern Bahamas have produced a great number of grain types and sedimentary features that perfectly match those in ancient deposits. This notwithstanding, we believe that the "Bahamian model" of carbonate plat~ forms should be used with great caution when the large-scale patterns and long-term stratigraphy are concerned. Both long-term evolutionary trends as well as random changes dictated by extraneous factors (such as eustacy), have shaped the stratigraphy and structure of the Bahama archipelago. The Holocene appears only as one of many modes in which the platforms have grown.

The Plio--Pleistocene sequence revealed by the drill is an example of change imposed by outside factors. Without a change in overall setting, the depositional regime changed drastically in the Plioeene, probably due to the change in amplitude and frequency of sea-level fluctuations as proposed by Beach (1980). These controls are poorly understood and not predictable at this point. It is of interest though, to see a similar change from reefs and skeletal sands to oolite and vice versa at many places in the geologic record. For instance, the Friuli platform in the southern Alps changed from mid- Jurassic oolite rims to Late Jurassic reef rims (Bosellini et al., 1980). Con- versely, the Late Triassic (Rhaetian) reefs in the northern Alps turned into oolite shoals before they were drowned (Fabricius, 1967). The Devonian of Western Australia shows several intercalations of oolite shoals in otherwise skeletal bank facies (Playford, 1980, p.819). Comparative sedimentology of ancient and modern examples is needed to test if they have a common cause.

Sedimentation and facies patterns in the troughs are an example of change following evolutionary trends. We have mentioned above that trough sedi- mentation was in part controlled by height and declivity of slopes which we believe have changed in a systematic way through time. As slopes increased in height, they steepened and the locus of deposition shifted from slope to basin floor. The transport mechanism changed from slumping and mass-flow to turbidity currents. Ancient basins seem to have evolved along similar

19

trends but most seem to be less advanced than the Bahamas. The Golden Lane, the Guadeloupe Mountains, the Triassic of the Dolomites (Wilson, 1975), the Mississippian of the Rocky Mountains (Rose, 1976) and many other platforms show gradual steepening of the slope as the platform builds up, but the advanced stages, by-pass and erosional slopes so common in the Bahamas, are rarely ever reached. Superimposed on this long-term trend we see in the Quaternary of the Bahamas the extraneous effects of eustatic sea level such as frequent exposure horizons, the facies change in the Pliocene discussed above and the variation in sediment input into the troughs.

The long-term stratigraphic record of the Bahamas too, reveals a trend that seems to be tied to the evolution of the passive ocean margin of which the Bahamas are a part: clastic sediments, probably only partly marine, are followed by carbonates and (sulfate) evaporites, deposited on a large platform with highly restricted circulation; these in turn, give way to the present#day array of platforms and troughs with normal-marine and hemipelagic carbon- ates. A similar trend commonly underlies ancient passive margin sequences. Two examples, the Permo--Mesozoic sediments of the Mediterranean and the Devonian of Central Europe, both may illustrate this point {Table I). For the Tethyan sequence, Schlager and SchSllnberger (1975) pointed out that gradual changes over long periods of time are punctuated by rapid and funda- mental changes, termed stratigraphic turning points. We speculate that a similar pattern will emerge for the Bahamas. One turning point seems to be the partial drowning of the megaplatform and the establishment of shallow troughs in the mid- or Early Cretaceous.

Compared to other carbonate margins, the Bahamas are very long-lived and thus super-mature platforms. The carbonate megabank persisted for 50--60 Ma and the platform--trough stage lasts since ca. 100 Ma. In the Austro-Alpine Mesozoic, neritic, sheet-like carbonates persisted for ca. 10 Ma and the platform--trough stage lasted ca. 22 Ma, with a major reorganization after 7 Ma. The Devonian carbonate margin of Central Europe passed through the respective stages in comparable time: 17 Ma for the large carbonate bank and 13 Ma for the array of small platforms (stratigraphy after Krebs, 1974, absolute ages after Ziegler, 1978). Very few platforms, such as the southern Apennines (Triassic-Tertiary, D'Argenio et al., 1975), match the Bahamas in longevity. We should keep this in mind when using the Bahamian platform model as a key to the geologic past.

CONCLUSIONS

The Bahamas, widely used as a standard for interpretation of ancient car- bonate deposits, can perform this role very well for small-scale environments and their products. However, the simple analogue approach that looks for exactly congruent images has to be modified when considering the large-scale facies patterns and the edifice of the platforms as a whole within the frame of a passive ocean margin. Here we need to remember the "evolutionary trends" that govern the rise and fall of carbonate platforms on passive

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21

margins. The Bahamas are at a highly advanced stage of platform growth; they are deeply dissected by submarine erosion, their flanks are unusually high and steep and the size of individual platforms has shrunk considerably through time. Subsidence decreased by one order of magnitude between the Jurassic and the Neogene. As a consequence, periods of exposure due to eustatic fluctuations of sea level became longer and more frequent.

Modern sediment facies patterns in the troughs are largely a result of relief and thus strongly influenced by the fact that the Bahamas are super- mature passive margin platforms. Shallow-water facies patterns are more independent of these long-term trends but strongly shaped by the history of sea level in the more recent past. Because of the extremely rapid Holocene rise, the depositional systems on the banks are lagging behind present-day sea level.

ACKNOWLEDGEMENTS

We acknowledge support by NSF grants OCE 76-21932, EAR-76-15853, OCE-79-25573 and EAR 7927212. Additional support was provided by Amoco Production Company and Amoco International, Cities Service Oil Company, Continental Oil Company, Getty Oil Company, Marathon Oil Company, Phillips Petroleum Company, Shell Oil Company, Sohio Petroleum Company, Chevron Oil Company, Exxon Production Research Company, Gulf Oil Corporation, Pennzoil Producing Company, Royal Dutch/SheU and Union Oil Company of California.

R.E. Sheridan and D.K. Beach made important manuscripts available to us.

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