Cat Island platform, Bahamas: an incipiently drowned Holocene carbonate shelf

Sedimentology (1988) 35,805-819

Cat Island platform, Bahamas: an incipiently drowned Holocene carbonate shelf

L A U R A L. D O M I N G U E Z and H E N R Y T . M U L L I N S *

Department of Geology, Her0.y Geology Laboratory, Syracuse University, Syracuse, New York 13244, USA

A L B E R T C. H I N E

Department of Marine Science, University ojSouth Florida, St . Petersburg, Florida 33701, USA

ABSTRACT

Analyses of high resolution, seismic reflection profiles and surface sediment samples indicate that the Cat Island shelf is presently in an incipiently drowned state. This small carbonate bank is characterized by a thin ( < 4 m), coarse-grained, relict sediment cover, along with limited reef development, and a relatively deep (20-30 m) margin indicating that it has been unable to ‘keep-up’ with Holocene sea-level rise.

Early flooding at relatively high rates of sea-level rise (4 m kyr- ’, 5-8 x lo3 yr BP) in conjunction with small bank size and relatively low elevation, led to a reduced rate of carbonate accumulation and incipient drowning. The shelf edge currently lies beneath the zone of maximum carbonate production and exposes the interior shelf to open marine conditions which may result in permanent drowning if it is unable to ‘catch-up’ with continued sea-level rise. Sediment facies patterns are largely oriented perpendicular or oblique to the shelf edge and appear to be controlled by shelf circulation patterns focused by bank-margin reentrants.

In comparison with most of the northern Bahamas, the Cat Island shelf was flooded earlier and at relatively higher rates of Holocene sea-level rise which led to selective drowning, implying that carbonate platforms need not drown synchronously over widespread areas as commonly thought. The potential rock record of this incipient drowning event would be a thin, open-marine sand sheet of highly degraded cryptocrystalline and aggregate grains associated with poorly developed reefs.

INTRODUCTION

Recent study of small Bahamian bank tops has revealed unexpected patterns of sedimentation not predicted by earlier models derived from the much larger Great Bahama Bank (Dravis, 1977). Because of their size, small platforms are more susceptible to drowning events (Hine & Steinmetz, 1984) which can place them out of the euphotic zone (Kendall & Schlager, 1981; Schlager, 1981) and effectively shut down the subtidal ‘carbonate factory’ (James, 1984).

Drowning of carbonate platforms occurs when the rate of relative sea-level rise exceeds vertical accumulation and the platform becomes submerged below the euphotic zone (Kendall & Schlager, 1981). Although drowned carbonate platforms are common in the geologic record (Wilson, 1975), potential modern

* Please direct all correspondence including requests for reprints to Dr Mullins.

analogues are not well known. Overall, drowned platforms present a ‘paradox’ because modern coral growth rates appear to exceed known rates of relative sea-level rise caused by long-term geological processes (Schlager, 1981). According to Schlager (1981), rapid pulses of relative sea-level rise or reduction of benthic growth by deterioration of the environment (Hallock & Schlager, 1986) are, at this time, the only plausible explanations of carbonate platform drowning. Schla- ger (198 1) has also previously pointed out that portions of the modern Bahamas, including the Cat Island platform, appear to be partially drowned.

Our investigation utilizes a relatively closely spaced ( N 5 km) grid of high-resolution seismic reflection profiles and bottom samples to define the Holocene depositional sequence across the Cat Island shelf. The 3-D picture provided by this grid of seismic lines and

805

806 L. L. Dominguez, H . T. Mullins and A . C. Hine

samples is the most detailed study yet undertaken for any Bahamian banktop. Our results reveal the history of Holocene shelf evolution and contribute new insights on drowned carbonate platforms in general while providing a modern analogue.

GEOLOGICAL SETTING

The Bahama Banks consist of a series of shallow- water carbonate platforms separated by deep-water basins (Fig. 1) within the easterly trade wind belt. The Cat Island platform is a relatively small, leeward- oriented (west facing) bank located on the eastern fringe of Great Bahama Bank (Fig. 1). It consists of a shallow-water( < 30 m) shelf(approximately790 km2) bordered on the east and south by reef-rimmed Pleistocene aeolianite that constitutes Cat Island (Lind, 1969). East of the island, submarine slopes drop precipitously to abyssal depths along the erosional Bahama Escarpment (Freeman-Lynde & Ryan, 1985). To the west of the shelf is Exuma Sound, a deep-water, closed-seaway characterized by steep marginal escarpments (Ball et al., 1969).

The Cat Island shelf was virtually unstudied before this investigation was undertaken. A generalized facies map of the Bahamas by Enos (1974) displayed the presumed distribution of surficial grain types on the Cat Island shelf. However, this map relied on idealistic concepts of Purdy (1963a, b) to predict sediment types and distributions on banks never directly investigated (Dravis, 1977).

METHODS

Approximately 3 15 km of high-resolution seismic reflection profiles were obtained with an EG & G Uniboom system (Fig. 2) (0.4-14.0 kHz) operated at 300 J . In addition, analogue side-scan sonographs (100 kHz) were locally collected simultaneously with uniboom profiles (Fig. 2). Navigation was by Loran- C cross-checked by radar triangulation. A P-wave velocity of 1-47 km s - ' was used for near-surface seawater (Enos & Perkins, 1977), and 1.75 km s f l was assumed for unconsolidated carbonate sediment (Dobrin, 1976).

Fig. 1. General index map of northwestern Bahama region illustrating location of Cat Island platform. Black areas are portions of banktops in water depths greater than 10 m suggesting drowned margins (based on Schlager, 1981 ; modified after Hine & Steinmetz, 1984).

Incipiently drowned carbonate platform 807

/ , / , I , X P R O B E S I T E SCUBA D I V E 0 S H I P E K GRAB 0 D I V E R OBSERVATION D I V E R GRAB

Fig. 2. Geophysical trackline and bottom sample map for Cat Island shelf superimposed on generalized bathymetry.

Seventy-seven surface sediment samples were obtained (Fig. 2) by divers and a Shipek grab sampler. Divers recorded observations at 35 sites, including one SCUBA dive, and sediment thickness was checked at 21 diver stations using a 3 m stainless-steel probe.

Sediment subsamples were treated with bleach to remove organic matter and wet-sieved for grain-size. Sand fractions were impregnated in epoxy, thin sectioned for point count (300 per section), analysed by a multivariate statistical cluster program, and subjected to principal component analysis (Davis, 1986) for objective partitioning of group identities.

Bulk sediment, as well as sand and mud fractions, were analysed by X-ray diffraction to quantitatively determine carbonate mineralogy (Milliman, 1974) by measuring peak areas using a calibration curve for aragonite and calcite. The amount of magnesium carbonate (MgCO,) within the magnesian-calcite phase was determined by measuring the shift of the calcite peak position (Goldsmith, Graf & Heard, 1961). In addition, selected samples were studied using scanning electron microscopy (SEM).

RESULTS

Seismic reflection data

Our bathymetric map of the Cat Island shelf (Fig. 3) shows that the shelf margin typically lies at water

depths of 20-30m before a precipitous drop into Exuma Sound. In the south, the 8 m contour extends close to the shelf-slope break whereas in the north this contour is displaced landward due to two large reentrants. Subtidal sand shoals (‘spits’) also extend from cuspate forelands along the west-central edge of Cat Island producing nearshore bathymetric highs perpendicular to the strike of the margin.

Our sediment isopach map reveals that there is relatively little (0-4 m) unlithified sediment on the Cat Island platform (Fig. 4). Thicker sediment deposits occur along subtidal sand shoals and the shelf edge. A structure contour map (Fig. 5) on top of the pre- Holocene rock surface shows that this surface lies <28 m below present sea-level. Overall, it has a similar morphology to present day bathymetry (Fig. 3) except that the deeper-water embayments which extend onto the platform are more pronounced.

Depositional patterns and physiography of the shelf margin vary along strike of the platform as seen in E- W oriented seismic profiles. Profiles in the north (Fig. 6) reveal sand bodies at the shelf edge that increase in thickness along the margin and have prograded over earlier reefs. To the south, there is increasing reef development at the shelf edge (Figs 7 & 8) with the best developed reefs on profile CI-11 (Fig. 8) located in the southernmost part of the study area. A series of as many as five marginal terraces, developed in the pre-Holocene rock surface, are pronounced on profile CI-9 at water depths between 10 and 20 m (Fig. 7).

L. L. Dominguez, H. T. Muitins and A . C. Hine

k J- - ATLANTIC OCEAN 4

T

- - ~ NAUTICAL MILES f -- - - - BATHYMETRY (rn) KILOMETERS

/ / / \

Fig. 3. Bathymetric map of Cat Island shelf based on geophysical tracklines shown in Fig 2; contour interval=2 m. Note embayed nature of contours

, Fig. 4. Holocene sediment thickness (isopach) map of Cat Island shelf Note overall paucity of unconsolidated sediments and rock ridge outcrop in central portion of the platform

Evidence for large-scale bedforms in the northern study area are best illustrated on profile CI-02 (Fig. 6) . The sand shoals in profile CI-02 have an asymmetric shape with their slipfaces oriented toward the west, indicating offbank migration. Other sand shoals also have a sharp asymmetry but their direction of migration appears to be toward the N W.

Space shuttle imagery

Areas of benthic floral cover, sand accumulations, shallow submergent topography and general margin morphology have been identified and mapped (Fig. 9)

from a colour photograph taken aboard U.S. space shuttle mission STS-5 (November 1982) at an altitude of 1300 km. Large bedforms occur at the shelf edge and across the northern part of the study area. The deeper-water (> 10 m) shoals are oblique to the shelf edge and have lateral extents of up to 5 km. Shoals in shallower water (< 10 m) are about a kilometre in lateral extent and are oriented both parallel and oblique to the shelf edge (Fig. 9). Other sand accumulations occur in the form of cuspate foreland subtidal sand shoals and ebb tidal deltas north of emergent cays.

Darker patches on the photograph of the inner shelf

Incipiently drowned carbonate platform

30-

809

-\--04 I! u 68316 PROFILE C I - 0 2 0 500 rn

, / , /

Fig. 5. Structure contour map on top of lithified Pleistocene surface using sea-level as datum. Note the large, embayed nature of this rock surface.

WEST I I - I

EAST h

E 0 W v) I- - n

W W a b

>

* - - I

E LL W b a 3 2

N

I ASYMMETRIC SAND SHOALS 1 toz 15

are areas of benthic floral cover composed of sea- margin collapse structures (Mullins, Gardulski & grass, algal, and possibly microbial mats. This cover Hine, 1986). is most dense and widespread along the southern inner shelf, between a rock ridge and Cat Island. The deep, Surface sediments leeward margin of the Cat Island platform displays a

(a) Grain size broad cuspate or ‘scalloped’ morphology. There are at least two arcuate, amphitheatre-like embayments 5- Sediments on the Cat Island shelf consist mostly of 10 km across (Figs 3 & 9) that may be partsof platform sand and gravel, with the dominant grain size

810 L. L. Dominguez, H. T. Mullins and A . C. Hine

- WE S T r 1 I

EAST

Fig. 7. Photograph (top) of a portion of uniboom seismic reflection profile CI-09 (Fig. 2) and line drawing interpretation (bottom) illustrating marginal reef development, middle to outer shelf terraces, and Holocene sand bodies. M, refers to first water-bottom multiple reflection.

WEST EAST

E

I I-

w

- a n

d Y I-

J r ROCK OUTCROP

\ HOLOCENE SANDS

N

ROFILE C I - l l

3’:1 -.04

Fig. 8. Photograph (top) of a portion of uniboom seismic reflection profile CI-11 (Fig. 2) and line drawing interpretation (bottom) illustrating Holocene shelf margin reef in about 20-30 m of water and thin Holocene sand sheet across the outer shelf. M,-M3 refer to water bottom multiples.

represented by the sand fraction which ranges from 48 to 97%. The surprising paucity of mud (1-6%) on the Cat Island shelf is in sharp contrast to muddy facies that occupy much of the larger Great Bahama Bank and that previously predicted for the Cat Island shelf (Enos, 1974).

(b) Carbonate mineralogy The mineralogy of sediments on the Cat Island shelf consists mostly of aragonite and magnesian-calcite, as well as minor calcite, with aragonite as the major mineral. Cross-shelf trends of most parameters (illus-

trated by sand percent aragonite) display elongate contour lobes that protrude onto the platform perpendicular to the shelf edge (Fig. 10; Dominguez, 1986).

Mole % MgCO, values greater than 18% suggest that most of the magnesian calcite is derived from red algae and/or benthonic forams (Scholle, 1978).

Petrographic observations

The surficial sands of the Cat Island shelf have been organized into six categories based on petrographic grain counts :

Incipiently drowned carbonate platform 81 1

- ________- ---. W I N D W A R D F R I N O l N G REEF

- 5 tm

Fig. 9. Line drawing interpretation of colour space shuttle photograph of the Cat Island platform. Note cuspate (‘scalloped’) nature of platform margin, zone of stabilized sea grasses, relict as well as active bedforms, tidal deltas, and subtidal sand ‘spits’.

AS- SC

Fig. 10. Map of weight % aragonite in the sand fraction of surface samples from the Cat Island shelf. Note large ‘embayed’ nature of contours and that the vast majority of the sand fraction consists of aragonite.

(a) Cryptocrystalline grains (range 30-73%; mean 48%)

These are the most abundant and widespread grain type on the Cat Island shelf. They are well represented in most samples and are particularly common at relatively shallow water depths.

(b) Aggregate grains (range 5-44%; mean 23%)

The majority of the aggregate components consist of subrounded cryptocrystalline grains as well as various skeletal grains. Matrix in the aggregates is usually cryptocrystalline, but some aggregates are cemented by acicular aragonite (Fig. 11).

(c) Benthonic foraminifera (range 1-32%; mean 7%)

These grains are mostly peneropolids and milliolids and are the only recognizable skeletal allochems of sufficient abundance to warrant a separate category. The overall abundance of forams and quality of test preservation increases towards the shelf edge.

(d) Other skeletal grains (range 3-47%; mean 13%)

These allochems include (in decreasing relative abundance) whole or fragmented corals, molluscs, green algae, echinoderms, bryozoans and red algae. Abun- dance and degree of preservation of skeletal grains increases with water depth toward the shelf edge.


Fig. 11. SEM photomicrographsof aggregate grain from the Cat Island shelf: (A) Overview of grain at low magnification; bar scale= 100 pm.Note abundance of variable grain types; (B) Close-up view of (A) illustrating peloids bound into aggregate; bar scale = 10 pn; (C) Close-up view of (B) illustrating acicular aragonite cement; bar scale = 10 km.

(e) Carbonate lithoclasts (range 1-23%; mean 8%)

These grains were distinguished from aggregates by their coarse crystalline cement, particles which are compositionally different from coexisting sediments, as well as truncated cement and broken grains. Most component grains of lithoclasts are surficially coated and were not recognized outside of lithoclastic particles.

(f, Unidentifiablegrains (range 0-1 1%; mean 2%)

These include a relatively low percentage of grains that could not be recognized with confidence.

An unusual grain type, recovered only at map location 37 (Fig. 2), consists of granule to small pebble sized, spheroidal to elliptical calcareous grains, which had a ‘sponge-like’ consistency when collected. SEM photomicrographs show an organic matrix between


grains with associated threads that coat, as well as penetrate, grain surfaces (Fig. 12). Because of their organic binding and lack of cement, though, these grains appear to have a low preservation potential.

Sediment facies

A sediment facies map, based on statistical analysis of grain count data, is presented as Fig. 13, and the average percent of constituent grain types for each of the five sedimentary facies is listed in Table 1.

The cryptocrystalline facies is restricted to the inner to mid-shelf (Fig. 13) and occurs in protected areas at water depths less than 10m. Such environments provide a stable location in which grains are repeatedly bored and filled with micritic carbonate and ultimately degraded beyond recognition.

The lithoclasticfacies is concentrated along subtidal sand shoals adjacent to cuspate forelands (Fig. 13) where sediment thickness is generally greater than 2 m. This facies trends perpendicular to the western margin of Cat Island and reflects a locally eroding coastline.

The aggregate facies extends seaward from central Cat Island, expands near the shelf edge, and is

oriented normal to the bank margin (Fig. 13). It represents an environment that provides sufficient winnowing of fines yet is protected enough to keep grains from active transport (Winland & Matthews, 1974). This facies forms the transitional boundary between the northern and southern portions of the Cat Island shelf (Fig. 13).

The skeletal facies occurs along the shelf edge, parallel with the trend of the bank margin where water depths are greater than 10m and sediment thinkness is in excess of 2 m (Figs 4 & 13). Sand-size sediment dominates this facies where more than 20% of the carbonate fraction is magnesian calcite. The skeletal allochems are locally generated in association with poorly developed reefs.

The cryptocrystalline-skeletal facies occurs in the mid-shelf region of the Cat Island shelf (Fig. 13) and is transitional between the cryptocrystalline and skeletal facies indicating gradual changes in allochem abundance. This facies does not occur at water depths greater than 14 m.

Unlike the bank margin-parallel/subparallel facies patterns predicted by Enos (1974) for the Cat Island shelf, our sedimentary facies map (Fig. 13) shows facies belts that, with the exception of shelf margin

Fig. 12. SEM photomicrographs of sponge-like, mucilaginous bioclast. Note binding of grains by external (A), as well as internal (B), organic matter; bar scales= 10 pm; sample from site 37 (Fig. 2).


TLANTIC OCE

- -- NAUTICAL MILES - - -

KILOMETERS

I I \ / \

AGGREGATE LITHOCLASTIC CRYPTOCRYSTALLINE SKELETAL CRYPTOCRYSTALLINE-SKELETAL

Fig. 13. Surface sediment facies map of Cat Island shelf based on grain counts and statistical analysis of 77 samples (Fig. 2). Note that with the exception of the shelf margin skeletal sands, facies patterns are normal, or oblique to shelf edge.

skeletal sands, are oriented perpendicular or oblique to the shelf edge. This distribution is likely the result of circulation patterns controlled by scallop-shaped, deep-water, reentrants that extend onto the Cat Island shelf.

Radiocarbon dates

Three bulk sediment samples, one each from the inner (7 m), middle (9m) and outer (22m) shelf (map numbers 24, 26, & 29, respectively, Fig. 2) were selected for radiocarbon dating, along with two coral samples from the SCUBA site (Fig. 2). Results show that inner (25 10 50 yr) and middle (2490 f 60 yr) shelf samples are substantially older than outer shelf skeletal sands that date at 990f60yr. Although

absolute 14C dates of bulk samples are always difficult to interpret, these data, combined with our grain type data, suggest that inner and middle shelf sands are largely relict and in part reworked. The outer shelf date further suggests that there is a significant component of reworked or relict carbonate in the skeletal sand facies as well. However, contemporary ages were determined for coral samples indicating that there is some modern coral growth along deep (20-30 m) bank margin reefs in the southern portion of our study area.

DISCUSSION

The most striking differences between the leeward Cat Island margin and other Bahamian examples are

Table 1. Sediment facies constituent grain types rA). Grain constituents Sediment facies

Aggregate Lithoclastic facies Crypto- Skeletal Cryptocrystalline/ facies crystalline facies skeletal

facies facies

Cry ptocrystalline 39.3 37.9 62.5 36.2 50.3 Aggregates 35.0 25.2 20.8 15.7 20.7 Benthonic Foraminifera 5.0 5.3 3.7 8.8 8.0 Skeletal (other) 12.4 14.6 5.8 35.5 13.6 Lithoclasts 6.3 15.1 6.3 2.6 5 5 2 Unidentified 1.9 1.7 0.9 1.2 1.6


the type and thickness of Holocene sediments. The degraded nature of Cat Island shelf sediments, their relatively small volume and old radiocarbon ages, all argue that most sediments are relict from an earlier, lower stand of Holocene sea level. Elsewhere in the Bahamas leeward margin sand bodies are as much as 10-15 m in thickness (Hine, 1983) compared with just 4 m for the Cat Island shelf margin. That this difference is not a simple function of a more efficient ofFbank sediment transport system along the Cat Island shelf, is evidenced by the paucity of bank- derived sands along its adjacent deep-water slope in Exuma Sound (Crevello et al., 1984).

Because of deep-water re-entrants, the Cat Island shelf margin also has a different physiography from other studied leeward margins which may account for the difference in sediment distribution. This shelf has facies patterns that have an oblique to perpendicular (rather than parallel) orientation to the shelf edge. In addition, the Cat Island shelf has a relatively small area, and relatively deep-water. The size of the shelf is important because it controls the available area of carbonate sediment production (Palmer, 1979) which, in turn, dictates the amount of sediment available for seaward progradation and platform aggradation. According to Schlager (1981) the zone of maximum carbonate sediment production is at water depths less than 10 m. If one considers the maximum widths of Bahamian platforms in less than 10 m water (Fig. 1) and their associated maximum thickness of leeward Holocene marginal buildups, it becomes apparent that platform size, especially width, controls the area of potential carbonate production and thus the potential for upward and outward growth.

Shelf width may also affect sediment grain size and type by controlling wave action in winnowing shelf sediments. Mud-size sediment is conserved on larger, wider platforms because shallow bank margins and extensive areas of shallow banktops tend to dampen the effects of waves and thus reduce winnowing. Current winnowing may well be responsible, in part, for the removal of mud on the narrow Cat Island shelf and setting up an environment which is conducive to cryptocrystalline and aggregate grain formation as well as overall low growth potential.

Flooding history

A flooding history for the Cat Island shelf was developed based on our depth to Pleistocene bedrock (structure contour) map (Fig. 5 ) and the Bermuda

Holocene sea-level curve* of Neumann (1971 ; Fig. 14). Initial Holocene flooding of the bank oc- curred ~ 8 . 1 ka. Within 500 years, N 100 km2, or 12% of the shelf, was submerged at a flooding rate of 4 m/ kyr (Figs 14 & 15). During this stage, shallow-water depositional environments ‘started up’ (Kendall & Schlager, 1981) and reefs were established on the Pleistocene rock surface along the shelf margin. Continued flooding at the same rate for the next 500 years (7.6-7.1 ka) increased the area for potential

v) LT W c W 5

cI4 YEAR x 10’ - 9 8 7 6 5 4 3 2 I

5

v) 10 LT W c- W 5

15

BERMUDA SEA LEVEL

20 HI S T O R Y 20

7

BERMUDA SEA LEVEL HI S T O R Y

AFTER A.C. NEUMANN (1971)

25

Fig. 14. Holocene sea-level curve for Bermuda based on Neumann (1971). Note changes in rate of sea-level rise over time. Shaded patterns correspond to times of map patterns of platform flooding in Fig. 15.

shallow-water carbonate production to a total of r 181 km2 or 41% of the shelf (Fig. 15). The area of potential carbonate production was enhanced first in the northern half of the shelf (Fig. 16) during this interval and may represent the time when offbank sediment transport began to overwhelm marginal reefs as leeward margin sand bodies developed (Hine & Neumann, 1977). Sea-level rise continued at a somewhat slower rate (3 m/kyr) for the remainder of shelf flooding. Between 6.3 and 5.6 ka, the area

* There is no available sea-level curve solely for the Bahamas.

816 L. L. Dominguez, H . T. Mullins and A . C . Hine

- - - NAUTICAL MILES

FLOODING HISTORY - - K i t O M F T F R I - - -

Fig. 15. Map perspective of Holocene flooding history on a structure contour (Fig. 5) base for the Cat Island shelf. Shaded areas correspond to times on sea-level curve in Fig. 14. Note topographic controls of flooding and that northern area was flooded prior to the southern portion of study area; ka = thousands of years.

NORTHERN SHELF

41 5 6 4 8

6 3

SOUTHERN SHELF -

I k r n -30

w

I km

Fig. 16. Cross-sectional perspective of the Holocene flooding history for the northern (top) and southern (bottom) Cat Island shelf. Note earlier flooding of the northern shelf by comparing time lines, particularly the 6.3 ka line.

flooded increased to about 73% and by 4.8 ka about 96% of the Cat Island shelf had been flooded (Figs 15 & 16).

platform is presently incipiently drowned. Analogous areas in the Bahamas, such as Cay Sal Bank (Hine & Steinmetz, 1984) and western Little Bahama Bank (Wilber, 1981) that are also partially drowned, were

Implications for drowned carbonate platforms rapidly flooded by early Holocene sea-level (Table 2). Combined, these data indicate that incipient drown-

In comparison with most other Bahama Banks (Table 2), the Cat Island platform was flooded earlier and at a higher rate by rising Holocene sea-level. This we believe is the primary reason why the Cat Island

ingoflarge carbonate platforms, such as the Bahamas, need not occur simultaneously over widespread areas as commonly assumed (Kendall & Schlager, 1981; Bova & Read, 1987). Rather, different portions of

Incipiently drowned carbonate platform

Table 2. Bahama Banks Holocene flooding rates.

817

Area Initial flooding Rate* Source

Cay Sal Bank Northern GBB Northeastern GBB Eastern GBB Western LBB -outer margin -platform interior Eleuthera Bank

Cat Island Platform -outer shelf -inner shelf

8-12 ka 4-8 ka

5 ka 7-8 ka

10-6 ka 6-3.5 ka 5-6 ka

8.1-7.1 ka 7 .14 .8 ka

6 m kyr-’ 1.5 m kyr-]

~

-

3.6 m kyr-l 2.8 m kyr-’

-

4 m kyr- 3 m kyr-

Hine & Steinmetz (1 984) Hine & Steinmetz (1984) Harris (1979) Taft et al. (1968), Palmer (1979)

Wilber (1981)

Martin & Ginsburg (1965); Taft, e t a / . (1968); Dravis (1977)

this study

* Based on Bermuda Sea Level Curve (Neumann, 1971). GBB=Great Bahama Bank; LBB=Little Bahama Bank

platforms can respond independently to changes in sea-level, controlled by relative size and initial elevation which may vary tectonically. In support of this, shallow subsurface stratigraphic study of small isolated carbonate platforms in the southeastern Bahamas (Pierson, 1982) has shown that these isolated banks have responded as independent structural/ tectonic blocks that affected relative sea-level change and sedimentation during the Plio-Pleistocene.

Although the Cat Island shelf has had a relatively rapid flooding history, evidence of marginal reef development on the pre-Holocene rock surface in seismic profiles indicates that shallow-water depositional environments were able to initially ‘start-up’. It appears that the deceleration of sea-level rise about 7.1 ka and differential elevation allowed depositional environments to become established earlier along the northern shelf (Fig. 16). Soon after this, though, of iank sediment transport was initiated and the reefal rim was inundated with sediment as the margin became sand-dominated. As sea-level continued to rise and further submerge the rim, the area of the shelf within lorn water depth was greatly reduced and of iank transport became insufficient to maintain up- building of the margin due to increased water depth and decreased sediment production. The growth potential of the rim was thus significantly reduced and the northern shelf entered into its incipiently drowned state.

The southern shelf area of the Cat Island platform appears to have escaped early drowning as a result of its higher initial elevation (Fig. 16), slower flooding rate ( N 3 m/kyr) and more protected environment. Once established, the reef rim was able to sustain growth because of the lack of offbank transport from

the unflooded platform interior. However, as sea-level rise continued, marginal reefs of the southern portion of the Cat Island platform were submerged to depths (20-30 m) below the zone of maximum production indicating their inability to ‘keep-up’.

If preserved in the rock record, the Holocene Cat Island shelf sequence would consist of a thin (< 4 m) sheet of packstones/grainstones dominated by degraded cryptocrystalline and aggregate grains with poorly developed reefs. However, the established criteria for recognition of ancient drowned carbonate platforms are shallow-water limestones abruptly ov- erlain by exposure hardgrounds and/or deep-water (pelagic) sediments (Schlager, 1981 ; Kendall & Schla- ger, 198 1). Such a sequence, though, will likely develop only from permanent, long-term drowning; more subtle, shorter-term, incipient drowning events would likely go undetected. Sequences in the rock record that are analogous to the Cat Island shelf, may be useful for interpreting early stages of relatively rapid sea-level rise and incipient drowning.

SUMMARY AND CONCLUSIONS

The Cat Island shelf is a small, leeward margin carbonate bank characterized by a thin, coarse- grained, relict, Holocene sediment cover of mostly cryptocrystalline and aggregate grains. It has open- and protected-leeward settings, as well as a relatively deep-water margin (20-30 m) and shelf interior. There is considerably less production and transport of shallow-water sediment on the Cat Island shelf as compared with the larger and more active


shallow-water platforms of the Little and Great Bahama Banks.

The northern and southern areas of the Cat Island shelf have had different responses to the Holocene transgression as a consequence of initial elevation and different flooding rates. The northern shelf had an initially lower pre-Holocene elevation surface and was flooded earlier, during a more rapid phase of Holocene sea-level rise ; the southern shelf interior was flooded later and at a slower rate. Facies patterns for the entire Cat Island shelf are largely perpendicular to the shelf margin and appear to be controlled by circulation patterns focused by platform margin reentrants.

Because of relatively rapid rates of early sea-level rise, narrow width, small size, and low elevation, the Cat Island shelf has been unable to produce enough sediment to maintain itself within the euphotic zone of maximum carbonate production. Relative to most of the rest of the northern Bahamas, the Cat Island shelf was flooded earlier and at greater rates of Holocene sea-level rise which have led to selective incipient drowning, implying that drowning of carbonate platforms need not be synchronous or widespread as commonly assumed. In the rock record, this incipient drowning event would likely be preserved in the form of a thin sand sheet dominated by cryptocrystalline and aggregate grains with poorly developed marginal sand bodies and reefs. If relative sea-level continues to rise, the Cat Island shelf may well become a permanently drowned carbonate platform.

ACKNOWLEDGMENTS

Shiptime aboard the R/V Bellows, operated by the Florida Institute of Oceanography, was provided for by an ARC0 Foundation faculty grant for Mullins. Laboratory work was supported by grants from the Syracuse University Senate Research Council and the American Association of Petroleum Geologists to Dominguez. We thank Anne Gardulski, Ed Hinchey, Dave Mearns, Phil Masters, Bronya Oldfield, Pam Hallock and the crew of the R/V Bellows for field assistance. The Johnson Space Center in Houston kindly provided space shuttle photographs, and radiocarbon dates were determined at the University of Texas. We also thank Ed Hinchey and John Dickey for help with scanning electron and petrographic microscopy, respectively. Sedimentology reviews by Gregor Eberli and Noel James are greatly appreciated.

REFERENCES

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(Manuscript received 27 October 1987: revision received 29 January 1988)

Cat Island platform, Bahamas: an incipiently drowned Holocene carbonate shelf

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