Co‐existence of skeletal and ooid shoals as a result of antecedent topography—Cat cay shoal complex, Bahamas

High‐resolution seismic data reveal an unexpected Pleistocene topography underneath the Cat Cay shoal complex along the western margin of Great Bahama Bank, illustrating how Pleistocene topography focuses tidal flow to create different types of grainstone shoals. The 1–3 km wide and 35 km long shoal complex is composed of the Cat Cay ooid shoal that is a laterally continuous 8 m thick ooid shoal and a sequence of 300–600 m wide and less than 6 m thick skeletal‐dominated tidal deltas south of Ocean Cay. The skeletal tidal deltas overlie an irregular Pleistocene surface, while the Cat Cay ooid shoal is situated on a flat Pleistocene surface east of a Pleistocene rock ridge. This finding challenges the assumption that an antecedent high is needed for ooid shoal initiation. The base of the Cat Cay ooid shoal is an up to 4 m thick skeletal‐peloidal unit that is similar in composition to the skeletal tidal deltas south of Ocean Cay but their deposition was followed by an up to 4 m thick accumulation of ooids. The Pleistocene ridge west of the Cat Cay ooid shoal allowed accumulation of mud and peloids (the nucleus source), while to the south, muddy sediment was winnowed away and no ooids formed. The evolution of the two shoal types is ultimately the result of the presence and absence of antecedent topography adjacent to the shoal system, resulting in variations of mud accumulations and the formation of the nucleus in the ooid shoal. The coeval occurrence of ooid and skeletal shoals in the same complex implies that in the rock record, a vertical succession from oolitic to skeletal shoals does not indicate an environmental change such as climate or an anoxic event but rather a change in flow conditions created by antecedent topography.

. One result of these studies was the suggestion that rock ridges were a prerequisite for the formation of ooid shoals. Many interpretations speculated that these rock ridges would be present beneath ooid shoals on the Great Bahama Bank driving enhanced bottom agitation and providing ideal conditions for ooid formation and accumulation (Ball, 1967;Harris, 1977;Newell et al., 1960;Purdy, 1961). Purdy (1961) first postulated the importance of bedrock topography and established the hypothesis that antecedent topography is a prerequisite for ooid accumulation. Subsequent studies using sub-bottom profiling in the Bahamas investigated the variable role of antecedent topography for sediment distribution along the windward and leeward margins of Little Bahama Bank and north-western Great Bahama Bank (Hine, 1977;Hine & Mullins, 1983;Hine & Neumann, 1977;Hine, Wilber, Bane, Neumann, & Lorenson, 1981;Wilber, Milliman, & Halley, 1990). These studies revealed the complexity of antecedent topography and the wide spectrum of reefs and sediment bodies covering its rocky surfaces. In particular, they documented the role of antecedent topography in driving initial reef growth or creating barriers to energy and sediment transport on or off the banks. Hine (1977), however, showed that the Lily Bank ooid sand belt at the northern margin of Little Bahama Bank was not linked to an underlying bedrock high. He pointed out that the relationship between sea-level rise, tidal flow regime, and antecedent topography and the evolution, distribution, and morphology of sand bodies is poorly known. More recent studies dealing with the morphological configuration of shoal complexes in the Bahamas document a close link between spatial patterns of tidal movements and granulometric parameters such as sorting and percentage of mud. These patterns are not random but vary systematically, following a hydro-geomorphological control that is determined by the antecedent and sedimentological topography (Rankey & Reeder, 2012;Rankey, Riegl, & Steffen, 2006). This control is most important for ooid sand bodies that occur close to bank embayments or as tidal deltas between Pleistocene highs where the tidal energy is focussed (Harris, 2010;Harris, Diaz, & Eberli, 2019;Rankey et al., 2006;Reeder & Rankey, 2009).
This study revisits the Cat Cay shoal complex on the leeward margin of Great Bahama Bank where inlets between Pleistocene islands focus the tidal wave to produce a narrow, elongated shoal complex with channels and tidal deltas ( Figure 1). Purdy (1961), using unpublished core information, sketched a profile across the Cat Cay ooid shoal depicting a Pleistocene rock ridge beneath the unconsolidated Holocene shoal. Here the analysis of the Pleistocene topography is extended south of Ocean Cay where a series of skeletal-rich tidal deltas form a discontinuous grainstone belt, hereafter called the Ocean Cay tidal deltas. The morphology of the antecedent topography represented by the Holocene-Pleistocene unconformity beneath the Cat Cay ooid shoal and the Ocean Cay tidal deltas is imaged by high-resolution seismic profiles. The paper assesses how the morphology of this surface influenced shoal geometry, thickness and evolution of ooid shoals of different morphology and composition along this margin. A particular motivation for this study is to explain the morphological and sedimentological differences and the implications between the northern Cat Cay ooid shoal and the skeletal Ocean Cay tidal deltas south of the ooid shoal ( Figure 1). This co-existence has implications for interpreting the cause of the successive oolitic and skeletal shoals in the rock record.
Recently the change from skeletal to ooid formation has been considered an indicator of an environmental crisis caused by anoxic events based on the fact that microbial carbonates are indicators of environmental change (Whalen, Day, Eberli, & Homewood, 2002). The ooids in the Cat Cay ooid shoal harbour, like all ooids on Great Bahama Bank, a highly diverse microbial community with capabilities of carbonate precipitation (Diaz, Piggot, Eberli, & Klaus, 2013;Diaz et al., 2014Diaz et al., , 2015Diaz, Eberli, Blackwelder, Phillips, & Swart, 2017;Edgcomb et al., 2013;O'Reilly et al., 2017). Microbial activities in tandem with the physicochemical conditions of the extracellular polymeric substances micro-domains control the organomineralization processes in ooids through two distinct avenues: biologically induced and biologically influenced mechanisms Harris et al., 2019). Although the contribution of microbial activities in the genesis of ooids is well documented and thus they could be considered microbial carbonates. Ooids, however, also occur adjacent to coral reefs. In the Cat Cay ooid shoal complex ooid shoals co-exist laterally to skeletal shoals. Thus, a case is made that the variable composition is not necessarily related to environmental change but rather to variable hydrodynamics due to different antecedent topography.

OCEANOGRAPHIC SETTING
The Cat Cay shoal complex is the only shoal located along the western leeward margin of Great Bahama Bank. The margin-parallel shoal complex is approximately 5-6 km inside the platform edge that borders the Straits of Florida ( Figure 1). It forms north-east of a slight indentation in the bank margin, which is a remnant of the platform configuration that existed during the Cretaceous but was modified throughout the Cenozoic (Eberli & Ginsburg, 1987. In the Late Cretaceous, the north-western Great Bahama Bank consisted of two banks, Andros Bank in the east and Bimini Bank in the west. The two banks were separated by the Strait of Andros. The Bimini Bank itself consisted of two elements that were divided by the Bimini embayment. Between the embayment and the Straits of Andros in the east existed a 25 km wide south-north trending platform. To the west of the embayment, the Bimini Bank was shorter, producing a corner in the platform. This ancient morphology is still visible as a slight indentation in the otherwise convex outward western Great Bahama Bank, although prograding Late Miocene to the early Pleistocene sequences filled the Bimini Embayment and extended the margin of Great Bahama Bank up to 25 km into the Straits of Florida (Eberli & Ginsburg, 1987. The Bimini Islands, Cat Cay and the Cat Cay shoal complex to the south are situated on top of this margin. The adjacent Straits of Florida is filled by the Florida Current that transports 31.0 ± 4.0 Sv of water northward (Beal et al., 2008). Frictional forces prevent the Florida Current from invading the shallow bank and as a result only tidal currents and wind-driven currents are measured on the platform in the area of the Cat Cay shoal complex (Cruz, 2008). Within the platform, the energy level at any given place is determined by tides and wind-driven currents and their interaction with antecedent topography. Boss (1994) mapped the Holocene-Pleistocene unconformity and the Holocene depositional geometry across northern Great Bahama Bank and concluded that the complex F I G U R E 1 Landsat image illustrating the morphologies of the Holocene shoal complex around Ocean Cay, including the Cat Cay ooid shoal in the north and the Ocean Cay tidal deltas in the south. (Inset) MODIS image shows the location of the study area on the western margin of Great Bahama Bank Holocene sediment accumulation pattern was controlled in part by the distribution of antecedent topography. For the same reason Holocene sediment thickness and accommodation created largely by sea-level rise are uncorrelated (Boss & Rasmussen, 1995).

| DATA AND METHODS
The data set for this study consists of approximately 120 line-km of high-resolution sub-bottom profile seismic data from 18 transects along and across depositional strike. Data were acquired using an X-STAR Full Spectrum TM Digital Sub-Bottom Profiler that transmits an FM pulse of 20 ms length with a linear sweep over a bandwidth from 2 to 10 kHz.
A suite of cores spaced 20-150 m apart around Ocean Cay reveals the Pleistocene section underneath the Holocene shoal complex and is tied to the seismic lines (Cruz, Eberli, & Byrnes, 2006). The depth of the top-Pleistocene surface observed in the cores is compared with a flat-lying seismic reflection in the lines close (<100 m) to the core locations and is used to establish the depth for this seismic horizon ( Figure 2). The interpretation of the top Pleistocene and sea bottom surfaces on each sub-bottom profile result in xyz files for these two horizons. PETROBRAS's in-house software with adjusted grid parameters was used to create surfaces for these two horizons along the survey area. Gridding is limited to the end points of each sub-bottom profile and constrained by the contours of rock islands (zero depth) to produce reliable surfaces and avoid excessive extrapolation in areas not covered by a seismic line. The topography of the subsurface Pleistocene bedrock and the thickness of Holocene deposits are mapped away from these 'known' data points across the seismic survey area, covering an area much larger than previous maps that were based on a limited number of shallow cores (Ball, 1967;Purdy, 1961).
Underwater observations provide ground truth for features imaged on the seafloor in the seismic data. Petrographic analysis (Tables 1 and 2) of bottom sediment samples assesses grain type variability within the shoal   Sorting (Friedman, 1962)   Sorting (Friedman,1962) Fietzke, Liebetrau, Eisenhauer, and Dullo (2005) was followed to provide the average age of surface sediments collected from bars with mobile sand waves within the Holocene shoal complex and from the adjacent areas (Table 3; Figures 3 and 4).

| THE HOLOCENE SHOAL COMPLEX
The Holocene shoal complex south of Cat Cay is 1-3 km wide and 35 km long. It includes the continuous Cat Cay ooid shoal, north of Ocean Cay, and a series of tidal deltas south of Ocean Cay ( Figure 1). North of Ocean Cay, the active Cat Cay ooid shoal is a continuous sand belt composed mostly of moderately well sorted, medium-sand sized (mean = 363 µm; one SD = 44 µm) ooids (47%), peloids (53%), and scattered skeletal grains. West of the active shoal, micritized skeletal sand with scattered ooids and peloids form either a layer with inactive ripples or a veneer of sediment a few centimetres thick overlying bedrock ( Figure 3). On its eastern flank, the active shoal has a sharp boundary from the bioturbated, fine to very fine and muddy peloidal sand stabilized by seagrass. In some parts, a final steep slip-face of the active shoal abuts against the seagrass-covered peloidal sands of the platform interior.
The U/Th age of ooid-peloid sand from the active part of the Cat Cay ooid shoal (samples 1-3) ranges from 2.7 to 3.2 ± 0.05 ky bp (Figure 1). Micritized skeletal grains mixed with ooids and peloids from stabilized adjacent areas seaward of the active shoal have an age of 4.2 ± 0.06 ky bp (sample 4), and the primarily peloidal sediments east of the active shoal (sample 5) are 5.8 ± 0.08 ky bp old. Based on these bulk ages (Table 3), it is likely that surrounding sediments are older than the ooids in the shoal crest.
The Ocean Cay tidal deltas south of Ocean Cay are approximately 3 km east of the platform margin, situated on the bankward side of a chain of rocky islands ( Figure 1). These small rocky islands are less than 1 km long, consist primarily of Pleistocene aeolianites, and extend approximately 18.5 km southward from Ocean Cay. Gaps between these islands form inlets that are 2-2.5 km wide and 8-10 m deep. These inlets are floored primarily with Pleistocene bedrock covered with sponges, hard and soft corals, or a veneer of skeletal sediments ( Figure 4). The tidal deltas develop platformward of the inlets and include elongated (2-3 km) and narrow (300-600 m) bars with orientations ranging from perpendicular to parallel to the shelf margin. These bars consist of moderately sorted, medium-sand sized (mean = 355 µm; one SD = 161 µm) skeletal and peloidal grains. The tidal delta bars terminate sharply against the sediment veneer of the inlet and against grass-covered bioturbated peloidal sediments of the platform  Table 3 for ages ( Figure 4). The composition of the tidal deltas is predominantly skeletal with a very small percentage of ooids (<15%). Oolitic bars are found only within the tidal delta nearest to Ocean Cay. Skeletal-rich sand and peloids from the Ocean Cay tidal deltas (samples 6-8) and adjacent stabilized areas (sample 9) range in age from 5.9 to 8.3 ± 0.10 ky bp ( Figure  1) and may be older than the sediments from the Cat Cay ooid shoal.

TOP-PLEISTOCENE SURFACE
Remote sensing images, sub-bottom profiles and direct underwater observation document the presence of a submerged Pleistocene rock ridge west of the Cat Cay ooid shoal (Figures 3, 5 and 6). This linear, subtidal ridge can be traced towards Ocean Cay where Pleistocene rocks, consisting of beach deposits and aeolianites, are exposed on the western side of the island. No sedimentary structures can be observed within the submerged Pleistocene rocks because they are covered by a thin veneer of sediment. The rock ridge west of the active part of the shoal is 1 km wide and at least 8 km long. It has about 4 m relief and is onlapped by Holocene strata (Figures 3 and 5). East of the rock ridge, the top Pleistocene forms a generally flat surface underneath the entire sand belt at a depth of ~7 to 8 m below present sea level (Figures 5 and 7).
Gridded maps of depth based on the sub-bottom profiles display the topography of the top-Pleistocene surface and thickness of Holocene sediments around Ocean Cay ( Figure  7). These maps illustrate contrasting characteristics between the areas north and south of Ocean Cay. The top-Pleistocene surface south of Ocean Cay consists of alternating high and F I G U R E 5 (A) Series of seismic lines displaying the Pleistocene top surface across and along the long axis of the Cat Cay ooid shoal. The top Pleistocene is mostly a planar surface except for the Pleistocene high forming a rock ridge towards the south-west (Lines B and D). This rock ridge extends ~8 km in a NNW-SSE-trending direction on the west side of the shoal and can also be observed in the remote sensing image (B). Lines H and J display the Holocene deposits (up to 8 m thick) that are incised by tidal channels. The edge of the ooid shoal on its west side is represented by a poorly defined horizon (Holocene discontinuity) shown in lines B and D. This horizon is interpreted to represent the transition between the skeletal sediments from stabilized areas westward and the oolitic-peloidal shoal low segments under the tidal deltas, indicating an undulating top-Pleistocene surface across this area (Figure 7). This undulating and irregular top-Pleistocene surface is at 6-7 m below present sea level and can be observed in Figure 7B where lighter blue areas occur and expand eastward of the islands (shown in black). The Pleistocene surface is thus, at least 1 m shallower than underneath the Cat Cay ooid shoal to the north ( Figure 7B).

| THE HOLOCENE SHOALS
The Cat Cay ooid shoal overlies the flat top-Pleistocene surface, forming a gently convex-up accumulation of Holocene sediments between Ocean Cay and Cat Cay (Figure 3). Along its southern half, the Cat Cay ooid shoal is ornamented with shallow (<2 m deep), linear to sinusoidal bars that vary in shape from symmetrical to asymmetrical parabolic bars (sensu Rankey et al., 2006) with few linear segments (Figure 8). The bars trend at angles to the long axis of the sand belt and alternate with generally deeper (but <3 m deep) stabilized areas. These bars are reminiscent of transverse bars (sensu Rankey & Reeder, 2012). The bar crests (<1 m deep at low tide) are approximately 200-300 m apart, separated by about 2 m deep-troughs. Most bars are adorned with sand waves (sensu Allen, 1980) with amplitudes of 0.4-1.2 m and wave lengths between 30 and 50 m. Sand waves are commonly asymmetrical and exhibit steep slipfaces oriented in the direction of the dominant tide ( Figures  8A through 9). Most sand waves are flood dominated but in some areas the flow is in the ebb direction ( Figure 8D). Ripples occurring on the sand waves and within the troughs are constantly changing their orientation, shape (from linear to undulatory crests) and size according to the 6-hr tidal cycle as flood or ebb-oriented flows wash across the sand belt. Some bar crests are exposed during spring low tides. The ripple wavelengths are between 10 and 60 cm and the amplitudes range between 4 and 15 cm. In the north half of the Cat Cay ooid shoal, the bars change to 'sigmoidal' flowparallel bars, broadly similar to the tidal sand ridges described by Dyer and Huntley (1999). The distance between the bars crests increases and can exceed 500 m. The flanks F I G U R E 6 (A) Seismic lines illustrating the contrasting Pleistocene topography below the Holocene shoal complex around Ocean Cay (B). Lines I, H and J show the Cat Cay ooid shoal (up to 8 m thick) above a mostly flat surface (except close to the rock ridge). In contrast, Line K across the Ocean Cay tidal deltas reveals an irregular Pleistocene topography that often forms the sea bottom and exhibits inlets. In Line L, the Pleistocene surface is overlain by thin (<6 m), and laterally discontinuous, Ocean Cay tidal deltas get steeper when juxtapositioning deeper (up to 3 m) tidal channels (Figures 3, 5 and 6). Some of these flow-parallel bars project bankward into ~2-3 m deep areas covered with seagrass. These elongated sand bodies or small noses were called 'spillover lobes' by Ball (1967) (Figure 8B,C). The north end of the sand belt is characterized by more abundant stabilized areas, usually covered by dense seagrass and cut by equally elongated tidal channels ( Figure 8A). In this part of the shoal, bathymetric changes are abrupt. Shallow-water areas (<1 m deep) with dense seagrass cover or ornamented with flow-parallel to linear bars pass sharply to deep (up to 5 m) channels across a 2-3 km transect.
Southwest of one tidal channel, a lobate form is imaged in sub-bottom profiles that shows asymmetrical sand waves at the surface with steep flanks towards the southwest (i.e. seaward) (Figure 9). A transparent interval (~1 m thick) underlies these foresets down to the bedrock. The foresets are at least 2 m high with cross-bed sets pointing west (Figure 9).
The Ocean Cay tidal deltas south of Ocean Cay are composed of a series of discontinuous, curvilinear and sigmoidal bars about 2-3 km long and 0.3-0.6 km wide, oriented at various angles to the shelf margin (Figures 4  and 6). The bars alternate with stabilized areas, covered by seagrass and inlets (up to 8-10 m deep) floored by rock (Figure 4). The bars have a strong asymmetry with a steeper flank facing the inlets and a gentle flank facing stabilized areas bankward or between the tidal deltas. Bars are ornamented by low-amplitude (<70 cm) sand waves with a crest-to-crest distance between of 60 and 150 m and current ripples with wavelengths between 10 and 30 cm and amplitudes up to 15 cm ( Figure 10). Amplitudes of these bedforms are generally smaller than those from the Cat Cay ooid shoal. The thickness of the Holocene deposits ranges from 0.1 to 6 m (Figures 4, 6 and 7). Most bar crests of the Ocean Cay tidal deltas are in more than 3 m water depth. These bedforms change orientation with each flow reversal as sediment is remobilized in each flood and ebb tide as observed in other tidal systems in the Bahamas (Gonzalez & Eberli, 1997). Many parts of the tidal deltas consist of stabilized areas on which inactive bedforms (in most cases, relict wide-spaced sand waves and linear to sinuous ripples) are commonly covered with a variable amount of seagrass and green algae. No mud is deposited in the inactive portion of the tidal deltas, indicating that fine-grained mud is being winnowed away by the tidal currents.

OF THE CAT CAY SHOAL COMPLEX
Pre-existing topography created by Pleistocene bedrock morphology determines the location of shoals and markedly influences their sedimentology and hydrodynamics (Harris et al., 2019;Rankey & Reeder, 2012). Various investigators of Holocene shoals in the Bahamas proposed that ooid shoals develop over an uneven topography and the presence of a limestone ridge is a prerequisite for shoal initiation (Harris, 1979;Purdy, 1961). Based on Purdy's (1961) interpretation, the working hypothesis before the seismic acquisition was that the Cat Cay ooid shoal was anchored on a shallow Pleistocene ridge (Figure11A); whereas, the skeletal Ocean Cay tidal deltas were situated on deeper Pleistocene bedrock. The seismic data, however, do not show a Pleistocene ridge underneath the presentday shoal. Instead, the Cat Cay ooid shoal is situated east of a rock ridge high, and above a flat Pleistocene surface (Figures 3, 5 and 11B). In addition, the new data document a 1-2 m shallower top-Pleistocene surface in the south than the north. In the south, below the Ocean Cay tidal deltas, the top Pleistocene is an irregular surface with high and low areas associated with inlets between Pleistocene islands west of the tidal deltas. North of Ocean Cay, a rock ridge that forms an elongated (~8 km) high is situated west of a flat top-Pleistocene surface. These two contrasting morphological characteristics of the top-Pleistocene surface underneath the shoal complex around Ocean Cay influence shoal geometry, sediment type and distribution, and thickness of Holocene deposits. The islands and inlets focus the flow across the bank and create the tidal deltas south of Ocean Cay. The focussing of the tidal currents from antecedent topography or sedimentary topography on the shoal itself has a major control on bedforms, shoal alignment and the size of the entire shoal complex in the Bahamas. The morphology of the shoal changes the tidal current strength locally and produces variations of sedimentary structures and bedforms on the shoal (Rankey et al., 2006). For example, variations of current velocity on a migrating tidal bar relate to the morphology of ripples (Gonzalez & Eberli, 1997). Likewise, various bedforms develop in response to the strength and direction of the tidal currents on the shoal (Rankey & Reeder, 2012). But the current focus is most influential for the formation of tidal deltas. Openings F I G U R E 8 Satellite images of the Cat Cay ooid shoal displaying the details of the shoal morphology and the ornamentation with sand waves.
(A) Tidal channels cutting through stabilized areas with seagrass cover (brown areas) and transverse shoulder bars between the channels. (B) A patchwork of small sigmoidal and longitudinal tidal sand ridges, transverse shoulder bars and troughs. The 'hooks' at the platform side of channels are flood tidal deltas that were called spill-over lobes by Ball (1967). Sand waves are oriented perpendicular to the sigmoidal bars. (C) Sigmoidal longitudinal tidal sand ridges and channels alternating with transverse shoulder bars with asymmetric and sinusoidal sand waves. (D) Ebb tide dominated portion of the shoal with transverse shoulder bars ornamented with sand waves pointing in an off-bank direction between Pleistocene islands along the platform margin restrict and concentrate tidal currents and promote the development of tidal deltas (Ball, 1967;Harris, 2010;Rankey et al., 2006;Reeder & Rankey, 2008, 2009). In the Abocos and in the Exumas, the size of ooid tidal deltas is related to the opening of the channel between the islands (Harris, 2010;Reeder & Rankey, 2009). In both locations flood and ebb tidal deltas are present. In contrast, the Ocean Cay tidal deltas to the south are mainly composed of skeletal particles and only the flood tidal delta developed ( Figure 10). The ebb flow is recorded only in the orientation of sand waves on top of transverse bars. Most of the data in previous studies of shoal morphology originated from satellite imagery and ground truthing with surface sediments. The incorporation of sub-bottom profiles in this study provides new insights into internal geometries of this shoal, in particular the 2 m high cross-bed sets within the shoal. These foresets dip towards the west (Figure 9). Based on examination of a suite of oriented cores that penetrated less than 4 m, Ball (1967) identified a basal set of large or medium scale cross-beds with the dip direction towards the platform interior to the east. Furthermore, Ball (1967) proposed an evolution of the shoal that began with a layer of skeletal-oolitic and burrowed muddy-peloidal sediments on top of the rock floor that were subsequently overlain by an oolitic sand belt. He estimated the thickness to be approximately 4 m but the seismic data reveal the top-Pleistocene surface about 8 m below present sea level. Some sub-bottom data display, however, a horizon that forms a discontinuity surface within the Holocene succession that might represent the transition between the bottom skeletal-rich sediments and the overlying oolitic deposits ( Figures 5 and 11). Unfortunately, the location of the cores is not given by Ball (1967) and a direct comparison of datasets is not possible.
Based on the observed Pleistocene topography, internal geometries of the shoal and the age of the surface sediment, the following evolution is envisioned (Figure 12). During initial stages of bank-flooding and increasing carbonate production, skeletal-peloidal sediments accumulated east of the continuous rock ridge of Pleistocene topography along the bank margin north of Ocean Cay ( Figure 12A). Because the off-bank transport was inhibited across this part of the leeward margin, sediment including mud was partially trapped east of the ridge. Trapped mud was reworked into peloids to form the nucleus of the ooids that accumulate in the nascent shoal ( Figure 12B). The modern-day morphology and sediment distribution ( Figure 12C) support this interpretation. At present, north of Ocean Cay, micritized skeletal grains and some ooids and peloids occur west of the ooid shoal; whereas, to the east, grains are predominantly peloids. Based on the U/ Th ages of bulk samples, the shoal has the youngest age and sediments on either side are 'older' or at least have older material admixed. Thus, it is reasonable to assume that skeletal grains and mostly peloids subsequently acted as nuclei for the ooids. It is possible that the discontinuity in the Holocene deposits shown in the seismic lines ( Figure 5) may represent this boundary between 'older' mixed skeletal-peloidal grains and the ooid shoal, supporting the interpretation that these mixed sediments accumulated before the younger ooid-peloidal shoal. South of Ocean Cay, the Pleistocene topography is different. Here the top-Pleistocene surface is irregular with a series of islands formed by aeolianites along the margin and depressions in between (Figures 6 and 7). The modern inlets probably are depressions in the Pleistocene topography that existed between the aeolianites along the margin. During the early stages of bank-flooding, these inlets probably played an important role in the evolution of the Holocene tidal complex along the bank margin.
Flow across this part of the bank margin was constrained inside these depressions focussing bankward transport of skeletal grains from the bank margin through the inlets to form the tidal deltas. Interpretation of seismic sections coupled with sediment dating suggest that four flood tidal deltas formed during the initial stages of bank flooding over a 6 m shallow and irregular top-Pleistocene surface south of Ocean Cay ( Figure 12A). This surface is covered by narrow and thin sand bodies composed primarily of skeletal grains. The relatively older (>5.9 ky bp) skeletal-rich sediments and the deeper (>3 m) bar crests in these tidal deltas suggest that the bars within the tidal deltas received little new material and progressively stabilized, not forming an ooid shoal over the previously deposited skeletal-rich bars ( Figure 12B). The comparison of the U/Th ages of bulk samples from the Ocean Cay tidal deltas with samples from the area north of Ocean Cay are similar, which can be interpreted that the skeletal-rich sediments in both areas were deposited nearly contemporaneously and before ooid production started on the Cat Cay ooid shoal. However, it is possible that the skeletal sediments in the tidal deltas (>5.9 ky bp) were deposited slightly before the peloidal-skeletal sediments (ranging from 4.2 to 5.8 ky bp) north of Ocean Cay underlying the younger ooid shoal.
Holocene sea-level curves for the Bahamas (Boardman, Neumann, & Rasmussen, 1989;Hine, 1977;Kindler & Bain, 1993) suggest an early stage of rapid sea-level rise from 8 to 3.5 or 4 ky bp, after which the rate of rise decreased. Based on these changes in rates of relative sea-level rise during bank flooding, the U/Th age of the sediments and the contrasting topography of the underlying shoals it would appear that during the initial Holocene sea-level rise, skeletal grains at the bank margin south of Ocean Cay were transported through the inlets by bankward flow, forming the tidal deltas. In contrast, north of Ocean Cay, sediments accumulated behind a continuous rock ridge, which made this part of the margin more protected than the area south of Ocean Cay. Likely, sedimentation east of this ridge filled available space and levelled it to the top of the ridge, resulting in a broad shallow area with skeletal-peloidal sand (Figures 11 and 12).
Around 4 ky bp, the rate of sea-level rise decelerated relative to the early stages of bank-flooding, creating an appropriate water depth and sufficient bottom agitation across this shallow area to encourage widespread ooid formation. Consequently, previously deposited grains acted as nuclei for ooids that deposited capping 'older' sediments and continued to accumulate as sea level rose, forming the elongated and thick ooid shoal north of Ocean Cay (Figures 11 and 12). Because of continuous ooid formation, sedimentation was able to keep up with sea-level rise during continued bank flooding in this area.
The thin skeletal-rich sediment layer and the extensive areas stabilized by seagrasses south of Ocean Cay provide evidence that the rate of sediment accumulation in the tidal deltas was not enough to keep up with rapid rates of relative rise in sea level during the early stages of bank-flooding ( Figures  5 and 7). The reduced sedimentation rate is probably the result of the off-bank transport of mud in this area. Skeletal sediment production itself was not enough to allow bars to expand laterally and vertically resulting in deeper (>3 m), thin (<6 m thick), narrow (0.3-0.6 km wide) and elongated (2-3 km) sand bodies ( Figure 12).
The control the rock ridge exerted on sediment accumulation north of Ocean Cay diminished during the late stage of shoal development as the accommodation space east of this ridge was nearly filled and the sea level is far above the ridge. Consequently, the Pleistocene islands northwest of the ridge and the shoal start to influence fluid flow across this northern part of the sand belt. The gaps between the islands focus the on-and off-bank flow associated with flood and ebb tides that result in channels in the northern portion of the Cat Cay ooid shoal. In the middle portion of the shoal, where Pleistocene islands are not present, no tidal channels formed across the ooid sand belt (Figure 12).

F I G U R E 1 1 Schematic cross-sections illustrating the Pleistocene bedrock and the Holocene sediments across the Cat Cay ooid shoal. (A)
Cross-section from Purdy (1961) showing an idealized Pleistocene ridge below oolite facies and top of the Pleistocene bedrock about 4 m deep. (B) Cross-section proposed in this study shows that the Pleistocene bedrock is deeper (~8 m below present sea level) than previously thought and is a planar surface below the ooid shoal 8 | CONCLUSIONS AND

IMPLICATIONS
The Holocene carbonate shoal complex south of Cat Cay on western Great Bahama Bank shows considerable variability in sediment attributes and shoal geometry as a result of the antecedent topography that influences the sediment distribution, morphology and thickness of the shoal complex. High-resolution seismic data from sub-bottom profiles reveal different antecedent Pleistocene topography under the skeletal part of the shoal and the ooid shoal. South of Ocean Cay, skeletal-rich tidal deltas are located over an irregular Pleistocene surface, while north of Ocean Cay, a laterally continuous ooid sand belt, the Cat Cay ooid shoal, is situated over a 1-2 m deeper and flat topography east of a rock ridge. This continuous Pleistocene ridge created a more protected area along the margin north of Ocean Cay and allowed thicker accumulation of Holocene sediments during an early stage of bank-flooding (~8 to 6 ky bp).
The unexpected relationship between the depth of the top-Pleistocene surface and the water depth of the modern shoal (top-Pleistocene surface ~6 m deep under skeletal tidal deltas at water depths >3 m and deeper [>8 m] Pleistocene surface under less than 2 m deep ooid shoals) questions the notion that ooid shoals start to form on top of shallow antecedent highs. In fact, the shoal systems at Ocean Cay clearly document that shoals do not need to have an antecedent topography as an anchor. Furthermore, they show that irregular antecedent topography promotes the formation of tidal deltas but not necessarily the formation of ooids.
Although the timing and reason for the coeval development of skeletal and ooid shoals in this shoal complex is based on an interpretation of the available data, it is clear that the system developed within the Holocene sea-level rise along the same margin. Thus, hydrodynamic differences rather than environmental factors enable the coexistence of skeletal and ooid shoals. Consequently, caution should be taken when implicating ooid formation as an indicator of environmental change in shallow-water carbonate environments.
The different composition and geometry between the tidal deltas and the adjacent ooid shoal are a result of the contrasting energy that is determined by the variations in the antecedent topography along the bank margin. In areas with initially lower energy, some mud accumulates, which in its pelleted form is the F I G U R E 1 2 Schematic showing the evolution of the Holocene shoal complex around Ocean Cay. (A) Early (between 8.0 and 3.5 ky bp) stage of bank-flooding with formation of flood tidal deltas south of Ocean Cay and a blanket of peloidal-skeletal sand east of a Pleistocene rock ridge. (B) Stabilized areas formed within the tidal deltas south of Ocean Cay; whereas, an ooid sand belt starts to form over a broad shallow area north of Ocean Cay between 3.5 and 1.0 ky bp. (C) Tidal channels and bars alternate in the northern part of the ooid sand belt influenced by the Pleistocene island westward of the shoal during the late stages of evolution of the sand belt source for the ooid nuclei, while in high energy tidal areas the delta mud is exported off the bank. The thereby reduced sedimentation rate renders portions of the shoal inactive as sea level continues to rise. Seagrass stabilizes these submerged areas but no mud cap develops in the inactive parts of the tidal deltas or the ooid shoal. This lack of muddy caps on inactive parts of ooid shoals is a common pattern in the shoals in the Bahamas. Thus, the composition, geometries and sedimentary topography that develops during the active stage of shoal formation is likely to be preserved in the rock record. This depositional topography will also be an important part of the antecedent during the subsequent sea-level cycle.