Abstract The Ambergris Shoal at the southeast end of the Caicos Platform is approximately 25 km long and composed of ooids. Ooids with nuclei of quartz grains were found in one sample. The coatings are up to 0.05 mm thick. Quartz does not occur naturally on the surface in Caicos. Therefore, coatings on quartz nuclei probably formed in the last 500 years when humans may have transported quartz sand to the platform. The quartz-cored ooids have radially-oriented crystals in their cortex, in contrast to tangentially-oriented needles in other ooids on the shoal. An “island” in the Caicos Mid-Platform Shoals is approximately 500 m long by 100 m wide and is now at sea level. Lithified Holocene grainstone beds along the margins of the “island” dip away from the middle of the “island” similar to beach deposits. Those dipping beds were truncated at sea level, and have fibrous cements similar to beachrock and other marine cements. The middle of the island also contains lithified grainstones at and slightly below sea level; however those grainstones have equant calcite cements supporting an interpretation that they precipitated from freshwater. The presence of freshwater cements suggests that eolian dunes covered the entire island in the past. A large storm (hurricane) apparently removed poorly lithified eolian sands above the water table, leaving meteoric-cemented rocks in the middle and marine-cemented rocks around the margins of the island.

Abstract Observations of the Caicos Platform indicate that shallow-marine and eolian carbonate sediments have equilibrated to sedimentological perturbations caused by Hurricane Donna in 1960 over a geologically short period of time (40 years). The north side of the Caicos Platform has several Holocene islands between the Pleistocene islands of North Caicos and Providenciales. In 1960, Hurricane Donna cut a channel ~700 m wide in one of the Holocene islands, Water Cay. For the next 35 years, tidal currents flowed on and off the platform through Donna Cut mixing platform interior sediments with open-marine sediments (coral, coralline algae, and other bioclasts). As a result, deposition of cross-bedded tidal bars composed of bioclasts and ooids immediately began to fill in the accommodation space created by the hurricane. By 1999, beach deposits had migrated laterally and completely filled Donna Cut from both sides of the channel. The depositional system is now apparently at steady state and characterized by meter high eolian dunes behind the beach.

Abstract Shallow core boring transects on West Caicos record the late Pleistocene evolution of the leeward margin of Caicos Platform. Throughout the Pleistocene and Holocene, this leeward margin progressively evolved from an open margin to one blocked by emergent topography, resulting in a dramatic shift in depositional facies. A leeward-margin barrier reef formed behind the protection of emergent oolitic grainstone ridges that persisted through the last two Pleistocene and the Holocene interglacial sea-level high stands. Soil crusts separate depositional sequences and define emergent periods. The youngest Pleistocene interglacial produced a classic shallowing upwards barrier reef that built up to a reef flat about 4 meters above present sea level. Pleistocene and Holocene reef sequences are each capped by oolitic grainstones that were spread across the reefs by longshore currents that also promoted beachspit accretion from ooid sources to the north and south. Ooid smothering of reef sequences occurred during the later part of each sea-level high stand. Older Pleistocene units are increasingly recrystallized and cemented, but the youngest retains most of the depositional mineralogy and has little marine or freshwater cement. Where not protected by preexisting emergent topography, the Pleistocene leeward margin sequence comprises repeated shallowing-upwards, burrowed to layered oolitic grainstone sequences. These grainstones retain much of their depositional mineralogy and are only weakly cemented. Well-cemented soil crusts formed during sea-level low stands, separate these oolitic sequences, and form permeability barriers that are responsible for ponding brines derived from overlying Holocene salinas. These brines promoted dolomitization of Holocene storm-derived grainstones below the salinas.

Abstract Outcrop and marine field work on the NW part of the Caicos platform illustrates Pleistocene-Holocene accretion of the Providenciales and West Caicos Islands, the effects of the Holocene transgression on the flooded substrate, and the differentiation of sedimentary provinces in relation to prevailing current direction and inherited topography. A marine field study of 2D seismic (CHIRP) sub-bottom profile data and surface sediment sampling on the shelf north of Providenciales and North Caicos, and on the platform interior south of Providenciales provide an image of the top-Pleistocene surface, the thickness of Holocene sediments, and the present distribution of facies, biogenic components, and grain sizes. A pronounced difference in the composition and grain sizes of Holocene sediments exists between the open shelf north and the platform interior south of Providenciales, but maximum thicknesses of approximately 2 meters are similar. On the back-reef shelf, peloidal nearshore sands grade seaward to coarse skeletal sands, rocky bottoms and reefs. On the platform interior, the sediments are peloidal and skeletal on or near beaches, coarse skeletal grains and rubble near patch reefs, and peloidal grainstones and mud-lean packstones elsewhere in the subtidal environment. Outcrop mapping and Uranium-Thorium age dating of carbonate material from Providenciales and West Caicos provide constraints on island growth and sequence development during the Pleistocene and Holocene. Providenciales has two discontinuous core ridges of eolian and subtidal deposits with ages ranging between 160 and 302 Ka. Cutting the island core and prograding away from it are shingles (up to 4 parasequences) of marine and eolian sediments deposited during 140-90 Ka representing the transgression and high stand of isotopic stages 5e and c. Comparison of the elevation of Pleistocene marine deposits (> 12 m) and published sea level curves for the area indicates the need for uplift or higher sea levels for the island. Holocene eolian ridges, beaches, and strandplains form a discontinuous outer shingle on the island. West Caicos is cored by two eolian cores with similar age sediments (219-136 Ka), followed by reef and beach growth (120-130 ka) and younger Holocene eolian ridges making up the east and northeast extension of the island.

Abstract The diverse modern carbonate environments and island outcrops on the Caicos Platform create an excellent locale for learning the fundamental principles of carbonate reservoir characterization. This study investigated aspects of early diagenesis at several classic Caicos field stops. The Pleistocene age, shallowing upward parasequence, outcropping as a vertical transition from subtidal reef and shoal to foreshore at Boat Cove, West Caicos, was sampled to describe texture, grain composition and sorting, cement, mineralogy, pore types and visual porosity. Early diagenetic products include the stabilization from aragonite to calcite, cementation and dissolution (fabric and non fabric selective). Pore types are dominantly interparticle with occasional isolated moldic pores and vugs. Measured porosity (11.1 to 33.3%) and permeability (0.8 to 1341 mD) are significantly lower than typical depositional values. Fractures at Boat Cove are dominantly mud filled and are potential baffles to flow. Waters were sampled from several locations including: salinas on West Caicos, Lake Catherine, Mid-Platform Shoals, Central Platform Trough and a variety of sub environments around North Caicos Tidal Flats. The presence of mesohaline to hypersaline brines suggest the potential exists for brine reflux in several Caicos environments. Numerical models of geothermal convection, illustrate subsurface early diagenetic environments and processes which can not be directly observed from the Caicos platform-top. Models predict geothermally driven calcite cementation around the platform margin and dissolution in the platform interior. Results from this study have been used to help understand early diagenesis in reservoir analogs, build a database of diagenetic fluids for reactive transport model studies and have been incorporated into Caicos field schools to enhance instruction of carbonate diagenesis and reservoir quality prediction.

Abstract For the past 30 some years, Caicos Platform has been an important area for studies of Holocene and Pleistocene carbonate successions and a destination for numerous geoscientists interested in learning about modern carbonate sedimentary systems. During the past few years there has been a renewed interest in understanding the geology of the platform, stemming in large part from recognition in the petroleum industry that more refined reservoir models of carbonate systems are needed both in exploration and development. The impetus for this workshop and this volume was a desire to bring together both present and past Caicos Platform workers with those not familiar with the Platform to share knowledge on the Holocene to Pleistocene sedimentology, diagenesis, platform evolution, and the applicability of the platform as an analogue for ancient isolated carbonate platforms. This volume includes a broad array of papers and is divided into five sections Introduction Holocene, Platform-Scale Surficial Sediment and Facies Distribution Holocene Subsurface Studies Holocene Sedimentology, Reservoir and Source-Rock Potential Pre-Holocene Platform Evolution Sedimentology, and Diagenesis In the Introduction section, Lloyd et al. ( How It All Began: The Early Days of the Turks and Caicos Field Seminars ) recount their search for carbonate seminar locations in the 1970s, prompted by logistical and security issues associated with the classic locations of the Florida Keys and Bahamas, and their identification of Caicos Platform as a new seminar destination. They also describe their early work sampling and documenting the major depositional environments. Larger-scale observations on Holocene sediment distribution and

Abstract Pleistocene and Holocene carbonate sediment bodies on Caicos Platform were created by a combination of major storm events and prevailing energy conditions. Major long-period oceanic swell events together with hurricane surges built pronounced coastal beach ridges on exposed north-facing margins and created elongate reef-rubble lobes within back-reef lagoons. Major hurricane surges have defined the location and geometry of Holocene platform-interior sand shoals; an elongate, leeward-margin shallow subtidal sand shoal; fanning, longitudinal sand ridges extending in from leeward margins; and coarse, skeletal gravel layers found towards the base of the platform-interior grainstone sequence. Hurricane events of all scales also have built extensive south-facing tidal flats and defined the depositional texture and fabric of accumulating Callianassa bioturbated, platform-interior grainstones and packstones. Within the framework of pre-existing and storm-generated topography/bathymetry, prevailing brisk, easterly Trade Winds and prevailing oceanic swells provided agitation for growth and expansion of ooid grainstone shoals and beach strand plains on the platform interior and margins, respectively. Prevailing conditions also defined the distribution and productivity of coral, seagrass, and other benthic communities. Although tidal currents provide important water renewal and create local sediment bodies, it is hurricanes, large oceanic swell events, and prevailing wind-wave and swell conditions that have defined sediment body forms, features, and textural components of Caicos Platform. Twenty to forty winter cold fronts pass through Caicos each year followed by brisk northerly winds. Unlike in the northern Bahamas, these have minimal impact on peritidal-flat sedimentation because the flats are found on the leeward-facing, south flanks of islands.

Abstract Because of the influences of strong easterly trade winds, Caicos Platform in the southeast Bahamas provides newer alternative models for shallow-marine carbonate deposition that can be directly applied to the exploration for subsurface carbonate plays. These winds promote widespread development of Holocene shallow-marine, platform-interior oolitic grainstones and isolated patch reefs. Sand bodies assume wind-parallel or wind-perpendicular orientations depending on preexisting topography and water depth; shoreline oolitic grainstones are common. Isolated patch reefs develop up to 40 kilometers inboard of open platform margins, and coalesce in seaward directions. Reefs and oolitic sands coexist because of wind–wave agitation. Trade winds also enhance the maturity of open-ocean, platform margin reefs. Isolated, kilometer-scale reef complexes exist along leeward platform margins when protected from off-bank sediment stress, with back-reef debris converted to oolite by wind-wave agitation. Grainstones are shed off leeward margins of this platform and may be potential reservoirs. Most of these Holocene relationships are reflected in outcropping and cored Late Pleistocene sequences on Caicos Platform, where a leeward margin barrier reef complex thrived coevally with oolitic grainstones. On Caicos Platform, reservoir potential exists over much of the platform, in contrast to classical northern Bahamian models, where the potential is confined to platform margins. Carbonate sedimentation driven by stronger easterly paleotrade winds, influenced by preexisting topography, creates potential for hydrocarbon plays in settings historically considered low-energy and nonprospective. The Caicos models now explain why Paleozoic and Mesozoic reef and/or oolitic reservoirs occur well inboard of platform margins (James Limestone Fairway Field in East Texas; Black Lake Field in Louisiana for example). Observations from Caicos Platform also explain why ancient leeward margin reefs developed and shed substantial quantities of carbonate sand from their margins (Golden Lane/Poza Rica Fields in Mexico).

Abstract The Caicos Platform has proven to be an area of continuing interest to researchers of modern carbonates, an important training venue, and a valuable modern analog for understanding facies patterns of subsurface isolated platforms. We hope to promote this interest by making readily available a set of processed satellite images and an offshore/onshore digital elevation model (DEM), along with examples of how this data can be visualized and used. The clearest satellite images of Caicos Platform from 1972 to 2005 were acquired and processed. Image processing was undertaken to maximize apparent water penetration and delineate submerged features. The primary images are 30-m Landsat TM complemented by older 57-m Landsat MSS, 15-m ASTER, and 0.6 m Quickbird. The processed scenes were georeferenced to a base image in GIS, creating a stack of co-registered images that can be effectively used for change detection. Masks were created for water and land to enable integration of different images and maps. Soundings, together with estimated water depths based on the spectral characteristics of a Landsat TM image from earlier work, were used to create a digital bathymetric map. This offshore depth map was integrated with an onshore DEM derived from NASA Space Shuttle elevation data (SRTM) for the islands. Drainage and possible offshore sediment flow were modeled using the onshore-offshore DEM—this new drainage map can be used for environmental and geological applications. Various satellite images and maps can be draped on the DEM within the GIS to provide perspective views. The transparency of draped layers can be modified, enabling different images and maps to be viewed together. To increase accessibility, improve learning, and promote spatially accurate feedback, the stack of images, color-coded DEM, and geologic maps were exported out of GIS using TerraGo’s GeoPDF for viewing with free Adobe Acrobat. Users of these GeoPDFs can annotate and interpret features, and then export their maps (points, lines, and polygons) as shapefiles for loading into a GIS. In addition, GIS layers were imported into GoogleEarth for global distribution and display as kmz files.

Abstract Modern analogs such as Caicos Platform can play an important role as conceptual facies models for characterization of a hydrocarbon reservoir, and also provide facies attribute information to be used as input in building reservoir models. In an attempt to gather such valuable data, the attributes for key reef-associated facies of the Caicos Platform are assessed from a Landsat image through facies mapping. Some key findings are: reef and reef apron occupy a smaller percentage of large platforms like Caicos than smaller ones, reefs seem to be patchier than aprons, fully aggraded reefs become somewhat wider as their length increases, reef width and apron width are directly related, and probabilities can be set for expected dimensions of reef and apron width.

Abstract Using Landsat 7 spectral data in conjunction with cluster analysis algorithms and field-collected sediment sample data, a surficial sediment texture map is created for the Caicos Platform. Cluster analysis utilizes statistical algorithms to group Landsat picture elements with similar spectral signatures into relatively homogeneous thematic classes. Carbonate sediment textures are subsequently assigned to each thematic class as part of a manual geologic interpretation that is calibrated using sediment data. Results indicate that the Caicos Platform is heavily grain-dominated. Geospatial calculations show that 7% of surficial sediments on Caicos are grainstone, 59% are mud-lean packstone, 18% are packstone, 5% are wackestone, and 1% are reef, with the remaining 10% characterized by exposed Pleistocene islands. Sediment distributions are highly asymmetric with most platform-scale facies variations following major depositional trends. At the 28.5 meter scale, the Landsat-derived facies map for Caicos demonstrates greater than 84% correlation with sediment data. This agreement indicates that the Landsat-derived facies map accurately characterizes the spatial dimensions and distribution of platform-scale depositional features, like grainstone shoals and tidal flats. Local-scale textural heterogeneity within individual depositional regimes is also identified. Surficial sediment distributions on the Caicos Platform are interpreted to be controlled by platform physiography and island orientation relative to the dominant hydrodynamic forces, such as the Antilles Current, easterly Trade Winds, and Atlantic swells. We also demonstrate that the coupling of statistical algorithms, Landsat data, and sediment data offers a powerful quantitative approach for investigating the spatial distribution of surficial sediments on modern carbonate platforms.

Abstract A combination of sediment-probing for thickness, push coring, and vibracoring has resulted in better documentation of the sedimentary facies, vertical successions, and Holocene evolution of northern and western Caicos Platform - adding to its value as an excellent modern analogue for ancient carbonate platforms and associated hydrocarbon reservoirs, especially those with a grain-rich interior. The Holocene history of sedimentation on northern and western Caicos Platform appears to have been dominated by aggradation of a complex arrangement of depositional environments that were greatly influenced by Pleistocene paleotopography (especially the location and topography of Pleistocene aeolian dunes that form the back-bone of the present-day islands) relative to dominant wind directions. Bank-top flooding began at least approximately 4,800 ybp, based on radiocarbon dates of peat and skeletal grainstone immediately overlying Pleistocene bedrock in cores from tidal flats of North Caicos Island. Tidal flat sediments of North, Middle, and East Caicos Islands accumulated in the lee of Pleistocene islands, whereas skeletal sands accumulated in the lee of aggrading reefs on the windward margins of the bank. Ooid sands formed locally within the platform interior and on beaches exposed to the easterly prevailing winds. Platform interior sediments are mostly a mix of pelletal, skeletal, and ooid grain-rich sediments reflecting extensive bioturbation and exposure of the platform to the winnowing effects of prevailing easterly winds. Mud-dominated platform-interior sediments are rare. Salinas locally formed between Pleistocene dunes on the western margin of the platform as marine waters flooded inter-dune lows of West Caicos at least 3,220 ybp, based on radiocarbon dating. Sediments in the East (Great) Salina consist of interbedded skeletal wackestone to grainstone, massive gypsum, and Scytonema mats reflecting alternating periods of marine influence and restriction. Sediments from marine environments are dominantly aragonite (> 80%) with lesser amounts of high- and low-Mg calcite, based on x-ray diffraction analysis, although subtle differences between marine environments are discernable. Gypsum is dominant in salina sediments, with lesser amounts of aragonite, low- and high-Mg calcite, halite, and dolomite. Nonstoichiometric dolomite was found in a few samples from tidal-channel levees on North Caicos and at the base of the salina sediments on West Caicos, but is volumetrically insignificant. Sediment accumulation rates range from 22 cm/1,000 yrs in gypsum-dominated salina sediments to a high of 106 cm/1,000 yrs for skeletal sands deposited behind the Providenciales Island barrier reef.

Abstract Reef-rimmed margins on carbonate platforms are notably heterogeneous. To explore along-strike changes of the north-western margin of Caicos Platform, B.W.I. (near Providenciales), a group of University of Miami and ExxonMobil personnel cooperatively collected field data, including sediment and bottom descriptions as well as Chirp subbottom profile data. The results illustrate the nature of sedimentologic and geomorphic variability along this part of the Caicos platform margin, and these changes are interpreted to reflect the changing nature of energy. The tidal deltas and discontinuous reef on the NW-facing margin suggest this margin is more tidally-influenced. In contrast, the NE-facing margin with a continuous reef and expansive sand apron is more wave-dominated and influenced by swells from the open Atlantic. These results illustrate the nature, scale, and causes of along-strike heterogeneity along one shelf margin, and likely will have ancient analogs.

Abstract Samples of aragonitic oolitic and grapestone sand from agitated shoal, platform and beach environments of Caicos Platform were assessed for grain durability in tumblers. After one week of tumbling with equal weights of 1-2 mm spherical glass beads, 2 to 7 percent of the oolitic sand had abraded to mud size. Additional durability assessments were made by tumbling only the carbonate sands to determine if they are capable of significantly abrading themselves in the absence of siliciclastic material. In samples that are mostly grapestone aggregates, 3-3.5 percent of the sand fraction was reduced to mud in one week. Samples containing 85 percent well-rounded, glossy oolitic grains produced .3-.4 percent mud from their sand fractions in one week. Scanning Electron Microscope analysis showed that grapestone breaks down by abrasion of the aragonitic marine cement between the constituent grains and by abrasion around pre-existing micro-bore structures and grain surface irregularities. The mud produced consists of broken aragonite needles, most less than three microns in length. The size of the mud component produced is extremely fine and may reflect the common milkiness associated with the waters of agitated shoals. This study suggests that in situ growth of ooids and grapestone grainstone sediment bodies is associated with the production of a significant amount of carbonate mud. This source of carbonate mud has been overlooked in both modern and ancient marine settings.

Abstract Ambergris ooid shoal, on the southeastern Caicos platform, is an elongate, asymmetric, 20 km long spit-like feature to the west of Ambergris Cays. The purpose of this study is to describe the sedimentology and morphology of the Ambergris complex, commonly held as the type example of a wind-influenced ooid shoal. The Ambergris shoal crest is bare well-sorted coarse sand-sized ooids, partly exposed at low tide and ornamented by low-amplitude sand waves with variable but systematically changing orientations. Flanking the shoal crest on the north side is a ~2 m deep rubbly shoulder of Holocene hardground and a deeper (~3 m), rippled sandy bottom which deepens to a burrowed bare sandy bottom (>4 m) with scattered reefs. The southern flank passes from the shoal crest to a slightly deeper (2-4 m) sandy bottom with abundant ooids and common clasts before passing to the burrowed sandy facies. The shoal is influenced by tidal and wave energy; the geometry of sand waves suggests it is not simply a wind-generated spit in the lee of the islands, however. Our on-going work continues to evaluate and quantify the roles of waves and currents on the geomorphic character of the system.

Abstract Geologic modeling of zones of early cavernous porosity development in carbonate reservoirs can be especially challenging beyond existing well control. As a result, dolines on Providenciales, Turks and Caicos Islands, were analyzed to evaluate the spatial density and metrics of these early diagenetic features, as well as their relationship to local fracture orientations. Dating of the adjacent host rock material indicates that these diagenetic features are well established in rocks deposited ~122,000 ybp (U/Th). In the dataset of 116 dolines, mean length is 5.16 ± 4.38 m, width = 3.40 ± 2.72 m, and area = 21.27 ± 32.57 m2. Regression analyses of aspect ratios reveal a good, predictable relationship between doline length and width. Approximately 60% of the dolines are less than 10 m2 in area. Numerical modeling of observed dolines using “change of support” spatial statistics establishes a density of 1.13 dolines/acre; a testament to the abundance of small heterogeneities in carbonates that can potentially impact fluid flow. Bivariant plots of area versus exceedance probability indicate three doline populations (0.1 to < 4 m2, 4 m2 to 44 m2, and > 44 m2), and calculations of associated fractal dimensions indicate some degree of scale invariance. Analyses of doline long axis azimuth reveal strong directional controls on doline development within the area of study, thereby establishing an early diagenetic anisotropy template. Early cavernous porosity development preserved in ancient carbonate successions is likely more widespread than would be resolved by traditional seismic methods, and the bulk of fluid flow may be controlled by relatively numerous, small dissolution features.

Abstract Developing Models and Analogs for Isolated Carbonate Platforms-Holocene and Pleistocene Carbonates of Caicos Platform, British West Indies - For the past 30 years, Caicos Platform has been an important area for studies of Holocene and Pleistocene carbonate successions and a destination for numerous geoscientists interested in learning about modern carbonate sedimentary systems. During the past few years there has been a renewed interest in understanding the geology of the platform, stemming in large part from recognition in the petroleum industry that more refined reservoir models of carbonate systems are needed both in exploration and development. The impetus for the workshop and the publication was a desire to bring together both present and past Caicos Platform workers with those not familiar with the Platform to share knowledge on the Holocene and Pleistocene Sedimentology, diagenesis, platform evolution, and the applicability of the platform as an analogue for ancient isolated carbonate platforms. This volume should serve as an intermediate-term documentation of research efforts and a spur for additional studies to better understand controls on sediment distribution, diagenesis, and the evolution of platform growth, furthering the Caicos Platform as an analogue for ancient, isolated, carbonate platforms.